Analytical and numerical study of near-field ignition of H2/air by injection of hot gas

Abstract

Ignition initiation by a turbulent hot jet involves complex transport and chemical processes with disparate and sensitive time scales. Its understanding is important for improved ignition in advanced and novel combustion engines and in ignition avoidance for explosion safety measures. This study is aimed at the modeling of turbulent hot jet ignition of stoichiometric hydrogen-air mixture with fast chemistry and short chemical time scales relative to the large-scale mixing time scales. The evolution of jet mixture fraction in the near-field shear layer of a suddenly-starting turbulent jet is analytically modeled and calibrated by large-eddy simulation of the reacting fluid. When integrated with a correlation for instantaneous chemical induction time, the model estimates the radial location and the timing of ignition as the jet travels in the streamwise direction. The estimation includes accounting for the role of the scalar dissipation rate in the suppression of ignition. The radial location of ignition migrates towards the jet centerline with time, migrating faster for higher injection temperatures and much slower for leaner ambient fuel-air ratios. The limitations of the present model are investigated by assessing the regions of mixing layer where strong diffusive transport collocates with the high production rate of active radicals. To complete the perspective into analytic modeling of transient jet ignition, other limitations associated with turbulent fluctuations and jet composition are discussed, as well as the role of near-field ignition relative to ignition at the jet tip or head vortex ring.