湍流燃燒模擬ppt課件(參考版)
2025-05-04 08:37本頁面
【正文】 由于煤的燃燒過程由揮發(fā)份燃燒和焦炭燃燒兩個階段組成，故燃料型的形成也由氣相氮的氧化（揮發(fā)份）和焦炭中剩余氮的氧化（焦炭）兩部分組成。 由于燃料中氮的熱分解溫度低于煤粉燃燒溫度，在 600－ 800℃ 時就會生成燃料型，它在煤粉燃燒 NOx產(chǎn)物中占 60－ 80％。 ? 由于燃料揮發(fā)物中碳?xì)浠衔锔邷胤纸馍傻?CH自由基可以和空氣中氮?dú)夥磻?yīng)生成 HCN和 N，再進(jìn)一步與氧氣作用以極快的速度生成，其形成時間只需要 60ms，所生成的與爐膛壓力 ,與溫度的關(guān)系不大。 ? 快速型 NOx是 1971年 Fenimore通過實(shí)驗(yàn)發(fā)現(xiàn)的。 隨著反應(yīng)溫度 T的升高，其反應(yīng)速率按指數(shù)規(guī)律增加。 在氮氧化物中， NO占有 90%以上，二氧化氮占 5%10%. (a).熱力型 燃燒時，空氣中氮在高溫下氧化產(chǎn)生，其中的生成過程是一個不分支連鎖反應(yīng)。 ? Discrete Ordinates Model (離散坐標(biāo)法 ) DO模型是所有四種模型是最為復(fù)雜的輻射模型，從小尺度到大尺度輻射計算都適用，且可計算非－灰度輻射和散射效應(yīng)，但需要較大計算量。 ? The Rosseland Model Rosseland模型是最為簡化的輻射模型，只能應(yīng)用于大尺度輻射計算。當(dāng)燃燒計算域的尺寸比較大時， P1模型非常有效。 ? P1模型 適用于大尺度輻射計算。h e te ro g e n e o u s s u rfa c e re a c ti o n熱輻射模型 ? Discrete Transfer Radiation Model (離散傳遞法 ) DTRM模型的優(yōu)點(diǎn)是簡單 , 且可以適用的計算對象的尺度范圍較大 , 其缺點(diǎn)是沒有包含散射和不能計算非灰的輻射。 turbulencechemistry interactions are important ? Realistic chemical mechanisms have tens of species, hundreds of reactions and stiff kiics (widely disparate time scales) ? Practical approaches ? Reduced chemical mechanisms ? Finite rate bustion 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 puted 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 particleparticle interaction or break up ? Turbulent dispersion modeled by ? Stochastic tracking ? Particle cloud (V5) ? RosinRammler or linear size distribution ? Particle tracking in unsteady flows (V5) ? Model particle separation, spray drying, liquid fuel or coal bustion, etc. Continuous phase flow field calculation Particle trajectory calculation Update continuous phase source terms ? Turbulent dispersion is modeled by an ensemble of MonteCarlo 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 putationally 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 multivariate 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 (. initial d,? etc.) ? Particles can escape, reflect or trap (release volatiles) at walls ? Eliminates (single cloud) or reduces (few clouds) stochastic tracking ? Decreased putational expense ? Increased stability since distributed source terms in gas phase BUT decrease
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