Heyl A, Bockhorn H
Institut für Chemische Technik, Universität Karlsruhe, Germany.
Chemosphere. 2001 Feb-Mar;42(5-7):449-62. doi: 10.1016/s0045-6535(00)00217-4.
The applicability of the laminar flamelet concept for the formation and destruction of nitric oxides in laminar and turbulent diffusion flames has been studied. In a first step, temperatures and species concentrations in an axisymmetric laminar diffusion flame have been calculated (i) by solving the detailed conservation equations and (ii) by applying the laminar flamelet concept. The main purpose of this step was the identification of differences between results from both approaches. It turned out that for highly temperature sensitive or relatively slow chemical processes, the inclusion of the full range of the prevailing scalar dissipation rates plays a major role for the calculated species concentrations. This behavior is obvious from the concept of the laminar flamelet model, where the scalar dissipation rate can be discussed in terms of the reciprocal of a residence time for attaining chemical equilibrium. In a second step, flamelet modeling of NOx formation was extended to a turbulent hydrogen diffusion flame. In both the steps, the flow fields of the flames were calculated by solving the Navier-Stokes equations in axisymmetric formulation using the SIMPLER algorithm. For the turbulent flow, Favre-averaged equations have been used and turbulence was modeled with the standard k-epsilon model including a correction term for axisymmetric systems. The averaging of the species concentrations was accomplished with presumed shape probability density functions (pdfs). The pdf of the mixture fraction was described with a beta-function whereas that of the scalar dissipation rate was assumed to be log-normal. Buoyancy effects have been taken into account. The calculated temperatures and concentrations were compared with data from different experiments.
研究了层流小火焰概念在层流和湍流扩散火焰中一氧化氮生成与破坏方面的适用性。第一步，通过求解详细的守恒方程以及应用层流小火焰概念，计算了轴对称层流扩散火焰中的温度和物种浓度。这一步的主要目的是识别两种方法结果之间的差异。结果表明，对于对温度高度敏感或相对缓慢的化学过程，包含所有主要的标量耗散率范围对计算出的物种浓度起着重要作用。从层流小火焰模型的概念来看，这种行为很明显，在该模型中，标量耗散率可以用达到化学平衡的停留时间的倒数来讨论。第二步，将氮氧化物生成的小火焰建模扩展到湍流氢扩散火焰。在这两个步骤中，通过使用SIMPLER算法求解轴对称形式的Navier-Stokes方程来计算火焰的流场。对于湍流，使用了Favre平均方程，并采用标准的k-ε模型对湍流进行建模，该模型包括轴对称系统的修正项。物种浓度的平均是通过假定形状概率密度函数（pdf）来完成的。混合分数的pdf用β函数描述，而标量耗散率的pdf假定为对数正态分布。考虑了浮力效应。将计算出的温度和浓度与来自不同实验的数据进行了比较。
