Experimental studies on interaction of water mist with class K fires

ARTICLESChinese Science Bulletin 2006 Vol, 51 No 24 3053- -3058more heat to minimize fuel costs may retard the dissi-DOI: 10.007114-06-2221-1pation of heat from the hot cooking oil in the event of afire. Since cooking oil fires have a different behaviorExperimental studies onfrom that of other types of flammable liquid fires, theyhave recently been classified as a special class of fireinteraction of water mist(class K)l1,10]Cooking oil hardly vaporizes at normal temperature,with class K fires，and the concentration of cooking oil vapor above oilsurface is far under the combustive concentration, so itFANG Yudong, ZHANG Yongfeng, LIN Lin,cannot be ignited. The evaporation rate of cooking oil .LIAO Guangxuan, HUANG Xin & CONG Beihuabecomes faster when it is heated up to about 310"C, andUniversity of Science & Technology of China, State Key Laboratory ofthe concentration of cooking oil vapor quickly reachesFire Science, Hefei 230026, Chinato the combustive concentration and is easy to be ig-Correspondence should be adressed to Fang Yudong (email: ydfang@nited. Auto ignition occurs when the cooking oil ismail.ustc.edu.cn)heated up to 360- 370C. The surface temperature ofReceived February 19, 2006; accepted April 13, 2006Abstract Interaction of water mist with cooking oilburning cooking oil ranges from 340 to 380C accord-fires is studied experimentally and theretically. Aing to the different species of cooking oil. Studies showLDV/APV system is used to measure the velocity andthat cooking oil fires come into complete combustiondiameter of water mist at different pressures in thestage within 8 s after it is ignited, accompanied by a lotexperiments, and the effect of water mist velocity andof smoke and strong heat radiation. Foam, powder anddiameter on fire extinguishment efficiency is investi-carbon dioxide can effectively extinguish the flamesgated. The experimental results show that water mistover the oil surface but it is very difficult for them tohas excellent surface cooling effect; it can control andcool the oil below its auto-ignition temperature and toextinguish cooking oil fires quickly without re-ignition.prevent it from re-ignition due to their limited coolingThe critical temperature (To) is calculated by energycapacity'". Wet chemical agents are the most com-balance equation, and the fire plume momentum ismonly used agents in commercial cooking areas. Theycalculated and compared with that of water mist inare effective in extinguishing cooking oil fires, but itorder to determine the critical velocity (Wwy) of firetakes a long time for a wet chemical agent to cool theextinguishment. This paper provides references fooil below its aut-ignition temperature, depending oncooking oil fires extinguishment with water mist.quantity of agents used. They may imitate skin and eyesnd bring up clean-up problems after fire extinguish-Keywords: water mist, cooking oil fires, re-ignition, combustion-ment. The system cost of wet chemical agents is rela-supporting.tively high89I1 IntroductionAccording to Montreal Protocol.3, the use of halonStatistics have shown that almost 50% of all acci-fire suppression systems should be ended by the begin-dental fires in hotels, restaurants and fast food outletsning of the 21* century. During the past several years,start in kitchens, and liquid cooking oil or fat is in-water mist technology has been developed and regardedvolved in a majority of these fires". From 1988 toas a promising substitute since it can extinguish fire1992, a total of 39182 kitchens fires occurred in Al-quickly with only a small quantity of water withoutberta of Canada, bringing 228 fire deaths, 2172 firedamaging environment and surrounding objectsl"1'.injuries, and $569 million loss2-”. Usually the averageRecently some engineer experiments of extinguishingburning rates of cooking oils are higher than those ofclass K fires with water mist are carried out", but fewother hydrocarbon oils, and cooking oil fires are morestudies have been done on the mechanism of extin-difficult to extinguish than other liquid fires89. Duringguishing class K fires with water mist. It is necessary tocooking, the oil is heated to a temperature up to itsthoroughly, investioate. the interaction of water mistflash point. Once the oil auto ignites, it is difficult towithc中国煤化工related principles socool the oil below its auto ignition temperature. Moderm as to|YHCNMHGortheuseofwaterenergyefficient fryers which are designed to retainmist in kitchens.www. scichina.com www.springerlink.com3053ARTICLES2 Experimental apparatus and characteristic ofby K type thermocouples. Four thermocouples werewater mistplaced above the pan every 0.2 m along the surfacecenter line. Data were acquired and processed auto-2.1 Experimental apparatusmatically by a computer. The experimental environ-The experimental systems include water mist firement temperature was 25C.suppression system, temperature data acquiring system,The experimental cases are summarized in Table 1.LDV/APV system, ISO9705 full-scale heat release rateEvery case was conducted for three times to get theplatform for multifunctional measurement and theaverage result. 500 mL cooking oil and 400 mL alcoholcombustion equipment, as shown in Fig. 1. The cook-were used in each case, the alcohol was burnt to heat uping oil was used as the combustibles in the experiment,the cooking oil when experiment began. For the cook-a 0.35 m diameter pan with a lip height of 0.02 m wasing oil fires, an 8 s of pre-burning was allowed to reachused for loading cooking oil 1.3 m under the nozle, steady burning conditions. Then water mist was appliedand alcohol was used to heat up the cooking oil toand the fire extinguishment time was recorded. Afterauto-ignite. A 7N nozzle was used to produce water fire was extinguished, the smoke vent was started upmist, with the working pressure ranging from 0.5 to 2and the experimental environment was cleaned up.Mpa. Using an LDVIAPV system, the velocity and di-2.2 Characteristic of water mistameter of water mist were measured. The heat releaserates of cooking oil fires and alcohol heat source wereThe distance between nozzle and fire is 1.3 m in themeasured by ISO9705. The temperature was measuredexperiment. The main area where water mist interactsSmoke sampling. HoodSmoke ventWater supplying pipeNozzle1.8mThermocouple treeCameraC]Fire.Data cllctinin card0.5 mqAlcohol |Plank户日ComputerFig. 1. Sketch map of experimental system.Table 1 Experimental casesCasePressure (MPa)Calefaction time (s)Preignition time (s)Heat release rate (kW)Fire extinguishment time (s)1858failure.0911(中国煤化工4:.52052:179YHCNMHG13054Chinese Science Bulletin Vol. 51 No. 24 December 2006ARTICLESwith fire positions is between 1.2 and 0.8 m under thetemperature decreased quickly, and the surface tem-nozzle, and a horizontal surface 1 m under the nozzle is perature of cooking oil decreased lttle by lttle. Whenselected as the sampling surface of characteristic meas-the fire was extinguished 45 s later, the temperature ofurement. The atomization angle of 7N nozzle is 50°, the cooking oil decreased to 220C (Fig. 4(a)). In case 3horizontal surface 1 m under the nozzle is a circle withthe jumping time of temperature shortened to 15 s at0.6 m diameter. The sampling point is positioned everythe initial stages of applying water mist, then the tem-0.1 m from the circle center along the radial. Fig. 2perature decreased quickly, and the fire was extin-shows that the vertical velocity of water mist increasesguished after 30 s (Fig. 4(b)). The jumping of tempera-with increasing pressure. Fig. 3 shows that the diameter ture was not obvious at the initial stages of applyingof water mist decreases with increasing pressure.water mist in case 3. The fire was extinguished 10 slater, the decreasing velocity of temperature rose+ 0.5 MPaquickly at 2 MPa pressure (Fig. 4(c)). The analysis-- 1 MPashows that the efficiency of water mist fire extinguish-. 2.0 MPament increases greatly with increasing pressure, and the置surface cooling effect of water mist is also enhanced.The reason is that high pressure water mist has ade-quate momentum to overcome the resistance of fire21plume and reach the combustion area, and the flux ofwater mist is excessive, so the evaporation and heatabsorption capability of water mist are strong enough toquickly decrease temperature. When the heat that100200300400transferred to the oil is not enough to support combus-Distance from center line (mm)tion, the oil temperature will reduce quickly till it is .Fig. 2. Distribution of vertical velocity.extinguished. When the pressure is decreased, watermist can not reach the combustion area by its own mo-x10-mentum, almost all the water mist in combustion area-0-0.5 MPa140善- 1.0MPa Igoes out with the ascending fire plume, the surface●1.5 MPacooling mechanism of water mist can not work effec--0- 2.0 MPal120tively, so at the initial spraying stages, the temperaturedecreases very slowly and jumps up and down, the as-cend ability of fire plume is weakened when the air100 .around the fire is cooled by water mist, at this time wa-ter mist can reach to the combustion area and extin-80guish the cooking oil fires.From the above analysis, we can reach the conclu-60sion that spray coverage, water flow rate and spraymomentum are three most important factors for extin-guishing cooking oil fires. Firstly, water mist coverageFig. 3. Distrbution of particle diameter.must be large enough to cover the entire oil surface andextinguish the whole flames over the oil surface at once,3 Results and discussionotherwise the flames that are not attacked by water mist3.1 Temperature variational principles of fires andwill not be extinguished, and the heat released by thenon-extinguished flames will counteract the coolingcooking oileffect of the water mist on cooking oil. Secondly, theThe fire extinguishment of case 1 is unsuccessful, soamount of discharged water must be sufficient enoughwe will not discuss it here. Fig. 4 gives the fire tem-to abs flame and cool the oilperature curve of cases 2- 4. In case 2, the temperature below中国煤化工dly, water mist mustdid not decrease quickly at the initial stages but jumpedhaveTYHC N M H Getrate the fire plumeup and down with water mist applied. After 30 s theand reach the fuel surface.www.scichina.comwww.springerlink.com3055ARTICLES800 Discharge-a- 0.6 m above cooking oil800- 0.6 m above cooking oil-0- 0.4 m above cooking oilDischarge700-0- 0.2 m above cooking oil-4- 0.2 m above cooking oil-*一cooking oil。600600- cooking oil50500400-,Extinguishing豆300300-Extinguishing200Discharge stop- Discharge stop100 t100--100b)160 180 200 220 240 260 28016080200 220 240 260 280Time (S)Time ()一o- 0.6 m above cooking oil- o- 0.2 m above cooking oil-★一cooking oil400. Extinguishing100| (C)180 200 220 240 260 280Fig. 4. Temperature of cooking oil and flame with water mist applied. (a) Case 2; (b) case 3; (C) case 4.water:3.2 Calculation of critical To for fire extinguishmentWhen cooking oil burns in a deep fat fryer, heat isQn=eσT°+K,二502+m。[Cpw(T%s -Tw)+Lw], .8transferred to the oil from flame by convection and ra-diation. Heat is lost from the oil by the evaporation owhere ε is emissivity of cooking oil. Usually Eof liq-water droplets and by heat transfer through the fryerl".uid combustible is between 0.8 and 1 (E is taken as 0.9),The fire can be extinguished by water mist mainly byσ is Stefan Boltzmann constant (5.67X 10.8 W/m2.K),cooling the fuel surface. The energy balance equationTis are the flame temperature at the fuel surface. Usu-of cooking oil surface isally the liquid flame temperature at the fuel surfaceS=(f。SH。-4)m; +Qe -Q，(1)remains at about burning point when fire is extin-where△H。is the heat of combustion of cooking oil (40guished, so Trs is taken as 380C in the calculation, TioMJ/kg)"4, fc is the fraction of△H。transferred from theis the fuel temperature 0.015 m below the surface, δisflame to cooking oil (15%)!'s;, me is the buming rate ofthe thickness of the heating layer below the fuel surfacecooking oil (0.01 kg/s. m), LAv is latent heat of evapo-(0.03 m), and K; is the heat conductivity of the fuelration of the fuel (0.4 MJ/kg), Qe is the external(0.145X 10-% MJ/s.m. C)4, mw is the dischargingheating flux transferred to the fuel (0.018 MJ/s), and QLrate中国煤化工vw is latent heat ofis the heat loss from the fuel surface, including heatevapMHCNMHG/kg)4. The energywritten as .loss to open space, to the depth of the fuel, and to the3056Chinese Science Bulletin Vol. 51 No. 24 December 2006ARTICLESs=(f.AH。- 4v)m; +Qp0.3 mx0.3 ming oil fire plume area is 0.2 kg/s xπx(0.8 m)2{eσr°+K, T -502+m.[C%(T。-I.)+Lw]0.0093 kg/s. Formula (4) shows that water mist can8reach the fuel surface when the following condition isWhen s≥0, sufficient heat is available to maintainsatisfied:0.0093 kg/sxvwy >0.043 kg . m/s> Vvwy > 4.6 m/s.the flame on the oil surface and the combustion con-tinues; however, when S < 0, the heat will not be suffi-Fig. 1 shows that the vertical velocity of water mistcient to produce fuel vapour to support the flame, re-is about 4.8 m/s at 2 MPa pressure which is bigger thansulting in the extinction of the flamel,.7. We can cal-the max fire plume velocity, so water mist can over-culate the To using formula (3) with all the parameterscome fire plume and reach the cooking oil surface.above. When Ti≤205"C, S<0, the fire will be extin-Consequently the fire is extinguished effectively.guished. Fig. 4 shows that the cooking oil temperature3.4 Combustion- supporing effect of water mistremains at about 200'C as fire is extinguished. TheIt was found in the experiment that cooking oil firesexperimental results are in good agreement with theo-were strengthened by water mist at the initial sprayingretical calculation. We can draw the conclusion by ex-stages. The cooking oil burnt violently for a short timeperiments and theoretical calculation that the fire willas water mist was applied, and the size of fire got morebe extinguished as To is decreased to 200°C by waterthan doubled. The temperature also increased andmist.jumped up and down. This phenomenon was weakenedwith increasing pressure. The size and temperature of3.3 Calculation of critical Vwy, for fire extinguishmentfire decreased quickly till the fire was extinguished.To keep the water mist from carrying away by theThe combustion-supporting effect was analyzed infire plume, the water mist momentum must be at leastthe light of the physical and chemical mechanisms 01equal in magnitude in the opposite direction to the fireinteraction of water mist with fires. Usually the effectplume momentum. Usually the fire plume is mainlyof water mist on fires mainly includes four aspects"composed of combustion productsh:(1) gas phase cooling or heat extraction from the fire;(2) reduced oxygen levels as water vapor displaces(4)oxygen near the seat of fire; (3) radiant heat attenuation;Mim≥M。，(4) enhanced the combustion by strengthening thewhere Mwy and My are the 'y' components of themixing of oxygen and fuel. The first three efects resultwater droplets and fire plume momentum, respectively.in the extinguishment of fire, and the last one causesThe maximum upward velocity in a fire plume, ur, isthe combustion-supporting.In addition, once carbon monoxide is produced inachieved by the following formulal5]:uy=kQ2 ,(5)the combustion, it was very hard to oxidize gaseouscarbon monoxide, but the oxidation of carbon monox-where Qc is the heat release rate of cooking oil fires.ide becomes easy when water mist is applied, because aThe average heat release rate of cooking oil fires in thelot of hydroxyl is produced by the following reaction:experiment is 169 kW. In continuous flame area: 0.9< k02→20<1.6; in intermission flame area: 1.6 20Hancy plume area: 1.4CO2 + Hcombustion products equals the combustion rate of fuel,and the cooking oil combustion rate is 0.01 kg/s, so theH+O2→0H+Omomentum of fire plume in unit time is 0.025 kg . m/s <中国煤化工2mry < 0.043 kg. m/s. The spraying diameter of waterThease with an increasemist is 1.6 m, the water mist discharging rate is 0.2 kg/s,in cMHC N M H Glowing reaction oc-and the water mist mass above the 0.3 mX0.3 m cook-currs:www.scichina.com www.springerlink.com3057ARTICLESH2O+C→CO+H2Acknowledgements The authors would like to thank Prof.The hydroxyl produced in the above reaction decom-Qin Jun for his useful suggestion, discussion and help. Thispose into a large amount of hydrogen and hydroxy bywork was supported by the China National Key Basic Re-search Special Funds (Grant No. 2001CB409600), the Na-following reaction:tional Natural Science Foundation of China (Grant No.H2 +O2→20H50323005) and the Anhui Development Fund of Person withAbility (Grant No. 2003Z019).H2 +M→2H+MHydrogen and hydroxyl are both lively roots. TwoReferenceslively hydrogen atoms were produced by consuming ahydrogen molecule, and at the same time two hydroxylslLiu Z G, Andrew K, Don C, et al. Extinguishment of cooking oilare produced in the following reaction:fires by water mist fire supression systems. Fire Tech, 2004, 40:OH+H2→H2O+H309-3332 Wijayasinghe M s, Makey T B. Cooking oil: A home fire hazard inH+O2- >0H+OAlberta, Canada. Fire Tech, 1997, 33: 140- 1660+H2 -→OH+H3 ANSUL Inc. Hybrid fire sppression technology for restaurantIt was obvious that in the combustion process watercooking equipment. Whitc Paper 1015, 1999mist and hydrogen produced in the reaction play the4 Suh J, Atreya A. The efet of water vapor on counterflow dffusion flames. In: Proceedings of International Conference on Firerole of activator. When the concentration of water Va-Research and Engineering. Orlando, 1995. 103- 108por exceeds a value, the effect of physical extinguish-Koscki H, Natsume Y, Iwata Y. Combustion of high flash pointment dominated and the effect of chemical catalysis arematerials. In: Proceedings of 7th Intemational Fire and Materialsweakened, so there is a critical value at which the effectConference, San Francisco, 2001. 339-349of chemical catalysis is the strongeste. Former re-6 Council of Canadian Fire Marshals and Fire Commissioners. Firesearches indicated that this value was about 30%，atLosses in Canada. Annual Report, 1997which the effect of chemical catalysis of water mist7 Hui Y H. Bailey's Industrial Edible Oil and Fat Products: Oils anddominated, and beyond which, the effect of physicalOil Seeds. 5th Ed. New York: John Wiley & Sons Inc. 1996extinguishment of water mist dominated.8 Edwards N. A new class of fire. Fire Prev, 1998, 310: 8-99 Vellkert C. Out of the frying pan. Fire Prev, 1998, 314: 24-264 Conclusions10 Voelkert C. The new class K NFPAI, 1999 718We reach the following conclusions:11 QinJ, Yao B, Chow W K. Experimental study of supressing(1) The atomization angle, spray momentum andcooking oil fire with water mist using a cone calorimeter. Hospflux of water mist are the key factors that determineManag, 2004, 23: 545- 556effectiveness of water mist extinguishing K class fires.2 Grant G Brenton J, Drysdale D. Fire suppression by water sprays.The fire extinguishment efficiency increases with theProgr Energ Combust Sci, 200, 26: 79- 130increasing pressure.13 Kim A, Mawhinney J, Su J. Wate-mist system can replace halon(2) Water mist extinguishes cooking oil fires mainlyfor use on clectrical equipment. Canad Consult Eng, 1996, 5: 30-by surface cooling effect; it effectively cools down the35oil surface and prevents re-ignition; water mist effec-DiNenno J P, Beyler L C. The SFPE Handbook of Fire Protectiontively destroys the stability of combustion by strikingEngineering. 2nd ed. Quincy: National Fire Protection Association,Boston: Society of Fire Protection Engineers.1995the root of fire, thus accelerating the process of fire15 Anthony H, Takashi K. Characteristics of Pool Fire Buming.extinguishment.Gaithersburg: NIST, MD 20899(3) Calculations show that fire will be put out when16 NFPA10. Standard for Portable Fire Extinguisher. Quincy: NaTr is decreased to 200C. When using water mist totional Fire Protection Association. 1998extinguish class K fires, one of the premises is that the17 NFPA17. Standard for Wet Chemical Extinguishing Systems.momentum of water mist should be bigger than that ofQuincy: National Fire Protection Association. 1994fire plume.18 Rashbash D J. Relevance of fire point theory to the asessment of(4) Water mist strengthens the combustion of cook-= In: Proceedings of Interna-ing oil at the initial stage of applying water mist. The中国煤化nsh Maran BEBephenomenon is weakened with the increasing pressure.TYHCNMHG3058Chinese Science Bulletin Vol. 51 No. 24 December 2006