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

<jats:p>We have investigated the properties of transverse sonic hydrogen jets in high-temperature supersonic crossflow at jet-to-crossflow momentum flux ratios<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline1" /><jats:tex-math>$J$</jats:tex-math></jats:alternatives></jats:inline-formula>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<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline2" /><jats:tex-math>$\text{OH}^{\ast }$</jats:tex-math></jats:alternatives></jats:inline-formula>chemiluminescence imaging were used to investigate the global flame structure, penetration and ignition points;<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline3" /><jats:tex-math>$\text{OH}$</jats:tex-math></jats:alternatives></jats:inline-formula>planar laser-induced fluorescence imaging over several planes was used to investigate the instantaneous reaction zone. It is found that<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline4" /><jats:tex-math>$J$</jats:tex-math></jats:alternatives></jats:inline-formula>indirectly controls many of the combustion processes. Two regimes for low (<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline5" /><jats:tex-math>${<}1$</jats:tex-math></jats:alternatives></jats:inline-formula>) and high (<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline6" /><jats:tex-math>${>}3$</jats:tex-math></jats:alternatives></jats:inline-formula>)<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline7" /><jats:tex-math>$J$</jats:tex-math></jats:alternatives></jats:inline-formula>are identified. At low<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline8" /><jats:tex-math>$J$</jats:tex-math></jats:alternatives></jats:inline-formula>, 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<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline9" /><jats:tex-math>$\text{OH}$</jats:tex-math></jats:alternatives></jats:inline-formula>occurs over broad regions. At high<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline10" /><jats:tex-math>$J$</jats:tex-math></jats:alternatives></jats:inline-formula>, the flame is anchored at the upstream recirculation region and remains attached to the wall within the boundary layer where<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline11" /><jats:tex-math>$\text{OH}$</jats:tex-math></jats:alternatives></jats:inline-formula>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<jats:inline-formula><jats:alternatives><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="gif" xlink:type="simple" xlink:href="S0022112015004541_inline12" /><jats:tex-math>$J$</jats:tex-math></jats:alternatives></jats:inline-formula>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.</jats:p>