《湍流燃烧模拟》PPT课件.ppt

湍流燃烧数值模拟,主 要 内 容,第一部分,湍流燃烧认识,在能源、动力、航空和航天等工程领域，经常遇到的实际燃烧过程几乎全部都是湍流燃烧过程. 在湍流燃烧中，湍流流动过程和化学反应过程有强烈的相互关联和相互影响。,湍流燃烧的认识, 湍流通过强化混合而影响着时平均化学反应速率，同时化学反应放热过程又影响着湍流，如何定量地来描述和确定这种相互作用是湍流燃烧研究的一个重要内容. 。,湍流燃烧的认识, 组份方程和能量方程中的源项是化学反应源项； 化学反应中组份的生成(消耗)率或能量的释放速率是反应物浓度和反应流体温度的强非线性函数； 由于湍流影响，化学反应中组份浓度和温度以及化学反应速率都是随时间而脉动的，因此在湍流燃烧的数值模拟中，不仅面临着湍流流动所具有的问题以及脉动标量的输运方程如何处理的问题，还面临着湍流燃烧所特有的，与脉动量呈确定的强非线性函数关系的脉动标量即时平均化学反应速率的模拟。 湍流燃烧模拟最基本的问题是反应速率的时均值不等于用时平均值表达的反应速率.,火焰正常传播,其中：p为火焰前沿法向移动的分速度； n为可燃混气在火焰前沿法向移动的分速度。,火焰传播速度,火焰传播速度,其中：up为火焰前沿法向移动的分速度； un为可燃混气在火焰前沿法向移动的分速度。 如果火焰传播速度和可燃混气的流动速度方向一致，取负号，反之，取正号。 火焰传播速度：火焰相对于无穷远处的未燃混合气在其法线方向上的速度,湍流火焰研究方法,一类为经典的湍流火焰传播理论，包括皱折层流火焰的表面燃烧理论与微扩散的容积燃烧理论。 另一类是湍流燃烧模型方法，是以计算湍流燃烧速率为目标的湍流扩散燃烧和预混燃烧的物理模型，包括几率分布函数输运方程模型和ESCIMO湍流燃烧理论。,分布函数P(f)的概念,间隔t的时间分数，即几率。,处在,范围内的时间,在空间任何一点上混合物分数的统计分布,其中混合分数是由于瞬态质量分数f随时间脉动而形成的; 也称为瞬态质量分数f的几率分布密度，简写为PDF,几率 产生某个值的可能性,Rosin-Rammler分布,此处R表示滴径大于doi的液滴重量占液滴总重量的百分数,第二部分,湍流燃烧模型,湍流燃烧唯象模型,旋涡破碎模型（eddy-break-up model) 拉切滑模型 (stretch-cut-and-slide model) 几率密度函数的输运方程模型 - Spalding的ESCIMO湍流燃烧理论,旋涡破碎模型(EBU),Eddy-Break-up (EBU) 基本思想 在湍流燃烧区充满了已燃气团和未燃气团，化学反应在这两种气团的交界面上发生，认为平均化学反应率决定于末燃气团在湍流作用下破碎成更小气团的速率，而破碎速率与湍流脉动动能的衰变速率成正比。,基本思想(Spalding, 1976 ) 把湍流燃烧区考虑成充满末燃气团和已燃气团；气团在湍流的作用下受到拉伸和切割，重新组合，不均匀性尺度下降；在未燃气和已燃气界面上存在着连续的火焰面，它以层流火焰传播速度向末燃部分传播。,拉切滑模型 (stretch-cut-and-slide model),气团尺度的变化过程,考虑一个单位厚度的流体块，设其中每层流体块的平均厚度为，则该流体块中一共有1/层流体。在湍流作用下各层流体的厚度不断减小，流体块内的流体层数不断增加。,ESCIMO理论,E，engulfment：卷吞，描述在大尺度湍流作用下，一种流体被另一种流体卷吞的过程。 S，stretching：拉伸，描述迭在一起的流体层长度增加、厚度减小的过程。 C，coherence：粘附，描述流体层不愿分离的一种趋势，认为两层流体一旦由于卷吞碰到一起，那么在传输、拉伸和化学反应的过程中都不会分开，它们互相粘附在一起。 I，interdiffusion和化学上的interaction：相互扩散和化学反应，描述在流体层受拉伸的过程中，发生在流体层内部及其交界面上的扩散和化学反应。 MO，moving observer：运动观察者，意味着为了描述相互扩散和化学反应，把坐标系取在流体层上，与流体一起运动。,以上模型是从唯象的角度(如拟序结构、流场结构及涡的输运和破碎)来考察湍流燃烧过程。 但从研究湍流燃烧的角度看，更为合理的是基于流体力学的角度。,湍流燃烧气相模型,Generalized Finite-Rate Model(通用有限速率模型) Non-Premixed Combustion Model（非预混燃烧模型） Premixed Combustion Model（预混燃烧模型） Partially Premixed Combustion Model(部分预混燃烧模型) Composition PDF Transport Combustion Model（组分概率密度输运燃烧模型）,有限速率模型,Chemical reaction process described using global mechanism. Transport equations for species are solved. These equations predict local time-averaged mass fraction, mj , of each species. Source term (production or consumption) for species j is net reaction rate over all k reactions in mechanism: Rjk (rate of production/consumption of species j in reaction k) is computed to be the smaller of the Arrhenius rate and the mixing or “eddy breakup” rate. Mixing rate related to eddy lifetime, k /. Physical meaning is that reaction is limited by the rate at which turbulence can mix species (nonpremixed) and heat (premixed).,有限速率模型,求解反应物和生成物输运组分方程，并由用户来定义化学反应机理。 反应率作为源项在组分输运方程中通过阿累尼乌斯方程或涡耗散模型。 有限速率模型适用于预混燃烧、局部预混燃烧和非预混燃烧。 应用领域：该模型可以模拟大多数气相燃烧问题，在航空航天领域的燃烧计算中有广泛的应用。,Generalized Finite Rate Model: Summary,Advantages: Applicable to nonpremixed, partially premixed, and premixed combustion Simple and intuitive Widely used Disadvantages: Unreliable when mixing and kinetic time scales are comparable (requires Da 1). No rigorous accounting for turbulence-chemistry interactions Difficulty in predicting intermediate species and accounting for dissociation effects. Uncertainty in model constants, especially when applied to multiple reactions.,非预混燃烧模型-PDF Model,Applies to nonpremixed (diffusion) flames only Assumes that reaction is mixing-limited Local chemical equilibrium conditions prevail. Composition and properties in each cell defined by extent of turbulent mixing of fuel and oxidizer streams. Reaction mechanism is not explicitly defined by you. Reacting system treated using chemical equilibrium calculations (prePDF). Solves transport equations for mixture fraction and its variance, rather than species transport equations. Rigorous accounting of turbulence-chemistry interactions.,非预混燃烧模型-PDF模型,该模型不求解单个组分输运方程，但求解混合组分分布的输运方程。各组分浓度由混合组分分布求得。 PDF模型尤其适合于湍流扩散火焰的模拟和类似的反应过程。在该模型中，用概率密度函数PDF来考虑湍流效应。 该模型不要求用户显式地定义反应机理，而是通过火焰面方法(即混即燃模型)或化学平衡计算来处理，因此比有限速率模型有更多的优势。 应用领域：该模型应用于非预混燃烧（湍流扩散火焰），可以用来计算航空发动机的环形燃烧室中的燃烧问题及液体/固体火箭发动机中的复杂燃烧问题。,Mixture Fraction Definition,The mixture fraction, f, can be written in terms of elemental mass fractions as: where Zk is the elemental mass fraction of some element, k. Subscripts F and O denote fuel and oxidizer inlet stream values, respectively. For simple fuel/oxidizer systems, the mixture fraction represents the fuel mass fraction in a computational cell. Mixture fraction is a conserved scalar: Reaction source terms are eliminated from governing transport equations.,Systems That Can be Modeled Using a Single Mixture Fraction,Fuel/air diffusion flame: Diffusion flame with oxygen-enriched inlets: System using multiple fuel inlets:,60% CH4 40% CO,21% O2 79% N2,f = 1,f = 0,35% O2 65% N2,60% CH4 40% CO,35% O2 65% N2,f = 1,f = 0,f = 0,60% CH4 20% CO 10% C3H8 10% CO2,21% O2 79% N2,f = 1,f = 0,f = 1,60% CH4 20% CO 10% C3H8 10% CO2,PDF Modeling of Turbulence-Chemistry Interaction,Fluctuating mixture fraction is completely defined by its probability density function (PDF). p(V), the PDF, represents fraction of sampling time when variable, V, takes a value between V and V + V. p(f) can be used to compute time-averaged values of variables that depend on the mixture fraction, f: Species mole fractions Temperature, density,PDF Model Flexibility,Nonadiabatic systems: In real problems, with heat loss or gain, local thermo-chemical state must be related to mixture fraction, f, and enthalpy, h. Average quantities now evaluated as a function of mixture fraction, enthalpy (normalized heat loss/gain), and the PDF, p(f). Second conserved scalar: With second scalar in FLUENT, you can model: Two fuel streams with different compositions and single oxidizer stream (visa versa) Nonreacting stream in addition to a fuel and an oxidizer Co-firing a gaseous fuel with another gaseous, liquid, or coal fuel Firing single coal with two off-gases (volatiles and char burnout products) tracked separately,PDF Model: Summary,Advantages: Predicts formation of intermediate species. Accounts for dissociation effects. Accounts for coupling between turbulence and chemistry. Does not require the solution of a large number of species transport equations Robust and economical Disadvantages: System must be near chemical equilibrium locally. Cannot be used for compressible or non-turbulent flows. Not applicable to premixed systems.,Partially Premixed Combustion Model(部分预混燃烧模型),该模型针对预混和非预混燃烧都存在的湍流反应流动。 通过求解混合分数方程和反应过程参数来确定火焰峰面的位置。,The Laminar Flamelet Model,Temperature, density and species (for adiabatic) specified by two parameters, the mixture fraction and scalar dissipation rate Recall that for the mixture fraction PDF model (adiabatic), thermo-chemical state is function of f only c can be related to the local rate of strain,Extension of the mixture fraction PDF model to moderate chemical nonequilibrium Turbulent flame modeled as an ensemble of stretched laminar, opposed flow diffusion flames,将湍流火焰看成嵌入湍流流场内的局部具有一维结构的薄层流火焰。 湍流燃烧的层流小火焰模型是一种基于快速反应假设的模型，在火焰面内以分子扩散和输运过程为主。,Laminar Flamelet Model (2),Statistical distribution of flamelet ensemble is specified by the PDF P(f,c), which is modeled as Pf (f) Pc (c), with a Beta function for Pf (f) and a Dirac-delta distribution for Pc (c) Only available for adiabatic systems in V5 Import strained flame calculations prePDF or Sandias OPPDIF code Single or multiple flamelets Single: user specified strain, a Multiple: strained flamelet library, 0 a aextinction a=0 equilibrium a= aextinction is the maximum strain rate before flame extinguishes Possible to model local extinction pockets (e.g. lifted flames),模型的应用,层流小火焰模型在湍流预混燃烧和湍流扩散燃烧中表达形式有很大不同； 在湍流扩散燃烧中，输运方程中无化学反应源项，可以唯一地确定燃烧状态的守恒标量是混合分数，摄动参量是标量的耗散率； 在湍流预混燃烧的层流小火焰模型中，输运方程中无源项的可以唯一确定燃烧状态的守恒标量是描述火焰面位置的标量，摄动变量为使层流火焰面皱褶的变形率。,层流小火焰模型的优点,层流小火焰模型的快速反应假定较切合实际燃烧系统； 大的标量耗散率或火焰面变形率又可引致灭火，因此，具有预报着火、灭火的能力； 层流燃烧的数值模拟可以考虑详细的化学反应动力学过程和分子输运过程，因此，该模型具有良好的发展前景和实用价值。,不论是在湍流扩散燃烧中,还是在湍流预混燃烧中,Premixed Combustion Model（预混燃烧模型）,主要针对纯预混湍流燃烧问题，在这些问题中，反应物和生成物由火焰峰面隔开，该模型通过求解各种反应过程参数来预测火焰峰面的位置; 该模型为考虑湍流对燃烧的影响，引入了一个湍流火焰速度。,The Zimont Model for Premixed Combustion,Thermo-chemistry described by a single progress variable, Mean reaction rate, Turbulent flame speed, Ut, derived for lean premixed combustion and accounts for Equivalence ratio of the premixed fuel Flame front wrinkling and thickening by turbulence Flame front quenching by turbulent stretching Differential molecular diffusion For adiabatic combustion, The enthalpy equation must be solved for nonadiabatic combustion,第三部分 Fluent软件中的湍流燃烧模拟,Applications,Wide range of homogeneous and heterogeneous reacting flows Furnaces Boilers Process heaters Gas turbines Rocket engines Predictions of: Flow field and mixing characteristics Temperature field Species concentrations Particulates and pollutants,Temperature in a gas furnace,CO2 mass fraction,Stream function,Aspects of Combustion Modeling,Dispersed Phase Models,Droplet/particle dynamics Heterogeneous reaction Devolatilization Evaporation,Governing Transport Equations,Mass Momentum (turbulence) Energy Chemical Species,Combustion Models,Premixed Partially premixed Nonpremixed,Pollutant Models,Radiative Heat Transfer Models,Gas phase combustion Generalized finite rate formulation (Magnussen model) Conserved scalar PDF model (one and two mixture fractions) Laminar flamelet model Zimont model Discrete phase model Turbulent particle dispersion Stochastic tracking Particle cloud model Pulverized coal and oil spray combustion submodels Radiation models: DTRM, P-1, Rosseland and Discrete Ordinates Turbulence models: k-, RNG k-, RSM, Realizable k- and LES Pollutant models: NOx with reburn chemistry and soot,Combustion Models Available in FLUENT,Modeling Chemical Kinetics in Combustion,Challenging Most practical combustion processes are turbulent Rate expressions are highly nonlinear; turbulence-chemistry interactions are important Realistic chemical mechanisms have tens of species, hundreds of reactions and stiff kinetics (widely disparate time scales) Practical approaches Reduced chemical mechanisms Finite rate combustion model Decouple reaction chemistry from turbulent flow and mixing Mixture fraction approaches Equilibrium chemistry PDF model Laminar flamelet Progress variable Zimont model,Discrete Phase Model,Trajectories of particles/droplets/bubbles are computed in a Lagrangian frame. Exchange (couple) heat, mass, and momentum with Eulerian frame gas phase Discrete phase volume fraction must 10% Although the mass loading can be large No particle-particle interaction or break up Turbulent dispersion modeled by Stochastic tracking Particle cloud (V5) Rosin-Rammler or linear size distribution Particle tracking in unsteady flows (V5) Model particle separation, spray drying, liquid fuel or coal combustion, etc.,Turbulent dispersion is modeled by an ensemble of Monte-Carlo realizations (discrete random walks) Particles convected by the mean velocity plus a random direction turbulent velocity fluctuation Each trajectory represents a group of particles with the same properties (initial diameter, density etc.) Turbulent dispersion is important because Physically realistic (but computationally more expensive) Enhances stability by smoothing source terms and eliminating local spikes in coupling to the gas phase,Particle Dispersion: The Stochastic Tracking Model,Coal particle tracks in an industrial boiler,Particle Dispersion: The Particle Cloud Model,Track mean particle trajectory along mean velocity Assuming a 3D multi-variate Gaussian distribution about this mean track, calculate particle loading within three standard deviations Rigorously accounts for inertial and drift velocities A particle cloud is required for each particle type (e.g. initial d,r etc.) Particles can escape, reflect or trap (release volatiles) at walls Eliminates (single cloud) or reduces (few clouds) stochastic tracking Decreased computational expense Increased stability since distributed source terms in gas phase BUT decreased accuracy since Gas phase properties (e.g. temperature) are averaged within cloud Poor prediction of large recirculation zones,Coal/Oil Combustion Models,Coal or oil combustion modeled by changing the modeled particle to Droplet - for oil combustion Combusting particle - for coal combustion Several devolatilization and char burnout models provided. Note: These models control the rate of evolution of the fuel off-gas from coal/oil particles. Reactions in the gas (continuous) phase are modeled with the PDF or finite rate combustion model.,热辐射模型,Discrete Transfer Radiation Model (离散传递法) DTRM模型的优点是简单, 且可以适用的计算对象的尺度范围较大, 其缺点是没有包含散射和不能计算非灰的辐射。提高模型中射线的数量可以提高DTRM模型的精度，但计算量也明显增加。 P-1模型 适用于大尺度辐射计算。对比DTRM模型，其优点在于计算量更小，且包含散射效应。当燃烧计算域的尺寸比较大时，P-1模型非常有效。另外P-1模型可应用在较为复杂的计算域中。 The Rosseland Model Rosseland模型是最为简化的辐射模型，只能应用于大尺度辐射计算。其优点是速度最快，需要内存最少。 Discrete Ordinates Model (离散坐标法) DO模型是所有四种模型是最为复杂的辐射模型，从小尺度到大尺度辐射计算都适用，且可计算非灰度辐射和散射效应，但需要较大计算量。,NOx Models,NOx consists of mostly nitric oxide (NO). Precursor for smog Contributes to acid rain Causes ozone depletion Three mechanisms included in FLUENT for NOx production: Thermal NOx - Zeldovich mechanism (oxidation of atmospheric N) Most significant at high temperatures Prompt NOx - empirical mechanisms by De Soete, Williams, etc. Contribution is in general small Significant at fuel rich zones Fuel NOx - Empirical mechanisms by De Soete, Williams, etc. Predominant in coal flames where fuel-bound nitrogen is high and temperature is generally low. NOx reburn chemistry NO can be reduced in fuel rich zones by reaction with hydrocarbons,氮氧化物的产生机理,1 热力NOx： 燃烧用空气中的N2在高温下氧化而生成的氮氧化物； 2 快速NOx ：碳化氢系燃料在燃烧时分解，其分解的中问产物和N2反应生成的氮氧化物； 3 燃料NOx ：燃料中的有机氮化合物在燃烧过程中氧化生成的氮氧化物。,在氮氧化物中，NO占有90%以上，二氧化氮占5%-10%.,(a).热力型 燃烧时，空气中氮在高温下氧化产生，其中的生成过程是一个不分支连锁反应。其生成机理可用捷里多维奇(Zeldovich)反应式表示。 随着反应温度T的升高，其反应速率按指数规律增加。当T1500时,T每增加100,反应速率增大6-7倍。,快速型NOx是1971年Fenimore通过实验发现的。在碳氢化合物燃料燃烧在燃料过浓时，在反应区附近会快速生成NOx。 由于燃料挥发物中碳氢化合物高温分解生成的CH自由基可以和空气中氮气反应生成HCN和N，再进一步与氧气作用以极快的速度生成，其形成时间只需要60ms，所生成的与炉膛压力0.5次方成正比,与温度的关系不大。,(b).瞬时反应型(快速型),由燃料中氮化合物在燃烧中氧化而成。 由于燃料中氮的热分解温度低于煤粉燃烧温度，在600800时就会生成燃料型，它在煤粉燃烧NOx产物中占6080。 在生成燃料型NOx过程中，首先是含有氮的有机化合物热裂解产生N，CN，HCN和等中间产物基团，然后再氧化成NOx。 由于煤的燃烧过程由挥发份燃烧和焦炭燃烧两个阶段组成，故燃料型的形成也由气相氮的氧化（挥发份）和焦炭中剩余氮的氧化（焦炭）两部分组成。,(c)燃料型NOx,Soot modeling in FLUENT,Two soot formation models are available: One-step model (Khan and Greeves) Single transport equation for soot mass fraction Two-Step model (Tesner) Transport equations for radical nuclei and soot mass fraction concentrations Soot formation modeled by empirical rate constants where, C, pf, and F are a model constant, fuel partial pressure and equivalence ratio, respectively Soot combustion (destruction) modeled by Magnussen model Soot affects the radiation absorption Enable Soot-Radiation option in the Soot panel,Combustion Guidelines and Solution Strategies,Start in 2D Determine applicability of model physics Mesh resolution requirements (resolve shear layers) Solution parameters and convergence settings Boundary conditions Combustion is often very sensitive to inlet boundary conditions Correct velocity and scalar profiles can be critical Wall heat transfer is challenging to predict; if known, specify wall temperature instead of external convection/radiation BC Initial conditions While steady-state solution is independent of the IC, poor IC may cause divergence due to the number and nonlinearity of the transport equations Cold flow solution, then gas combustion, then particles, then radiation For strongly swirling flows, increase the swirl gradually,Combustion Guidelines and Solution Strategies (2),Underrelaxation Factors The effect of under-relaxation is highly nonlinear Decrease the diverging residual URF in increments of 0.1 Underrelax density when using the mixture fraction PDF model (0.5) Underrelax velocity for high bouyancy flows Underrelax pressure for high speed flows Once solution is stable, attempt to increase all URFs to as close to defaults as possible (and at least 0.9 for T, P-1, swirl and species (or mixture fraction statistics) Discretization Start with first order accuracy, then converge with second order to improve accuracy Second order