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

Abstract

Three-dimensional direct numerical simulations of turbulent lifted hydrogen jet flames in heated coflows are performed with a detailed H-2/air chemical mechanism to understand their ignition dynamics and stabilization mechanisms. Turbulent lifted jet flames with four different coflow temperatures, T-c, between 750 K and 1100 K are investigated by examining the instantaneous/time-averaged values and conditional means of heat release rate and species critical to ignition, and by performing a displacement speed analysis and a local combustion mode analysis with an indicator, alpha. Although T-c at 950 K is higher than the autoignition limit, the flame is primarily stabilized by flame propagation rather than autoignition, while at 1100 K, flame stabilization is found to be highly affected by autoignition. The local combustion mode analysis further reveals that at 950 K, even if a local ignition mode with vertical bar alpha vertical bar < 1 first appears in the near field of the jet, it develops into a local extinction mode with alpha < -1 as local temperature decreases due to the excessive mixing of heated coflow and cold H-2 within vortical structures, which inhibits the ignition kernel development upstream of the flamebase. At 1100 K, however, a local ignition mode prevails upstream of the flamebase. To further identify the effect of a vortex on the early development of an ignition kernel in a mixing layer between the heated coflow and cold H-2, a series of two-dimensional DNSs are performed, varying several vortex parameters and air temperature, as a reference for the more complicated corresponding 3-D turbulent DNS cases. The results substantiate that the development of a vortex in the mixing layer tends to retard the autoignition within the vortex, especially when its temperature is slightly above the autoignition limit. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.