Computational Analysis of Unsteady Flow in a Partial Admission Supersonic Turbine Stage

Abstract

Turbines used in upper stage engine for a rocket are sometimes designed as a supersonic turbine with partial admission. This study deals with numerical investigation of supersonic partial admission turbine in order to understand influences on the unsteady flow pattern, turbine losses and aerodynamic forces on rotor blades due to partial admission configuration. Two-dimensional CFD analysis is conducted using “Numerical Turbine” code. Its governing equation is URANS (Unsteady Reynolds Averaged Navier-Stokes Simulation) and fourth-order MUSCL TVD scheme is used for advection scheme. The unsteady simulation indicates that strongly non-uniform circumferential flow field is created due to the partial admission configuration and it especially becomes complex at 1st stage because of shock waves. Some very high or low flow velocity regions are created around the blockage sector. Nozzle exit flow is rapidly accelerated at the inlet of blockage sector and strong rotor LE shock waves are created. In contrast, at rotor blade passages and Stator2 blade passages existing behind the blockage sector, working gas almost stagnates. Large flow separations and flow mixings occur because of the partial admission configuration. As a result, additional strong dissipations are caused and the magnitude of entropy at the turbine exit is approximately 1.5 times higher than that of the full admission. Rotor1 blades experience strong unsteady aerodynamic force variations. The aerodynamic forces greatly vary when the Rotor1 blade passes through the blockage inlet region. The unsteady force in frequency domain indicates that many unsteady force components exist in wide frequency region and the blockage passing frequency component becomes pronounced in the circumferential direction force. Unsteady forces on Rotor2 blades are characterized by a low frequency fluctuation due to the blockage passing.