Local statistics of turbulent spherical expanding flames for NH<sub>3</sub>/CH<sub>4</sub>/H<sub>2</sub>/air measured by 10 kHz PIV

In this paper, we present high-speed particle image velocimetry measurements to quantify combustion-induced turbulence and turbulent stretch rate effects on flame propagation in a fan-stirred constant-volume combustion vessel. Three stoichiometric fuel/air mixtures including methane/air, methane/hydrogen/air and ammonia/methane/air were tested. The expansion rate of flame radius is found to scale with a half-power law of the mean-radius based Reynolds number. Measurements of velocity in the reactant zone show that the gas mean and rms velocity increase within the flame brush then gradually decrease when approaching the detection edge. Combustion enhances the measured non-reacting flow turbulence by ∼64 % within the reactant side of the flame brush of this expanding spherical flame. The evolution of rms velocity is found to be insensitive to pressure but sensitive to initial turbulent intensity and laminar flame speed, scaling with Reynolds number in a similar manner as the rate of expansion of the flame radius. Hydrogen blending shows the largest absolute value of rms velocity increase, but ammonia blending leads to the largest increase in rms velocity relative to laminar flame speed. The pdfs of 2D curvature are broadened as u′ and p increase but gradually saturate at high u′ for all mixtures, those of stretch rate are widened by flame speed. The temporal evolution of statistical flame stretch rates suggests the tangential stretch rate is larger than and anti-correlated with the normal stretch, and that there is a strong positive correlation between instantaneous flame propagation rate and the rms strain rate. Finally, the mean flame curvature and mean stretch rate are found to be near zero as flame propagates outwardly.

在本文中，我们采用高速粒子图像测速技术进行测量，以量化燃烧诱导的湍流以及湍流拉伸率对风扇搅拌定容燃烧容器中火焰传播的影响。对三种化学计量比的燃料/空气混合物进行了测试，包括甲烷/空气、甲烷/氢气/空气以及氨/甲烷/空气。发现火焰半径的膨胀率与基于平均半径的雷诺数的二分之一次方定律成比例。对反应区速度的测量表明，气体平均速度和均方根速度在火焰锋面内增加，在接近探测边缘时逐渐降低。在这种膨胀球形火焰的火焰锋面的反应侧，燃烧使测量到的非反应流湍流增强了约64%。发现均方根速度的演变对压力不敏感，但对初始湍流强度和层流火焰速度敏感，其与雷诺数的比例关系与火焰半径的膨胀率相似。掺氢显示出均方根速度增加的绝对值最大，但掺氨导致均方根速度相对于层流火焰速度的增加最大。二维曲率的概率密度函数随着\(u'\)和\(p\)的增加而变宽，但对于所有混合物在高\(u'\)时逐渐饱和，拉伸率的概率密度函数因火焰速度而变宽。统计火焰拉伸率的时间演变表明，切向拉伸率大于法向拉伸率且与法向拉伸率呈反相关，并且瞬时火焰传播率与均方根应变率之间存在很强的正相关性。最后，发现随着火焰向外传播，平均火焰曲率和平均拉伸率接近零。