CAREER: Influence of Unsteady Stretch on Premixed Flame Kernel Growth

摘要CTS-9702277这是研究火花产生的火焰核在不同生长阶段与已知速度和长度尺度的单一、明确定义的涡流的相互作用，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。目的是根据各种动力学参数，如等效比和路易斯数，确定哪些涡流对核生长最有利，哪些最有利，哪些最有害。众所周知，在低到中等湍流水平下，预混反应物的燃烧速度和可燃性极限有明显的增加，但随着湍流强度的增加，涡流的长度尺度与反应区相比变小，这种情况会发生逆转，因为涡流现在能够局部熄灭火焰前缘。先进的激光诊断仪用于测量温度、速度场和反应状态。检测羟基和甲基自由基，定位反应区，指示反应状态；它们的消失表明火焰已经熄灭。温度通过瑞利散射和粒子氧化氮平面激光诱导荧光（PPLIF）测量，粒子图像测速（PIV）用于测量二维速度场，从而确定局部流场对火焰核施加的局部瞬时应变。在内燃机中，火花产生火焰内核，火焰内核可以将其能量传递给燃料-空气混合物，从而产生一个强有力的、自我维持的火焰锋，或者，也可以通过流体动力应变和曲率（统称为拉伸）的组合来熄灭（熄灭）。在局部熄灭的情况下，核可以存活，但表现出严重的整体燃烧效率下降和污染物排放增加。核生长速率的扰动是发动机循环变化的主要原因。因此，理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.