燃烧室和燃烧器课件：Chapter 4 Gaseous Fuel Combustion

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1、燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Chapter 4 Gaseous Fuel Combustion(6h)4.1 Diffusion Flame（扩散火焰）（扩散火焰）4.1.1 Round Free jet flame 4.1.1.1 Properties of free jet flame 4.1.1.2 Properties of free jet 4.1.1.3 Turbulent free jet flame 4.1.2 Confined and swirl jet flame4.2 Premixed Flame（预混火焰）（预混火焰）4.2.1 Combusti

2、on modes 4.2.2 Laminar Flame Speed and relevant analysis 4.2.3 Laminar Flame Speed measurement 4.2.4 Turbulent Premixed Flame燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用What is a diffusion flame?For a diffusion flame,the fuel and air are supplied separately until the flame is formed.The counterpart of the diffusion f

3、lame is the premixed flame.For a premixed flame,the fuel and air are mixed uniformly before it is ignited to form a flame.燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用4.1.1.1 Properties of free jet flame1.Flame structureFlame front:locus of points where the equivalence ratio equals unity燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用What

4、do we care about the diffusion jet flame?Blow out limitFlame shape(length,boundary)Laminar and turbulent flameInvestigation by ExperimentsCorrelations(from non-reactive to reactive flows)Solution to control equations燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用2.Blow-off problem（吹熄）（吹熄）Transition from laminar to turbu

5、lent flameThe transition Reynolds number is different for different fuels and generally located between 200010000燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Stability regimes（稳定模式）（稳定模式）for free jet diffusion flame of ethylene-air mixture燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用3.Flame length,Lf1).Laminar flame Proportional to volu

6、metric flowrate.For circular-port,038fQ GfDLLaminar ethylene jet diffusion flame燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用2).Turbulent flame The length of the flame is proportional to jet diameterfLD燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用4.1.1.2 Properties of free jet燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Formation 燃烧与燃烧室 热热 能能 工工 程程 系系 授课专

7、用Simplified model燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Entrainment and self-similarity Entrainment（引射引射）Self-Similarity（自模化自模化）燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Experimental correlations for self-similarityVelocity at center line000.9720.29muaxudRadial velocity profile 21.51murubJet expansion half angle（射流半扩张角）3.4tgawh

8、ere a=0.070.08,andHalf jet width（射流半宽）bx tg000,mmmmmmuTTCCTTCCuuTTCCuTTCC燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Theoretic Analysis by Conservation equations Ideas&Assumptions:2-D steady round jet fuel flow into still air incompressible,non-reactiveconstant pressure,constant temperatureno body forceonly radial di

9、ffusion of momentum and species is importantaxial velocity is much greater than radial velocityThe conservation equations can be simplified as follow:(turbulent flows)燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用()()0rurvxr()()uuuTxrrrrurvr()()iiiYYYTxrrrrurvDDr()()()()()T TT TT TTxrrrrurvaarwhere,D,and a are laminar

10、kinetic viscosity,diffusivity,and thermal diffusivity,respectively.The subscript T denotes turbulent.燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Initial and boundary conditions are as follow:1)If rb,and 0 x ,/()/0,0()/oxoxoxoxFFuurTTTTrYYYYrYYr2)If r=0,and 0 x ,/()/,0()/moxoxox moxoxoxFF mFuuurTTTTTTrYYYYYYrYYYr燃烧与燃烧

11、室 热热 能能 工工 程程 系系 授课专用3)If x=0,and 0 r d0/2,4)If x=,and 0 r ,00,0oxoxoxFFuuTTTTYYYYY,/()/0,0()/oxoxoxoxFFuurTTTTrYYYYrYYr燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用 Now the problem is how to solve the equations.Take the momentum equation for example.Solution:According to the continuity equation,we obtain,00,0oxOxFoxF

12、YYTTYuuTTYY()()rurvxr It can be seen that each conservation equation and its boundary condition have the same form.Then,their solutions should be similar with each other and take on the following form(if Pr=Le=1):燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Integrate the continuity equation twice.First,integrate from

13、r=0 to r=b,one can obtain:10(1)bbbxvrudr Second,integrate from r=0 to r=b/2,one can obtain:/22/20(2)bbbxvrudr Rewrite the momentum equation as follow(how?)2()()()()ruruvuTxrrrr()()uuuTxrrrrurvr燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Then,integrate the momentum equation twice.First,integrate from r=0 to r=b,one ca

14、n obtain200bxru drThat means the jet momentum should be a constant(equals to the value at the exit plane),i.e.Second,integrate from r=0 to r=b/2,one can obtain22200402(3)bduru dr/22/2/20()(4)bubTbxrru drruvr|燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Assume a simple linear profile for velocity as follow1,murubrb And

15、 substituting the assumption into equations(1)(4)yields222161/26300212()(5)()(6)(7)()(8)mmmu bbbxu bbbxmu bTxvvu bu d 燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Combining equations(7)and(8),we obtain Since T,and assumimg T=cumb(Prandtl mixing length hypothesis，混合长度理论)Integrating the equation from x=0 to x=x yields22

16、008()1()Tmxuu d00132(1 8)(9)muxudc002326(18)(10)bxddcSubstituting it into equation(7)yields燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用 000011332232(11)(1 8)1(1 8)uxxrudddccConclusions:1)The maximum jet velocity um decreases with distance in x direction2)Jet width 2b increases with distance in x direction3)Jet veloci

17、ty at any point decreases with both x and r0002321 81 0.125(10)bxxdddc Substituting equations(9)and(10)into the linear profile assumption yields Take c0.0128(by experiments),and let 2b=d0 for x=0,then燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Entrainment（引射）Phenomenon:when the jet flows forward,the still surrounding

18、 air will be entrained to flow with the jet,which is called entrainment.According to above calculation,the total mass flow at any section x downstream can be derived as follow:0000002313182223182btotalrdxcdmurdrurdrxcd燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用0200000002/2233331 81 81 8222bbduurdrr drxxxcccddd320020

19、00222332331 81 822u bubxxccddd2000131 822xu dcd00000023131 822231 82btotalrdxcdmurdrurdrxcd燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用2000013442etotalxmmmu dcdThus we can obtain0031162emxcmdIt should be noted that the above jet velocity profile is a simplified distribution,so it is not true for near jet exit region.

20、The initial mass flow rate is:20004mudThen the entrainment air flow rate is燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用000.321emxmdExperiments show that when Re2.5104,and x/d06,the total mass flow rate at downstream section x is:000.32totalxmmdThen the ratio of entrainment air to jet mass flow rate is For more genera

21、l situation with 2 different fluids,0000.321emxmd燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Similarly,the solution for laminar round jet is as followThe solutions for other parameters are as follow00018Re(1)muxud00028Re6(1)bxdd00000011882Re3Re(1)1(1)uxxruddd,0,00oxOxFoxFYYTTYuTTYYu燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Further a

22、ssumptions:Thermal properties are constant for different temperature and speciesChemical reaction has no impact on flowNo heat loss by radiationThen the conservation equations become()()0rurvxr()()uuuTxrrrrurvr()()iiiiYYYR rTxrrrrurvDDr()()()()()pppFcTTcTTcTTR QrTxrrrrurvaar4.1.1.3 Turbulent free je

23、t flame燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Using Zeldovich form as follow.Assume the reaction is as simple as (1g)F+(g)OX (1+)g P+QPand define,()F OOXFOXBYYY11,11()(1)F PFPOXBYYY,11()OXYO POXPBYY(),pcTTF TFQBY(),pcT TO TOXOXQBYYA little bit different(source term in species continuity and energy equations)from

24、 that without combustion燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Now the flame shape can be derived by simple analysis.Assume the species concentrations of both fuel and oxygen are zero at flame front due to fast reaction,i.e.,If r=rf,then YF,f=YO,f=0then one can obtain BF,O,f=-YOX,and BF,O,0=-(+YOX,)That is to sa

25、y,0,0,0,0,00(,)F OF PO PF TO TF OF PO PF TO TBBBBBuBBBBBuF x r,0,(,)F O fOxF OOxBYffBYF xrThen one can get the Zeldovich form,which can be solved as燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用For laminar flame 000000,11882Re3Re,(1)1(1)fffxrxoxuudddfoxYYFor turbulent flame 0000,11332232,(1 8)1(1 8)fffxrxoxuudddfoxYccY

26、000023,8Re8,0Re1(1)(1)ffxfoxdxoxdrYYd00023,323,21(1 8)1 8fffrxdoxdxoxdYcYc燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用This equation gives the flame shape（火焰形状）denoted by xf and rf.A qualitative plot of the solution defined by the equation is as followLet rf=0,we obtain the length of the flame,L.For turbulent flame30,

27、28TOXLdcYFor laminar flame0,0Re8LOXLYd燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用where Re0 is the Reynolds number at the exit plane,Re0=u0d0/.Conclusions:(based on the simple linear profile（线性分布）assumption)1)The length of turbulent diffusion flame has nothing to do with the volumetric flowrate of fuel,but rather dep

28、ends on the diameter of the nozzle.2)The length of laminar diffusion flame is proportional to the volumetric flowrate of fuel.3)Since LL/LT=(3/2)0.5cRe0,and c0.0128(for round jet),assuming Re0=1000(laminar)yields LL/LT=15.6767,which means the length of laminar diffusion flame is much longer than tha

29、t of turbulent flame.燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用4.1.2 Confined and swirl jet flameConfinedJet（受限射流）燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用The wall confinement will influence the entrainment air flow rate.For the case of the ratio of confinement diameter to jet diameter approximately to be 10,assume that the entra

30、inment air flow is totally recirculated,then it can be derived that(suppose the jet expansion angle is 9.7)0000.470.52eammdRin which R is the radius of the confinement.5.85pXR燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用 Swirl number（旋流数）For a free swirl jet,both the axial flux of angular momentum G and the axial forc

31、e Gx are constant through out the downstream flowfield,i.e.,0()2RGwrurdrconst0022RRxGu urdrprdrconstSwirl jet燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Thus,we define swirl number,S,as follow:where R is the radius of the nozzle.The common methods to generate swirl are as follow:xGRGSTangential entryMovable blockCurv

32、ed vane燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Profile of the streamline function for S=2.2The classification（分类）of swirl flow:1)S0.2,weak swirl(applicable boundary layer assumption)2)0.2S0.6,intense swirl(strong reverse pressure gradient)For axial velocity profile,if S0.4,the profile is Gauss type;if 0.4S0.6,cen

33、tral recirculation zone occurs near the nozzle燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用S=0S=1.1Effect of swirl on flame length燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用本本 节节 要要 点点扩散火焰、结构特点、火焰锋面扩散火焰、结构特点、火焰锋面圆形自由射流结构特点圆形自由射流结构特点圆形自由射流的广义圆形自由射流的广义Reynolds比拟比拟圆形自由射流速度场的简化求解圆形自由射流速度场的简化求解圆形自由射流扩散火焰形状、火焰长度估算圆形自由射流扩散火焰形状、火焰长度估算受限射流结构特点

34、受限射流结构特点旋转射流结构特点、旋流数、强弱旋流差异旋转射流结构特点、旋流数、强弱旋流差异燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用4.2 Premixed Flame4.2.1 Combustion modes4.2.2 Laminar Flame Speed（层流火焰传播速度）and relevant analysis4.2.3 Laminar Flame Speed measurement4.2.4 Turbulent Premixed Flame燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用4.2.1 Combustion modesThere are 2 modes

35、 of combustion:deflagration and detonation.Deflagration is a subsonic wave sustained by a chemical bustion wave,flame,a n d d e f l a g r a t i o n h a v e b e e n u s e d interchangeably.Detonation（爆震）is supersonic wave sustained by a chemical reaction.燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Further consideratio

36、n by reactive fluid dynamicsAssumptions:1-D steady flow No viscosity,and no body force Ideal gases Constant specific heat capacity and molecular weight Reaction scale is much less than the flow scale(diameter)No friction and heat loss by duct wall The coordinate system is fixed to the wave as follow

37、燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用where c,x are sound speed and axial velocity,respectively;and the subscripts 1 and 2 denote the states of before and after the combustion wave.Conservation equations are as follow Continuity:1x1=2x2=m0.5=constant (1)Momentum:p1+12x1=p2+22x2=constant (2)Energy:h1+2x1/2=h2+2x

38、2/2=constant (3)For constant cp,equation(3)can be rewritten as cpT1+2x1/2+Q=cpT2+2x2/2 (4)Rayleigh line is obtained from equations(1)and(2)as follow211121ppm or,using the specific volume as2121ppm 燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用It is a straight line through the point(p1,1)in the p plot Increasing m cause

39、s the line to steeppen.In the limit of an infinitive m,the Rayleigh line would be vertical;while in the opposite limit of m=0,the line becomes horizontal.Since these two extremes contain all possible m,no solutions are possible in the two quadrants labeled A and B.Regions A and B are physically inac

40、cessible.燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Hugoniot curve is obtained from equation(4),state relationships,and Rayleigh line equation as followLet=Q1/p1,Hugoniot curve is plotted as(for k=1.4)2121121112112()()()ppkkppQ燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用It can be seen from above figure that increasing the heat releas

41、e would cause the curve smoother and moving outer.Plot both Rayleigh and Hugoniot in a single figure with origin state(1,1)燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Since any real process going from state 1 to state 2 must satisfy both the Rayleigh and Hugoniot relations,then the points between C and D on the Hugon

42、iot curve are unrealizable.For upper branch of Hugoniot curve,there is a limiting Rayleigh line that is just tangential to the Hugoniot at point B.This point of tangency is designated the upper Chapman-Jouguet point.Similarly,the point E is the lower C-J point.The four limiting Rayleigh lines(2 tang

43、ential,a vertical,and a horizontal lines)divide the Hugoniot into 5 parts.The physical characteristics associated w i t h e a c h p a r t a r e a s f o l l o w.燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Partcharacteristicx2(relative to wavefront)Above BStrong detonationsubsonicB-CWeak detonationsupersonicC-Dinaccess

44、ible-D-EWeak deflagrationsubsonicBelow EStrong deflagrationsupersonicNote:most real flames are located in the part D-E.How to determine the Ma number in different area?According to Rayleigh line2122211111ppmv 燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用One can obtain21221121111ppv Since the sound speed c1 is defined

45、as21111/ckRTkpThen the Ma number before combustion is obtained211221111ppM ak For upper left area,p2p1 and 21,then11Ma So this area is called detonation area.Similarly,the Ma number of lower right area is less than 1,so it is called deflagration area.燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用The typical properties

46、of normal shocks,detonations,and deflagrations are as follow燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Other propertiesSpeed of Detonation WaveStructure of Detonation WaveTransition from Deflagration to Detonation Please read the textbook by yourself燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用4.2.2 Flame speed and relevant analysis F

47、lame speed(also called flame velocity,or laminar flame speed层流火焰传播速度层流火焰传播速度)is the velocity at which flame move through the combustion wave and towards the unburned reactants in the direction normal to the wave surface.Analyze the flame speed directions of the following flames:燃烧与燃烧室 热热 能能 工工 程程 系系

48、 授课专用 Flame Thickness is defined as the small distance,over which the temperature profile goes from T1(incoming)to T2(burned),although the thickness of the visible flame zone is only less than half of.Detailed structure of a flame front燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Conservation equations for flame speed

49、Boundary conditions are as followLuuSm.pconst()iidYdYdiidxdxdxuDR()dTddTpfdxdxdxucR Q燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Combining continuity with energy equation yieldswhich is called the laminar flame propagation equation（层流火焰传播方程）（层流火焰传播方程）Now the problem is how to solve the equation,00iimiixTTYYxTTYd Yd T

50、d xd x ()pLfdTddTS cR Qdxdxdx燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用The theory of Zeldovich,Frank-Kamenetskii,and Semenov(also called Two-Parts Approximation分区近似分区近似)Assume the flame is made up of two parts.The preheat part is a zone of little reaction,and the reaction part is the zone in which the reaction and

51、diffusion terms dominate and the convective term can be ignored.Thus,in the preheat zone,the energy equation reduces to(1)LpdTddTS cdxdxdxBoundary conditions are,0;dTxTTdx 0,iixxTT燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用In the reaction zone,the energy equation reduces toIntegrating equation(1)from T to Ti yields:

52、220(2)fd TR QdxBoundary conditions are,00,miidTxTTdxxxTT()(3)iLpixdTS c TTdxIntegrating equation(2)from Ti to Tm yields:2(4)miiTfTxdTR QdTdx燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Since Then,one can obtainiixxdTdTdxdx2222(5)()miTfTLpiR QdTScTTAssume mmiTTffTTmiR QdTR QdTTTTTEquation(5)can be re-written as2228(6)(

53、)mTfTLpmR QdTScTT燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Define a=/cp,and letSuppose no heat loss,then one can have the approximation()mTffmTR QdTQRTTThen equation(6)becomes8(7)()fLpmaQRScTT()()fafpmQmmmc TTi.e.,0(1)()pmQGc TTwhere G0 is the stoichiometric air flow rate燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Substituting the r

54、esult into equation(7)yieldsmiTTd Td x()LpidTScTTdx08(1)fLaGRSAccording to the definition of flame thicknessAnd the above equation(3)()()mipLiLTTac STTSOne can obtain燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Impacts of initial temperature,T The increase of T will result in the increase of Tm,and further the increas

55、e of reaction rate Rf.That is to say,the increase of T causes the increase of SL.T has little impact on flame thickness.nLSTand n=1.52.1-C2H2+air;2-C3H8+air;3-CH4+air燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Impacts of initial pressure,p22,nLSpC3H8+air,room TStoichiometric CH4+air,room Tapwhere a=0.71燃烧与燃烧室 热热 能能 工

56、工 程程 系系 授课专用Impacts of stoichiometry,(F),or%Flame speed will be maximum around=1 because of the highest flame temperature.C3H8+air,p=1atm,various T燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Impacts of stoichiometry on flame thicknessCH4+air,p=1atm燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Impacts of oxygen concentration,%The increas

57、e of oxygen concentration results in higher flame temperature,thus yields higher flame speed.P=1atm,room T燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Impacts of inert gas type燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Impacts of thermal propertiesAccording to previous analysis,the change of thermal diffusivity,a,heating value,Q,and c

58、hemical reaction rate,R,would result in the change of flame speed and thickness.!It is interesting to note that for the majority of fuels the value is somewhere around 0.4m/s.The reaction zone for most conventional flames is of the order of 0.2-0.7mm thick.This is probably because most complex fuels

59、 are pyrolysed to smaller sized molecules(methane,hydrogen,1 or 2 carbon radicals)before combustion.燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Impacts of fuel type In general,for hydrocarbon fuel,the flame speed of acetylene（乙炔）is greater than that of olefins(alkene烯烃)and paraffins(alkane烷烃),while the flame speed of

60、 olefins is greater than that of paraffins燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用4.2.3 Flame speed measurement Bunsen burner method 本生灯法Special designed inner curve燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用sin(/2)LSwwhere/2 had better be taken at the central portion of the cone described by Schlieren edge or inner shadow cone燃烧

61、与燃烧室 热热 能能 工工 程程 系系 授课专用Cylindrical tube method圆柱管法LASuFwhere A is the cross section area of the quartz glass tube(inner passage),while F is the surface area of the flame front燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用本本 节节 要要 点点燃烧模式种类及特点燃烧模式种类及特点层流火焰传播速度定义、方程推导与求解层流火焰传播速度定义、方程推导与求解影响层流火焰传播速度的因素及影响规律影响层流火焰传播速度的因素及影

62、响规律火焰厚度定义及其影响因素火焰厚度定义及其影响因素常温常压下常用燃料的层流火焰传播速度取值范常温常压下常用燃料的层流火焰传播速度取值范围围测量层流火焰传播速度的方法及计算公式测量层流火焰传播速度的方法及计算公式燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用4.2.4 Turbulent premixed flames（自学）（自学）,fffSSSTTTRRR()pLfdTddTS cR QdxdxdxFor laminar flameFor turbulent flameThe turbulent flame speed SL is a function of some correl

63、ation termsHow would the flame speed change?Greater or smaller?First,let look back to the change of the flame shape of a Benzene lamp燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Definition:the turbulent flame speed is defined asTmSAwhere is the reactant flowrate,is the time-smoothed flame are

64、a,and is the unburned gas density.m A燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Classification Small scale turbulence(),Re),Re6000 Weak and intense turbulence is the flame speed,is the integral scale,which characterizes the largest eddy size,and q is the average kinetic energy(or the root-mean-square velocity fluctu

65、ation)燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用Small scale turbulence(),Re),Re6000Weak turbulence(vSL)No longer continuous flame surface.Thick flame.Karlovitz(1950)assumed that the turbulent flame speed should be the sum of diffusion speed of turbulent fluid element and the laminar flame speed,ST=SD+SL and2DLSu S Then 2TLLSSu S 燃烧与燃烧室 热热 能能 工工 程程 系系 授课专用本本 节节 要要 点点湍流火焰传播速度的量级湍流火焰传播速度的量级何谓小尺度湍流火焰？大尺度？何谓小尺度湍流火焰？大尺度？湍流火焰传播速度为什么较大？湍流火焰传播速度为什么较大？火焰表面皱褶理论的基本思路？火焰表面皱褶理论的基本思路？