The effects of water injection on flame surface topology and local flame propagation characteristics have been analyzed for statistically planar turbulent n-heptane spray flames with an overall (i.e., liquid + gaseous) equivalence ratio of unity using carrier-phase direct numerical simulations. Most fuel droplets have been found to evaporate as they approach the flame even though some droplets can survive until the burnt gas side is reached, whereas water droplets do not significantly evaporate ahead of the flame and the evaporation of water droplets starts to take place in the reaction zone and is completed within the burnt gas. However, the gaseous-phase combustion occurs predominantly in fuel–lean mode although the overall equivalence ratio remains equal to unity. The water injection has been found to suppress the fuel droplet-induced flame wrinkling of the progress variable isosurface under the laminar condition, and this effect is particularly strong for small water droplets. However, turbulence-induced flame wrinkling masks these effects, and thus, water injection does not have any significant impact on flame wrinkling for the turbulent cases considered here. The higher rate of evaporation and the associated high latent heat extraction for smaller water droplets induce stronger cooling effects, which weakens the effects of chemical reaction. This is reflected in the decrease in the mean values of density-weighted displacement speed with decreasing water droplet diameter. The weakening of flame wrinkling as a result of injection of small water droplets is explained through the curvature dependence of the density-weighted displacement speed. The combined influence of cooling induced by the latent heat extraction of water droplets and flame surface flattening leads to a decrease in volume-integrated burning rate with decreasing water droplet diameter in the laminar cases, whereas the cooling effects are primarily responsible for the drop in burning rate with decreasing water droplet diameter in the turbulent cases.

中文翻译：

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液态水喷射对火焰表面拓扑结构和喷雾火焰传播特性的影响：直接数值模拟分析

使用载相直接数值模拟，对总体（即液体+气体）当量比为一的统计平面湍流正庚烷喷雾火焰，分析了注水对火焰表面拓扑和局部火焰传播特性的影响。人们发现大多数燃料液滴在接近火焰时会蒸发，尽管有些液滴可以存活到到达燃烧气体侧，而水滴在火焰之前不会显着蒸发，并且水滴的蒸发开始发生在火焰中。反应区并在燃烧气体内完成。然而，气相燃烧主要发生在贫燃料模式下，尽管总体当量比保持等于1。人们发现，在层流条件下，注水可以抑制燃料滴引起的进度变量等值面的火焰起皱，并且这种效果对于小水滴尤其强烈。然而，湍流引起的火焰起皱掩盖了这些影响，因此，对于此处考虑的湍流情况，注水对火焰起皱没有任何显着影响。较小水滴的较高蒸发速率和相关的高潜热提取会产生更强的冷却效果，从而削弱化学反应的效果。这反映在密度加权位移速度平均值随着水滴直径的减小而减小。由于注入小水滴而导致的火焰起皱的减弱可以通过密度加权位移速度的曲率依赖性来解释。在层流情况下，水滴潜热提取和火焰表面扁平化引起的冷却的综合影响导致体积积分燃烧速率随着水滴直径的减小而降低，而冷却效应是燃烧率下降的主要原因在湍流情况下，速率随水滴直径的减小而变化。