Study on Flow Inside Diffusers for Centrifugal Turbomachines : Report 2, Diffusers with Larger Area Enlargement than that of the Logarithmic Spiral

Abstract

A series of diffusers for centrifugal turbomachines were derived, which had larger area enlargement than that of the logarithmic spiral. The flow inside them was investigated. A calculation method for boundary layers was given as an extension of the method for diffusers with the shape of logarithmic spiral. Influences of area enlargement, shape, and aerodynamic parameters were discussed. (1) Features of secondary flows and boundary layers are qualitatively equal to those of logarithmic spiral diffusers. Boundary layer development on convex wall is very rapid. (2) Influence of area enlargement is expressed by coefficients g_h and g_L. By the introduction of these coefficients, equations for boundary layer and performance are obtained. (3) For diffusers with large area enlargement, efficiency is generally higher than for the logarithmic spiral ones. The limit of enlargement is given when one keeps the boundary layer at the convex wall outlet just before separation. (4) Influence of inlet momentum thickness upon performance is large in cases of large enlargement, but those of channel depth and Reynolds number are small in these cases.