Local equilibrium temperature as a measure of stretch and preferential diffusion effects in counterflow H₂/air premixed flames

The effects of stretch and preferential diffusion on the structure and extinction of hydrogen-air premixed flames are investigated numerically. A symmetric double-flame configuration in a counterflow is adopted as a model problem. A local total enthalpy and enthalpies of formation of local mixture and corresponding equilibrium composition are discussed in conjunction with the energy status of local mixtures. Based on these, a local adiabatic equilibrium temperature is proposed as a measure of energy loss and gain caused by preferential diffusion, flame stretch, and reaction incompleteness. Results show that the extinction mechanism of hydrogen-air premixed flames can be explained based on the reaction incompleteness and energy loss and gain. The reaction incompleteness can be quantified by the difference between flame temperature and local equilibrium temperature and the energy gain by the difference in local equilibrium temperatures between free stream and burned gas. The effects of stretch on reaction incompleteness and energy gain are also discussed. It has been found for the symmetric counterflow flames that lean hydrogen flames have energy gain through preferential diffusion; however, continuous increase in reaction incompleteness with stretch causes extinction. While for rich mixtures, even though reaction incompleteness nearly levels off in the high stretch regime, energy loss by preferential diffusion leads to extinction.