Investigation of Transient Flamelets Behavior in Turbulent Diffusion Flames

湍流扩散火焰被认为是层流小火焰团的集合，这些小火焰团处于非稳定应变状态，具有正或负曲率，并受优先扩散的影响。本文研究了空气侧或燃料侧非定常撞击微细空气或燃料射流时的非定常层流逆流扩散火焰。用激光瑞利散射法进行的二维时变温度测量显示了火焰的局部熄灭和再点火过程。考虑到详细的化学动力学和多组分扩散的时间相关数值计算很好地预测了实验证据的特征，并揭示了控制火焰相对于火焰曲率和优先扩散的瞬时行为的过程。(1)微喷流撞击处观察到局部火焰的瞬时熄灭。在熄火过程中，瞬时火焰可以在较高的拉伸率下瞬时存活，不存在稳定火焰。(2)微喷流冲击局部熄灭后观察到再燃现象。重燃后的温度明显高于原始火焰的温度。这一高温是由于H^2浓度与凹曲率有关的优先扩散引起的。(3)重燃是在形成高度未反应的可燃预混气体并随后的火焰传播之后发生的。(4)微燃料射流冲击熄灭后几乎没有观察到重燃。这是由于与凸曲率有关的优先扩散引起的H^2稀释。(5)揭示了具有拉伸/压缩和周期起伏的火焰结构。

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.