Preliminary study on interaction of water mist with diffusion flame of liquid fuels

Vol. 13, No. 3. March 2003PROGRESS IN NATURAL SCIENCEPreliminary study on interaction of water mist with diffusion flameof liquid fuels”LIU Jjianghong", LIAO Guangxuan, LI Peide and YAO BinX9 A(State Key Laboratory of Fire Saley Science, Universily of Sciene and Technology of China, Hetei 230026. China)Reeived July 29, 2002; revised October 18, 2002AbstraetThe chetnical and physical interaction mechanisms of the water mist with dffusion flame of liquid fuels are ivestigatede.The dffererce of the thermograms and the thermal field isogroms between ethanol flame and kerosene flame with the water mist applica.tion is explained. With lhe waler mist ppliain, the diferece bewe ethanol and ketosene in heat release rate, O2 and co concentra-tions of their combustion products, and the termperature of their smoke are analyzed. At the same time. the interaction mechanism of thewater mist with diffusicon flame is presented and their relautionship to the fuel species and t the concentration of water mist is desribed.Keywords: water mist, difuson fame, lterncto mechanism.Since the first version of the Montreal Protocolmist is applied and it seems that this cannot be causedwas introduced in 1987， water mist for fire extin-only by physical effect. Some results suggest that theguishment and control have been widely recognized aschemical reactions are changed and enhanced withina halon 1301 replacement'. Water mist refers tothe flames by water vapor91. The present work is tofine water droplets in which 99% of the volume ofstudy the effect of water mist on suppressing or en-the spray is in drops with diameters less than 1000hancing the diffusive flames in confined space.micronss[21. Extinguishment mechanisms of watermist include thermal cooling， oxygen displacement,1Experimental apparatusfuel surface cooling and attenuation of radiative heatThe experimental apparatus is shown in Fig. 1.transfer. Factors that contribute to the success or fail-ure of a water mist system include droplet size, veloc-The fuel sample is contained in a circular stainlessity, spray pattern, momentum, geometry and othersteel pan with height of 10 mm and inner diameter ofcharacteristics of the protected area and the type of150 mm. The pool was mounted on a steel stand 600fuel3]. So it is very important to study the interac-mm above the ground to minimize the effect of sur-tionof water mist with flames. Most of flames arerounding ground surfaces on the behavior of the fire.diffusive and occur in confined space with proper ven-A downward-directed pressure nozzle was positionedtilation control, and the fuel always varies. The studyon a square steel plate 300 mm over the fuel sample.of the interaction of water mist with a diffusive flameThe nozzle was operated at pressure of 0. 5 MPa,in the confined space will. enhance the knowledge ofwhich could be altered by adjusting the pressure regu-such processes and be useful for developing the waterlators to meet different requirements. The water mistmist fire suppression system, improving the fire sup-was injected into the pool fire downward directly, andpression and control efficiency and extending their ap-the relative flow rate was about 1.0 mL/s. The coneplications.angle of the nozzle was 60*，and the volume mean di-ameter off the mists was about 80 μm. During the ex-The interaction of water mist with a diffusiveperiment period, the TVS 2000ST infrared thermo-flame in confined space has been investigated in dif-gram system and a thermal video system were used toferent aspectsl4~ 8but the enhancement effect ofrecord the thermogram and visualize the thermal fieldwater mist on combustion in some cases is still notof the flame before and after the injection of waterunderstood. Some recent experiments show that themist respectively .flame by a certain kind of fuel will be enhanced to alarger scale and its temperature will rise when waterSome K-type thermal couples of 0.5 mm diame-中国煤化工●Supported by the Major State Basic Ressearch Develpment Program of China (200“W E-mail: lh@ustc. edu. cnYHCNMHGProgress n Natural Science Val. 13 No.3 2003238ter were arrayed alongthe centerline and radius toignition started, and the fire was allowed to burn forcalibrate the TVS apparatus. The radiant heat flux of100s to make quasi-steady burning before the spraythe flame with and without the application of waterinjection. All the raw data were saved and processedmist was also measured by a thermogauge. All of theautomatically by a computer.systems began to receive the data after the automaticNozleComputer工Camera headFlameProcessor一tDisk1Realtime displayand recordHigh pressure Water tanknitrogenFig. 1. Schematic of the experimental apratus.2 Results and discussion(c)) and thermal field isograms ((d), (e), (f)) of .the kerosene flame with water mist application. ItFig. 2 shows the thermograms ((a), (b) anddemonstrates that the kerosene flame is enhanced to a(c)) and thermal field isograms ((d), (e) and (1))larger scale and temperature rises at the beginning ofof the ethanol flame with and without the applicationthe injection of water mist, the high temperature areaof water mist. It is obvious that the flame was sup-doubled after 35s, even after 60 s, the flame struc-pressed to a smaller scale and the temperature loweredture remained and the combust intensity kept high.rapidly due to the application of water mist, and afterThese experiments suggest that water mist indeedabout 60 s，the temperature lowered to below thechanges the chemical reactions within the flames. Ibolling point， and then the fire was extinguishedcan also be explained by the experimental results ofquickly. This phenomenon might be caused by physi-who observed the heat release rate olcal suppression effect of water mist，and can be ex-kerosene, O2 and CO concentrations of kerosene com-plained by the experiments of Yao'. One of his exbustion products, and the temperature of smoke withperiments was conducted in the cone colorimeter withthe water mist application generated by 4 differenta litle modification. The heat release rate of ethanol,water pressure <0.1, 0.2, 0.4 and 0.7 MPa). OurO2 and CO concentrations of ethanol combustionexperimental results demonstrated that when the wa-products, and the temperature of smoke were mea-ter pressure was under 0.2 MPa, the heat release ratesured with the water mist application generated by 5and the CO concentration of kerosene combustiondifferent water pressure (0.1, 0.2, 0.4, 0.7, 1.0products increased with the increase of the waterMPa). The experimental results demonstrated thapressure,and the smoke temperature of kerosenerose, while the O2 concentration decreased with theproducts were increased with increase of water mistincrease of the water pressure. When the water pres-while the heat release reduced and smoke temperaturesure was higher than 0.2 MPa, the heat release rateof ethanol lowered.products decreased with the increase of the waterFig. 3 shows the thermograms ((a)， (b) and中国煤化工that under the condi-1) Yao. B. A simulative study on the interaction of waler tmist with a diff.MC N M H G of Since and TchobogyoChina, 1999239Progress in Naturail ScienceVal. 13 No.3 2003tion without the water mist application). The smokerelated to the concentration of the water mist andtemperature of the kerosene rose as well while the 02support Jail Suh' s conclusion that there is a turningconcentration increased with the increase of the waterpoint of the enhancement of water vapor to physicalpressure (but still lower than that under the conditionsuppression effect on the flame (in this case, it is nearwithout the water mist applications). These indicate0.2 MPa water pressure).that the chermical enhancement of the water mist is((2)100-x1”460DWduoWdhanlWathieng(团)(e)(0rig, 2. Trrrvogrums and thermal fidc loerums o ehuncl lane. 1n) nnd (d)。wibeu. te splerion ef wexer uit. () amd (e),wihthe weer mist for 40s.《0) und《f，with he water nist for 56。.(0)b1e200xx7watncemWh (en)Fig. 3. Thermograms and thermal field isograms of kerosene flame. (a) and (d), without the aplication of water mist; (b) and (e),with the water mist for 35s; (c) and (f), with the water mist for 60s.Yao' s work gives another example, e.g. whenconcentration and carbon particles in the smoke. Asthe water mist was applied, the radiant heat flux ofto the flame temperature, it is lower for keroseneethanol flame decreased while that of kerosene flamethan ethanol, it makes no contribution to the en-increased.hancement of the combustion, so CO might be theThen, why can the water mist enhance themain reason. Kerosene combustion generates moreCo than ethanol. Dry CO is hard to be oxidized (onlychemical reactions inside the kerosene flames?when1000 K，atThe main differences between the combustion ofwhich 1中国煤化工、)，pour whehethanol and kerosene are the flame temperature, COwater miHCNMHGfCObecomes240Progress in Natural Science Vol. 13 N.3 2003easier because more (H radical will be generated by3 Conclusions and future workthe following reaction;0+ H2O→2OH.A series of experiments were performed to inves-tigate the interaction mechanism of flame with waterThen the following reactions will occur:mist using ethanol and kerosene. It was found thatOH+ CO- +CO2+ H,the interaction mechanism is related to the fuel speciesH+ O2→0H+ O, .for dfferent fuels have different flame temperature,CO+ O→CO2.product components ( especially the CO concentra-tion) and smoke components. In the case of ethanol,When the combustion intensity increases rapid-physical suppression effect is dominant, but in thely， the flame and smoke temperature will increasecase of kerosene, chemical reaction is dominant. Therapidly, then the following reaction may occur:reason that the water mist can enhance the chemicalH2O+ C→CO+ H2.reaction is because it can help generate extremely acThe H2 can bring more H and OH through thetive radical H and OH. Combustion enhancement isalso related with the amount of the water mist, therefollowing reactions:may be a turning point at which the effect of water onH2+O2 +20H,the flame changes from dominant chemical to domi-H2+ M-→2H+ M.nant physical.H and OH are extremely active radicals, one HFurther work will consider: (1) Performing theradical can generate three H radicals and one OH cansame experiments using the water mist at differentbring two OH in the fllowing chain branching reac-concentrations to investigate the relationship betweentions;the concentration and the effect; (2) using a specialOH+ H2→+H2O+ H,apparatus to detect the H and OH radical concentra-H+ O2→0H+ O,tions in the flame; (3) doing the same experimentsO+ H2→OH+ H.using different fuels, and with some additives whichThe global combustion reaction rate can be ex-can help suppress flame (for example NaCl).pressed in terms of the rates of the elementary reac-Referencestions given above, some of them are shown in Table1 Alpert, R. L. Incentive for use of misting sprays as a fire suppres-110]. The rate constants are expressed in the Arrhe-sion flooding agent. Water Mist Fire Suppression Workshop, NIS-nius form:TIR 5207， National Instiute of Standerds and Technology,Kk=AkT~texp( - En/RT).Gaithersburgh, 1993， s.2 National Fire Protection Association. NFPA 750 Standard for Wa.Table 1. The elementary reaction rate cofficientster Mist Fire Supression Systems. NFPA, Quincy. MA. USA.ReactionAl_N，2000.H+ O2 +OH+O1.20x1017 -0.91 69.100+ H: +OH+H1. s0x 10'2.00 31.60fire suppression research and development 1996. Internal ReportNo.718, National Research Council Conada, 1996.OH+ Hr +HO+H1. 00x 108.1.60 13.804 Grant, G. et al. Fire sppessicsn by water sprays. Progres in En-O+ H20→20H1. 49x10101.14 71.14ergy and Combustion Science, 2000, 26: 79.H+ O2+ M→HO2+ M2.00x1018 -0.80 0.005 Kim, M. B. et al. Burning rate of a pool fire with downward-di-OH+CO-CO2+H440x10.1.50 -3.10rected sprays. Fire Safety Journal, 1996， 27; 37.a) Catalytic eficiencies are taken from Ref. [5]6 Bac Kim Myung, et al. Extinction limit of a pool fire with a waterIt is obvious that, in the process of kerosene7 Yao, B. et al. Interaction of water mists with dfsion flames in acombustion, the water mist ( and H2 which may beconfined space. Fire Safety Journal, 1999, 33(2): 129.8 Liu. J. H. etal. Experimental study on the pool fire suppressiongenerated during the reaction) has catalyzed thwith water mists. Intermational Symposium on Safety Sei. & Tech,chemical reaction. But when the water mist addition2000, 861exceeds a certain amount, the physical suppression ef-9 Jeeil Suh, et al. The efet of water vapor on counterflow difusionfect will produce, the combustion enhancement willflames. Internstional Conference on Fire Research and Engineer-decrease, so there should be a turning point at which中国煤化工nd einctcn of methane airthe concentration of water mist and the chemical en-_me, 1988, 73: 23.hancement reach the top.FHCNMHG'