Effect of non-ambient pressure conditions and Lewis number variation on direct numerical simulation of turbulent Bunsen flames at low turbulence intensity
Effect of non-ambient pressure conditions and Lewis number variation on direct numerical simulation of turbulent Bunsen flames at low turbulence intensity
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DOI：
10.1016/j.combustflame.2021.111500
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发表时间：
2021-09
影响因子：
4.4
通讯作者：
R. Rasool;N. Chakraborty;M. Klein
中科院分区：
文献类型：
--
作者：
R. Rasool;N. Chakraborty;M. Klein
采用直接数值模拟（DNS）数据库，对大范围特征刘易斯数下的高压湍流预混本生火焰的瞬时火焰阵面结构进行了分析。稀轻燃料的高压湍流预混火焰可能具有热扩散和Darrieus-Landau不稳定性。由于这些不稳定性的影响被强烈的湍流所掩盖，本分析的重点是位于褶皱和波纹状火焰制度的边界处的火焰。火焰形态的特征在于平均曲率和高斯曲率的概率密度函数（PDF）分布的偏斜度和峰度。虽然偏度本身不足以作为区分热扩散不稳定性和Darrieus-Landau不稳定性的标志，但已经发现高斯曲率的过度峰度可能可以用于区分这两种不稳定性。此外，内截止长度和分形维数已被计算用于表征火焰尺度和参数化的燃烧因子模型。研究表明，压力的增加和刘易斯数的减小会导致火焰不稳定性的增加，从而导致分形维数的增加和内截止尺度的减小。对于刘易斯数约为1的高压火焰，内截止尺度与由Matalon和Matkowsky（1982）的理论分析确定的火焰不稳定性的临界波长成很好的比例。对于小的亚单位刘易斯数，火焰变得无条件不稳定和临界波长失去了意义，而内截止尺度继续减少与刘易斯数和分形维数继续增加到约7/3的限制。目前的研究结果与文献中的实验观察和火焰不稳定性的理论分析非常一致。
The instantaneous flame front structure of high pressure turbulent premixed Bunsen flames has been analyzed for a wide range of characteristic Lewis numbers using a new Direct Numerical Simulation (DNS) database. High pressure turbulent premixed flames of lean-light fuels are likely to feature both thermo-diffusive and Darrieus–Landau instabilities. As the effects of these instabilities are eclipsed by intense turbulence, the present analysis focuses on flames located at the border of the wrinkled and corrugated flamelets regimes. The flame morphology has been characterized by the skewness and kurtosis of the probability density function (PDF) profiles of the mean and Gaussian curvatures. While skewness alone is not sufficient as a marker to distinguish between thermo-diffusive and Darrieus–Landau instabilities, it has been found that excess kurtosis of Gaussian curvature possibly can be used to distinguish between both instabilities. Further, the inner cut-off length and the fractal dimension have been computed for characterization of flame scales and for parameterization of wrinkling factor models. It has been observed that increasing pressure and decreasing Lewis number give rise to flame instabilities which results in an increased fractal dimension and a decreased inner cut-off scale. For high pressure flames with Lewis numbers around unity, the inner cut-off scale scales very well with the critical wavelength for flame instabilities determined from the theoretical analysis by Matalon and Matkowsky (1982). For small sub-unity Lewis numbers, the flames become unconditionally unstable and the critical wavelength loses its meaning, while the inner cut-off scales continues to decrease with Lewis number and the fractal dimension continues to increase up to a limit of about 7/3. The present findings are in excellent agreement with experimental observations from literature and theoretical analysis of flame instabilities.