Effects of a Recess on Coaxial Cryogenic Injections at Supercritical Pressure

Effects of recess lengths on coaxial cryogenic jets under a supercritical pressure are numerically studied. A hybrid large-eddy simulation/Reynolds-averaged Navier–Stokes methodology is applied to capture unsteady jet behaviors of recessed injectors. For the injector with a moderate recess length, the dense-core length of the inner jet is shorter than that with a nonrecessed injector, demonstrating that recessing improves the mixing. The flowfields show that the vortex structures in an inner mixing layer are strongly developed within the recessed region, and the entrainment of the outer jet into the inner jet is strengthened. However, a further increase in the recess length turns to the increase of the dense-core length and deteriorates the inner jet decay along the centerline. This deterioration is caused by the separation of the outer jet flow on the wall in the recessed region. The turbulence generation in the outer mixing layer is suppressed because of the separation, resulting in less enhancement of the mixing in the downstream region. Therefore, the study demonstrates that there is an approximate recess length for optimum jet mixing. Finally, a correlation relationship is provided between the dense-core length and the momentum flux ratio from a comprehensive comparison of existing experimental and numerical data.