Experimental and Numerical Studies of Flame Extinction: Validation of Chemical Kinetics

摘要

Laminar flame speeds have been traditionally used for partially validating flame kinetics. The accurate determination of laminar flame speeds, however, requires in most cases sophisticated experimentation and is, in general, an indirect measurement. Additionally, flame front instabilities can render the concept of laminar flame speed irrelevant. This is the case of Le < 1 flames, with the most profound example being the ultra-lean H_2/air flames than can experience severe thermo-diffusional instabilities and can develop cellular structures. Thus, for such flames the validation of flame kinetics cannot be done based on laminar flame speeds. In the present investigation, a methodology is presented for validating flame kinetics by experimentally determine accurate extinction strain rate data and by comparing them with the predictions of direct numerical simulations. The approach is demonstrated for hydrocarbon/air and H_2/air flames even under ultra-lean conditions. The latter is possible, as while the unstretched lean H_2/air flames are cellular, the stretched ones are not. Thus, comparisons of experimental extinction conditions with the predicted ones are feasible. Such comparisons revealed serious discrepancies for ultra-lean H_2/air flames that correspond to a kinetics regime that is of relevance to hydrocarbon ignition. Subsequently, the H_2 reaction model was optimized against the available combustion data and the current extinction stretch rate data to improve model predictions. Comparisons of extinction conditions for hydrocarbon/air flames also revealed noticeable discrepancies for a number of kinetics mechanisms.