On the flame stabilization of turbulent lifted hydrogen jet flames in heated coflows near the autoignition limit: A comparative DNS study

摘要

Three-dimensional direct numerical simulations of turbulent lifted hydrogen jet flames in heated coflowsare performed with a detailed H_2 /air chemical mechanism to understand their ignition dynamics and stabilizationmechanisms. Turbulent lifted jet flames with four different coflow temperatures, T_c , between750 K and 1100 K are investigated by examining the instantaneous/time-averaged values and conditionalmeans of heat release rate and species critical to ignition, and by performing a displacement speed analysisand a local combustion mode analysis with an indicator, α. Although T_c at 950 K is higher than theautoignition limit, the flame is primarily stabilized by flame propagation rather than autoignition, whileat 1100 K, flame stabilization is found to be highly affected by autoignition. The local combustion modeanalysis further reveals that at 950 K, even if a local ignition mode with α < 1 first appears in thenear field of the jet, it develops into a local extinction mode with α < ?1 as local temperature decreasesdue to the excessive mixing of heated coflow and cold H_2 within vortical structures, which inhibits theignition kernel development upstream of the flamebase. At 1100 K, however, a local ignition mode prevailsupstream of the flamebase. To further identify the effect of a vortex on the early development ofan ignition kernel in a mixing layer between the heated coflow and cold H_2 , a series of two-dimensionalDNSs are performed, varying several vortex parameters and air temperature, as a reference for the morecomplicated corresponding 3-D turbulent DNS cases. The results substantiate that the development of avortex in the mixing layer tends to retard the autoignition within the vortex, especially when its temperatureis slightly above the autoignition limit.