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Transition from pulled to pushed fronts in premixed turbulent combustion: Theoretical and numerical study

机译：预混湍流燃烧从拉力前沿到推力前沿的转变：理论和数值研究

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This paper extends a previous theoretical study (Sabelnikov and Lipatnikov, 2013) of the influence of countergradient transport (CGT) on the speed of a statistically stationary, planar, 1D premixed flame that passively propagates in homogenous turbulence in the form of a traveling wave, i.e. retains its mean thickness and structure. While two particular models of the mean rate of product creation were addressed in the previous article, with the shape of the rate as a function of the Favre-averaged combustion progress variable being concave in both cases, the present paper deals with a more general model that subsumes both concave functions and functions with an inflection point, i.e. a point where the function changes from being concave to convex or vice versa. In this more general case, transition from pulled (flame speed is controlled by processes localized to the flame leading edge) to pushed (flame speed is controlled by processes within the entire flame brush) flames can occur both due to interplay of the nonlinear reaction term and a nonlinear convection term associated with CGT and due to the change of the shape of the reaction term in the absence of CGT. Explicit pushed traveling wave solutions to the studied problem are theoretically derived and conditions under that developing flames approach either pushed or pulled traveling wave solution are obtained by analyzing the governing equations at the flame leading edge and invoking the steepness selection criterion which highlights traveling wave with the steepest profile at the leading edge. Other analytical results include conditions for transition from pulled to pushed premixed turbulent flames, dependence of flame speed on the magnitude of the CGT term and the shape of the mean reaction rate, analytical expressions for the mean thickness of the pushed flames and turbulent scalar flux within the pushed flames. All these theoretical findings are validated by results of unsteady numerical simulations of the initial boundary value problem with steep initial wave profiles. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
机译：本文扩展了先前的理论研究（Sabelnikov和Lipatnikov，2013年），研究了反梯度输运（CGT）对统计平稳的平面一维预混火焰的速度的影响，该火焰以行波的形式在均匀湍流中被动传播，即保留其平均厚度和结构。虽然在上一篇文章中讨论了两个特定的平均产品创造速度模型，但在两种情况下，该速度的形状与Favre平均燃烧进度变量的函数关系均呈凹形，但本文采用的是更通用的模型它既包含凹函数又包含具有拐点的函数，即拐点从凹变为凸或反之亦然的点。在这种更普遍的情况下，由于非线性反应项的相互影响，从拉动（火焰速度由火焰前沿控制的过程控制）到推动（火焰速度由整个火焰刷内部的过程控制）的过渡都可能发生。以及与CGT相关的非线性对流项，并且是由于在没有CGT的情况下反应项的形状发生了变化。从理论上推导了所研究问题的显式推行波解，并通过分析火焰前沿的控制方程并调用陡度选择准则来突出发展行进的火焰的条件。最陡峭的轮廓在前缘。其他分析结果包括从拉动预混湍流到推动预混湍流过渡的条件，火焰速度对CGT项的大小和平均反应速率的形状的依赖性，推挤火焰的平均厚度和内部湍流标量通量的解析表达式被推的火焰。所有这些理论发现均通过具有陡峭初始波剖面的初始边界值问题的非稳态数值模拟的结果进行了验证。 （C）2015年燃烧研究所。由Elsevier Inc.出版。保留所有权利。

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