A series of two dimensional (2D) detailed numerical simulations of premixed cylindrical expanding ethanol/air flames in a constant volume encloser under elevated pressures and equivalence ratios are performed to investigate cellular flame instability. The results demonstrate that at pressure of 10 atm and temperature of 358 K, ethanol/air flame cellular instability increases non-monotonically with equivalence ratio (ϕ) from 0.8 to 1.6 and has the most intense instability at ϕ = 1.2. The trend is similar in both theory and simulation, while the latter overpredicts the critical flame radius compared with the theory. At the equivalence ratio of 1.2, the flame instability increases monotonically with pressure from 2 to 20 atm. The 2D simulation results are theoretically analysed by Peclet number, logarithmic growth rate of disturbance, flame thickness and critical flame radius for the onset of flame instability. It is found that hydrodynamic (DL) instability is insensitive to ϕ. In contrast, thermal-diffusion (TD) instability is overwhelming and changes dramatically with increasing ϕ mainly due to molecular diffusion. Therefore, the destabilising effect of logarithmic growth rate on the ethanol/air flame surface with increasing ϕ is actually due to the weakening stable effect of TD instability. The almost constant critical Peclet number and drastically reduced flame thickness lead to a great decrease in the critical flame radius with pressure rising both in theory and 2D simulations. It is concluded that the onset of instability advances with higher pressure and the acceptable quantitative comparison in terms of critical flame radius was observed between simulation and theory.

中文翻译：

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乙醇/空气火焰不稳定性的数值和理论研究

在升高的压力和当量比下，对恒定体积外壳中的预混圆柱形膨胀乙醇/空气火焰进行了一系列二维 (2D) 详细数值模拟，以研究细胞火焰的不稳定性。结果表明，在 10 atm 的压力和 358 K 的温度下，乙醇/空气火焰细胞不稳定性非单调增加，当量比 (ϕ) 从 0.8 到 1.6，并且在 ϕ = 1.2 时具有最强烈的不稳定性。理论和模拟的趋势相似，而后者与理论相比高估了临界火焰半径。当当量比为 1.2 时，火焰不稳定性随压力从 2 到 20 atm 单调增加。二维模拟结果通过佩克莱特数、扰动的对数增长率、火焰不稳定性开始时的火焰厚度和临界火焰半径。发现流体动力学 (DL) 不稳定性对 ϕ 不敏感。相比之下，热扩散 (TD) 不稳定性是压倒性的，并且主要由于分子扩散而随着 ϕ 的增加而显着变化。因此，随着φ的增加，对数增长率对乙醇/空气火焰表面的不稳定效应实际上是由于TD不稳定的稳定效应减弱。几乎恒定的临界 Peclet 数和急剧减小的火焰厚度导致临界火焰半径大大减小，理论和 2D 模拟中的压力都在上升。得出的结论是，在模拟和理论之间观察到在临界火焰半径方面不稳定性的开始随着更高的压力和可接受的定量比较而发展。