Examination of differential diffusion effects in spatially-developing supersonic mixing layer hydrogen flames

Abstract

Differential diffusion (DD) plays a crucial role in the fundamental understanding of combustion process, particular in the context of hydrogen or hydrogen-blended fuel flames. This paper intends to address whether the impact of DD on the flame stabilization in the turbulence-dominant supersonic flow can be negligible and if not, to elucidate its mechanisms. To this end, a spatially-developing supersonic non-premixed mixing layer hydrogen flame is simulated by large eddy simulations. Three distinct flow-chemistry interaction patterns: laminar flow-chemistry, transition-chemistry, and turbulence-chemistry are well designed within the mixing layer to examine the dependence of the DD effect on turbulence and its implications for flame stabilization. Results show that the importance of DD in flame-base zones of transition-chemistry and turbulence-chemistry interaction patterns is more pronounced than in laminar flow-chemistry one, even though their turbulence effects are more significant. The DD effect is observed to shorten the flame lift-off length and shift the leading point dynamic from a low-frequency to a high-frequency mode. Further “budget analysis” of transport- and chemistry- effect of DD shows that although the transport contribution of DD diminishes in turbulence-dominant flow-chemistry interaction patterns, the chemistry contribution of DD, i.e., sensitivity of the ignition delay time (IDT) to DD, is increased due to the low mixture temperature. Specifically, even a minor increase in the concentration of certain radicals, such as H, caused by DD can result in a significant reduction in IDT. This is primarily responsible for DD shortening the flame lift-off length within transition-chemistry and turbulence-chemistry interaction patterns. In laminar flow-chemistry pattern, DD facilitates the reaction to withstand high strain rates and in turn reducing the flame lift-off length.