Numerical investigation on thrust efficiency dropping phenomenon of annular expansion–deflection nozzles

Abstract

The flow patterns in a large-expansion-ratio annular expansion–deflection (ED) nozzle are numerically investigated, focusing on the flow mechanisms underlying the sudden thrust efficiency (η) dropping phenomenon. The objective of the present work is to provide a physical understanding of the ED nozzle flow physics responsible for the nozzle performance variation. The simulations are performed by solving the Reynolds averaged Navier–Stokes equations in combination with the one-equation Spalart–Allmaras turbulence model. The Advection Upstream Splitting Method is used to discretize the convective fluxes, and the second-order central scheme is used to discretize the viscous fluxes. The present investigation demonstrates that two flow mechanisms exist, which can cause the sudden η dropping: one is supersonic flow impingement at the nozzle exit, and the other is the formation of the internal normal shock. The pintle arc radius downstream of the throat plays an important role in determining the size and redirection of the supersonic core flow region, thus influencing the overall nozzle performance. A large radius dimension gives a higher η value at the first rising stage, a large dropping nozzle pressure ratio (NPR), and a large mode transition NPR. Interestingly, a cap shock is proved possible in the current large-expansion-ratio ED nozzle, in which the near-shroud shock acts as the internal shock in the traditional thrust-optimized or parabolic nozzles.