Enhanced burning rates in hydrogen-enriched turbulent premixed flames by diffusion of molecular and atomic hydrogen
Enhanced burning rates in hydrogen-enriched turbulent premixed flames by diffusion of molecular and atomic hydrogen
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DOI：
10.1016/j.combustflame.2021.111740
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发表时间：
2021-12
影响因子：
4.4
通讯作者：
M. Rieth;A. Gruber;F. Williams;Jacqueline H. Chen
中科院分区：
文献类型：
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作者：
M. Rieth;A. Gruber;F. Williams;Jacqueline H. Chen
为了更好地理解快速扩散、低Lewis数氢物种(H2和H)的分子扩散对反应物混合物总燃烧速率的局部影响，对燃料贫化湍流预混富氢火焰进行了直接数值模拟。虽然通常假设分子扩散的重要性随着湍流强度的增加而降低，但我们的分析表明，相反，即使在高的卡洛维茨数(Ka≫1)下，分子和原子氢的扩散仍然是速率控制过程。在不同的反应物温度和压力下，对氢-空气和氢/氨-空气湍流预混火焰在薄反应区和分布反应区的化学动力学和组分迁移进行了详细的三维数值模拟和分析。结果表明，在所分析的所有火焰中，H_2和H_2的扩散都有显著的影响。特别是，来自湍流反应锋面火焰元素附近的低进动变量的氢扩散的强度在高压下通过增加空间梯度而大大增强，从而导致局部当量比浓缩和超绝热条件加速和加强火焰锋面。此外，对于较高的反应物温度，氢原子从反应区向未燃烧区域的反向扩散，从其周围的负弯曲反应层传输到未燃烧区域，有利于在反应前沿的后缘发生局部自燃事件。在某些条件下，由差异扩散引起的这些事件可以成为控制反应物总消耗速率的主导过程。
Direct numerical simulations (DNS) of fuel-lean turbulent premixed hydrogen-enriched flames are analyzed to improve understanding of local effects of molecular diffusion of fast-diffusing, low-Lewis-number hydrogen species (H 2 and H) on the overall burning rates of reactant mixtures. Although it is often assumed that the importance of molecular diffusion decreases with increasing turbulence intensity, our analysis reveals that, on the contrary, diffusion of molecular and atomic hydrogen can remain the rate-controlling processes even at high Karlovitz numbers (Ka≫ 1). Three-dimensional DNS with detailed chemical kinetics and species transport of turbulent premixed hydrogen-air and hydrogen/ammonia-air flames in the regimes of thin and distributed reaction zones are performed and analyzed at different reactant temperatures and pressure levels. The DNS data reveal a significant impact of H 2 and H diffusion in all flames analyzed. In particular, the magnitude of H 2 diffusion, occurring from low progress variable near the flame elements of the turbulent reaction front that exhibit convex (positive) curvature towards the fresh mixture, is greatly enhanced at high pressure through increased spatial gradients, resulting in localized equivalence-ratio enrichment and super-adiabatic conditions that accelerate and strengthen the flame front. Moreover, for higher reactant temperatures, back diffusion of atomic hydrogen, transported from the reaction zone into unburnt regions from negatively curved reaction layers surrounding them, facilitates the occurrence of localized spontaneous ignition events at the trailing edges of reaction fronts. For certain conditions these events, induced by differential diffusion, can become the dominant process controlling the overall rate of reactants consumption.