Effects of Recess Length on Supercritical Kerosene/Oxygen Combustion in a Bi-Swirl Injector

This study examines how recess length affects supercritical kerosene/oxygen combustion in a bi-swirl injector using Reynolds-averaged Navier–Stokes simulations at 200 bar. Five recess ratios (RR = 1.3–3.1) are analyzed to assess their impact on flame stabilization, swirl decay, pressure drop, thermal loading, and combustion efficiency. Real-fluid behavior is captured using the SRK equation of state with high-pressure transport-property corrections. Results show that RR 1.3–2.5 support a diffusion flame anchored at the recess tip, whereas RR 3.1 produces a lifted premixed flame, accompanied by significant fuel slip and reduced efficiency. Within the diffusion-flame regime, increasing recess ratio weakens swirl intensity and shifts the recess-tip peak-temperature location outward. Pressure-drop trends reveal a peak at RR 2.2 due to the strongest flame-induced obstruction, while shorter flame lengths correlate with higher downstream wall temperatures. Overall, RR 2.5 provides the most balanced performance, offering stable diffusion-flame behavior, moderate pressure losses, and high combustion completeness. These findings clarify how geometric variation influences flame stabilization, heat transfer, and hydraulic losses in bi-swirl injectors, providing insight for high-performance preburner design.