Strained Premixed Laminar Flames Under Nonadiabatic Conditions

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Abstract
The method of activation energy asymptotics is used to describe the behavior and characteristics of nonadiabatic flamelets involving counterflowing reactants and products under the assumption of a unity Lewis number. For moderate and low rates of strain the results are analogous to those obtained in earlier applications of the method, namely reaction zones which are maintained in a first approximation by a diffusive-reactive balance. Indeed for low rates of strain many features of flamelet behavior are independent of the extent of the nonadiabaticity since the reaction zone is insulated from the stream of altered enthalpy by a diffusive-convective zone near the stagnation point. Two limiting processes are considered. One pertains to nearly adiabatic flamelets and exposes the principal qualitative results of this study. The second pertains to the full, nonadiabatic case. Flamelets with product streams having elevated enthalpies are shown to possess essentially the same features as the adiabatic flamelets studied earlier. On the contrary, flamelets with cooled product streams involve complex behavior depending on the extent of the nonadiabaticity and the rate of strain. In particular the continuous behavior shown to prevail in an earlier study when adiabatic flamelets are subjected to varying rates of strain is replaced if the product stream is suitably cooled by abrupt transitions involving altered locations of the reaction zone and altered rates of creation of product. These transitions are interpreted in terms of extinction and ignition. A special treatment is provided for flamelets subjected to high rates of strain and thus involving reaction zones with diffusion, convection and chemical reaction all operative. Density variations of practical interest are considered in the numerical examples.