Ignition, Flame Structure and Near-Wall Burning in Transverse Hydrogen Jets in Supersonic Crossflow

The work aims at investigating near-wall ignition and flame structure in transverse underexpanded hydrogen jets in high-enthalpy supersonic crossflows generated in an expansion tube. Crossflow conditions are held fixed at $M\mbox{=}2.4$, $p=40\ kPa$ and $T\approx1400\ K$, while jet-to-crossflow momentum flux ratios $J$ in the range $0.3-5.0$ are considered. Schlieren and $OH^{*}$ chemiluminescence imaging are used to characterize flow structure, ignition and flame penetration, while the instantaneous reaction zone is identified with planar laser-induced fluorescence imaging of OH on side- and plan-view planes. The upstream separation length is found to scale as $J^{0.44}D$ ($D$ jet diameter). Similarly, the ignition point $x_{ig}$ strongly depends on $J$: $x_{ig}$ tends to a limiting value of $\sim22D$ as $J\rightarrow0$, and the flame is anchored in the upstream recirculation region and lee-side of the jet for $J>3$. Flame penetration is well described by the traditional form $k\left(x/DJ\right)^{m}$ where both $k$ and $m$ are found to depend on $J$ but these parameters reach a limiting value of $k\approx1$ and $m\approx0.3$ for $J>2$. The roles of the unsteady bow shock, the separation and recirculation regions on the near-wall ignition, stabilization and mixing at large $J$ are discussed.