ON THE EQUATIONS FOR FLASHOVER FIRE IN SMALL COMPARTMENTS

Transcription

1 International Journal on Engineering Perormane-Based Fire Codes, Volume 3, Number 4, p , 2001 ON HE EQUAIONS FOR FLASHOVER FIRE IN SMALL COMPARMENS R. Huo, X.H. Jin and C.L. Shi State Key Laboratory o Fire Siene, Uniersity o Siene and ehnology o China, Heei, Anhui, China W.K. Chow Department o Building Series Engineering, he Hong Kong Polytehni Uniersity, Hong Kong, China (Reeied 21 Noember 2001; Aepted 30 Noember 2001) ABSRAC Equations or studying lashoer ire were studied experimentally in this paper. Experiments were arried out in a hamber inside a ull-sale burning test hall o the joint projet between he Hong Kong Polytehni Uniersity and the Uniersity o Siene and ehnology o China. hree orrelation equations on estimating the ritial heat release rates or lashoer were onsidered. he two-layer zone model CFAS on prediting lashoer was also studied. Results are useul in understanding the possibility o lashoer in small shops suh as those o a abin design, while arrying out hazard assessment with ire saety engineering. 1. INRODUCION It is essential to estimate the probable ire enironment in a ompartment while using engineering perormane-based ire odes [e.g. 1,2] or proiding appropriate ire saety. O whih, the lashoer phenomenon is o great interest and the Authority should hae a lear understanding on that or approing ire saety design. Literature inluded the thermal lashoer in a ompartmental ire analyzed by homas et al. [3] based on explosion theory. For the rate o heat gained o the smoke layer higher than the rate o heat lost, it is possible to bring about lashoer. Single zone model was analyzed by Babrauskas [4], and Quintiere and MCarey [5]; and with Computational Fluid Dynamis (CFD) by Lokwood and Malalasekera [6]. Analysis o nonlinear phenomenon on heat release equation was reported by Bishop et al. [7] and Graham et al. [8]. On the other hand, ire models are ommonly used in hazard assessment. here are dierent iews [e.g. 9] on how good the equations aailable or estimating the ire enironment and ire models an predit. Although there are many experimental data aailable in the literature on assessing or justiying the models, systemati studies on experimental ires in this part o the world are absent. Carrying out ull-sale burning tests would be good in proiding those inormation, and assessing the aailable equations and models or estimating the ire enironment suh as smoke temperature. In iew o that, an experimental burning hall [10] was built as a joint projet between the Uniersity o Siene and ehnology o China (USC) and he Hong Kong Polytehni Uniersity (PolyU). A series o ull-sale burning tests on lashoer ires were perormed. Results are useul or understanding the possibility o lashoer in small retail shops, inluding the abin design [11] whih is ommonly used in airport terminals and railway stations. he ire enironment in the small shops must be learly understood beore putting in appropriate ire series installation. 2. WO-LAYER ZONE MODEL A two-layer zone model [e.g. 12] as shown in Fig. 1 is ommonly used in ire hazard assessment. Conseration o enthalpy gies the rate o enthalpy inrease o smoke layer E & s as: d E & s s = Cpms dt (1) where C p is the speii heat apaity o smoke, m s is the mass o smoke layer and s is the smoke layer temperature. E & s an be expressed in terms o the enthalpy gain E & g and enthalpy lost &E l as: E& s = E& E& (2) g l where E & g omes rom the enthalpy inlux rate o the plume E & p ; and &E l is expressed in terms o the 158

2 International Journal on Engineering Perormane-Based Fire Codes enthalpy lost through the wall E &, eiling jet E & o and radiation E & sr : E & = E& + E& + E& l o sr with E & o, E & and E & sr expressed in terms o the mass o gas m& g, surae area A u and A l o the upper and lower layers, wall temperature w, emissiity o smoke ε, and Stean-Boltzman onstant σ o alue Wm -2 K -4. E & = m& C (3) o g p s E & = A h ( ) (4) u s w E & 4 4 εσ(a + A )( ) (5) sr = u l s Mass onseration on the entire system gies the mass rate o smoke lowing out o the upper part o the ent m& g in terms o the air intake rate m& a at the lower part o the ent and pyrolyzed rate o the uel m& : g m & = m& + m& (6) a here should be suiient air or ombustion beore lashoer and so the ire is uel-ontrolled. E & p an be expressed in terms o the ombustion eiieny η, alorii alue o the uel H, and onetie ration o heat release rate to the plume λ as: w E & = λ ηm& H (7) p For liquid uel as used in this experimental study, pyrolyzed rate m& is expressed in terms o the alorii alue o the uel H, pool ire area A and total radiatie heat lux o the lame q & as: A & = & (8) m q tot H where q & tot is expressed in terms o the radiatie heat lux o the wall q & wr, the re-radiated heat lux o the lame q & rr, the radiatie heat lux rom the lame q & pr and the thermal radiation lux rom the smoke q &. tot sr q & = q& + q& + q& q& (9) pr wr sr In this paper, q & wr and q & rr are relatiely smaller than q & pr and q & sr, and so being negleted in the alulation. Flashoer may our i E & g is greater than &E l. One lashoer ours, the ire is hanged rom uel-ontrolled to entilation-ontrolled. Under this situation, E & p annot be alulated by equation (7). rr tot Ė Ė p Ė o Ė sr Fig. 1: wo-layer zone model 159

3 International Journal on Engineering Perormane-Based Fire Codes he eets o entilation on deeloping o a ompartment ire were studied by Kawagoe [13]. With large olume o experimental data, m& was ound to be related to the entilation ator A H, where A and H are the area and height o a ertial opening o the ire room. his was simpliied urther by Babraukas [4]. Further analysis arried out by homas et al. [3,14] and Quintiere et al. [15,16] led to seeral orrelation equations. Note that the rates o air intake and smoke lowing out would be hanged beore and ater lashoer. For a pre-lashoer ire, m& g was ound by Quintiere [16]: 2 m & 1/ 2 o o g = C1ρoA H 2g 1 3 (10) where C 1 is a low oeiient ound to be about 0.7 by Bishop et al. [7], g is the aeleration due to graity o 9.8 ms -2, o is the initial air temperature o the ompartment (taken as 300 K) and is the aerage temperature o the smoke layer. Ideal gas law gies the air density ρ at as: P ρ = (11) R P an be taken as the atmospheri pressure o 101,325 Pa and R is the ideal gas onstant o 287 Jkg -1 K -1. In iew o equation (10), m& g depends not only on the entilation ondition, but also on the smoke temperature. For a post-lashoer ire, entilation ator as: a 2 1/ 2 m& a depends on the m & = C A H (12) where C 2 is the proportionality onstant o alues lying between 0.4 to 0.61 kgs -1 m -5/2. m& also depends on the entilation ator as: 3 1/ 2 m & = C A H (13) where C 3 is about 0.09 kgs -1 m -5/2. Putting equations (12) and (13) into equation (6): g 4 1/ 2 m & = C A H (14) where C 4 is o alue about 0.6 kgs -1 m -5/2. 3. PREDICION OF CRIICAL HEA RELEASE RAES FOR FLASHOVER here are many equations deeloped or prediting the ritial heat release rates or lashoer. In at, systemati estimation on the smoke temperature was reported by Walton and homas [17]. he three equations listed below are ommonly used: Babrauskas Equation [4] aking the smoke temperature rise o 575 C (with ambient temperature o 20 C) as lashoer, the ritial heat release rate or lashoer Q & (in kw) is: Q & = 750A H Comparing with experimental results, twothirds o the data o Q & were lying between 450 A H to 1050 A H. MCarey, Quintiere and Harkeleroad (MQH) Equation [15] Upper layer temperature rise was itted by oer 100 sets o experimental data: = 480 gc ρ A p o Q& o H 2 / 3 h ka gcpρoa H (16) aking o 575 C as the riterion or lashoer, and putting in numerial alues o C p, ρ o and o, Q & is expressed in terms o the aailable heat transer area o the room A and the heat transer oeiient h k : ( h A A H ) 1/ 2 Q & = 740 (17) homas Equation [14] k Heat balaning o the smoke layer with simpliiation gies two terms on heat lost &Q l and m g C p ( o ), Q & is gien by: Q & = m g C p ( o ) + &Q l (18) Data analysis gies: Q & = 378A H + 7.8A (19) 1/ 3 160

4 International Journal on Engineering Perormane-Based Fire Codes 4. EXPERIMENAL SUDIES A hamber o length 4 m, width 3 m and height 3 m onstruted with double layers o ire-rated gypsum board o 7 mm thik was used or the experimental study. A door o width 1.6 m and height 2.2 m was onstruted as shown in Fig. 2. It was plaed in the PolyU/USC burning hall [10] o length 24 m, width 18 m and height 30 m. Diesel pool ires o three dierent sizes were tested: S1: Square tray o size 1 m by 1 m S2: Square tray o size 0.5 m by 0.5 m C2: Cirular pan o diameter m he uel was put aboe a weighing deie or measuring its transient mass during the burning proess. hree thermoouple trees A, B and C were plaed inside the hamber or measuring the smoke temperature. hermoouple trees A and B were plaed lose to the wall. here were ie thermoouples plaed at 5 m, 40 m, 80 m, 120 m and 165 m ertially below the eiling. hermoouple tree C had 10 thermoouples arranged in a -shape, with ie thermoouples plaed horizontally at the same height with a separation distane o 40 m rom eah other. Another ie thermoouples were plaed ertially with distribution the same as thermoouple trees A and B. All data rom the weighing deie and thermoouples were transmitted to a personal omputer or proessing. A metre stik was used to measure the smoke layer interae with suitable arrangement o illumination. Heat release rate Q & o the ire was estimated by the burning area A b with heat release rate per area Q & o taken as 1986 kwm -2 : Q & = α A b Q & o (20) where α is the ator on ombustion eiieny, taken as 0.7 in this paper. From the hanges o the heat release rate ure, Q & is estimated rom the mass loss rate M & dis (in kgs -1 ) and the alorii alue H dis o diesel o about 46 MJkg -1 : Q & = αm& dis H (21) dis A typial ariation o the diesel mass M dis (in kg) with time t (in s) is shown in Fig. 3 with the ollowing ure itted: M dis = t (22) Six tests with our on a irular pan and two on the square trays were arried out with a summary shown in able 1. he mass o diesel burnt was rom 6 kg to 6.4 kg. he air temperatures outside the hamber but inside the hall aried rom 8.5 C to 15 C. Fig. 2: he hamber 161

5 International Journal on Engineering Perormane-Based Fire Codes Mass o diesel M dis / kg From mass loss rate Steady burning Fitted line ime t / s Fig. 3: Heat release rate o uel Smoke temperature / o C ime t / s Fig. 4: ypial smoke layer temperature able 1: Summary o results est Fire Mass o diesel burnt / kg Hall temp. / C Ventilation height H / m Ventilation ator A H / m 5/2 Height to neutral plane o the ent / m Height aboe neutral plane o the ent / m Burning time / s Aerage temp. o room at steady burning / C Flashoer ourred? Critial heat release rate Q & / kw 1 C to No C to No C to > 600 Yes C to > 600 Yes S to No S to > 600 Yes

6 International Journal on Engineering Perormane-Based Fire Codes he entilation height o the hamber aried rom 0.72 m to 2.2 m, giing the entilation ator arying rom 0.98 m 5/2 to 5.22 m 5/2. he uel was burnt with the height o the neutral plane obsered and reorded. he burning times were rom 267 s to 1281 s. Whether lashoer ourred or not was obsered with the ritial heat release rate or lashoer measured. A typial ariation o smoke layer temperature in the hamber is shown in Fig. 4. From this study, a modiied equation an be reommended: Q & = a A H + b A (24) where a = 378 kwm 5/2 and b = 7.8 kwm -2 or small openings with entilation ator rom 0 m 5/2 to 6 m 5/2 ; and a = 17.4 kwm 5/2 and b = 43.3 kwm -2 or big openings with entilation ator rom 15 m 5/2 to 7.5 m 5/2. 5. DISCUSSION he ritial heat release rate Q or lashoer was measured and ompared with the three orrelation equations as shown in Fig. 5. Experimental data with lashoer ourred are also shown. From the experimental studies, the heat release rate predited by CFAS [12] at 10 minutes (or 600 s) was taken as Q and shown also in the igure. Note that or large openings, homas equation gies: Q & = 17.4A H A (23) 6. CONCLUSION hree orrelation equations on estimating the ritial heat release rates or lashoer to our in a hamber are assessed by ull-sale burning tests. hese are equations due to Babrauskas [4], MQH [15] and homas [14]. Results are also ompared with those predited by the two-layer zone model CFAS [12]. From the studies, an equation was proposed or alulating the ritial heat release rate or lashoer in a small hamber with the entilation ator or large openings taken into aount. his will be helpul in estimating the possibility o lashoer in small hambers suh as a retail shop. Appropriate ire series installation to be proided an then be onsidered. (a) Small opening Fig. 5: Critial heat release rates 163

7 International Journal on Engineering Perormane-Based Fire Codes (b) Large opening Fig. 5: Critial heat release rates ACKNOWLEDGEMEN he projet is ully supported by the President and Deputy President o the PolyU with aount number 1-A078. REFERENCES 1. G.V. Hadjisopholeous, N. Bénihou and A.S. amin, Literature reiew o perormane-based odes and design enironment, Journal o Fire Protetion Engineering, Vol. 9, No. 1, pp (1998). 2. W.K. Chow, A preliminary disussion on engineering perormane-based ire odes in the Hong Kong Speial Administratie Region, International Journal on Engineering Perormane- Based Fire Codes, Vol. 1, No. 1, pp (1999). 3. P.H. homas, M.L. Bullen, J.G. Quintiere and B.J. MCarey, Flashoer and instabilities in ire behaior, Combustion and Flame, Vol. 38, pp (1980). 4. V. Babrauskas, Estimating room lashoer potential, Fire ehnology, Vol. 16, No. 2, pp (1980). 5. J.G. Quintiere and B.J. MCarey, he burning o wood and plasti ribs in an enlosure: Vol. 1, NBSIR , Final report, Noember (1980). 6. F.C. Lokwood and W.M.G. Malalasekera, Fire omputation: he lashoer phenomenon, wenty-seond Symposium (International) on Combustion (1988). 7. S.R. Bishop, P.G. Holborn, A.N. Beard and Drysdale, Nonlinear dynamis o lashoer on ompartment ires, Fire Saety Journal, Vol. 21, pp (1993). 8.. Graham, G. Makhiladze and J.P. Roberts, On the theory o lashoer deelopment, Fire Saety Journal, Vol. 25, No. 3, pp (1995). 9. A.N. Beard, Fire models and design, Fire Saety Journal, Vol. 28, No. 2, pp (1997). 10. W.K. Chow, Y.Z. Li, E Cui and R. Huo, Natural smoke illing in atrium with liquid pool ires up to 1.6 MW, Building and Enironment, Vol. 36, pp (2000). 11. M. Law, Fire and smoke models - their use on the design o some large buildings, ASHRAE ransations, Vol. 96, Part 1, pp (1990). 12. R.D. Peaok, P.A. Reneke, R.W. Bukowski and V. Babrauskas, Deining lashoer or ire hazard alulations, Fire Saety Journal, Vol. 32, pp (1999). 13. K. Kawagoe, Fire behaiour in rooms, Report no. 27, Building Researh Institute, okyo, Japan (1958). 14. P.H. homas, esting produts and materials or their ontribution to lashoer in rooms, Fire and Materials, Vol. 5, pp (1981). 164

8 International Journal on Engineering Perormane-Based Fire Codes 15. B.J. MCarey, J.G. Quintiere and M.F. Harkeleroad, Estimating room temperatures and the likelihood o lashoer using ire data orrelations, Fire ehnology, Vol. 17, No. 2, pp (1981). 16. J.G. Quintiere, Growth o ire in building ompartments, Fire Standards and Saety, ASM SP 614, Amerian Soiety or esting and Materials, pp (1976). 17. W.D. Walton and P.H. homas, Estimating temperatures in ompartment ires, Setion 2, Chapter 2, he SFPE handbook o ire protetion engineering, 1st edition, NFPA SFPE 88, Quiny, MA, pp (1988). 165