A Large Eddy Simulation Study on Hydrogen Microjets in Hot Vitiated Crossflow

In this paper, the results of a large eddy simulation (LES) study on hydrogen microjets (d(j) similar to 0.5 mm) injected into a hot (1600 K) vitiated crossflow at different angles, namely, normal (90 deg) and inclined jet (30 deg), are presented. The goal is to explore the effects of injection angle on coherent turbulent structure formations, flame-vortex interactions, and wall heat flux contributions. The LES identifies the presence of the horseshoe vortex, the shear layer vortices (SLV), and the counter-rotating vortex pair (CVP), along with the shedding of spanwise-symmetry hairpin vortices in both the normal and inclined jets. The structures in the latter, however, appear more convoluted. In the near field, the SLV-induced flow is found to play a key role in the mixing and flame propagation in the windward side of the jet that is stabilized through the auto-ignition process along the front edge of both the normal and inclined jets close to their exits. The flame-shear layer offset phenomenon is also noticed on the windward side of the jets. In the far field, the CVP is found to be the dominating mechanism in the entrainment of the hot vitiated crossflow by the reacting jet and large-scale mixing. Its induced counter-rotating flow field give rises to the flame propagation and the heat and species transfer from the windward to the leeward side of the jet near the injection wall. In the wake region, the combustion and its byproducts persist in closer proximity to the jet exit of the normal case because of the presence of a much stronger recirculation zone behind the jet. Accordingly, higher wall heat fluxes are obtained in this region for the normal jet. The mean wall heat flux values of both the normal and inclined jets decrease and approach each other with moving away from the jet exit in the streamwise direction. The findings indicate that the CVP-induced flow drastically increases heat transfer to the near wall region, resulting in a spanwise-symmetry heat flux profile with double peaks in downstream. The results of the present LES study are compared to the experimental data available in the literature by considering instantaneous hydroxide (OH) fields and mean wall heat fluxes.