Swirling airflow through a nozzle: Choking criteria

The choking criteria, thrust, and specific impulse of swirling airflow through a choked nozzle are investigated both numerically and experimentally. The effects of swirl are examined at matched nozzle reservoir pressure as well as matched mass flow. A convergent nozzle is used to generate the underexpanded airflow. It was found that the throat velocity itself (and not any of its components) is choked in a swirling flowfield. Therefore, the limiting tangential Mach number is unity, and the application of swirl always results in a reduction in the axial Mach number component. The velocity is choked all over the flow cross section at the nozzle throat with similar swirling and nonswirling sonic lines. Since the mass flow rate through nozzle is primarily a function of throat density and axial Mach number, the reduction in the latter with swirl explains the observed reduction in mass flow at matched reservoir pressure. Greater pressures, on the other hand, result in higher throat densities, which compensates for the reduced axial Mach number, and the mass flow rate can be kept constant at its nonswirling value. It was also found that the distribution of subsonic Mach number (and not any of its components) in a swirling flow is solely dependent on cross-sectional area, similar to nonswirling flows; i.e., nonswirling and swirling flows have the same subsonic Mach number profile. In terms of thrust and specific impulse, the application of swirl at matched nozzle reservoir pressure results in the expected reductions in discharge coefficient, thrust, and specific impulse. At matched mass flow, however, the application of swirl results in the enhancement of both thrust and specific impulse. This is attributed to the considerable degree of underexpansion associated with the swirling flow as a result of the higher nozzle reservoir pressure with swirl.