Comprehensive Investigation of Boundary Layer Flashback Limits and Wall Temperature Effects in Turbulent Hydrogen Flames

Boundary layer flashback of hydrogen-air mixtures in a turbulent channel flow configuration is investigated using large eddy simulations. The numerical framework is validated with suitable data from DNS calculations obtained from the literature. The influence of the key parameters equivalence ratio, gas temperature, and wall temperature on the flashback behavior is examined at atmospheric conditions. Due to the utilization of LES, a large variation of the key parameters could be studied not available in the literature before. Especially the wall temperature effects are analyzed in detail, elucidating how the wall temperature affects the flow-flame interaction. The variations in equivalence ratio, gas and wall temperatures demonstrated a strong positive correlation between the laminar flame speed and flashback propensity, whereas the channel bulk velocity played a minor role. In very lean and very rich fuel-air gas mixtures wall cooling can prevent flashback and wall heating can cause flashback, while for moderately lean to rich and near stoichiometric mixtures flashback occurred for all cases considered. A deeper insight into the flow-flame interaction in the nearwall region revealed that the relation between the quenching distance and sub-laminar-flamespeed region, where the flow velocity is below that of the laminar flame speed, plays an important role for the occurrence of flashback. Furthermore, the distinct flame behavior at different wall temperatures is attributed to the modification of combustion properties due to the local temperature change in front of the flame, leading to a modified flow field, subsequently influencing the flame propagation.