Effect of Swirl on Shock Structure in Underexpanded Supersonic Airflow

Abstract

This study examines the effect of imparting swirl to underexpanded supersonic-nozzle airflow on shock structure under matched mass flow conditions. A convergent nozzle with swirling capabilities is used to generate the underexpanded airflow. Fuel is injected coaxially at the nozzle throat. Nonreacting conditions are considered, wherein fuel is simulated by mixtures of helium, argon, and krypton inert gases. It was found that the effects of swirl and nozzle reservoir pressure interfere destructively from the point of view of shock-structure axial compactness. Increasing reservoir pressure stretches the shock structure axially, whereas swirl shrinks it. On the other hand, constructive interference was observed from the point of view of radial jet expansion; both result in greater jet diameter. The application of swirl was found to weaken the shock structure at matched reservoir pressure but to strengthen it at matched mass flow. It was also found that fuel injected at low subsonic Mach numbers into the supersonic airflow has to propagate initially with a negative shear angle; i.e., the cross-sectional area of fuel-rich core flow converges first, before this core flow reaches a throat after which it propagates supersonically. This behavior was found to be advantageous, as it results in reduced shock-structure strength.