Ignition, flame structure and near-wall burning in transverse hydrogen jets in supersonic crossflow

Abstract

We have investigated the properties of transverse sonic hydrogen jets in high-temperature supersonic crossflow at jet-to-crossflow momentum flux ratios$J$between 0.3 and 5.0. The crossflow was held fixed at a Mach number of 2.4, 1400 K and 40 kPa. Schlieren and$\text{OH}^{\ast }$chemiluminescence imaging were used to investigate the global flame structure, penetration and ignition points;$\text{OH}$planar laser-induced fluorescence imaging over several planes was used to investigate the instantaneous reaction zone. It is found that$J$indirectly controls many of the combustion processes. Two regimes for low (${<}1$) and high (${>}3$)$J$are identified. At low$J$, the flame is lifted and stabilizes in the wake close to the wall possibly by autoignition after some partial premixing occurs; most of the heat release occurs at the wall in regions where$\text{OH}$occurs over broad regions. At high$J$, the flame is anchored at the upstream recirculation region and remains attached to the wall within the boundary layer where$\text{OH}$remains distributed over broad regions; a strong reacting shear layer exists where the flame is organized in thin layers. Stabilization occurs in the upstream recirculation region that forms as a consequence of the strong interaction between the bow shock, the jet and the boundary layer. In general, this interaction – which indirectly depends on$J$because it controls the jet penetration – dominates the fluid dynamic processes and thus stabilization. As a result, the flow field may be characterized by a flame structure characteristic of multiple interacting combustion regimes, from (non-premixed) flamelets to (partially premixed) distributed reaction zones, thus requiring a description based on a multi-regime combustion formulation.