抄録
Laminar flame propagation in a stratified charge with a step function distribution of equivalence ratio was studied in a spherical combustion vessel. The stratification was created by trapping a methane air mixture at fuel equivalence ratio φ_1 in a small spherical soap bubble at the center of the vessel ; the remaining of the vessel was filled with the mixture at fuel equivalence ratio φ_2. The charge was ignited at the vessel center by a focused laser beam. Results were obtained at various combinations of φ_1 and φ_2. The results show that the flame has a memory of the previous history - it takes a substantial amount of time for the flame speed to relax from its steady state value at φ_1 to that at φ_2. A 1-D, time-dependent numerical simulation of the flame propagation in a methane/air charge with step-stratification was used to explain the observations. For a rich-to-lean flame transition, the gradient transport of heat and radicals from the previously burned gas tend to give a "back-support" to enhance the flame speed over the nominal lean value for a homogeneous charge. Thus the usual notion of laminar flame speed being determined by the unburned mixture condition does not apply to a stratified charge in many practical situations such as in a stratified charge engine.
