Numerical Analysis of a Vortex Controlled Diffuser

A numerical study of a prototypical vortex controlled diffuser is performed. The basic diffuser geometry consists of a step expansion in a pipe of area ratio 2.25:1. The incompressible Reynolds averaged Navier-Stokes equations, employing the RNG based κ − ∈ turbulence model, are solved. Results are presented for bleed rates ranging from 1 to 7 percent. Diffuser efficiencies in excess of 80 percent were obtained. These results are in good qualitative agreement with previous experimental work. The results do not confirm previous suggestions that the increases in effectiveness of the VCD over a step expansion result from an inhibition of flow separation due to the generation and downstream convection of extremely high levels of turbulence generated in the region of the bleed gap. The results do indicate that the effectiveness of the diffuser is a consequence of the turning of the flow toward the outer wall due to the influence of the low pressure vortex chamber. Calculations employing the RNG based turbulence model were able to capture the abrupt increase in diffuser effectiveness that has been shown experimentally to occur at low bleed rates. Calculations employing the standard κ − ∈ model were unable to predict this occurrence.