Turbulent Premixed Hydrogen/Air Flames at High Reynolds Numbers

Abstract

Measurements of mean and fluctuating reaction progress variable and streamwise velocities are reported for turbulent premixed hydrogen/air flames burning at relatively high Reynolds numbers (up to 1800. based on streamwise r.m.s. velocity fluctuations and integral length scales). A round-jet geometry was used with the flame surrounded by a hot combustion-gas environment at atmospheric pressure. Mixing-limited combustion was achieved: values of r.m.s. velocity fluctuations, normalized by the laminar burning velocity (ü/SL), exceeded 15 in some instances. Test conditions included fuel-equivalence ratios of 0.3-3.6 and burner exit Reynolds numbers (based on exit diameter) of 7000-40000 with fully-developed turbulent pipe flow at the burner exit. It was found that effects of diffusive-thermal (preferential-diffusion) phenomena were important, for both stable (fuel-equivalence ratios greater than 1.8) and unstable conditions, even at the present high Reynolds numbers; for example, flame surfaces were more spiked and effective turbulent burning velocities for unstable conditions were twice as large as burning velocities for stable conditions having comparable normalized turbulence levels (ü/SL). The measurements were used to evaluate contemporary turbulence models of premixed flames that are based on the laminar flamelet concept and which allow for effects of flame stretch: predictions yielded only fair agreement with measurements, partly due to the fact that the models ignore effects of finite laminar flame speeds and diffusive-thermal (preferential diffusion) phenomena that are found to be particularly important for hydrogen/air flames.