3. Fundamentals of Fire Behavior and Building Construction
Fire (exothermic oxidatoo reactoon roddces heat aod light. There are two ty es of combdstoon
Flaming combuston (gaseods combioatoo where both fdel aod oxidizer are gasesn;
Smoldering combuston (solid fdel aod gaseods oxidizer (e.g. oxygeo io airnn.
Maoy fres start as smolderiog fres. The difereoce comes dde to the oatdre aod cooditoo of the
fdel, aod the availability of oxygeo. Both cadse a difereoce io rate at which heat is beiog roddced.
Basic Combustion
Cooditoos for fre are a combdstble fdel, ao oxidizer, eoergy aod a self-sdstaioiog chaio reactoo.
Removiog ooe of the fodr, cadses the fre extogdish. However, the reseoce of all fodr, doeso’t
meao that there will be a fre (e.g. the fdel mdst be reseot io a sditable hysical staten.
Flaming Fire
The colodr of a fame is determioed artally by elemeots io the reactog mixtdre; yellow (iocom lete
combdstoon or blde (com lete combdstoon. The reactoos of a flaming lfirei are oxidatve (oxygeo is
the oxidizern. Liqdid fdels cao roddce ooly famiog fres; they caooot bdro themselves aod oot
sd ort smolderiog.
Solid fdels ofeo roddce famiog fres, accom aoied by a
glowiog fre. Charcoal formed afer a famiog fre cao
smolder. Coal has less volatle com ooeots aod so gives of
less combdstble gases thao wood. The fgdre illdstrates the
famiog combdstoo of a solid or liqdid. The heat radiates
ioto the sdrface of the fdel, raisiog its tem eratdre,
eva oratog (liqdidn or yrolyziog (solidn it. The molecdles
that exist dde to this, are dissociated or toro a art by
chemical cooditoos io the fame (e.g. combioe with oxygeon
(reiducing cooditoosn. The plume of hot gases (aod soot
(solidn aod aerosols (liqdidn roddced dde to iocom lete
combdstoon rises by buoyancy. This draws oxygeo ioto the
fame; maiotaioiog the reactoo. Entrainment is the drawiog
of air ioto the bdoyaot ldme.
With oxygeo cooceotratoos <15%, the combdstoo rate will
decrease. The higher the tem eratdres, the lower the oxygeo cooceotratoo cao dro aod stll
sd ort fames; a cooceotratoo of 5-8% is eoodgh for a flammable liuuid with a tem eratdre of
1000 oC.
Structure of Flames
, Most fames are diffusion lflameis; gases or va ors that sd ort the fame difdse odtward from the
sdrface of the fdel aod the oxygeo difdses toward the fdel. If the
cooceotratoos of fdel va ors aod oxygeo
are correct, the mixtdre cao be igoited.
The rate at which fdel is delivered to the
fame is de eodeot oo the total heat fdx
reachiog the fdel sdrface, aod the lateot
heat of va orizatoo (liqdidsn or of
gasifcatoo (solidsn. Io combdstoo
ex losioos, the fdel va ors aod air are mixed together aod theo
igoited io a preimixeid lflamei.
A small fame is called a laminar lflamei; the zooes of high fdel cooceotratoos, dissociatoo, mixiog,
aod combdstoo are defoed io layers. The high tem eratdre arodod the caodle (dde to the airfown
allows the combdstoo of the soot created by the yrolysis of wax to take lace at very high
tem eratdres. Wheo soot esca es the high-tem eratdre zooe, it has beeo (almostn com letely
coosdmed aod tdroed ioto CO 2. If the fame becomes too large, the airfow caooot be cootaioed,
cadsiog the high-tem eratdre zooe to o eo, allowiog some soot to esca e dobdroed. Uobdroed soot
is a sigo of a turbuleint fre.
Smoldering Fire
Characterisatoo of a smolderiog fren abseoce of fame, aod reseoce of hot materials oo the sdrface.
The colodr of the glow is related to its tem eratdre. Glowing combuston is a ooo-self-sdstaioiog
cooditoo (dsdally dde to forced ventlatonn, smoldeiring is self-sdstaioiog solid gas combdstoo.
The ddratoo of high tem eratdres of smolderiog cao be looger thao that of famiog fres. The ratos
of oxidizer aod fdel io a smolderiog fre are oot of direct coocero. The ro ortoos of fdel, oxygeo,
aod foal roddcts are called the stoichiometry of the reactoo. Perfectly balaoced chemical reactoos
(com lete reactoo of all startog materials aod oo wasten are called stoichiomeitric or ideial lreiactons.
Smolderiog combdstoo cao cootode io ao oxygeo-defcieot (<5%n atmos here, io art dde to high
tem eratdres at the oxidiziog sdrface. If oxygeo is sd lied, a famiog fre cao start. Flamiog fres
mostly reach smolderiog io their later hases. The smolderiog material here is oot the origioal fdel,
bdt a derived fdel resdltog from the earlier ortoos of the fre.
Io iovestgatoo; the smolderiog hase geoerally re reseots the ioci ieot stage or the last art of the
rocess. As ao ioci ieot fre it cao be rolooged aod cadse a lot of damage. Mdch of the ost-fre
cleaod or overhaul however, is directed at seekiog odt smolderiog s ots aod eosdriog that they do
oot rekiodle ioto a destrdctve fre later. Exce toos have to be coosidered. The rate of s read of a
smolderiog fre is 10-100x slower thao that of flame spread across solid fdels.
Fires that a ear to start at tmes wheo there is oo obviods sodrce or reasoo for igoitoo are ofeo
atribdted to a rolooged igoitoo mechaoism. Proof of sdch mechaoism reqdires coosideratoo of
the followiogn
Ioital sodrce of igoitoo;
Evideoce of rolooged smolderiog (adeqdate fdel massn;
Natdre of the fdel;
Cadse that cadsed the fre to reach the o eo-fame state.
