Theoretical analysis of the backdraft phenomena induced by liquid fuel

ARTICLESChinese Science Bulletin 2006 Vol. 51 No. 3 364- -368the experiments, a dynamical model of backdraft phe-DOI: 10.1007/s11434-006-0364-5nomena induced by gas fuel (methane) was estab-lished9. Furthermore, from the view of system controlTheoretical analysis of thesurface and catastrophes theory they analyzed typicalevolution of the eigenvalue and the dimensionlessbackdraft phenomena inducedtemperature excess of hot smoke layer with the dimen-sionless time, evolution of the dimensionless energyby liquid fuelgain and loss rate with dimensionless temperature ex-cess, as well as the dimensionless temperature excessGONG Jian, YANG Lizhong, CHEN Xiaojunwith concentration of combustible gas in the hot gas& GUO Zaifulayer. However, there is no comparison between theo-State Key Laboratory of Fire Science, University of Science and Tech-retical analysis and experimental results. As yet there isnology of China, Hefei 230026, Chinano research of dynamical model of backdraft phe-Correspondence should be adesedd to Yang Lizhong (emal: yanglz@ nomenon induced by liquid fuel. In the present paper, austc.edu.cn)dynamical model of hot smoke layer temperature ver-Abstract A dynamical model of temperature ofsus time is established based on experimental results ofhot smoke layer is quantitatively established based backdraft induced by liquid fuel in ref. [8]. The dy-on the whole backdraft procedure induced by liquidnamical model consists of the whole procedure of ex-fuel. The whole procedure consists of the preburn fireperiment including the preburn fire (the first period), .(the first period), the secondary fuel injection (the the secondary fuel injection (he second period) andsecond period) and backdraft development (the thirdbackdraft development (the third period), taking intoperiod). The model considers enthalpy loss of liquidaccount energy loss rate of volatilization from liquid tofuel volatilization and hot smoke layer mass gain. Ingas, mass increment of hot smoke layer during the ex-this paper, simulative results of the model are wellperiment and enthalpy loss rate resulting from the masscompared with experimental results, and simulativeincrement. Additionally, we present a temperatureresults of the model are analyzed. Furthermore,simulation of the whole backdraft experiment and makecombustion efficiency under limited ventilation anda comparison between simulation results and experi-practical combustion reaction rate are worth investi-mental results.gating.Keywords: liquid fuel, backdraft, volatilization, dynamical model, 2 Dynamical model of the backdraft phenomenontemperature.induced by liquid fuel1 IntroductionThomas et al."0) idealized the energy of a hot smokeThe backdraft phenomenon is defined as an explo-layer for a room fire bydE,sion caused by the inrush of fresh air from any sourcedt= G(T,t)- L(T,t),1)or cause (sudden open of door or crash of glazing), intoa burning building, where combustion has been taking where G(T, ) and L(T, t) are energy gain rate and en-place in a shortage of air. Before the supply of fresh air,rgy loss rate of hot smoke layer, En is energy of hotthere is a considerable amount of combustibles in hot smoke layer, T is temperature and t is time. Therefore,smoke layer, therefore, a deflagration or a flashover T is governed by the forms of G and L which are func-with a fireball would take place if the combustibles aretions of T andt.mixed with fresh air under the effect of fire source.On the basis of the qualitative description, tempera-Because of its concealment, abruptness and violent ture of hot smoke layer can be defined asdevastation, the occurrence of backdraft phenomena indT_口_ G-L-Hroom fires is a hazard that threatens the safety of people,2)especially firefighters- 4!中国煤化工In recent years, the method to study backdraft phe-where EYHCN MH G is specifc heatnomenon was mainly limited in the experimental way. at constant prvovuvI Lv. Ounv iwyvr, and m 1S masshe experiments included backdraft phenomena in- of hot smoke layer. The compartment is assumed to beduced by gas fuel'and by liquid fuel8l. Based onairtight.364Chinese Science Bulletin Vol. 51 No. 3 February 2006ARTICLES2.1 Dynamical model of *the first period67 s.G,L and H are denoted byMass of hot smoke layer can be defined asG=0,(11) .m=ρV +m x△t,(3)L=Qw +Q,(12)where ρ is density of hot smoke layer, V is volume ofhot smoke layer, m is fuel supply rate of the first peH= imnXcp3X(T-Ta),(13)riod, and Ot1 is time of fuel injection in the first period,where Q，is energy loss rate of volatilization of liquidwhich increases with the progress of experiments.fuel, Cp3 is specific heat at constant pressure of fuel gas.tionsltl.G,L and H are described by the following equa-Qw is defined by eq. (5), Tw is given byG=q xmxH。,(4Tw =U.(T-Ta2)+Ta2.(14)Uc is defined in eq. (9). The difference from the firstL=Qw = A,[εσ(T4-Tt)+h,(T -T.)],(5period is Ta2 here, which is the temperature of steelH=mxcp2X(T-Ta),(6boundary.where Q1 is combustion efficiency of the first period,Qw is given byHe the effective heat of combustion of No. 2 diesel,Q, =Hg xi,(15)Qw the energy loss rate through the walls, Aw the areawhere Hg the heat of vaporization of No. 2 diesel. Bof room exposed to hot smoke layer, ε the hot smoke cause boiling point of No.2 diesel is 210- -235°Cl18,layer emissivity, σ Stefan-Boltzman constant, Tw thethere are two assumptions: that is, all the liquid fuel istemperature of walls, ht the convection heat transfervolatilized and liquid fuel volatilization rate equals fuelcoefficient, Cp2 the specific heat at constant pressure ofsupply rate.carbon dioxide and water steam mixture based 01stoichiometric combustion of No.2 diesel, and Tal the2.3 Dynamical model of the third periodtemperature of ambient air.Tm, Aw are defined asm=ρV +mXt +m2 Xtz,(16)Tw =U。(T -Ta)+Ta，(7wheret= 290sl8),t=231 s8.Aw =2x(LrXWr + WxXHR +HpXLR),(8G,L and H readwhere LR, Wk, Hr are length, width and height of room,G=Q2xxXin,xHg,(17)respectively. Hot smoke layer is assumed to fill withL=Qw = A,[εσ(7° -Tt)+h,(T -T.)]，the whole compartment.Uc is surface termperature factor, given by12]H=in。XCp1 x(T-Tn)，(18)U =e(9) ”where Q2 is combustion efficiency of the third period, xwhere β= 0.539, Bi = 0.338 are constants. kw Pw. Cwis fuel-ventilation control function, i, is mass com-are thermal conductivity of wall, density of wall andbustion rate of combustible gas, and ma is mass flowspecific heat at constant pressure of wall. .rate of air into room via door. Some parameters in L are2.2 Dynamical model of the second perioddefined as eqs. (7)- -(9).Mass of hot smoke layer can be expressed asx is given bym=ρV +minXt +mn2X Ol2,(10)[1ing/mng≥S,,(19)where t1= 920s8，in2 the average fuel supply rate oflim。/(mjS,)mng/in,