Computational study on reducing flow asymmetry in over-expanded planar nozzle by incorporating double divergence

摘要

Rocket nozzles accelerate combustion products or high pressure gases to supersonic/hypersonic velocities. The planar nozzles have rectangular cross-sections. At low altitude, the ambient pressure is higher than the exiting jet pressure, which leads to flow separation from the nozzle wall. In planar nozzles, asymmetric flow separation can be observed at low Nozzle Pressure Ratios. Asymmetric flow separation can lead to undesirable side forces. In the present study, numerical analysis is performed on the flow through planar nozzles. Nozzle geometries with different divergence angles and different double divergence having same area ratio are considered for flow analysis. The flow analysis is performed using commercial software ANSYS Fluent. All geometries are studied with similar boundary conditions. The numerical analysis is done on two-dimensional planar models. Reynolds Averaged Navier-Stokes, and Unsteady Reynolds Averaged Navier-Stokes equations are solved with realizable k-epsilon model turbulence model. The divergence angle of the nozzle having the same area ratio had a significant effect on the side load. It is found that as divergence angle increases the magnitude of side load decreases and flow becomes symmetry at low Nozzle Pressure Ratios. The design Mach number of the inner nozzle and extension length of double divergent nozzle is crucial for reducing the asymmetry of the flow. Shock reflection inside the nozzle due to discontinuity could be controlled by changing the divergence angle of the nozzle. The performance of the nozzle was seen to be augmented by introducing double divergence. The side load was profoundly affected by double divergence. It was also observed that, some double divergent nozzle had adverse side load. It was observed that asymmetry, unsteadiness, and side load of the flow was completely absent at inner nozzle having design Mach number of 1.3 for the double divergent nozzle having smaller extension length. (C) 2020 Elsevier Masson SAS. All rights reserved.