A Numerical Study on the Estimation of Heat Release Rate Based on the Flow Field through the Doorway

Transcription

1 A Numercal Study on the Estmaton of Heat Release Rate Based on the Flow Feld through the Doorway SUNG CHAN KIM School of Fre and Dsaster Preventon Kyung Il Unversty 33, Buho-r, Hayang-eup, Gyungsan-s, Gyungbook, , Republc of Korea ABSTRACT The present study has been conducted to examne the possblty of estmaton of heat release rate usng doorway flow n the real structure fre. As a prelmnary study to quantfy the heat release rate based on the flow feld through the openng, a seres of CFD calculatons for the ISO-9705 room has been performed and compared wth experments. Pror to the full numercal smulaton, the grd ndependence test was performed to optmze the mesh sze, and then the doorway flow calculated by the CFD analyss s compared wth PIV and b-drectonal probe measurements. The cal and ectve heat release rate was obtaned by ntegraton of ndvdual cell values through the doorway and the cal heat release rate by the CFD model was matched well wth the nomnal heat release rate wthn 8 %. The ectve heat release rate through the doorway shows reasonable agreement wth the nomnal heat release rate havng maxmum dscrepancy of 20 % when consderng the heat loss effect. In ths matter, ths work mproves our understandng of heat release rate measurement by doorway flow. KEYWORDS: heat release rate, CFD, compartment fres. NOMENCLATURE LISTING A area (m 2 ) Greek c p specfc heat (kj/kg K) α thermal dffusvty (m 2 /s) h heat transfer coeffcent (W/m 2 K) δ thckness (m) H c heat of combuston (kj/kg) ρ densty (kg/m 3 ) k thermal conductvty (W/m K) subscrpts m mass flow rate (kj/kg) a ambent q heat flux (W/m 2 ) g gas heat release rate () w wall t tme (s) L lower layer T temperature (K) U upper layer Y mass fracton (g/g) s sold INTRODUCTION Heat release rate n the feld of fre protecton engneerng s regarded as a prmary parameter to understand the fre phenomena and assess the fre hazards and desgn of fre protecton system. Many mportant parameters such as fuel burnng rate, yeld of toxc gases, smoke producton, and so on are drectly related wth the heat release rate from fre, hence, the heat release rate s generally recognzed as a measure of hazardness n most fre scenaro. Because the thermal energy released from a fre depends on the amount and type of burnng materal and burnng condton, the heat release rate of varous burnng materal has a wde range of ts magntude and transent characterstcs n real fre. Therefore, the measurement of heat release rate has been consdered as one of the most challengng ssue among the fre quanttes. Among the several methods of quantfyng the heat release rate, the smplest way s to measure the mass loss rate and consder the heat of combuston of the burnng materal. However, t has dffculty to quantfy for the case of ncomplete combuston of fuel vapor from the combustble materal and the case havng the mult-layered composte materal. More advanced approach to quantfyng the heat release rate s to measure the thermal energy released from fre or the change of cal composton durng the combuston process. The oxygen consumpton prncple s based on the assumpton that the amount of heat FIRE SAFETY SCIENCE-PROCEEDINGS OF THE TENTH INTERNATIONAL SYMPOSIUM, pp COPYRIGHT 2011 INTERNATIONAL ASSOCIATION FOR FIRE SAFETY SCIENCE / DOI: /IAFSS.FSS

2 released per unt mass of consumed oxygen s approxmately constant for most common burnng materals and has been wdely appled to heat release rate measurement, especally n real scale fre test [1,2]. The key parameter need to determne the heat release rate by the oxygen consumpton method s to measure the oxygen defct durng the combuston process [3]. Ths means that the combuston gases from fre and entraned ar should be collected through the hood and exhaust duct system and measured the exhaust mass flow rate and the oxygen and gas concentratons at a samplng locaton. Therefore, the scale of the fre test s restrcted by the hood sze and exhaust flow rate of oxygen consumpton calormeter. However, we occasonally need to perform the fre test n a real buldng envronment, not only at real-scale [4] and, t s not easy to estmate the heat release rate n a real structural fre. In ths case, the measurement of heat release rate usng the tradtonal oxygen consumpton calormeter s almost mpossble due to the lmts of sze of hood and the fre test faclty. The present study ams to examne the possblty of the heat release rate measurement n real buldng envronment based on the fre drven flow and entraned ar flow through the openng of the fre compartment. As the frst part of the whole project, the present study has been performed a prelmnary approach to apply the tradtonal measurement technque for quantfyng the heat release rate for the smple geometrc compartment. A seres of CFD (computatonal flud dynamcs) calculaton has been appled to evaluate the heat release rate through the openng of the fre compartment and the process of the present study s dvded nto followng three steps. Analyss of grd senstvty on the fre drven flow n the compartment Valdaton of the CFD model wth Rodney s experments for the doorway flow. Evaluaton of the cal and ectve heat release rate usng doorway flow It s expected that ths study contrbutes to enhance the understandng of heat release rate measurement and become helpful n accomplshng the advanced measurement technque for the real fre test. External open boundares Fg. 1. Satc of the computatonal doman of the ISO-9705 compartment. NUMERICAL SIMULATION As a prelmnary study to quantfy the heat release rate based on the flow feld through the openng, the present study performs a seres of CFD calculaton for the ISO-9705 room fre and examnes the 1526

3 applcablty of the heat release estmaton method through the doorway flow. Pror to the full numercal smulaton, the grd ndependence test s performed to ensure the valdty of the mesh sze n the study. Then, the doorway flow calculated by the CFD analyss s compared wth the prevous research that had been recently conducted by Rodney Bryant n NIST. Base on the valdty of the numercal results, the heat release rate calculated by doorway flow s drectly compared wth the nomnal heat release rate whch s gven from the fuel mass flow rate. The experments were conducted n an ISO 9705 room measurng 2.4 m 3.6 m n plane and 2.4 m n heght. The doorway openng measurng 0.8 m wde by 2.0 m hgh was centrally located on the one of the 2.4 m 2.4 m walls and ts depth was 0.3 m. The natural gas fre was located on the floor n the center of the room and a 30.5 cm square burner wth heght of 30 cm was used for the fre tests. The detals of the fre experments were gven by Bryant [5,6]. Fgure 1 shows the computatonal doman corresponds to the fre compartment and extended computatonal regon for outsde of the doorway. The CFD calculatons were carred out usng CFX verson 12.1 [7] whch s a commercal computatonal flud dynamcs (CFD) code. Hgh Reynolds number k-ε model wth buoyancy modfcaton and P-1 radaton model wth sotropc spectral model were appled to smulate the turbulent flow and radatve heat transfer n enclosure fre, respectvely. The absorpton coeffcent for the gas was calculated as the mass weghted average of the partcpatng gas speces. The fre s modeled by the combuston of CH 4 n the eddy dsspaton model and a 2-step rreversble reacton mechansm [8] for CH 4 combuston was used. CH 4 + (3/2) O 2 CO + 2 H 2 O CO + (1/2) O 2 CO 2 The heat flux at the wall boundary n CFD model s calculated usng the ectve heat transfer coeffcent. w c w n q h T T (1) where h c s a heat transfer coeffcent, T w s the external wall boundary temperature, and T n s the temperature n the neghborng cell on the nternal wall boundary. Celng and sde walls are assumed to have unform thckness of 2.5 cm and the external wall heat transfer coeffcent and ambent temperature was consdered as 10 W/m 2 K and 298 K, respectvely. Open boundares on the extended regon are set to be 0.0 Pa of pressure boundary condton. Grd Independence Test Pror to the full CFD smulaton, the grd ndependence test was performed to ensure the feasblty of the mesh sze n the study. The grd ndependence tests were conducted for dfferent mesh and fre sze to examne the effect of grd sze on the predcton results of fre drven flow n the compartment door. The fre szes for the grd senstvty analyss were 34 and 511. The computatonal doman ncludng nsde and outsde of the fre compartment s dvded by hexahedral type meshes usng the ICEM-CFD whch s a commercal CAD and grd generaton program. The grd tests were carred out for the cells of 52,000, 70,000, 96,000, 136,000 and 200,000. Fgure 2 dsplays the comparson of the velocty magntude profles at the centerlne of the doorway. The vertcal profles of velocty magntude, except near the neutral heght of the doorway, were n good agreement for the overall tested mesh sze. For the 34 fre, the enlarged fgure of Fg. 2a shows a smlar trend at the neutral heght of the doorway for the cases of grd cells more than 96,000, whle the flow drecton s opposte for the coarse grd less than 70,000. Ths means that the predctons wth coarse grd show lower neutral heght at the center lne of the doorway for 34 fre. For the 511 fre, the calculated velocty profle was qute dfferent from the mesh sze near the neutral heght. However, the calculated velocty profle was well matched for the case of the number of grd cell more than 136,000. Based on the grd ndependence tests, the present study determned the optmum grd resoluton of 136,000 grd cells for ensurng the grd ndependency and the effcent computaton. 1527

4 heght [m] heght [m] heght [m] heght [m] N=52K 0.5 N=70K 0.5 N=96K 0.3 N=136K 0.3 N=200K U [m/s] Fre Fre 511 Fre N=52K N=70K N=96K N=136K N=200K U [m/s] (a) (b) U [m/s] U [m/s] Fg. 2. Comparson of the vertcal velocty profle at the centerlne of the doorway wth dfferent mesh sze for the; (a) 34 fre; (b) 511 fre. RESULTS AND DISCUSSION Numercal Valdaton In order to valdate the CFD results for the doorway flow n the fre compartment, the present study compares the velocty profle between CFD calculaton and Rodney s experments for the doorway flow of ISO-9705 room. Fgure 3 shows that the vertcal profles of the velocty predcted by CFD model are compared wth those of the velocty measurement from the b-drectonal probes and PIV at the centerlne of the doorway. The predcted velocty profles of the CFD model are well matched wth the experments for the overall heght and gve heat release rate. As seen n Fg. 3, the results calculated usng the CFD model are n better agreement wth the PIV measurements than those measured usng b-drectonal probes. (a) 34 (a) (b) 160 (b) (c) 511 (c) Fg. 3. Comparsons of vertcal velocty profle at the center lne of the doorway for the heat release rate of: (a) 34 ; (b) 160 ; (c)

5 Fgure 4 represents the temperature comparson between the CFD calculaton and temperature measurement usng an asprated thermocouple for the quas-steady state fre whch s defned by a relatvely constant temperature readng n the upper layer nsde the compartment durng the fre test. The dscrepancy between the CFD predctons and experments was less than 20 ºC for the lower heat release rate than 160, whle the CFD model under-predct more than 100 ºC for the 511 fre. Consderng the overall performance, the CFD model shows acceptable results for the velocty profle and upper layer temperature n spte of smplcty and lmtatons of the CFD model used n ths study. Fg. 4. Comparson between the predcted and measured temperature at a pont n the compartment for a quas-steady state fre condton. Heat Release Rate Calculaton by Doorway Flow Based on the valdaton of the CFD model, the present study examnes the heat release rate usng the ntegratng method of oxygen depleton and ectve heat release rate through the openng. The cal heat release rate s defned by the rate of generaton of cal heat from fre and usually determned from the oxygen consumpton method [3]. o c, O2 O2 O2 H m m (2) The cal heat release rate s drectly calculated by the oxygen depleton mass flow rate n the system and average heat release per unt mass of oxygen. The man parameters to determne the heat release rate n Eq. 2 are the oxygen mass flow rate of ncomng ar flow nto the system and combuston product from the system. In the compartment fre, the neutral heght at the doorway openng s consdered as a reference heght between the upper part of hot smoke flow from the compartment and the lower part of ar flow nto the compartment. The ndvdual oxygen mass flow rate can be obtaned by the oxygen mass fracton and mass flow rate n the ndvdual cells and the mass flow rate of ncomng and outgong flow can be calculated by the each layer at the doorway. The sgn of the ncomng flow nto the compartment s assumed to be negatve. da (3) o mo my 2 O2 A L L O my 2 O da (4) U m 2 A U 1529

6 my O2 da H c, O2mYO H c O, 2 2 da (5) A A For better understandng of cal heat through the doorway, ths study ntroduces ndvdual cell at the doorway as follows; for the q, (6) q, H c, O2m YO 2, Fgure 5 shows the calculated feld of q for the nomnal heat release rate of 34 and 160 fre at the cross secton of the doorway. Because the sgn of q s determned by the flow drecton through the doorway, q s postve n the upper part of the doorway whle negatve n the entraned ar flow nto the room. Therefore, the ntegraton of q represents the cal heat release rate calculated by the dfference of oxygen mass flow rate between ncomng and outgong flow through the doorway. The calculated cal heat release rate based on the doorway flow for the nomnal heat release rate of 34 and 160 fre are about 33 and 166, respectvely. For the 511 fre, the calculated cal heat release rate s about 548 and the dscrepancy between the nomnal and the calculated heat release rate do not exceed 8 %. Ths means that the calculaton method based on the oxygen consumpton method for the doorway flow may be qute effectve to evaluate heat release rate. q, (b) oxyxgen (a) depleton (a) 34 heat release rate [] q, [], door , door 166 (b) oxyxgen (b) (b) 160 depleton heat release rate Fg. 5. Contour plots of the cal heat release rate at the cross-secton of the doorway: (a) 34 ; (b) 160. In contrast to the cal heat release rate, the ectve heat release rate n an ndvdual cell s manly determned from the gas temperature rse and mass flow rate. q m C T (7), p, where the m and ΔT represent the mass flow rate and the temperature rse of combuston product-ar mxture, respectvely. The ectve heat release rate through the doorway s obtaned by the ntegraton of the ectve heat release rate n the ndvdual cell through the doorway. N da m, C p, ( Tg, Ta ) A 1 q (8) The ectve heat release rate due to the ncomng ar flow s neglgble due to small temperature dfference through the lower layer of the doorway. Fgure 6 demonstrates the calculated feld of q for the nomnal heat release rate of 34 and 160 fre at the cross secton of the doorway. As expected, 1530

7 q s close to zero n the lower part of the doorway whle relatvely hgh n the upper part of the doorway. The ectve heat release rate for the 34 and 160 fre are 22 and 110 and ths means that the fracton of ectve heat release rate through the doorway are about 0.65 and 0.7, respectvely. For the 511 fre, q was 363 and ts fracton s approxmately 71 %. Therefore, fracton of heat loss n the fre compartment s approxmately 30 % of the generated heat from the fre. For the quas-steady state fre, we can assume that the heat release rate n the compartment s equvalent wth the sum of rate of heat loss due to gas flow through openng and the rate of heat loss to the compartment boundares. loss (9) In order to examne the heat loss n the fre compartment, a well known smple formula s appled to estmate the heat loss to the compartment boundares [9]. The heat loss to the boundares nvolves many heat transfer modes and the domnant heat transfer process s conductve heat loss to the sold wall. The heat loss term can be wrtten as follows; [W] q, [W] q, , door 22 (a) nomnal heat (a) 34 (a) release rate of 34 (b) nomnal (b) 160 (b) heat release rate of 160 Fg. 6. Contour plots of the ectve heat release rate at the cross secton of the doorway: (a) 34 ; (b) , door 110 h A ( T T ) (10) a loss k T g where h k s the effectve heat transfer coeffcent, A T s the nteror surface area n the compartment, T g and T a denote the upper layer and ambent temperature. For the effectve heat transfer coeffcent, McCaffrey et al. defned h k as follows; h k kc t k s ( t t ) ( t t ) p p (11) here C s s the specfc heat, δ s the thckness of the sold materal, and the thermal penetraton tme, t p, can be gven as 2 t (12) p

8 heat release rate by doorway flow [] heat release rate by doorway flow [] where α s the thermal dffusvty. The materal propertes of calcum slcate board are lsted n Table 1. Table 1. Typcal thermal propertes for the calcum slcate board. Thermal conductvty, k Specfc heat, C s Densty, ρ Thermal dffusvty, α (W/m K) (J/kg K) (kg/m 3 ) (m 2 /s) In Eq. 10, the mean gas temperature n the upper layer s not easly determned n CFD model and experment. The present study assumes that the measured temperature n Fg. 4 s used as T g to calculate heat loss to the boundary wall. The heat loss calculated by Eq. 10 s compared wth ntegraton of heat loss to the wall boundary wth CFD macro functon and summarzed n Table 2. Table 2. Calculated heat loss n the fre compartment for the nomnal heat release rate. loss calculated by Eq. 10 loss calculated by the CFD model The heat loss n CFD model s calculated by the local temperature n the upper layer of the fre compartment. As seen n Table 2, the heat loss calculated by the measured temperature at a pont of upper layer over-estmate than those of CFD model for the 34 and 160 fre, whle under-estmated for the 511 fre. Except the case of 34 fre, the result of Eq. 10 shows an acceptable agreement wth CFD model. Fgure 7 summarzes the calculated heat release rate by doorway flow for the gven fre sze. The ectve heat release rate has approxmately 30 % dscrepancy comparng wth nomnal heat release rate, whle the cal heat release rate usng oxygen consumpton through the doorway was less than 8 %. However, the overall dscrepancy of the ectve heat release rate ncludng heat loss term was less than 10 % except the case of 34 fre wth heat loss calculated by emprcal correlaton of Eq HRR by OC HRR by OC Convectve HRR Convectve HRR loss +, eqn Heat loss by eqn () Convectve HRR + Heat loss by CFX loss, CFX Convectve HRR Convectve HRR + Heat loss by eqn () Convectve HRR + Heat loss by CFX nomnal heat release rate [] Fg. 7. Comparson of the estmated heat release rate based on the cal and ectve heat release rate wth and wthout heat loss for the gven nomnal heat release rate nomnal heat release rate [] 1532

9 SUMMARY AND CONCLUSIONS As a prelmnary study of the heat release rate estmaton n real structural fre, the cal and ectve heat release rate based on the doorway flow of ISO-9705 room was nvestgated usng a CFD model valdated wth experments. The CFD model was valdated usng expermental measurements for the vertcal velocty profle at the centerlne of the doorway and temperature n the upper layer nsde the fre compartment. The velocty profles for the CFD results were n good agreements wth the measured velocty usng PIV technque and b-drectonal probe. The predcted temperature by the CFD model was well matched wth experments wthn 20 ºC for the case of the heat release rate less than 160, whle under-predcted more than 100 ºC for the 511 fre. But ths dscrepancy can be consdered as acceptable results for performng the man objectve of the present study. In order to evaluate the cal heat release rate, the oxygen consumpton method was rearranged for the doorway flow based on the reference heght of the doorway. The cal heat release rate was obtaned by ntegratng of q at ndvdual cells across the doorway and the predcted was well matched wth the nomnal heat release rate wthn 8 %. The dscrepancy of the predcted ectve heat release rate by the CFD model was approxmately 30 % wthout consderng heat loss to sold boundares, but t was less than about 20 % wth consderng the heat loss effect. Ths means that the measurement of ectve heat release rate through the openng may be useful way to estmate the rate of energy released n the structural fre. In order to apply ths approach to real structural fre, we stll need to study for varous fuel type and fre sze but also have many challengng works such as feld measurement of velocty and oxygen concentraton. Nevertheless, ths work shows the possblty of estmaton of heat release rate usng doorway flow. ACKNOWLEDGEMENT The author s grateful to Dr Rodney Bryant for helpful dscusson on the velocty measurement n a enclosure fre and ths research was supported by Basc Scence Research Program through the Natonal Research Foundaton of Korea (NRF) funded by the Mnstry of Educaton, Scence and Technology ( ) REFERENCES [1] Babrauskas, V., Heat Release Rates, The SFPE Handbook of Fre Protecton Engneerng (3 rd ed.), DNenno P.J. (ed.), Natonal Fre Protecton Assocaton, uncy, MA 02269, 2002, p [2] Huggett, C., (1981) Estmaton of Rate of Heat Release by Means of Oxygen-Consumpton Measurement, Fre and Materals 4: 61-65, [3] Rodney, R.A., Ohlemller T.J., Johnsson, E.L., Hamns, A., Grove, B.S., Guthre, W.F., Maranghdes, A., and Mulholland, G.W., The NIST 3 Megawatt uanttatve Heat Release Rate Faclty Descrpton and Procedures, Natonal Insttute of Standards and Technology Report NISTIR 7052, Gathersburg, MD, 2004, 5 p. [4] Km, M.B., Han, Y.S., Cho, B.I., and Do, K.H, (2009) A Full-Scale Fre Test n an Apartment House, Journal of Korean Insttute of Fre Scence 23: [5] Bryant, R.A., (2009) A comparson of gas velocty measurements n a full-scale enclosure fre, Fre Safety Journal, 44: , [6] Bryant, R.A. (2009) The Applcaton of Stereoscopc Partcle Image Velocmetry to Measure the Flow of Ar nto an Enclosure Contanng a Fre, Experments n Fluds 47: , [7] ANSYS Inc., (2009) ANSYS CFX-Pre User s Gude 12.1 [8] Westbrook, C.K., and Dryer, F.L., (1981) Smplfed Reacton Mechansm for the Oxdaton of Hydrocarbon Fuels n Flames, Combuston Scence and Technology 27: 31-43,