CAREER: Influence of Unsteady Stretch on Premixed Flame Kernel Growth

点击翻译按钮获取中文摘要

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.