When a premixed flame propagates in a turbulent flow, not only does turbulence affect the burning rate (e.g., by wrinkling the flame and increasing its surface area), but also the heat release in the flame perturbs the pressure field, and these pressure perturbations affect the turbulent flow and scalar transport. For instance, the latter effects manifest themselves in the so-called countergradient turbulent scalar flux, which has been documented in various flames and has challenged the combustion community for approximately 35 years. Over the past decade, substantial progress has been made in investigating (a) the influence of thermal expansion in a premixed flame on the turbulent flow and turbulent scalar transport within the flame brush, as well as (b) the feedback influence of countergradient scalar transport on the turbulent burning rate. The present article reviews recent developments in this field and outlines issues to be solved in future research.

中文翻译：

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对预混湍流火焰热膨胀效应理解的最新进展

当预混火焰在湍流中传播时，湍流不仅会影响燃烧速度（例如，通过使火焰起皱并增加其表面积），而且火焰中的热量释放会扰乱压力场，这些压力扰动会影响湍流和标量输运。例如，后一种效应在所谓的逆梯度湍流标量通量中表现出来，这种现象已在各种火焰中得到记录，并且已经对燃烧界提出了大约 35 年的挑战。在过去的十年中，在研究 (a) 预混火焰中的热膨胀对火焰刷内的湍流和湍流标量传输的影响以及 (b) 反梯度标量传输的反馈影响方面取得了实质性进展关于湍流燃烧速度。