Numerical simulation of the flow in a radial compressor channel

摘要

We tested a finite volume solver of the Navier-Stokes equations. The solver is the by Volvo Aero Corporation developed VolSol. In the process, differences in the flows in two channel geometries of a radial compressor were investigated. The compressor is the VT600 gas turbine. The main modelling difficulty of the compressor is the high pressure ratio ((approx) 8.5). A high pressure ratio can cause instability in simulations as well as in reality. The flows were compared using the k-(epsilon) turbulence model. Unfortunately, the viscous simulations were not possible to get running successfully during the course of the work. Therefore, the main conclusion of this work is that VolSol 2.6 is not easily applicable to high pressure ratio radial compressor flow. Since no successful viscous modelling has been done, no conclusions can be drawn regarding the differences in the flow between the geometries. Instead, some conclusions about the experienced difficulties are dealt with: The oscillations in mass flow caused by the inherent instability of high pressure ratio compressor flow is a major problem. As a remedy, non-reflecting boundary conditions can be used. At the outlet of the impeller, a supersonic radial flow speed has to be avoided, because otherwise the outlet pressure boundary condition has no influence on the flow. Speeds exceeding sonic at the outlet boundary can be avoided by modelling a contraction of the outlet. Inviscid calculations were used to get an initial flow field for the viscous simulations. Some iterations without a turbulence model but with viscosity were run to get a velocity gradient at the wall. Otherwise the step to the steep velocity gradient of the turbulent flow was too high. The most important drawbacks of the k-(epsilon) model for this flow case are: The model is unable to model separated boundary layers correctly and is also unable to predict the turbulence energy correctly in a flow over a curved surface. 14 refs, 7 figs, 2 tabs