Investigation of Transient Flamelets Behavior in Turbulent Diffusion Flames

紊流扩散火焰应该是层流小火焰的集合，这些小火焰应具有不稳定的应变，具有正曲率或负曲率，并受优先扩散的影响。本文研究了由空气或燃料侧微空气或燃料射流不稳定撞击而产生的瞬态层流扩散火焰。激光瑞利散射法二维随时间变化的温度测量显示了局部火焰熄灭和重燃过程。考虑了详细的化学动力学和多组分扩散的时变数值计算很好地预测了实验证据的特征，并揭示了火焰曲率和优先扩散方面主导火焰瞬态行为的过程。(1)微射流撞击处出现瞬态局部火焰熄灭。在消光过程中，瞬态火焰可以在相当高的拉伸速率下瞬间存活，而稳定火焰则不能存在。(2)微射流撞击局部消光后重燃。重燃后的温度明显高于原火焰的温度。这种高温是由H^2浓度引起的，这是由于相对于凹曲率的优先扩散。(3)高水平不反应性的可燃预混气体混合物的形成和随之而来的火焰传播引起重燃。(4)微射流撞击消光后几乎看不到重燃现象。这是由于H^2的稀释由于与凸曲率相关的优先扩散。(5)火焰具有拉伸/压缩和周期性波动的结构。

The turbulent diffusion flame is supposed to be an ensemble of the laminar flamelets which should strained unsteadily and have positive or negative curvature, and be affected by preferential diffusion. In this study, transient laminar counterflow diffusion flames strained by unsteadily impinging micro air or fuel jet from air or fuel side were investigated. Two-dimensional time-dependent temperature measurements by laser Rayleigh scattering method showed the local flame extinction and reignition process. Time-dependent numerical computations taking into account detailed chemical kinetics and multicomponent diffusion predicted well the characteristics of the experimental evidence and revealed the processes dominating the transient behavior of the flame with respect to the flame curvature and preferential diffusion.(1) Transient local flame extinction is observed where the micro jet impinges. During the process of the extinction, the transient flame can survive instantaneously under the quite high stretch rate where the steady flame cannot exist.(2) Reignition is observed after the local extinction by the micro air jet impingement. The temperature after reignition becomes significantly higher than that of the original flame. This high temperature is induced by the H^2 concentration due to the preferential diffusion in relation to the concave curvature.(3) The reignition is induced after the formation of combustible premixed gas mixture of high level of unreactedness and the consequent flame propagation.(4) The reignition is hardly observed after the extinction by micro fuel jet impingement. This is due to the H^2 dilution due to the preferential diffusion in relation to the convex curvature.(5) Flame structure with stretch/compression and periodic fluctuation was revealed.