Modeling and simulations of flame mitigation by fine water spray

Transcription

1 Alied Modeling and simulations of flame mitigation by fine wate say Liyaynen Alexey, Phd student St-Petesbug State Polytechnic Univesity JASS 2009

2 Agenda Alied Intoduction and objectives Model descition Gas hase modeling Liquid hase modeling Modeling of injection Results of flame suession by fine wate say Effect of initial say disesion

3 Intoduction Alied Advantages: Wate is envionmentally fiendly and toxically safe extinguishing agent Chea, clean, available Effective if otimized (may event unaccetable damage to otected oety ) A ossible halon (CF 3 B, CF 2 BCl, ) substitute (such FEA ae ohibited) FEA Fie Extinguishing Agent 3

4 Objectives Alied Undestanding fundamental mechanisms of the inteaction of fine wate say and tubulent diffusion flame Develoment of the aoiate mathematical model of a evaoating say Incooation of the model into the existing in-house softwae - Fie3D Comutational study of inteaction of wate say and buoyant tubulent diffusion flame D V50 =200 μ 4

5 Alied Fine wate say fie suession Initial dolet size is a key aamete that switches extinguishment egimes Fine wate say is secial: behaves simila to the gas (total flooding) extinguishing agents Fine say This ange of dolet sizes has not been thooughly investigated Coase say High essue fine wate say can be moe efficient Why and when? 5

6 Alied Otimum use of fine wate say Two oosite equiements: efficient delivey is needed fo wetting of flame suface aid dolet evaoation is equied fo cooling of the flame Wate say hysics: two dolet delivey egimes Lage dolets, coase say gavity mode (weak lumesay inteaction, dolet enetation is detemined by dolet diamete) Small dolets, fine say momentum mode (stong lume-say inteaction, enetation may not occu, it is detemined by the atio of say and lume momentums) Otimal solution can t be univesal it deends on a ossible fie scenaio, geomety etc. Need in caeful CFD modeling and costly simulations 6

7 Model descition Alied Eule-Lagange aoach : gas hase multicomonent eacting mixtue disesed hase lage numbe of dolets Full inteaction assumed (two-way couling): dolet-gas heat tansfe dolet evaoation (mass tansfe) momentum exchange dolet disesion due to tubulence 7

8 Gas hase modeling Alied Navie-Stokes system based on Fave-aveaged comonent, momentum and enthaly tansot equations Modified k-ε, eddy beak-u and themal adiation models ae used as closing elationshis Low Mach numbe flow is consideed Finite volume technique fo discetization Langman et al, 2007 A momentay image B hoto with exosue (analogue to aveaging) 8

9 Liquid hase modeling Alied Lagangian aoach to model evaoating say - multile discete dolets ae tacked along thei tajectoies in gasflow with given chaacteistics Momentum, mass, enegy consevation equations ae consideed fo gous of simila dolets (aticles) Dolet movement due to gavity and dag foces du dt, i 3ρC = 4d ρ D u, i u~ i ( u, i u~ ) + i g i 1 ρ ρ dx dt, i = u, i C D = 24/ Re ( Re / 3 ),Re,Re < 1000 >

10 Dolet disesion Paticle tansit time though single examined eddy Inteaction time τ u = u ~ + u u ' = 2k 3 τ = D ln 1 τ τ D l u t u *, if l, if l ( τ, 2 3), t = min l t k 4 3 / 2 l = 3 / t C μ k t t < τ > τ D D u u ε u u * * Alied Tubulence may have a consideable effect! Heat tansfe fom and to the caying gas Dolet heating q q, conv + q, ad m C dt dt = q + Δh va 0 ( T dm ) dt g ~ = q = πd μ C ( T T ), conv Nu P, g Nu P = Re 1 / 2 P 1 / 3, T, T < T = T boil boil Latent heat of evaoation Δh va ( T ) = Δhva, boil + T T boil C ( T ) dt 10

11 Dolet mass loose due to evaoation Alied dm dt = Sh πd μ g ln 1 P q Δhva ( Tboil ) ( + B ) m, T, T < T = T boil boil Sh B m = = Re Y va, sat 1 Y 1 / 2 Sc ( T ) Yva ( T ) va, sat 1 / 3, Inte-hase exchange & M & V & H n dm = Δ V dt n u u~ dm = m u ΔV τ D dt ~ n T T = mc hva ΔV τ T ( T ) dm dt - is the vao enthaly at dolet temeatue 11

12 Modeling of injection Alied Sinkle is modeled hee as a oint souce Rosin-Rammle distibution to model olidisesity R ( ) d = ex ln 2 dv 50 d γ Velocity vectos unifomly distibuted inside the cone R ( d ) d, мкм 12

13 Alied Fine wate say flame suession Schwille & Luetow exeiment 15 kw, 18 cm diamete bune, methane d v50 = mm l/min 120º cone angle, 1.6 m height Say cone is much wide than the flame base 13

14 Flame without wate say Alied Comaison with exeiments Caloific owe 15kW Isosufaces of temeatue T=200 C Tansient simulation of the gas flame ae then used as initial conditions fo case with say 14

15 Alied Fine wate say flame suession d V50 = mm 7.5 l/min d V50 = mm 7.5 l/min d V50 = mm 3.0 l/min Coase say: say angle evaoates inside the flame small dos deflected Fine say: Naowed jet, votex ing Vao cloud does not enetate but suounds the flame 15

16 Effect of initial say disesion Alied Say dynamics and flame-say inteaction is vey sensitive to initial say disesion Coase say, d V50 = mm 15 kw, 7.57 l/min Fine say, d V50 = mm 15 kw, 7.57 l/min Fine say suesses flame faste with smalle wate suly ate because of: (i) faste evaoation; (ii) highe and moe focused momentum 16

17 Alied Effect of initial say disesion The fine say oduces the amount of vao which is by moe than an ode of magnitude geate than that oduced by the coase say Fine say extinguishes the flame within few seconds afte the nozzle activation 17

18 Conclusion Alied Model of evaoating say is develoed and incooated into CFD softwae Mechanisms of say-flame inteaction ae identified and demonstated Fine wate say causes faste flame extinguishment with smalle wate flow ate 18

19 Alied Thank you! Discussion 19