Evaluation of mean species mass fractions in premixed turbulent flames: A DNS study

Evaluation of mean species mass fractions in premixed turbulent flames: A DNS study
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DOI：
10.1016/j.proci.2020.05.006
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发表时间：
2020-07
期刊：
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影响因子：
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通讯作者：
A. Lipatnikov;V. Sabelnikov
A. Lipatnikov;V. Sabelnikov
中科院分区：
其他
文献类型：
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作者：
A. Lipatnikov;V. Sabelnikov

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Dave和Chaudhuri（2020）从一个精益的，复杂的化学反应中获得的直接数值模拟（DNS）数据，分析了与预混湍流燃烧的薄反应区区域相关联的氢-空气火焰（通过调整燃烧过程变量的五种替代定义c），以检查三种不同的模型，（i）基于小火焰范例和（ii）目的是在湍流燃烧的应用CFD研究中评估各种物质的平均浓度。如果使用概率密度函数（PDF）P（c）直接平均化物质摩尔分数和密度的层流火焰分布，则预测所有考虑的物质的平均摩尔分数和平均密度。通过从DNS数据中提取P（c）并基于氢质量分数定义c来获得最佳预测。这些预测表明，在预混湍流火焰中的各种物质的平均摩尔分数，可以在后处理阶段的CFD研究通过adoptingP（c），在主要阶段的模拟，平均小火焰库进行评估。当以这种方式应用时，小火焰范例甚至对于贫氢-空气火焰和甚至在Karlovitz数高达13时都是有用的。如果将相同的PDF应用于来自相同小火焰库的平均反应速率，则不能很好地预测物种的平均产生/消耗速率，例如，对于自由基H，O，OH，HO 2和H2 O2，如果使用氢质量分数定义。假设条件速率<Wn| c>可以使用条件摩尔分数|c>，温度<T| c>，密度<ρ| c>也不支持，例如对于基团O和OH。第一种方法（直接平均浓度曲线）和其他两种方法（平均反应速率）的预测能力之间的这些差异归因于浓度（分别为速率）的弱（高度）非线性依赖性。
Direct Numerical Simulation (DNS) data obtained by Dave and Chaudhuri (2020) from a lean, complex-chemistry, hydrogen-air flame associated with the thin-reaction-zone regime of premixed turbulent burning are analyzed (by adapting five alternative definitions of combustion progress variablec) in order to examine three different models that (i) are based on the flamelet paradigm and (ii) aim at evaluating mean concentrations of various species in applied CFD research into turbulent combustion. Mean mole fractions of all considered species and mean density are predicted if the laminar-flame profiles of species mole fractions and density, respectively, are directly averaged using a Probability Density Function (PDF)P(c). The best predictions are obtained by extractingP(c) from the DNS data and definingcbased on hydrogen mass fraction. These predictions suggest that mean mole fractions of various species in a premixed turbulent flame can be evaluated at a post-processing stage of a CFD study by adoptingP(c), obtained at the major stage of the simulations, to average a flamelet library. When applied in such a way, the flamelet paradigm is useful even for lean hydrogen-air flames and even at Karlovitz number as large as 13. If the same PDF is applied to average reaction rates from the same flamelet library, the mean rates of production/consumption of speciesnare poorly predicted, e.g. for radicals H, O, OH, HO2, and H2O2ifcis defined using hydrogen mass fraction. A hypothesis that conditioned rates <Wn|c> can be predicted using conditioned mole fractions <Xn|c>, temperature <T|c>, and density <ρ|c> is not supported either, e.g. for radicals O and OH. These differences between predictive capabilities of the first approach (directly averaging concentration profiles) and two other approaches (averaging reaction rates) are attributed to weakly (highly) non-linear dependencies of the concentrations (rates, respectively) onc.