Analysis of high-speed combustion regimes of hydrogen jet in supersonic vitiated airstream

Abstract

Highly-resolved reactive simulations of a hydrogen jet injected into a transverse supersonic flow of vitiated air at Mach 2 are conducted. The operating conditions are chosen to be relevant of scramjet operative conditions and are representative of experiments conducted at ONERA Palaiseau Research Center. Hydrogen injection in the supersonic vitiated airstream leads to the formation of a bow shock, which interacts with the boundary layer and gives rise to separation zones. In the resulting recirculation zones, only small amounts of OH radicals are produced. Combustion stabilization and development take place further downstream. Heat release is significant in the vicinity of the wall within a sonic region where the equivalence ratio is around two. Farther from walls, combustion takes place at supersonic speeds but features lower levels of heat release rates. Supersonic turbulent combustion regimes are analyzed in detail: first in a standard set of coordinates (u′/SL0,Lt/δL0) using data collected in regions featuring a sufficiently large degree of premixing. Then, they are analyzed in the turbulent Reynolds and Damköhler numbers sub-space (Ret,Da). In this representation, three distinct Damköhler number definitions based on (i) production rate of H2O, (ii) flame propagation characteristic time, and (iii) ignition delay time are considered. These various representations put into evidence turbulent combustion regimes featuring significant finite-rate chemical kinetics effects. The whole set of computational results confirms that (i) the use of models based on the fast chemistry hypothesis is questionable for such conditions, (ii) taking into account finite-rate chemistry effects is essential, and (iii) ignition processes play a key role in combustion stabilization. The manuscript ends with some perspectives and challenging issues for future works.