CAREER: Influence of Unsteady Stretch on Premixed Flame Kernel Growth

CTS-9702277研究了不同发展阶段的火花火焰核心与已知速度和长度尺度的单个明确涡旋的相互作用，以确定现代内燃机中燃料-空气混合物的最佳流体动力流场。对于不同的动力学参数，如当量比和刘易斯数，它的目的是确定哪些涡流最有益，哪些涡流最有益，哪些涡流在核生长方面最具破坏性。众所周知，在低到中等湍流水平下，预混反应物的燃速和可燃性极限显著增加，但随着湍流强度的增加和涡旋的长度尺度变得比反应区小，这种情况发生了逆转，因为涡流现在能够局部熄灭火焰前锋。先进的激光诊断技术用于测量温度、速度场和反应状态。检测到羟基和亚甲基自由基，以定位反应区并指示反应状态；它们的缺失表明火焰前锋熄灭。温度测量采用瑞利散射和种子氮氧化物平面激光诱导荧光(PPLIF)，而粒子图像测速仪(PIV)用于测量二维速度场，从而确定局部流场对火核的局部瞬时应变。在内燃机中，火花产生的火核可能会将其能量转移到燃料-空气混合物中，从而形成强劲的、自我维持的火焰锋面，或者，也可以通过流体动力应变和曲率的组合来熄灭(熄灭)，统称为拉伸。在局部消光的情况下，籽粒可能存活下来，但表现出卵壳燃烧效率的严重退化和污染物排放的增加。内核生长速度的摄动是发动机循环间变化的主要原因。因此，了解这一现象是发动机提供平稳、高效和清晰性能的关键。

ABSTRACT CTS-9702277 This is study of the interactions of a spark-generated flame kernel at various stages of growth with a single, well-defined eddy of known velocity and length scale in order to identify the optimal hydrodynamic flowfield for the fuel-air mixture in a modern internal combustion (IC) engine. It is intended to identify which eddies are most beneficial and which are most beneficial and which a re most destructive in terms of kernel growth for various kinetics parameters such as equivalence ratio and Lewis number. It is known that at low to moderate turbulence levels there is a marked increase in the burning rate and in the flammability limits of the premixed reactants, but that this reverses as turbulence intensity increases and the length scale of the eddies become small compared to the reaction zone, since the eddies are now able to locally quench the flamefront. Advanced laser diagnostics are used to measure temperature, velocity field, and state of reaction. Hydroxyl and methylidine radicals are detected to locate the reaction zone and to indicate the state of the reaction; their absence indicates a quenched flamefront. Temperature is measured by Rayleigh scattering and by seeded nitric-oxide planar laser-induced fluorescence (PPLIF), while particle-image velocimetry (PIV) is used to measure the two-dimensional velocity field and thereby determine the local, instantaneous strain executed on the flame kernel by the local flowfield. In an internal combustion engine, the spark generates a flame kernel that may transfer its energy to the fuel-air mixture resulting in a vigorous, self-sustaining flame front, or, alternatively, may be quenched (extinguished) by a combination of hydrodynamic strain and curvature, collectively termed stretch. In the case of localized extinction, the kernel may survive but exhibit a severe degradation of ovepall combustion efficiency and in increase in pollutant emissions. Perturbation of kernel growth rate is the major cause of cycle -to-cycle variations in engines. Thus understanding of this phenomenon is the key to engines that give smooth, efficient, and clear performance.