July 2, 2024 • 46 mins
Unlock the secrets of fire investigation and fire science with our expert guest, Rory Hadden from Edinburgh University.
Rory takes us through his captivating career as a reader at the School of Engineering, sharing insights into his cutting-edge research and the renowned fire engineering group at Edinburgh.
Discover the vital mechanisms of heat transfer—conduction, convection, and radiation—that are crucial for interpreting fire scenes and understanding fire dynamics.
Rory uses compelling examples, such as the heating of a sprinkler bulb and compartment fire flashover, to illustrate these concepts.
You'll learn why mastering these mechanisms is essential for effective fire investigation and communication of fire dynamics, making complex scenarios easier to decipher.
You'll hear about the influential figures in the field, like Dougal Drysdale, and gain an understanding of the University's esteemed fire investigation course that promises significant credentials and membership opportunities with the Institute of Fire Engineers.
Grasp the pivotal role of oxygen in fire behavior and the importance of stoichiometric mixtures in combustion processes. Rory explains how varying oxygen concentrations can drastically alter flame behaviors and temperatures, and discusses the implications of flammability limits and ignition in real-world scenarios.
We’ll also explore the multifaceted realm of fire science education, highlighting the need for a solid foundation in chemistry, heat transfer, and fluid mechanics.
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Episode Transcript
Available transcripts are automatically generated. Complete accuracy is not guaranteed.
Speaker 1 (00:03):
Hi, welcome to CSI on Fire, the podcast that takes
you behind the scenes of thefire investigation community.
I'm your host, mike Moulden,and episode after episode, we'll
attempt to excavate the oftendifficult but always fascinating
world of the fire investigator.
Welcome to CSI on Fire, yourfire investigation podcast.
(00:27):
This is episode, believe it ornot, episode 22.
I've got a fantastic guest ontoday.
I've got Rory Hayden fromEdinburgh University.
Rory, welcome to the podcast.
Thanks for coming on.
Cool.
Thanks, mike.
Nice to be here.
It's a pleasure, mate.
It's a pleasure Now.
You and I first met I think itwas 2019 when you taught me on a
fire investigation course.
We'll get into that a littlebit more, but tell us about your
(00:48):
background.
How have you ended up here onthe podcast?
What's your background in fireinvestigation and fire science?
Speaker 2 (00:54):
Sure, so I have the job title of reader at the
School of Engineering at theUniversity of Edinburgh and it's
okay, because I don't know whatthat means either as a job
title.
But basically it means I'm kindof your regular academic who
does teaching of undergraduatestudents, master's students, phd
students, but also research, soworking on research projects,
and in my case that covers awhole range of topics from
(01:16):
fundamental fire science orlooking at ignition and flame
spread and things like that, allthe way through to application
specific things in the builtenvironment and also with
wildfires in the naturalenvironment, so quite a broad
spectrum of activity.
Of course that sits within thelarger fire engineering group at
Edinburgh, so that includescolleagues working on a whole
(01:36):
host of different topics withdifferent applications in the
built environment, in thenatural environment, around fire
science, fire engineering and,of course, fire investigation.
Sure, and I mean Edinburgh, Idon't know fire engineering and,
of course, fire investigation.
Speaker 1 (01:46):
Sure, and I mean Edinburgh.
I don't know I'll tell some ofthe listeners, but Edinburgh is
renowned because of DougalDrysdale and his famous book,
which everyone should have anddoes have in their thing, and I
remember Dougal was on thatcourse.
I went on the fireinvestigation course, which is a
short course for the EdinburghUniversity In 2019, I met you
and Ricky, and Dougal was thereand a few a number of others.
Mike Wisecar, who also had onthe podcast, was there as well.
(02:09):
So, yeah, fantastic course.
Thoroughly recommend it.
The thing about that course aswell also allows you into gives
you associate membership for theIFE as well, the Institute of
Fire Engineers yeah, so the examwe do at the end of that course
you can use with the IFE as atrade-off with their exam.
Speaker 2 (02:25):
I suppose is a way of doing that if you are going for
one of those qualifications ormemberships.
Mike, I don't know the detailsof that, to be honest, but look
on our website and we can tellyou.
Speaker 1 (02:36):
Look on your website, look on the IFE website.
They'll tell you but what I'msaying is basically a
well-established course and it'sgot some serious credentials
behind it, some seriousindividuals behind it.
One of the reasons I got you ontoday, to be honest with you,
rory is that my fire science ispretty lacking.
I'm not a chemist by any shapeor way.
A lot of my colleagues are fireservice CSIs, engineers, et
(02:57):
cetera, and wouldn't necessarilyhave a chemistry background.
So I wanted to sort of gothrough really some sort of
really basic 101, if you like,definition.
So I'll let you expand howeveryou want to.
But what is combustion?
What is fire and what'scombustion?
Speaker 2 (03:11):
Excellent.
Let's start off with what iscombustion.
That's maybe the easiest placeto start.
And a combustion reaction ispretty simply defined as a
reaction between a fuel andoxygen, and that's one.
And when you say combustion, itnormally also implies some kind
of use from that reaction.
So where that combustionreaction or that combustion
process is useful we tend torefer to it as that, and that
(03:33):
would be, for example, in a carengine or if you're heating with
fires.
We'd talk about combustion inthat context.
Fire, on the other hand, isbasically, I think, quite useful
.
The way to think about it is isthat combustion process, but
not being useful, but insteadcausing some kind of damage or
some kind of harm?
And, of course, you can pickholes in all these definitions,
because is a campfire causingany harm?
(03:53):
Well, probably not.
But I think a combustion systemis one which is engineered and
designed to deliver power fromthat combustion reaction,
whereas a fire is something thatis less controlled and is often
more destructive, but it's justnot as controlled as a
combustion process.
But what underpins that?
The science is the same.
In the end you have some fuelreacting with oxygen that is
(04:14):
generating carbon dioxide, water, other products of combustion
and liberating a lot of heat ora lot of thermal energy, great.
Speaker 1 (04:22):
And what's the definition of a flame?
What would you describe as aflame?
Cool, I think, a flame that's areally good question.
Speaker 2 (04:28):
A flame is basically the manifestation of that
combustion process.
What you see as a flame I meanyou have blue flames, right, I
mean you have yellow flameswhat's happening there is
effectively the same thing youhave fuel reacting with air.
Really, the difference that yousee in terms of the color of the
flame is due to how much fuelrelative to the air that you
have the blue flame.
(04:49):
What you're seeing actuallyyour eyes are seeing there is
molecules that are excited sothat they give off energy and
that is apparent to us as bluelight.
If you have a diffusion flame,then you generate a lot more
soot and a lot more solid carbonparticles and when they get hot
they give off yellow-orangelight.
So in both cases what you'reseeing is the flame is light, so
(05:10):
your eyes are perceiving.
You might also feel heat comingfrom that flame, so you'll get
radiation coming from that flame.
But all of that that you seeand that you perceive is that
chemical process of fuelreacting with oxygen, and it's
just the differences in thedetails of how that reaction is
progressing and what that flameactually looks like in the end.
Speaker 1 (05:30):
So a flame is very simply just a manifestation of a
combustion reaction okay, andif we go back to the good old
days, I'm not sure they still doit, but when I went to school
we used to have a bunsen burnerand we used to open up the
bottom and we'd get a nice blueflame and that's what we call
pre-mixed flame, is that right?
So you're mixing the oxygenbefore it gets to that ignition
point.
Am I right in thinking that?
Speaker 2 (05:51):
Yeah, with the Bunsen burner, exactly as you've
described, when you open thehole at the bottom of the barrel
, the air is able to enter thathole.
It's entrained into that holeby themoving flow of fuel gases.
And that means by the time youget to the top of the barrel of
the Bunsen burner you've got amixture of fuel and air that is
very well mixed indeed.
(06:11):
So when it burns it burns withthat really characteristic blue
flame.
So yeah, that's a pre-mixedflame.
The opposite of a pre-mixedflame is when you close that
hole at the bottom of the Bunsenburner, then you have pure fuel
issuing as a jet from the topof the barrel and that then has
to mix with air from thesurroundings and the process of
(06:32):
that is usually dominated bydiffusion, so the diffusion of
oxygen from the air into thatstream of fuel.
So we call the kind ofcharacteristic yellow flame that
results from that a diffusionflame.
And the diffusion flame isnormally of a lower temperature
than a pre-mixed flame and ithas that yellow color again
because it's generally lesscomplete combustion than in the
(06:55):
pre-mixed flame and that givesyou the production of soot and
that is what gives you thatyellow color, as the soot heats
up and infandesses great, let'smove on to.
Speaker 1 (07:04):
Sorry apologies, I'm just going to keep hitting you
with words, but pyrolysis.
Then let's go through.
Pyrolysis.
What's pyrolysis?
Speaker 2 (07:10):
Awesome.
I think pyrolysis is one of themost important phenomena in
fire science to understand.
Because when you think aboutwhat makes fire scientists
different from combustionscientists, if you're dealing
with a combustion process, so ifyou're designing an internal
combustion engine, then the fuelthat you have is already quite
(07:31):
simple the gasoline you put inthe fuel tank.
We know what that is, it'salready a liquid, it's well
defined.
When you inject it into thecylinder of the engine, it
vaporizes and then it oxidizesin the gas phase.
And that is true of any fuel.
And if you're designing acentral heating boiler for use
in the UK, that's going to bepowered by natural gas.
So again, you know exactly whatthe fuel is entering that
system.
(07:51):
You know exactly the chemicalcomposition of that.
You know exactly how muchoxygen you need to supply to
react, to get the mostefficiency out of that system.
Because the fuels arerelatively simple.
They're either in the liquidphase or they're in the gas
phase.
The problem with fires, or atleast the challenge I see with
fires, is we're normally dealingwith solid fuels that are
(08:11):
burning.
I mean, I'm sure everyone'sdone an investigation where a
liquid fuel or a gaseous fuelwas what was important, but many
fires.
If you're dealing with domesticfire, fire on a commercial
premises, a lot of the stuffthat burns is solid, solid fuels
.
The world around us is made ofsolids and pyrolysis is the
process that you turn that solidwhich doesn't burn directly.
You've got to turn that into agas and pyrolysis is that
(08:34):
process.
And it's a process, I argue,that is central to fire science
and to the understanding of firephenomena, because getting your
solid material into the form ofa gaseous fuel is a key step in
any fire process and that iswhat pyrolysis does.
And the word pyrolysis is it'sa very simple word has two parts
(08:55):
pyro, which you might have somefamiliarity with what that
means, but that generally isit's associated with heat or
with energy of some kind, butnormally with heat.
And then lysis is another wordthat comes from the Greek, which
means to cut or to chop.
So pyrolysis or pyrolysis isthe chopping of something by
heat.
But what's that?
(09:15):
Something that's being choppedis the molecules that form the
solids.
So pyrolysis literally meansyou're going to chop the
molecules from being big, longpolymer molecules that are
solids into shorter moleculesthat are gases.
From my perspective, pyrolysisis one of the most fascinating
things to study in fire sciencebecause it is right at the heart
(09:35):
of how you turn whatevermaterial is present in a room or
on a fire scene into the gasesthat can burn, and it needs
relatively large amounts ofenergy for that to happen, and
that's often manifested by hightemperatures.
We're talking certainly on theorder of 300, 400 degrees
Celsius for materials to beginto pyrolyze and then, once you
get that pyrolysis, you cansustain the burning.
(09:57):
So problems as diverse asignition and flame spread, even
smoldering, combustion are alldependent on this pyrolysis
process.
So it can be phenomenallycomplicated and people like
myself will spend their entirecareers maybe studying pyrolysis
.
But I think the main importanceof it is actually looking at
what are some of thesimilarities between different
materials and the pyrolysisbehavior, and that's something I
(10:19):
think that any fireinvestigator should be thinking
about.
When they see a material that'spresent in the back of your
mind, you should be thinkingwell, that's a timber and
probably we're looking at apyrolysis temperature of around
300 degrees Celsius for that.
Could it get that hot in here?
How easy would it be for thatto be the first material ignited
, heating it up to thattemperature?
So for me you've gone kind ofquite quickly to one of the most
(10:42):
important processes for firespread, fire growth, fire
development, which is how do youturn those solid fuels into
gases so they can burn, and thatprocess is pyrolysis.
Speaker 1 (10:51):
There's obviously the three phases of heat transfer,
so can we just go into those andhow those three kind of affect
that pyrolysis, the mechanisms,if you like.
Speaker 2 (11:01):
Cool, yeah, so heat transfer, I think, is for me
again, if you like, cool, yeah.
So heat transfer, I think, isfor me, again, one of the most
important processes when itcomes to interpreting a fire
scene, but also when it justcomes to understanding fire
spread, fire development moregenerally, because a lot of the
time what you see at the end ofa fire might be a result of heat
transfer.
Right, the fire patterns thatyou might explore, some of them
(11:23):
are going to be, of course, dueto ventilation and other explore
.
Some of them are going to be,of course, due to ventilation
and other processes, but many ofthem are going to be due to
heat transfer.
And many of the observationsthat are made in a fire scene
where is there heat damage andwhatnot is all due to the heat
transfer.
Of course, with heat transfer,there's three mechanisms
conduction, convection andradiation, and of these I think
it's useful to separate a littlebit why we're interested in
(11:47):
them.
So conduction is heat transferwithin a solid.
That's usually where we seeconduction in most fire
applications.
So that is, if I'm trying toheat up a fuel so that it
pyrolyzes, what's happening tothe energy as it arrives at that
material is what's important,and the way the energy is
transferred through the materialis going to be by conduction,
(12:08):
and there are lots of equationsthat we can solve for conductive
heat transfer.
But the main application of itin the sense of fire
investigation and fire scienceis the thing that we're heating
up is a solid and therefore tounderstand how it heats up we
need to understand conduction,the other two modes of heat
transfer, radiation andconvection.
Those are usually how we getenergy to the surface of that
(12:29):
material that we're trying toheat up.
Convection is the transfer ofthermal energy by the movement
of a fluid.
Usually in a fire scenario thatfluid is the smoke and the hot
gases produced by the fire, andradiation is the transfer of
thermal energy byelectromagnetic radiation.
So that is, when you hold yourhands up to a campfire, you're
(12:49):
feeling the radiation.
If you somehow hold your handabove a candle, what you feel
there is convection and that'squite a nice little
demonstration.
I think if you hold your handon the side of a candle it
doesn't feel very hot at all,not much radiation from a candle
.
Hold your hand above it and itfeels really hot really quickly,
even just from a small tealight, because of the effects of
convection the way I kind oftry to explain it to students is
(13:12):
conduction is what's happeninginside my solid.
Convection and radiation arehow my solid is being heated up
and for fire applications.
I think that that's usuallyquite a simple way to look at
things.
All these modes of heat transferare actually really well
understood, especially radiation, because that's how we study
the universe around us.
So the people in physics thedepartment is just across the
way from me here they know loadsabout radiation.
(13:34):
It's how we understand theuniverse, it's how we
communicate with radio waves andeverything like that.
So we know a lot aboutradiation.
Convection is harder, the heattransferred by the motion of
fluids.
But we know on what it dependsand probably don't have time to
go in too much detail here.
But thin things, so things thathave got relatively small
dimensions, respond very rapidlyto convective heating.
(13:56):
That's why often you will seeif there's been an explosion.
You'll see scorch marks only onvery fine fuels or sometimes on
that old fattened kind ofbubbly wallpaper, toilet paper,
toilet paper.
Yeah, these thin fuels willrespond well to convective
heating, whereas radiation tendsto be more of a.
(14:17):
You need relatively largesurfaces, relatively flat areas
to respond significantly toradiation, because these three
modes of heat transfer arealways in some kind of
equilibrium.
If you heat something uprelative to the ambient, it
starts to cool down Always.
We're looking at these energybalances when it comes to how
things get hot.
But I think any fireinvestigator needs to be very
clear in their mind about thedifferent modes of heat transfer
(14:37):
and where they become relevantin a fire scene.
And to give a very simpleexample, if I'm not talking for
too long on this the growth of acompartment fire to flashover.
In the beginning of that you'vegot a small fire in a
compartment.
The smoke and the hot gasesrise up to the ceiling and as
they hit the ceiling they spreadout radially.
They form the so-called ceilingjet.
(14:58):
If you have a sprinkler upthere, then the hot gases are
going to flow past thatsprinkler bulb, around that
sprinkler bulb and they're goingto heat up that sprinkler bulb.
And the main mechanism ofheating there is convection the
flow of smoke past the sprinklerbulb.
And the sprinkler bulb is a fewmillimeters in diameter.
It's going to be heated reallyeffectively by convection and
hopefully activate.
(15:19):
It hits the activationtemperature and it will activate
and your fire will go out Ifthe sprinkler doesn't activate
or it's not present, then asthat smoke layer builds up in
the compartment it increases intemperature, it has a relatively
high emissivity, which is tosay a large fraction of the
energy is radiated from the suitin the smoke that will radiate
onto the compartment contentsand that will be one of the main
(15:42):
drivers to flashover isradiation from that smoke layer
onto the materials in thecompartment.
Again, just understanding therelative importance of the modes
of heat transfer for thescenario that you're interested
in I think is quite importantbecause the reality is heat
transfer is always happening,it's always on and it's always
(16:02):
conduction, convection andradiation that we need to think
about.
But there are some scenarioswhere one of those modes of heat
transfer is the most important.
And if you figure out which oneis the most important, then
that makes it much easier tounderstand the problem, makes it
much easier to communicate thatproblem to whoever you need to
communicate it to, whetherthat's in the court or somewhere
else.
And I think it helps tosimplify problems rather than
(16:25):
make problems more complex.
And I think that's one of thehuge advantages of mental
understanding of combustionscience, fire, dynamics and
whatever is is.
It allows you to take a verycomplicated problem and simplify
it in a way that can help youcommunicate that.
Speaker 1 (16:41):
Fantastic, fantastic.
And moving on to the next thing, which is a question for me,
really, in that heat releaserate you can have a fire, you
can have a.
Am I right in thinking thatheat release rate is really
about the amount of heat that'sreleased over a period of time?
So you can have two samples,two materials.
They would burn.
If one burns quicker than theother, you get a higher heat
(17:01):
release rate.
Am I right in thinking that ornot?
Speaker 2 (17:08):
Almost right, mike.
I think there's a nuance thatwe need to put on that.
The heat release rate is.
You're absolutely right in whatyou say there.
The definition is the amount ofenergy that is released in a
period of time.
Heat release rate has got unitsof joules per second, so joules
is energy, seconds is time, sothe amount of energy per second,
and of course that's oftenreferred to as watts right, it's
a unit of power in the end,like a 60 watt light bulb.
All those things are basicallyobsolete now.
That means that every secondit's turning 60 joules of
(17:32):
electrical energy into heat andlight.
That's what that means.
And in a fire it's turning 60joules of electrical energy into
heat and light.
That's what that means.
And in a fire it's basicallythe same thing.
If I have a fire with a heatrelease rate of 100 watts,
that's quite a small fire, butthat means every second 100
joules of chemical energy that'sstored in the fuel is being
converted largely into thermalenergy.
There's a bit of light andmaybe some sound as well from
that, but mostly it's beingconverted into thermal energy.
(17:55):
And it's a really importantdefinition, though, because one
of the things that people Ithink often struggle with and
this is in kind of generalunderstanding of fires is what
is the size of a fire?
How do you measure how big itis?
We all kind of have some kindof intuitive sense.
You look at some flames and yougo, oh geez, that's a huge fire
, I'm going to run away.
Or you look at some flames andyou say, well, that's a little
(18:15):
tiny fire, I'm going to stamp onit and put it out.
I think, qualitatively, prettymuch everyone has a good sense
of that.
Big fire, small fire.
But of course, from a disciplineof fire investigation, fire
science, fire engineering, weneed to quantify that.
And the way we quantify it isby measuring or evaluating in
some way the energy that isreleased per unit time, so the
(18:40):
joules per second from thecombustion process, and that is
what we call the heat releaserate.
And a fire of 100 kilowatts isa pretty small fire and people
are quite surprised.
Actually the numbers get quitebig quite quickly.
100 kilowatts is that bad.
You easily put that out with afire extinguisher if you kind of
know what you're doing, but bythe time you get up to a
megawatt, well, that's a prettyscary thing to be around, but
nevertheless, I mean, that'sprobably about the size of a
couch burning a megawatt.
So just to give a sense of howthese things vary 100 kilowatts
(19:04):
might be a paper basket, onemegawatt kind of onto a couch.
That's how we measure the sizeof a fire and that's really
important because the amount ofenergy that's being released is
what drives everything else.
It drives the temperature ofthe smoke.
It drives the quantity of smokethat is produced.
Heat release rate scales reallywell with loads of the
parameters that we're interestedin.
But what heat release rateisn't is temperature, and I
(19:26):
think it's worth pointing thisout.
If you take two candles, youlight one candle, then it has a
heat release rate of whatever afew watts probably and also has
a temperature that is fixedright.
You can measure thattemperature.
It's maybe 1200 degrees Celsius, something like that.
If I light a second candle andI hold them next to each other
so that the flames merge, I'vegot a flame that is twice the
(19:47):
size, but the temperature isstill the same.
The two candles have atemperature of about 1200
degrees Celsius.
When I put them together, thetemperature isn't changing, but
the heat release rate ischanging, and I think that's a
really important definitionbecause the temperature of a
flame is determined by many,many factors.
But actually in the end, if youhave a diffusion flame, so you
(20:07):
have that yellowish kind oforangey flame, the temperature
is usually always around 1200degrees Celsius, maybe a little
bit more, maybe a little bitless.
There's not really such a thingas a hot fire or a cold fire
from that perspective.
You might have a broke fire ora small fire, a large heat
release rate or a small heatrelease rate, and obviously a
fire with a larger heat releaserate it's going to feel hotter.
(20:29):
You're going to perceive thatas being hotter because there's
more radiative heat transferfrom those flames or there's
more convective heating of theenvironment, so everything feels
hotter.
But actually an objectivemeasurement of the flame
temperature of the fire is goingto give you the same answer.
So I think that's one of thethings that really important to
be able to separate andcommunicate is the difference
between temperature and heatrelease rate.
Speaker 1 (20:50):
I'm going to go on a little side tinder, but that's
sort of.
One of the myths in fireinvestigation is that ignitable
liquid fire will burn hotterthan, or was, until it's been
disproved, will burn hotter thana normal fire.
And that's what you're saying.
There is, the temperature isactually the same, it's just the
rate of burn, the rate ofburning.
Is that right?
Speaker 2 (21:07):
Yeah.
So rate of burning is anotherterm and if we just take that
one first, I guess the rate ofburning it scales like with the
rate of heat release or the heatrelease rate.
Because if I know the rate ofburning, that has units of
kilograms per second, it's massper unit time, the rate of
burning.
And if I know the rate ofburning of something and I know
the heat of combustion of thatobject, that material that is
(21:29):
burning, I can multiply thosetwo terms and that will give me
the heat release rate.
So I can actually, if I knowthis term, the heat of
combustion, which for a liquidfuel is usually very easy to
find out.
For pure materials it's quitesimple there's no such thing as
a heat of combustion for a couchor for a table or for a
computer, but those are made ofmaterials and you can probably
(21:50):
make a guess at what it would be.
But if you know the rate ofburning and you multiply by the
heat of combustion, you get theheat release rate.
But that again doesn't reallychange the temperature of the
fire.
It might change the temperaturewithin the compartment and the
environment around that fire,but the fire doesn't change.
So, going back to your liquidfuels.
I think one of the things therethat is different if you
compare a liquid fuel fire to asolid fuel fire, is simply the
(22:14):
rate of increase of the heatrelease rate.
I mean, you ignite the petrolor a liquid fuel and you get
basically the maximum rate ofheat release immediately.
If you ignite a tear orsomething else, right, the fire
has to grow, it has to develop.
Everything is much slower.
I think that's where most ofthe differences come from,
unless you're doing some exoticchemistry.
Maybe if you've got a methanolfire, that's going to be hotter
(22:35):
than if you have a gasoline fire.
But I mean, I think that's alevel of nuance that is not
going to be that generallyapplicable.
Speaker 1 (22:42):
Okay, and that leads me on to another sort of area
that I wanted to touch on.
Is that obviously a normal fire, in normal atmospheric
conditions, just reacts with air, with 20, whatever percent of
air?
Why does fires involving oxygen?
Why do they burn?
Am I right in saying they burnhotter?
Speaker 2 (22:59):
absolutely okay why is that amazing question?
I think the first thing we haveto kind of recognize when it
comes to this is just howreactive oxygen is.
We kind of take it for grantedbecause we're surrounded by it.
20, 21 percent more or less ofthe atmosphere we breathe is
oxygen, and oxygen is anincredibly reactive molecule.
(23:20):
It really wants to combine withother elements to form other
products.
We think about it because we'reso familiar with it.
But just kind of take a lookaround you at the.
Even at room or roomtemperatures my bike chain gets
rusty and that's because themetal of the chain is reacting
with oxygen and rusting.
Now I live in Edinburgh, right,so it's not that hot here.
(23:41):
So it's a slow process, but ithappens.
If I leave things unattended weget this oxidation process,
oxygen reacting with materialsat low temperatures.
Most other things don't reactat these sorts of low
temperatures every day.
Oxygen is super reactive.
That's the most important thingto remember.
But the other thing is, when welook at air, 20 percent of air
(24:03):
is oxygen.
The other 80 percent or 79percent is nitrogen and nitrogen
is basically completely inertfor most practical reasons when
it comes to fires and when youhave a fuel, that fuel has got
an amount of energy in it.
For each mass of fuel unit mass, one kilogram gram, whatever of
the fuel there's an amount ofenergy and that's the heat of
(24:24):
combustion.
Again, the question is when youburn that fuel you release that
energy, you turn the chemicalenergy into thermal energy.
Where does it go?
What actually happens to allthat thermal energy?
And the answer is it heats upthe products of combustion.
I start at room temperature.
I've got my fuel and my air,then I react to them and then at
the end I have hot products ofcombustion.
(24:45):
So all of the energy from thecombustion process that's
released, all the energy that ismeasured as the heat of
combustion of that material, isreleased and heats up those
products of combustion.
Now, when I'm burning a fuel inair, some of the air that's
being heated up is nitrogen.
It's not like I'm an activeparticipant in the reaction,
(25:05):
it's just there.
When I burn a fuel in air, thetemperature rise that I get is
proportional to the amount ofproducts of combustion.
A big chunk of those productsof combustion is nitrogen and
that's not doing anything.
That's absorbing that energyand heating up the nitrogen.
If I increase the oxygenconcentration, then I've got
more oxygen and that is areactive thing.
So I react it with more fueland I release some energy.
(25:25):
But there's now proportionallyless nitrogen to heat up.
So I've got a smaller heat sinkfrom the nitrogen and that
means I achieve higher flametemperatures.
So that's why, if you'reburning things in an increased
oxygen environment, it gets veryexciting very quickly.
I mean, we do some flame spreadexperiments here in the lab and
if we do it in an airenvironment, we have flame
(25:47):
spread rates I think on theorder about 0.3 millimeters per
minute.
Okay, we increase the oxygenconcentration from 20% to 40%
and we go from 0.3 millimetersper minute to three millimeters
per minute.
So we increase the flame spreadrate by 10 times, by double the
oxygen concentration.
So fires are really sensitiveto this.
And it works the other way too.
(26:09):
If you decrease the oxygenconcentration just a little bit,
you will get to the point wherethe flame can't sustain itself
and will have what we callquenching.
Sometimes you might see it asextinction, and that's the
process that you're using ifyou're applying a carbon dioxide
extinguisher onto a fire, or ifyou're trying to smother a fire
in a ship's cargo hold byinjecting inert gas.
(26:29):
It's that process.
So oxygen is really important.
It's really reactive and arelatively small increase in the
oxygen will give you hugeincreases in the fire behaviors
and flame temperatures and so onfantastic, okay, brilliant.
Speaker 1 (26:40):
That was awesome.
Okay, let's move on to lowerexplosive, higher explosive
flame limits, etc.
Just take me through that andmaybe stoichiometric mix yeah,
okay.
Speaker 2 (26:51):
So I mean the easiest place to start here is the
stoichiometric mixture.
Now that sounds like a veryfancy word, but actually all it
means is, if you've got astoichiometric mixture, it means
from a chemical point of view,from the point of view of the
chemistry, when you've got fuelplus oxygen or fuel plus air
goes to carbon dioxide, waterand nitrogen.
If you've got the stoichiometricmixture, what it means is
(27:12):
you've got the perfect amount ofoxygen or air to react with all
of the fuel, so you've got aperfectly balanced combustion
reaction and you will release,in theory, the maximum amount of
energy.
You will achieve the highestflame temperatures again,
because you don't have anyexcess.
There's no spare fuel, there'sno spare oxygen, so you release
the most energy and you heat theminimum amount of products of
(27:35):
combustion.
So that's what thestoichiometric mixture is.
It just is, when you've got aperfectly balanced ratio of fuel
to oxygen, you produce carbondioxide and water, and then your
nitrogen as well.
The lower explosive limit is, Ithink, maybe the next easiest
one to discuss, because thestoichiometric mixture will burn
easily If you go to the lowerexplosive limit then, what
(27:56):
you're doing is you're addingmore and more air or more and
more oxygen into the mixture andthat's decreasing the
proportion of fuel.
You go from your stoichiometricmixture and then you add more
and more air, so proportionally,the amount of fuel you have in
that mixture is decreasing.
At some point you'll have notenough fuel to sustain a
combustion process becauseyou've basically effectively
(28:19):
only got air.
There's not enough fuelmolecules in there to propagate
the chain reaction of thecombustion process.
The temperatures are too lowfor the chemistry, for the
chemical reactions to happen atthe rate that they need to.
You won't have a fire and formost materials most practical
materials that's on the order ofa few percent.
So methane, natural gas, thelower explosive limit is about
(28:39):
5% methane in air.
Then if you go to otherhydrocarbon fuels, as you go to
longer molecules, that numberdecreases down to about 1.8% or
something like that so smallnumbers which has important
applications for safety becauseit means you don't need much
fuel in an air environment topotentially have an explosion to
(29:00):
have that process occur.
And what's happening there isagain.
It's this balance between theenergy that you can release from
the combustion process and howthat energy is transferred
through the mixture in order tosustain the chemical reaction.
I think the good thing for mostfire investigators to think
about is that lower explosivelimit is usually for most normal
things.
And of course there's alwaysexceptions and people can put in
(29:21):
the comments whatever they likeabout best exception they can
find to what I'm saying, butit's usually a small number.
Like I say, methane is aboutfive.
One of the exceptions I canthink of are carbon monoxide and
hydrogen.
They're weird in terms of theirflammability limits, quite wide
and also starting at differentvalues, but for most common
fuels it's a few percent and theupper flammability limit is the
(29:41):
opposite.
So I take my stoichiometricmixture where everything is
perfectly balanced, and thistime I proportionally just
increase the amount of fuel thatI have available and eventually
I will increase the amount offuel to a point where there's
too much well, there's notenough oxygen, too much fuel,
and then the mixture won'tsustain the combustion process.
Again, the temperatures are toolow and the chemistry just
(30:02):
doesn't work.
That's called the upperflammability limit or the upper
explosive limit and again,that's usually on the order of
well.
For methane, to go back to that,it's 15% and it's usually on
the order of kind of 5% to 10%for hydrocarbon fuels.
And again, why does that matter?
(30:41):
It's a good question gas,because those explosive limits
are a few percent.
The low explosive limit is onepercent, that kind of order of
magnitude.
So it means I don't actuallygenerate that much pyrolysis gas
from my solid material to beable to sustain a flame.
And I think, again, that's aninteresting link between what we
understand from nice mixturesof gases and simple combustion
(31:04):
and simple fuels back to thesolid fuel.
Is that link between pyrolysisand the amount of gaseous fuel
you need to sustain a flame?
It's quite a powerful tool fromthat point of view.
Again, for the investigator, itdoesn't just apply to
explosions, the low landabilitylimits, it actually applies also
to all the processes aroundburning of solids, whether it's
ignition, flame spread or whathave you.
Speaker 1 (31:25):
Sure, but those limits though.
Are we looking at a point inspace, or are we looking at a
cubic metre compartment?
Speaker 2 (31:33):
Again, really good question the literal meaning of
the data that you would find ina table.
If the flammability limit lowerflammability limit of methane
in air is 5% by volume, that isfor a well-mixed volume of gas.
That is all at thatconcentration, of course.
In reality if you've got a gasleak in a property, then you're
(31:53):
going to.
If the gas leak is at highlevel and your gas is lighter
than air, then probably you havea very fuel-rich band that
doesn't have enough oxygen toburn near the ceiling.
But somewhere you will have agradient and there will be that
flammable mixture.
And the issue with that isthere's always somewhere that
that flammable mixture willexist.
Right.
Physics tells you that you havediffusion and all these other
processes.
(32:13):
That means you will have thatflammable mixture and as soon as
it ignites there you get all ofthe processes around the
expansion of the gases, thefluid mechanics, the mixing of
the gases that you generate fromthat.
That means your nice stratifiedlayer is now all turbulent and
mixed up, so you actually have achance of then getting that
explosion.
Even though you're maybethinking that it's too rich,
(32:34):
it's too fuel rich up there toburn Somewhere, there will be
that kernel that is at the rightmixture, and then you will get
that propagation.
Speaker 1 (32:42):
And you need that.
Let's move on to sort ofignition sources and maybe we
can talk about the energyrequired for that and how
different materials needdifferent energies.
But you need an ignition sourcejust at that one area or that
one point.
Yeah, for these mixers, that'sit.
Speaker 2 (32:55):
I mean, the fire science runs out quite quickly
here so we can define theconditions that might lead to
ignition.
But if you have that kind ofstratified thing of if we go
back to the idea of a leak ofnatural gas, it's lighter than
air, sorry, so it will rise upto a ceiling and it will
accumulate there and you willhave all sorts of mixing and
entrainment processes.
I mean, really it's verydifficult to have any confidence
(33:18):
in what the composition of thatmixture is going to be.
So you end up with this idea of, if you're doing it from a fire
investigation point of view,thinking, well, what ignition
sources are there?
Is there anything in thatlocation that could cause
ignition?
Because any argument that youmight try to put of, oh well, in
this region it's going to betoo fuel rich and in this region
it's going to be too lean, Ithink you're having to learn
that kind of stuff in thoseapplications, unless you've got
(33:39):
some other information to drawon or some sophisticated
numerical modeling or somethinglike that.
But to your point on ignitionsources.
One of the things that is verydifficult to do in fire
investigation is to demonstrateis your ignition source
competent to ignite the mixture.
That is there Because you don'tknow what the mixture is most
of the time.
So that's a real challenge withthat.
But the thing I was taught along time ago now is you know
(34:01):
any flame will ignite anyflammable mixture because you've
got the ongoing chemistry andall the kind of ingredients that
you need to start a chemicalreaction with that.
But sparks are a bit different.
Some sparks are capable ofigniting mixtures and others
aren't.
And unless you know what yourmixture is, you kind of have to
err on the side of probably thespark did ignite the mixture.
I always find arguments aroundspark energies and things we
(34:23):
just don't know enough Pick aside right, and that's not where
we want to be, as I think of acommunity right.
We want to be objective andaccurate.
So that's tricky, that's anunknown, I would say, for a lot
of practical scenarios.
Speaker 1 (34:35):
Sure, I think we need to recognize that fire science
is pretty, fairly young in termsof general science, isn't it?
I mean, are well establishedand mr bacon and all that kind
of good stuff, but actuallystudying it's only been the last
less than a century, isn't itlast 40, 50 years?
Speaker 2 (34:52):
I kind of struggle with this because, yeah,
sometimes we are.
I don't know that age is theright way to describe it.
I think it's more like amaturity thing, because we have
been studying fire andcombustion processes, you know,
really since the industrialrevolution.
That's when we started reallyin earnest doing that and that's
a long time ago.
If you talk about somethinglike quantum physics, I mean
that really is only since maybethe kind of early middle part of
(35:14):
the 20th century.
And look where that's got us.
We're building atomic bombs,nuclear power stations and
whatever, because that was builton a very mature set of physics
and ideas and a large communityof people working on that
internationally.
I think fire science we havebeen studying it for a long time
and we've kind of borrowedideas from combustion science.
We look at material science andwe're kind of this weird
(35:36):
mongrel discipline of differentideas from different places and
that's really good because thatbrings good practice from these
other areas.
The downside on fire sciencestuff is it is extraordinarily
broad.
I mean the longer I spendteaching this stuff, the more
appreciation I have for that.
When I'm teaching theundergraduate students, I mean
they have to know chemistry,they have to know heat transfer
they have to know fluidmechanics and in isolation you
(35:59):
have to know them all whenthey're happening at the same
time.
The complexity of the problems,I think, is really
extraordinary.
Each individual part is quiteeasy on its own.
We know how to solve heattransfer problems in most
scenarios.
We know how to deal withcombustion chemistry.
I mean we build jet engines andwe've optimized that to reduce
the emissions, to increase theefficiency, all that stuff.
(36:19):
So we know how to do that.
And the fluid mechanics.
We're dealing with relativelysimple flows.
They're relatively lowvelocities and whatever.
We know all these individualcomponents.
But putting them together iswhat makes fire really difficult
.
And I think it's where you alsohave to be a little bit humble,
because I think I know quite alot about those things.
Over the years I've learnedquite a lot.
But ask me what the implicationis on the structural
(36:40):
performance of a steel beamthat's being heated up by the
fire.
I don't know.
That's a problem I have to handoff to one of my colleagues who
knows about that kind of stuff.
So I think it's almost toosimple to say, oh, we're a
relatively young discipline,because if you trace the history
you can draw a line all the wayback to the early studies of
thermodynamics and things.
What we are is a very smalldiscipline and everyone has
(37:03):
their own specialty within thatdiscipline.
So actually having some ofthese conversations is quite
difficult on a very technicallevel when you compare it to
something like some of thebranches of physics where you've
got hundreds or thousands ofscientists all with exactly the
same background talking aboutexactly the same problem.
We are people with verydifferent backgrounds trying to
solve quite a wide range ofproblems and I think very often
(37:24):
there's just a total mismatch inknowledge in some of these
areas and just people just knowdifferent stuff.
Speaker 1 (37:29):
So getting everyone's opinion yeah, and I think you
summed it up earlier on quitewell is that combustion science
is very different to the firescience, because combustions you
have a set of knowns, you knowthe fuel, you know how they're
going to interact, you know thatthey're pure or not pure,
whereas with fire sciences, bynature, it's chaotic.
There's so much going on.
Speaker 2 (37:47):
Yeah, absolutely, I mean if fire science would be
easy, if we knew how much fuelwas being injected, if you knew
how quickly something waspyrolyzing, if we could predict
that ahead of time, then that'dbe very, very easy problem to
solve.
The reality is we don't, andthat, for me, is the challenge
of fire science.
It's about figuring out how youturn your solid fuel into those
gas molecules that can thenreact in a flame.
(38:07):
We've seen.
Speaker 1 (38:08):
I'm from a practical point of view.
We've seen very similar fires.
Nist and the atf have done verysimilar cell burns with the
same materials but got differentresults because of different
humidity or different aireffects and stuff like that.
So it's's very difficult.
Speaker 2 (38:23):
Yeah, you take that kind of stuff and you think
about the extreme.
Example is wildfires, whereyou've got also the effects of
weather and climate and verycomplicated fuels to account for
as well.
I mean, I think what I alwayssay about fire science to our
undergrad students is if yougive me one material, a pure
material I can probably tell youquite a lot about that and
predict well how it's going toburn.
(38:43):
But a pure material, I canprobably tell you quite a lot
about that and predict well howit's going to burn.
But the minute you put thatmaterial into some kind of
system whether that's acompartment, a forest, the
engine room of a ship, the cargohold of a ship and there's
other things, there areboundaries and there are other
materials that are going tostart reacting Soon, as you get
(39:08):
to that scenario, it becomesreally, really difficult.
And I think that's where, from afire investigation point of
view from my experience, that isthe challenging part is I don't
have pure materials anymore,I've got systems of materials
that are interacting with eachother and with the environment
and the problem becomesextraordinarily complicated.
So you've got to have thosefundamentals pinned down really
well so you can begin tosystematically work through.
Well, what happens to the waythat this material burns?
When there is some airflow, orwhen it's also being heated by
this material, or when the fireservice has sprayed some water
on it?
What's that going to do to whatit looks like?
Speaker 1 (39:28):
so I think for me that's one of the big challenges
in fire investigation, anytimeI it's not very often to be fair
, but anytime I show up to afire scene and have a look
around, sure I mean, you knowthe geometry of fire and I
remember on the course that youtaught me on firmly thin
materials, firmly thick material, where the fire is within that
cell, if it's in the middle ofthe room or if it's up against
the wall or in the corner, andyou get that feedback and all
(39:49):
that.
It makes a massive difference.
So there's so many variables.
But listen, roy, I'm I'm reallysad to sort of cut you off for
you, because we've only got intoabout half of not even half of
what I wanted to get into.
But we're sort of running outof time.
But hopefully you'll come backon we can ask some of these
other questions.
But I've got loads of otherquestions.
But just to sort of summarizethis, there's, if I'm an FI and
(40:09):
I want to know more about firescience, which NFPA 921 says we
should have as one of theprerequisites at 1033, our kind
of Bible, if you like.
What would you recommend?
Where would you recommend we go?
Is there a textbook or is therea course?
I mean, obviously I wouldthoroughly recommend you, but I
know that the university startedrunning a new MSc.
Speaker 2 (40:32):
Is that something you'd recommend?
Yeah, so I mean you've kind ofmentioned three of the things I
would suggest.
So the first thing is grabyourself a copy of Introduction
to Biodynamics by DougalDrysdale.
I think that is still the mostgeneral, most easily accessible
textbook on these things and Ithink if you are presenting
reports or to the court, then Ithink that level is absolutely
sufficient for that kind ofapplication.
But reading a textbook on itsown, extraordinarily dry and
(40:54):
dull and I will confess I havenever read it cover to cover,
right Sat down, you know, on asun lounge or on holiday, and
flicked through, I've never donethat.
But I think I have read everypage in it at some point.
Get yourself out there and rollon a course.
I mean we obviously I'm goingto say about our fire
investigation short course thatwe run every year usually
happens at the beginning ofsummer, and keep an eye on our
website and stuff and you'll seewhen that's happening and in
(41:15):
that we cover practical firescience things but also fire
specific info about how toinvestigate the fire scene, the
legal aspects and things likethat.
If you're interested much morein the fire science side of
things, then we have an MSc atEdinburgh.
It's a one-year MSc, that is,two semesters of teaching and
then the final semester is adissertation.
(41:35):
Do some research there as well.
And the other thing that I canrecommend of course I'm going to
say those things right, becausethose are the Edinburgh things
the other thing I would saythat's really important in fine
investigation is to know whatyou know and to know what you
don't know and to be completelycomfortable when your knowledge
runs out.
To speak to someone, because youlearn a lot by speaking to
(41:57):
people and asking for help fromthe right people, and we're
always open for an email thatsays I've got this problem.
Can we have a chat?
Sure, no worries, pick up thephone and whatever.
We can have a discussion or anemail back and forth if you're
looking for information or help.
If it's a quick question, we'revery happy to try and answer
and point in the right directionfor these things, because I
(42:17):
mean, I don't know everything.
Ricky, my colleague, doesn'tknow everything.
Professor Bisbee here doesn'tknow everything, but what we do
have here is six or sevenacademics between us know a lot.
So it's not that I know and cansolve every problem.
So talk to people, know whatyou don't know and be very okay
with that, because it's muchbetter, I think, to say I'm not
sure about this than to havekind of an overconfidence in an
(42:39):
opinion or in a point of viewthat is not substantiated in an
opinion or in a point of viewthat is not substantiated.
The last thing I guess I amalways surprised at how many
fires there's a specific causefound.
For A lot of the time you'relike well, okay, I'm not so sure
about that.
There's maybe these othercauses that seem to not be
addressed very much, and I thinkthere's sometimes too much
emphasis on origin and cause.
(43:01):
Now, that is, of course, a veryimportant part of the process,
but what you've got to also dois figure out the fire
development that resulted fromthat, because as the
investigator you go to the scene, the fire's over, you've got to
turn the clock backwards.
So you're solving this inverseproblem and they are, by
definition, ill-posed right andthere's mathematical proofs that
you can do to show that there'smany initial solutions or
(43:22):
initial conditions can get youto a final solution.
So I think doing that kind ofjust logic test a lot of it
isn't even detailed science.
A lot of it is just sittingdown with a friendly colleague
or a friendly person and thinklook, what do you think about
this sequence of events?
Is there something I've missed?
Is it something I've not gotthere?
Because it's only by talkingand being open to other
solutions being proposed bycolleagues or collaborators that
(43:45):
you get to a good answer.
There's no shame in changingyour mind no, definitely not.
Speaker 1 (43:50):
it's always better to change it at the report stage,
having had it peer-reviewed, andrather than when you get to
court and somebody else has someinformation that you didn't
have or someone didn't gothrough that process or question
it, and every firm firm thatI've worked for they always had
a robust kind of a secondchecker and even a third checker
.
If it was a big case and it wasgoing to court, you'd have a
(44:10):
third review and there's noshame in being picked apart
because you know what you know.
You don only see what younecessarily see, and sometimes
it takes that third eye orsecond eye to say, well, have
you thought about this?
Or what about that particularpattern, or when did that window
break, for example, those sortsof things.
Speaker 2 (44:31):
Totally.
Yeah, absolutely.
It's easy to say to beopen-minded and we all know
ourselves.
You bring your own biases andyour own whatever.
But it's recognizing that andthen being open to the idea of
something different, becausethat makes everything better.
As you say, you're more likelyto get to the more likely
conclusion or more kind ofconsensual opinion, and the more
agreement you can find, thebetter.
And then you're arguing, if youneed to, over some other points
(44:53):
, but you can agree most of whatyou need to.
Speaker 1 (44:57):
Fantastic.
Thanks, roy.
I really appreciate your time.
Again, this isn't a sales pitchfor Edinburgh University, but
I've done the course and I cansay that from a fire science and
that sort of it was definitelythe best course I've been on in
relation to that kind ofunderstanding of the chemistry
and the fire science thermallythin, thermally thick materials
and how that's applied in fireinvestigation.
That short course wasabsolutely fantastic and I've
(45:18):
been after you for a long timeand we've managed to get you on.
I hope you'll come on againbecause there's loads that I
want to.
There's loads more I want tocover.
I'm sure my audience isthinking I wish you'd ask this
question.
I wish you asked that question.
But thanks ever so much.
We really appreciate your time.
Absolutely wonderful, mate.
Thanks for having me on.
Cheers, mate, thank you.
Hey, thank you for listening tocsi on fire.
(45:39):
Please don't forget to like,subscribe and suggest future
topics on our web page.
Remember factor non-verbal.
Take care, good hunting.
I hope to see you on the nextone.
Cheers.
Hi, welcome to CSI on Fire, the podcast that takes
you behind the scenes of thefire investigation community.
I'm your host, mike Moulden,and episode after episode, we'll
attempt to excavate the oftendifficult but always fascinating
world of the fire investigator.
Welcome to CSI on Fire, yourfire investigation podcast.
(00:27):
This is episode, believe it ornot, episode 22.
I've got a fantastic guest ontoday.
I've got Rory Hayden fromEdinburgh University.
Rory, welcome to the podcast.
Thanks for coming on.
Cool.
Thanks, mike.
Nice to be here.
It's a pleasure, mate.
It's a pleasure Now.
You and I first met I think itwas 2019 when you taught me on a
fire investigation course.
We'll get into that a littlebit more, but tell us about your
(00:48):
background.
How have you ended up here onthe podcast?
What's your background in fireinvestigation and fire science?
Speaker 2 (00:54):
Sure, so I have the job title of reader at the
School of Engineering at theUniversity of Edinburgh and it's
okay, because I don't know whatthat means either as a job
title.
But basically it means I'm kindof your regular academic who
does teaching of undergraduatestudents, master's students, phd
students, but also research, soworking on research projects,
and in my case that covers awhole range of topics from
(01:16):
fundamental fire science orlooking at ignition and flame
spread and things like that, allthe way through to application
specific things in the builtenvironment and also with
wildfires in the naturalenvironment, so quite a broad
spectrum of activity.
Of course that sits within thelarger fire engineering group at
Edinburgh, so that includescolleagues working on a whole
(01:36):
host of different topics withdifferent applications in the
built environment, in thenatural environment, around fire
science, fire engineering and,of course, fire investigation.
Sure, and I mean Edinburgh, Idon't know fire engineering and,
of course, fire investigation.
Speaker 1 (01:46):
Sure, and I mean Edinburgh.
I don't know I'll tell some ofthe listeners, but Edinburgh is
renowned because of DougalDrysdale and his famous book,
which everyone should have anddoes have in their thing, and I
remember Dougal was on thatcourse.
I went on the fireinvestigation course, which is a
short course for the EdinburghUniversity In 2019, I met you
and Ricky, and Dougal was thereand a few a number of others.
Mike Wisecar, who also had onthe podcast, was there as well.
(02:09):
So, yeah, fantastic course.
Thoroughly recommend it.
The thing about that course aswell also allows you into gives
you associate membership for theIFE as well, the Institute of
Fire Engineers yeah, so the examwe do at the end of that course
you can use with the IFE as atrade-off with their exam.
Speaker 2 (02:25):
I suppose is a way of doing that if you are going for
one of those qualifications ormemberships.
Mike, I don't know the detailsof that, to be honest, but look
on our website and we can tellyou.
Speaker 1 (02:36):
Look on your website, look on the IFE website.
They'll tell you but what I'msaying is basically a
well-established course and it'sgot some serious credentials
behind it, some seriousindividuals behind it.
One of the reasons I got you ontoday, to be honest with you,
rory is that my fire science ispretty lacking.
I'm not a chemist by any shapeor way.
A lot of my colleagues are fireservice CSIs, engineers, et
(02:57):
cetera, and wouldn't necessarilyhave a chemistry background.
So I wanted to sort of gothrough really some sort of
really basic 101, if you like,definition.
So I'll let you expand howeveryou want to.
But what is combustion?
What is fire and what'scombustion?
Speaker 2 (03:11):
Excellent.
Let's start off with what iscombustion.
That's maybe the easiest placeto start.
And a combustion reaction ispretty simply defined as a
reaction between a fuel andoxygen, and that's one.
And when you say combustion, itnormally also implies some kind
of use from that reaction.
So where that combustionreaction or that combustion
process is useful we tend torefer to it as that, and that
(03:33):
would be, for example, in a carengine or if you're heating with
fires.
We'd talk about combustion inthat context.
Fire, on the other hand, isbasically, I think, quite useful
.
The way to think about it is isthat combustion process, but
not being useful, but insteadcausing some kind of damage or
some kind of harm?
And, of course, you can pickholes in all these definitions,
because is a campfire causingany harm?
(03:53):
Well, probably not.
But I think a combustion systemis one which is engineered and
designed to deliver power fromthat combustion reaction,
whereas a fire is something thatis less controlled and is often
more destructive, but it's justnot as controlled as a
combustion process.
But what underpins that?
The science is the same.
In the end you have some fuelreacting with oxygen that is
(04:14):
generating carbon dioxide, water, other products of combustion
and liberating a lot of heat ora lot of thermal energy, great.
Speaker 1 (04:22):
And what's the definition of a flame?
What would you describe as aflame?
Cool, I think, a flame that's areally good question.
Speaker 2 (04:28):
A flame is basically the manifestation of that
combustion process.
What you see as a flame I meanyou have blue flames, right, I
mean you have yellow flameswhat's happening there is
effectively the same thing youhave fuel reacting with air.
Really, the difference that yousee in terms of the color of the
flame is due to how much fuelrelative to the air that you
have the blue flame.
(04:49):
What you're seeing actuallyyour eyes are seeing there is
molecules that are excited sothat they give off energy and
that is apparent to us as bluelight.
If you have a diffusion flame,then you generate a lot more
soot and a lot more solid carbonparticles and when they get hot
they give off yellow-orangelight.
So in both cases what you'reseeing is the flame is light, so
(05:10):
your eyes are perceiving.
You might also feel heat comingfrom that flame, so you'll get
radiation coming from that flame.
But all of that that you seeand that you perceive is that
chemical process of fuelreacting with oxygen, and it's
just the differences in thedetails of how that reaction is
progressing and what that flameactually looks like in the end.
Speaker 1 (05:30):
So a flame is very simply just a manifestation of a
combustion reaction okay, andif we go back to the good old
days, I'm not sure they still doit, but when I went to school
we used to have a bunsen burnerand we used to open up the
bottom and we'd get a nice blueflame and that's what we call
pre-mixed flame, is that right?
So you're mixing the oxygenbefore it gets to that ignition
point.
Am I right in thinking that?
Speaker 2 (05:51):
Yeah, with the Bunsen burner, exactly as you've
described, when you open thehole at the bottom of the barrel
, the air is able to enter thathole.
It's entrained into that holeby themoving flow of fuel gases.
And that means by the time youget to the top of the barrel of
the Bunsen burner you've got amixture of fuel and air that is
very well mixed indeed.
(06:11):
So when it burns it burns withthat really characteristic blue
flame.
So yeah, that's a pre-mixedflame.
The opposite of a pre-mixedflame is when you close that
hole at the bottom of the Bunsenburner, then you have pure fuel
issuing as a jet from the topof the barrel and that then has
to mix with air from thesurroundings and the process of
(06:32):
that is usually dominated bydiffusion, so the diffusion of
oxygen from the air into thatstream of fuel.
So we call the kind ofcharacteristic yellow flame that
results from that a diffusionflame.
And the diffusion flame isnormally of a lower temperature
than a pre-mixed flame and ithas that yellow color again
because it's generally lesscomplete combustion than in the
(06:55):
pre-mixed flame and that givesyou the production of soot and
that is what gives you thatyellow color, as the soot heats
up and infandesses great, let'smove on to.
Speaker 1 (07:04):
Sorry apologies, I'm just going to keep hitting you
with words, but pyrolysis.
Then let's go through.
Pyrolysis.
What's pyrolysis?
Speaker 2 (07:10):
Awesome.
I think pyrolysis is one of themost important phenomena in
fire science to understand.
Because when you think aboutwhat makes fire scientists
different from combustionscientists, if you're dealing
with a combustion process, so ifyou're designing an internal
combustion engine, then the fuelthat you have is already quite
(07:31):
simple the gasoline you put inthe fuel tank.
We know what that is, it'salready a liquid, it's well
defined.
When you inject it into thecylinder of the engine, it
vaporizes and then it oxidizesin the gas phase.
And that is true of any fuel.
And if you're designing acentral heating boiler for use
in the UK, that's going to bepowered by natural gas.
So again, you know exactly whatthe fuel is entering that
system.
(07:51):
You know exactly the chemicalcomposition of that.
You know exactly how muchoxygen you need to supply to
react, to get the mostefficiency out of that system.
Because the fuels arerelatively simple.
They're either in the liquidphase or they're in the gas
phase.
The problem with fires, or atleast the challenge I see with
fires, is we're normally dealingwith solid fuels that are
(08:11):
burning.
I mean, I'm sure everyone'sdone an investigation where a
liquid fuel or a gaseous fuelwas what was important, but many
fires.
If you're dealing with domesticfire, fire on a commercial
premises, a lot of the stuffthat burns is solid, solid fuels
.
The world around us is made ofsolids and pyrolysis is the
process that you turn that solidwhich doesn't burn directly.
You've got to turn that into agas and pyrolysis is that
(08:34):
process.
And it's a process, I argue,that is central to fire science
and to the understanding of firephenomena, because getting your
solid material into the form ofa gaseous fuel is a key step in
any fire process and that iswhat pyrolysis does.
And the word pyrolysis is it'sa very simple word has two parts
(08:55):
pyro, which you might have somefamiliarity with what that
means, but that generally isit's associated with heat or
with energy of some kind, butnormally with heat.
And then lysis is another wordthat comes from the Greek, which
means to cut or to chop.
So pyrolysis or pyrolysis isthe chopping of something by
heat.
But what's that?
(09:15):
Something that's being choppedis the molecules that form the
solids.
So pyrolysis literally meansyou're going to chop the
molecules from being big, longpolymer molecules that are
solids into shorter moleculesthat are gases.
From my perspective, pyrolysisis one of the most fascinating
things to study in fire sciencebecause it is right at the heart
(09:35):
of how you turn whatevermaterial is present in a room or
on a fire scene into the gasesthat can burn, and it needs
relatively large amounts ofenergy for that to happen, and
that's often manifested by hightemperatures.
We're talking certainly on theorder of 300, 400 degrees
Celsius for materials to beginto pyrolyze and then, once you
get that pyrolysis, you cansustain the burning.
(09:57):
So problems as diverse asignition and flame spread, even
smoldering, combustion are alldependent on this pyrolysis
process.
So it can be phenomenallycomplicated and people like
myself will spend their entirecareers maybe studying pyrolysis
.
But I think the main importanceof it is actually looking at
what are some of thesimilarities between different
materials and the pyrolysisbehavior, and that's something I
(10:19):
think that any fireinvestigator should be thinking
about.
When they see a material that'spresent in the back of your
mind, you should be thinkingwell, that's a timber and
probably we're looking at apyrolysis temperature of around
300 degrees Celsius for that.
Could it get that hot in here?
How easy would it be for thatto be the first material ignited
, heating it up to thattemperature?
So for me you've gone kind ofquite quickly to one of the most
(10:42):
important processes for firespread, fire growth, fire
development, which is how do youturn those solid fuels into
gases so they can burn, and thatprocess is pyrolysis.
Speaker 1 (10:51):
There's obviously the three phases of heat transfer,
so can we just go into those andhow those three kind of affect
that pyrolysis, the mechanisms,if you like.
Speaker 2 (11:01):
Cool, yeah, so heat transfer, I think, is for me
again, if you like, cool, yeah.
So heat transfer, I think, isfor me, again, one of the most
important processes when itcomes to interpreting a fire
scene, but also when it justcomes to understanding fire
spread, fire development moregenerally, because a lot of the
time what you see at the end ofa fire might be a result of heat
transfer.
Right, the fire patterns thatyou might explore, some of them
(11:23):
are going to be, of course, dueto ventilation and other explore
.
Some of them are going to be,of course, due to ventilation
and other processes, but many ofthem are going to be due to
heat transfer.
And many of the observationsthat are made in a fire scene
where is there heat damage andwhatnot is all due to the heat
transfer.
Of course, with heat transfer,there's three mechanisms
conduction, convection andradiation, and of these I think
it's useful to separate a littlebit why we're interested in
(11:47):
them.
So conduction is heat transferwithin a solid.
That's usually where we seeconduction in most fire
applications.
So that is, if I'm trying toheat up a fuel so that it
pyrolyzes, what's happening tothe energy as it arrives at that
material is what's important,and the way the energy is
transferred through the materialis going to be by conduction,
(12:08):
and there are lots of equationsthat we can solve for conductive
heat transfer.
But the main application of itin the sense of fire
investigation and fire scienceis the thing that we're heating
up is a solid and therefore tounderstand how it heats up we
need to understand conduction,the other two modes of heat
transfer, radiation andconvection.
Those are usually how we getenergy to the surface of that
(12:29):
material that we're trying toheat up.
Convection is the transfer ofthermal energy by the movement
of a fluid.
Usually in a fire scenario thatfluid is the smoke and the hot
gases produced by the fire, andradiation is the transfer of
thermal energy byelectromagnetic radiation.
So that is, when you hold yourhands up to a campfire, you're
(12:49):
feeling the radiation.
If you somehow hold your handabove a candle, what you feel
there is convection and that'squite a nice little
demonstration.
I think if you hold your handon the side of a candle it
doesn't feel very hot at all,not much radiation from a candle
.
Hold your hand above it and itfeels really hot really quickly,
even just from a small tealight, because of the effects of
convection the way I kind oftry to explain it to students is
(13:12):
conduction is what's happeninginside my solid.
Convection and radiation arehow my solid is being heated up
and for fire applications.
I think that that's usuallyquite a simple way to look at
things.
All these modes of heat transferare actually really well
understood, especially radiation, because that's how we study
the universe around us.
So the people in physics thedepartment is just across the
way from me here they know loadsabout radiation.
(13:34):
It's how we understand theuniverse, it's how we
communicate with radio waves andeverything like that.
So we know a lot aboutradiation.
Convection is harder, the heattransferred by the motion of
fluids.
But we know on what it dependsand probably don't have time to
go in too much detail here.
But thin things, so things thathave got relatively small
dimensions, respond very rapidlyto convective heating.
(13:56):
That's why often you will seeif there's been an explosion.
You'll see scorch marks only onvery fine fuels or sometimes on
that old fattened kind ofbubbly wallpaper, toilet paper,
toilet paper.
Yeah, these thin fuels willrespond well to convective
heating, whereas radiation tendsto be more of a.
(14:17):
You need relatively largesurfaces, relatively flat areas
to respond significantly toradiation, because these three
modes of heat transfer arealways in some kind of
equilibrium.
If you heat something uprelative to the ambient, it
starts to cool down Always.
We're looking at these energybalances when it comes to how
things get hot.
But I think any fireinvestigator needs to be very
clear in their mind about thedifferent modes of heat transfer
(14:37):
and where they become relevantin a fire scene.
And to give a very simpleexample, if I'm not talking for
too long on this the growth of acompartment fire to flashover.
In the beginning of that you'vegot a small fire in a
compartment.
The smoke and the hot gasesrise up to the ceiling and as
they hit the ceiling they spreadout radially.
They form the so-called ceilingjet.
(14:58):
If you have a sprinkler upthere, then the hot gases are
going to flow past thatsprinkler bulb, around that
sprinkler bulb and they're goingto heat up that sprinkler bulb.
And the main mechanism ofheating there is convection the
flow of smoke past the sprinklerbulb.
And the sprinkler bulb is a fewmillimeters in diameter.
It's going to be heated reallyeffectively by convection and
hopefully activate.
(15:19):
It hits the activationtemperature and it will activate
and your fire will go out Ifthe sprinkler doesn't activate
or it's not present, then asthat smoke layer builds up in
the compartment it increases intemperature, it has a relatively
high emissivity, which is tosay a large fraction of the
energy is radiated from the suitin the smoke that will radiate
onto the compartment contentsand that will be one of the main
(15:42):
drivers to flashover isradiation from that smoke layer
onto the materials in thecompartment.
Again, just understanding therelative importance of the modes
of heat transfer for thescenario that you're interested
in I think is quite importantbecause the reality is heat
transfer is always happening,it's always on and it's always
(16:02):
conduction, convection andradiation that we need to think
about.
But there are some scenarioswhere one of those modes of heat
transfer is the most important.
And if you figure out which oneis the most important, then
that makes it much easier tounderstand the problem, makes it
much easier to communicate thatproblem to whoever you need to
communicate it to, whetherthat's in the court or somewhere
else.
And I think it helps tosimplify problems rather than
(16:25):
make problems more complex.
And I think that's one of thehuge advantages of mental
understanding of combustionscience, fire, dynamics and
whatever is is.
It allows you to take a verycomplicated problem and simplify
it in a way that can help youcommunicate that.
Speaker 1 (16:41):
Fantastic, fantastic.
And moving on to the next thing, which is a question for me,
really, in that heat releaserate you can have a fire, you
can have a.
Am I right in thinking thatheat release rate is really
about the amount of heat that'sreleased over a period of time?
So you can have two samples,two materials.
They would burn.
If one burns quicker than theother, you get a higher heat
(17:01):
release rate.
Am I right in thinking that ornot?
Speaker 2 (17:08):
Almost right, mike.
I think there's a nuance thatwe need to put on that.
The heat release rate is.
You're absolutely right in whatyou say there.
The definition is the amount ofenergy that is released in a
period of time.
Heat release rate has got unitsof joules per second, so joules
is energy, seconds is time, sothe amount of energy per second,
and of course that's oftenreferred to as watts right, it's
a unit of power in the end,like a 60 watt light bulb.
All those things are basicallyobsolete now.
That means that every secondit's turning 60 joules of
(17:32):
electrical energy into heat andlight.
That's what that means.
And in a fire it's turning 60joules of electrical energy into
heat and light.
That's what that means.
And in a fire it's basicallythe same thing.
If I have a fire with a heatrelease rate of 100 watts,
that's quite a small fire, butthat means every second 100
joules of chemical energy that'sstored in the fuel is being
converted largely into thermalenergy.
There's a bit of light andmaybe some sound as well from
that, but mostly it's beingconverted into thermal energy.
(17:55):
And it's a really importantdefinition, though, because one
of the things that people Ithink often struggle with and
this is in kind of generalunderstanding of fires is what
is the size of a fire?
How do you measure how big itis?
We all kind of have some kindof intuitive sense.
You look at some flames and yougo, oh geez, that's a huge fire
, I'm going to run away.
Or you look at some flames andyou say, well, that's a little
(18:15):
tiny fire, I'm going to stamp onit and put it out.
I think, qualitatively, prettymuch everyone has a good sense
of that.
Big fire, small fire.
But of course, from a disciplineof fire investigation, fire
science, fire engineering, weneed to quantify that.
And the way we quantify it isby measuring or evaluating in
some way the energy that isreleased per unit time, so the
(18:40):
joules per second from thecombustion process, and that is
what we call the heat releaserate.
And a fire of 100 kilowatts isa pretty small fire and people
are quite surprised.
Actually the numbers get quitebig quite quickly.
100 kilowatts is that bad.
You easily put that out with afire extinguisher if you kind of
know what you're doing, but bythe time you get up to a
megawatt, well, that's a prettyscary thing to be around, but
nevertheless, I mean, that'sprobably about the size of a
couch burning a megawatt.
So just to give a sense of howthese things vary 100 kilowatts
(19:04):
might be a paper basket, onemegawatt kind of onto a couch.
That's how we measure the sizeof a fire and that's really
important because the amount ofenergy that's being released is
what drives everything else.
It drives the temperature ofthe smoke.
It drives the quantity of smokethat is produced.
Heat release rate scales reallywell with loads of the
parameters that we're interestedin.
But what heat release rateisn't is temperature, and I
(19:26):
think it's worth pointing thisout.
If you take two candles, youlight one candle, then it has a
heat release rate of whatever afew watts probably and also has
a temperature that is fixedright.
You can measure thattemperature.
It's maybe 1200 degrees Celsius, something like that.
If I light a second candle andI hold them next to each other
so that the flames merge, I'vegot a flame that is twice the
(19:47):
size, but the temperature isstill the same.
The two candles have atemperature of about 1200
degrees Celsius.
When I put them together, thetemperature isn't changing, but
the heat release rate ischanging, and I think that's a
really important definitionbecause the temperature of a
flame is determined by many,many factors.
But actually in the end, if youhave a diffusion flame, so you
(20:07):
have that yellowish kind oforangey flame, the temperature
is usually always around 1200degrees Celsius, maybe a little
bit more, maybe a little bitless.
There's not really such a thingas a hot fire or a cold fire
from that perspective.
You might have a broke fire ora small fire, a large heat
release rate or a small heatrelease rate, and obviously a
fire with a larger heat releaserate it's going to feel hotter.
(20:29):
You're going to perceive thatas being hotter because there's
more radiative heat transferfrom those flames or there's
more convective heating of theenvironment, so everything feels
hotter.
But actually an objectivemeasurement of the flame
temperature of the fire is goingto give you the same answer.
So I think that's one of thethings that really important to
be able to separate andcommunicate is the difference
between temperature and heatrelease rate.
Speaker 1 (20:50):
I'm going to go on a little side tinder, but that's
sort of.
One of the myths in fireinvestigation is that ignitable
liquid fire will burn hotterthan, or was, until it's been
disproved, will burn hotter thana normal fire.
And that's what you're saying.
There is, the temperature isactually the same, it's just the
rate of burn, the rate ofburning.
Is that right?
Speaker 2 (21:07):
Yeah.
So rate of burning is anotherterm and if we just take that
one first, I guess the rate ofburning it scales like with the
rate of heat release or the heatrelease rate.
Because if I know the rate ofburning, that has units of
kilograms per second, it's massper unit time, the rate of
burning.
And if I know the rate ofburning of something and I know
the heat of combustion of thatobject, that material that is
(21:29):
burning, I can multiply thosetwo terms and that will give me
the heat release rate.
So I can actually, if I knowthis term, the heat of
combustion, which for a liquidfuel is usually very easy to
find out.
For pure materials it's quitesimple there's no such thing as
a heat of combustion for a couchor for a table or for a
computer, but those are made ofmaterials and you can probably
(21:50):
make a guess at what it would be.
But if you know the rate ofburning and you multiply by the
heat of combustion, you get theheat release rate.
But that again doesn't reallychange the temperature of the
fire.
It might change the temperaturewithin the compartment and the
environment around that fire,but the fire doesn't change.
So, going back to your liquidfuels.
I think one of the things therethat is different if you
compare a liquid fuel fire to asolid fuel fire, is simply the
(22:14):
rate of increase of the heatrelease rate.
I mean, you ignite the petrolor a liquid fuel and you get
basically the maximum rate ofheat release immediately.
If you ignite a tear orsomething else, right, the fire
has to grow, it has to develop.
Everything is much slower.
I think that's where most ofthe differences come from,
unless you're doing some exoticchemistry.
Maybe if you've got a methanolfire, that's going to be hotter
(22:35):
than if you have a gasoline fire.
But I mean, I think that's alevel of nuance that is not
going to be that generallyapplicable.
Speaker 1 (22:42):
Okay, and that leads me on to another sort of area
that I wanted to touch on.
Is that obviously a normal fire, in normal atmospheric
conditions, just reacts with air, with 20, whatever percent of
air?
Why does fires involving oxygen?
Why do they burn?
Am I right in saying they burnhotter?
Speaker 2 (22:59):
absolutely okay why is that amazing question?
I think the first thing we haveto kind of recognize when it
comes to this is just howreactive oxygen is.
We kind of take it for grantedbecause we're surrounded by it.
20, 21 percent more or less ofthe atmosphere we breathe is
oxygen, and oxygen is anincredibly reactive molecule.
(23:20):
It really wants to combine withother elements to form other
products.
We think about it because we'reso familiar with it.
But just kind of take a lookaround you at the.
Even at room or roomtemperatures my bike chain gets
rusty and that's because themetal of the chain is reacting
with oxygen and rusting.
Now I live in Edinburgh, right,so it's not that hot here.
(23:41):
So it's a slow process, but ithappens.
If I leave things unattended weget this oxidation process,
oxygen reacting with materialsat low temperatures.
Most other things don't reactat these sorts of low
temperatures every day.
Oxygen is super reactive.
That's the most important thingto remember.
But the other thing is, when welook at air, 20 percent of air
(24:03):
is oxygen.
The other 80 percent or 79percent is nitrogen and nitrogen
is basically completely inertfor most practical reasons when
it comes to fires and when youhave a fuel, that fuel has got
an amount of energy in it.
For each mass of fuel unit mass, one kilogram gram, whatever of
the fuel there's an amount ofenergy and that's the heat of
(24:24):
combustion.
Again, the question is when youburn that fuel you release that
energy, you turn the chemicalenergy into thermal energy.
Where does it go?
What actually happens to allthat thermal energy?
And the answer is it heats upthe products of combustion.
I start at room temperature.
I've got my fuel and my air,then I react to them and then at
the end I have hot products ofcombustion.
(24:45):
So all of the energy from thecombustion process that's
released, all the energy that ismeasured as the heat of
combustion of that material, isreleased and heats up those
products of combustion.
Now, when I'm burning a fuel inair, some of the air that's
being heated up is nitrogen.
It's not like I'm an activeparticipant in the reaction,
(25:05):
it's just there.
When I burn a fuel in air, thetemperature rise that I get is
proportional to the amount ofproducts of combustion.
A big chunk of those productsof combustion is nitrogen and
that's not doing anything.
That's absorbing that energyand heating up the nitrogen.
If I increase the oxygenconcentration, then I've got
more oxygen and that is areactive thing.
So I react it with more fueland I release some energy.
(25:25):
But there's now proportionallyless nitrogen to heat up.
So I've got a smaller heat sinkfrom the nitrogen and that
means I achieve higher flametemperatures.
So that's why, if you'reburning things in an increased
oxygen environment, it gets veryexciting very quickly.
I mean, we do some flame spreadexperiments here in the lab and
if we do it in an airenvironment, we have flame
(25:47):
spread rates I think on theorder about 0.3 millimeters per
minute.
Okay, we increase the oxygenconcentration from 20% to 40%
and we go from 0.3 millimetersper minute to three millimeters
per minute.
So we increase the flame spreadrate by 10 times, by double the
oxygen concentration.
So fires are really sensitiveto this.
And it works the other way too.
(26:09):
If you decrease the oxygenconcentration just a little bit,
you will get to the point wherethe flame can't sustain itself
and will have what we callquenching.
Sometimes you might see it asextinction, and that's the
process that you're using ifyou're applying a carbon dioxide
extinguisher onto a fire, or ifyou're trying to smother a fire
in a ship's cargo hold byinjecting inert gas.
(26:29):
It's that process.
So oxygen is really important.
It's really reactive and arelatively small increase in the
oxygen will give you hugeincreases in the fire behaviors
and flame temperatures and so onfantastic, okay, brilliant.
Speaker 1 (26:40):
That was awesome.
Okay, let's move on to lowerexplosive, higher explosive
flame limits, etc.
Just take me through that andmaybe stoichiometric mix yeah,
okay.
Speaker 2 (26:51):
So I mean the easiest place to start here is the
stoichiometric mixture.
Now that sounds like a veryfancy word, but actually all it
means is, if you've got astoichiometric mixture, it means
from a chemical point of view,from the point of view of the
chemistry, when you've got fuelplus oxygen or fuel plus air
goes to carbon dioxide, waterand nitrogen.
If you've got the stoichiometricmixture, what it means is
(27:12):
you've got the perfect amount ofoxygen or air to react with all
of the fuel, so you've got aperfectly balanced combustion
reaction and you will release,in theory, the maximum amount of
energy.
You will achieve the highestflame temperatures again,
because you don't have anyexcess.
There's no spare fuel, there'sno spare oxygen, so you release
the most energy and you heat theminimum amount of products of
(27:35):
combustion.
So that's what thestoichiometric mixture is.
It just is, when you've got aperfectly balanced ratio of fuel
to oxygen, you produce carbondioxide and water, and then your
nitrogen as well.
The lower explosive limit is, Ithink, maybe the next easiest
one to discuss, because thestoichiometric mixture will burn
easily If you go to the lowerexplosive limit then, what
(27:56):
you're doing is you're addingmore and more air or more and
more oxygen into the mixture andthat's decreasing the
proportion of fuel.
You go from your stoichiometricmixture and then you add more
and more air, so proportionally,the amount of fuel you have in
that mixture is decreasing.
At some point you'll have notenough fuel to sustain a
combustion process becauseyou've basically effectively
(28:19):
only got air.
There's not enough fuelmolecules in there to propagate
the chain reaction of thecombustion process.
The temperatures are too lowfor the chemistry, for the
chemical reactions to happen atthe rate that they need to.
You won't have a fire and formost materials most practical
materials that's on the order ofa few percent.
So methane, natural gas, thelower explosive limit is about
(28:39):
5% methane in air.
Then if you go to otherhydrocarbon fuels, as you go to
longer molecules, that numberdecreases down to about 1.8% or
something like that so smallnumbers which has important
applications for safety becauseit means you don't need much
fuel in an air environment topotentially have an explosion to
(29:00):
have that process occur.
And what's happening there isagain.
It's this balance between theenergy that you can release from
the combustion process and howthat energy is transferred
through the mixture in order tosustain the chemical reaction.
I think the good thing for mostfire investigators to think
about is that lower explosivelimit is usually for most normal
things.
And of course there's alwaysexceptions and people can put in
(29:21):
the comments whatever they likeabout best exception they can
find to what I'm saying, butit's usually a small number.
Like I say, methane is aboutfive.
One of the exceptions I canthink of are carbon monoxide and
hydrogen.
They're weird in terms of theirflammability limits, quite wide
and also starting at differentvalues, but for most common
fuels it's a few percent and theupper flammability limit is the
(29:41):
opposite.
So I take my stoichiometricmixture where everything is
perfectly balanced, and thistime I proportionally just
increase the amount of fuel thatI have available and eventually
I will increase the amount offuel to a point where there's
too much well, there's notenough oxygen, too much fuel,
and then the mixture won'tsustain the combustion process.
Again, the temperatures are toolow and the chemistry just
(30:02):
doesn't work.
That's called the upperflammability limit or the upper
explosive limit and again,that's usually on the order of
well.
For methane, to go back to that,it's 15% and it's usually on
the order of kind of 5% to 10%for hydrocarbon fuels.
And again, why does that matter?
(30:41):
It's a good question gas,because those explosive limits
are a few percent.
The low explosive limit is onepercent, that kind of order of
magnitude.
So it means I don't actuallygenerate that much pyrolysis gas
from my solid material to beable to sustain a flame.
And I think, again, that's aninteresting link between what we
understand from nice mixturesof gases and simple combustion
(31:04):
and simple fuels back to thesolid fuel.
Is that link between pyrolysisand the amount of gaseous fuel
you need to sustain a flame?
It's quite a powerful tool fromthat point of view.
Again, for the investigator, itdoesn't just apply to
explosions, the low landabilitylimits, it actually applies also
to all the processes aroundburning of solids, whether it's
ignition, flame spread or whathave you.
Speaker 1 (31:25):
Sure, but those limits though.
Are we looking at a point inspace, or are we looking at a
cubic metre compartment?
Speaker 2 (31:33):
Again, really good question the literal meaning of
the data that you would find ina table.
If the flammability limit lowerflammability limit of methane
in air is 5% by volume, that isfor a well-mixed volume of gas.
That is all at thatconcentration, of course.
In reality if you've got a gasleak in a property, then you're
(31:53):
going to.
If the gas leak is at highlevel and your gas is lighter
than air, then probably you havea very fuel-rich band that
doesn't have enough oxygen toburn near the ceiling.
But somewhere you will have agradient and there will be that
flammable mixture.
And the issue with that isthere's always somewhere that
that flammable mixture willexist.
Right.
Physics tells you that you havediffusion and all these other
processes.
(32:13):
That means you will have thatflammable mixture and as soon as
it ignites there you get all ofthe processes around the
expansion of the gases, thefluid mechanics, the mixing of
the gases that you generate fromthat.
That means your nice stratifiedlayer is now all turbulent and
mixed up, so you actually have achance of then getting that
explosion.
Even though you're maybethinking that it's too rich,
(32:34):
it's too fuel rich up there toburn Somewhere, there will be
that kernel that is at the rightmixture, and then you will get
that propagation.
Speaker 1 (32:42):
And you need that.
Let's move on to sort ofignition sources and maybe we
can talk about the energyrequired for that and how
different materials needdifferent energies.
But you need an ignition sourcejust at that one area or that
one point.
Yeah, for these mixers, that'sit.
Speaker 2 (32:55):
I mean, the fire science runs out quite quickly
here so we can define theconditions that might lead to
ignition.
But if you have that kind ofstratified thing of if we go
back to the idea of a leak ofnatural gas, it's lighter than
air, sorry, so it will rise upto a ceiling and it will
accumulate there and you willhave all sorts of mixing and
entrainment processes.
I mean, really it's verydifficult to have any confidence
(33:18):
in what the composition of thatmixture is going to be.
So you end up with this idea of, if you're doing it from a fire
investigation point of view,thinking, well, what ignition
sources are there?
Is there anything in thatlocation that could cause
ignition?
Because any argument that youmight try to put of, oh well, in
this region it's going to betoo fuel rich and in this region
it's going to be too lean, Ithink you're having to learn
that kind of stuff in thoseapplications, unless you've got
(33:39):
some other information to drawon or some sophisticated
numerical modeling or somethinglike that.
But to your point on ignitionsources.
One of the things that is verydifficult to do in fire
investigation is to demonstrateis your ignition source
competent to ignite the mixture.
That is there Because you don'tknow what the mixture is most
of the time.
So that's a real challenge withthat.
But the thing I was taught along time ago now is you know
(34:01):
any flame will ignite anyflammable mixture because you've
got the ongoing chemistry andall the kind of ingredients that
you need to start a chemicalreaction with that.
But sparks are a bit different.
Some sparks are capable ofigniting mixtures and others
aren't.
And unless you know what yourmixture is, you kind of have to
err on the side of probably thespark did ignite the mixture.
I always find arguments aroundspark energies and things we
(34:23):
just don't know enough Pick aside right, and that's not where
we want to be, as I think of acommunity right.
We want to be objective andaccurate.
So that's tricky, that's anunknown, I would say, for a lot
of practical scenarios.
Speaker 1 (34:35):
Sure, I think we need to recognize that fire science
is pretty, fairly young in termsof general science, isn't it?
I mean, are well establishedand mr bacon and all that kind
of good stuff, but actuallystudying it's only been the last
less than a century, isn't itlast 40, 50 years?
Speaker 2 (34:52):
I kind of struggle with this because, yeah,
sometimes we are.
I don't know that age is theright way to describe it.
I think it's more like amaturity thing, because we have
been studying fire andcombustion processes, you know,
really since the industrialrevolution.
That's when we started reallyin earnest doing that and that's
a long time ago.
If you talk about somethinglike quantum physics, I mean
that really is only since maybethe kind of early middle part of
(35:14):
the 20th century.
And look where that's got us.
We're building atomic bombs,nuclear power stations and
whatever, because that was builton a very mature set of physics
and ideas and a large communityof people working on that
internationally.
I think fire science we havebeen studying it for a long time
and we've kind of borrowedideas from combustion science.
We look at material science andwe're kind of this weird
(35:36):
mongrel discipline of differentideas from different places and
that's really good because thatbrings good practice from these
other areas.
The downside on fire sciencestuff is it is extraordinarily
broad.
I mean the longer I spendteaching this stuff, the more
appreciation I have for that.
When I'm teaching theundergraduate students, I mean
they have to know chemistry,they have to know heat transfer
they have to know fluidmechanics and in isolation you
(35:59):
have to know them all whenthey're happening at the same
time.
The complexity of the problems,I think, is really
extraordinary.
Each individual part is quiteeasy on its own.
We know how to solve heattransfer problems in most
scenarios.
We know how to deal withcombustion chemistry.
I mean we build jet engines andwe've optimized that to reduce
the emissions, to increase theefficiency, all that stuff.
(36:19):
So we know how to do that.
And the fluid mechanics.
We're dealing with relativelysimple flows.
They're relatively lowvelocities and whatever.
We know all these individualcomponents.
But putting them together iswhat makes fire really difficult
.
And I think it's where you alsohave to be a little bit humble,
because I think I know quite alot about those things.
Over the years I've learnedquite a lot.
But ask me what the implicationis on the structural
(36:40):
performance of a steel beamthat's being heated up by the
fire.
I don't know.
That's a problem I have to handoff to one of my colleagues who
knows about that kind of stuff.
So I think it's almost toosimple to say, oh, we're a
relatively young discipline,because if you trace the history
you can draw a line all the wayback to the early studies of
thermodynamics and things.
What we are is a very smalldiscipline and everyone has
(37:03):
their own specialty within thatdiscipline.
So actually having some ofthese conversations is quite
difficult on a very technicallevel when you compare it to
something like some of thebranches of physics where you've
got hundreds or thousands ofscientists all with exactly the
same background talking aboutexactly the same problem.
We are people with verydifferent backgrounds trying to
solve quite a wide range ofproblems and I think very often
(37:24):
there's just a total mismatch inknowledge in some of these
areas and just people just knowdifferent stuff.
Speaker 1 (37:29):
So getting everyone's opinion yeah, and I think you
summed it up earlier on quitewell is that combustion science
is very different to the firescience, because combustions you
have a set of knowns, you knowthe fuel, you know how they're
going to interact, you know thatthey're pure or not pure,
whereas with fire sciences, bynature, it's chaotic.
There's so much going on.
Speaker 2 (37:47):
Yeah, absolutely, I mean if fire science would be
easy, if we knew how much fuelwas being injected, if you knew
how quickly something waspyrolyzing, if we could predict
that ahead of time, then that'dbe very, very easy problem to
solve.
The reality is we don't, andthat, for me, is the challenge
of fire science.
It's about figuring out how youturn your solid fuel into those
gas molecules that can thenreact in a flame.
(38:07):
We've seen.
Speaker 1 (38:08):
I'm from a practical point of view.
We've seen very similar fires.
Nist and the atf have done verysimilar cell burns with the
same materials but got differentresults because of different
humidity or different aireffects and stuff like that.
So it's's very difficult.
Speaker 2 (38:23):
Yeah, you take that kind of stuff and you think
about the extreme.
Example is wildfires, whereyou've got also the effects of
weather and climate and verycomplicated fuels to account for
as well.
I mean, I think what I alwayssay about fire science to our
undergrad students is if yougive me one material, a pure
material I can probably tell youquite a lot about that and
predict well how it's going toburn.
(38:43):
But a pure material, I canprobably tell you quite a lot
about that and predict well howit's going to burn.
But the minute you put thatmaterial into some kind of
system whether that's acompartment, a forest, the
engine room of a ship, the cargohold of a ship and there's
other things, there areboundaries and there are other
materials that are going tostart reacting Soon, as you get
(39:08):
to that scenario, it becomesreally, really difficult.
And I think that's where, from afire investigation point of
view from my experience, that isthe challenging part is I don't
have pure materials anymore,I've got systems of materials
that are interacting with eachother and with the environment
and the problem becomesextraordinarily complicated.
So you've got to have thosefundamentals pinned down really
well so you can begin tosystematically work through.
Well, what happens to the waythat this material burns?
When there is some airflow, orwhen it's also being heated by
this material, or when the fireservice has sprayed some water
on it?
What's that going to do to whatit looks like?
Speaker 1 (39:28):
so I think for me that's one of the big challenges
in fire investigation, anytimeI it's not very often to be fair
, but anytime I show up to afire scene and have a look
around, sure I mean, you knowthe geometry of fire and I
remember on the course that youtaught me on firmly thin
materials, firmly thick material, where the fire is within that
cell, if it's in the middle ofthe room or if it's up against
the wall or in the corner, andyou get that feedback and all
(39:49):
that.
It makes a massive difference.
So there's so many variables.
But listen, roy, I'm I'm reallysad to sort of cut you off for
you, because we've only got intoabout half of not even half of
what I wanted to get into.
But we're sort of running outof time.
But hopefully you'll come backon we can ask some of these
other questions.
But I've got loads of otherquestions.
But just to sort of summarizethis, there's, if I'm an FI and
(40:09):
I want to know more about firescience, which NFPA 921 says we
should have as one of theprerequisites at 1033, our kind
of Bible, if you like.
What would you recommend?
Where would you recommend we go?
Is there a textbook or is therea course?
I mean, obviously I wouldthoroughly recommend you, but I
know that the university startedrunning a new MSc.
Speaker 2 (40:32):
Is that something you'd recommend?
Yeah, so I mean you've kind ofmentioned three of the things I
would suggest.
So the first thing is grabyourself a copy of Introduction
to Biodynamics by DougalDrysdale.
I think that is still the mostgeneral, most easily accessible
textbook on these things and Ithink if you are presenting
reports or to the court, then Ithink that level is absolutely
sufficient for that kind ofapplication.
But reading a textbook on itsown, extraordinarily dry and
(40:54):
dull and I will confess I havenever read it cover to cover,
right Sat down, you know, on asun lounge or on holiday, and
flicked through, I've never donethat.
But I think I have read everypage in it at some point.
Get yourself out there and rollon a course.
I mean we obviously I'm goingto say about our fire
investigation short course thatwe run every year usually
happens at the beginning ofsummer, and keep an eye on our
website and stuff and you'll seewhen that's happening and in
(41:15):
that we cover practical firescience things but also fire
specific info about how toinvestigate the fire scene, the
legal aspects and things likethat.
If you're interested much morein the fire science side of
things, then we have an MSc atEdinburgh.
It's a one-year MSc, that is,two semesters of teaching and
then the final semester is adissertation.
(41:35):
Do some research there as well.
And the other thing that I canrecommend of course I'm going to
say those things right, becausethose are the Edinburgh things
the other thing I would saythat's really important in fine
investigation is to know whatyou know and to know what you
don't know and to be completelycomfortable when your knowledge
runs out.
To speak to someone, because youlearn a lot by speaking to
(41:57):
people and asking for help fromthe right people, and we're
always open for an email thatsays I've got this problem.
Can we have a chat?
Sure, no worries, pick up thephone and whatever.
We can have a discussion or anemail back and forth if you're
looking for information or help.
If it's a quick question, we'revery happy to try and answer
and point in the right directionfor these things, because I
(42:17):
mean, I don't know everything.
Ricky, my colleague, doesn'tknow everything.
Professor Bisbee here doesn'tknow everything, but what we do
have here is six or sevenacademics between us know a lot.
So it's not that I know and cansolve every problem.
So talk to people, know whatyou don't know and be very okay
with that, because it's muchbetter, I think, to say I'm not
sure about this than to havekind of an overconfidence in an
(42:39):
opinion or in a point of viewthat is not substantiated in an
opinion or in a point of viewthat is not substantiated.
The last thing I guess I amalways surprised at how many
fires there's a specific causefound.
For A lot of the time you'relike well, okay, I'm not so sure
about that.
There's maybe these othercauses that seem to not be
addressed very much, and I thinkthere's sometimes too much
emphasis on origin and cause.
(43:01):
Now, that is, of course, a veryimportant part of the process,
but what you've got to also dois figure out the fire
development that resulted fromthat, because as the
investigator you go to the scene, the fire's over, you've got to
turn the clock backwards.
So you're solving this inverseproblem and they are, by
definition, ill-posed right andthere's mathematical proofs that
you can do to show that there'smany initial solutions or
(43:22):
initial conditions can get youto a final solution.
So I think doing that kind ofjust logic test a lot of it
isn't even detailed science.
A lot of it is just sittingdown with a friendly colleague
or a friendly person and thinklook, what do you think about
this sequence of events?
Is there something I've missed?
Is it something I've not gotthere?
Because it's only by talkingand being open to other
solutions being proposed bycolleagues or collaborators that
(43:45):
you get to a good answer.
There's no shame in changingyour mind no, definitely not.
Speaker 1 (43:50):
it's always better to change it at the report stage,
having had it peer-reviewed, andrather than when you get to
court and somebody else has someinformation that you didn't
have or someone didn't gothrough that process or question
it, and every firm firm thatI've worked for they always had
a robust kind of a secondchecker and even a third checker
.
If it was a big case and it wasgoing to court, you'd have a
(44:10):
third review and there's noshame in being picked apart
because you know what you know.
You don only see what younecessarily see, and sometimes
it takes that third eye orsecond eye to say, well, have
you thought about this?
Or what about that particularpattern, or when did that window
break, for example, those sortsof things.
Speaker 2 (44:31):
Totally.
Yeah, absolutely.
It's easy to say to beopen-minded and we all know
ourselves.
You bring your own biases andyour own whatever.
But it's recognizing that andthen being open to the idea of
something different, becausethat makes everything better.
As you say, you're more likelyto get to the more likely
conclusion or more kind ofconsensual opinion, and the more
agreement you can find, thebetter.
And then you're arguing, if youneed to, over some other points
(44:53):
, but you can agree most of whatyou need to.
Speaker 1 (44:57):
Fantastic.
Thanks, roy.
I really appreciate your time.
Again, this isn't a sales pitchfor Edinburgh University, but
I've done the course and I cansay that from a fire science and
that sort of it was definitelythe best course I've been on in
relation to that kind ofunderstanding of the chemistry
and the fire science thermallythin, thermally thick materials
and how that's applied in fireinvestigation.
That short course wasabsolutely fantastic and I've
(45:18):
been after you for a long timeand we've managed to get you on.
I hope you'll come on againbecause there's loads that I
want to.
There's loads more I want tocover.
I'm sure my audience isthinking I wish you'd ask this
question.
I wish you asked that question.
But thanks ever so much.
We really appreciate your time.
Absolutely wonderful, mate.
Thanks for having me on.
Cheers, mate, thank you.
Hey, thank you for listening tocsi on fire.
(45:39):
Please don't forget to like,subscribe and suggest future
topics on our web page.
Remember factor non-verbal.
Take care, good hunting.
I hope to see you on the nextone.
Cheers.
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