Correlation of flow path length to total pressure loss in diffuser flows

Abstract

This article studies the effect of changing the flow path length on development of total pressure loss in diffuser flows. The studies incorporate quasi-one-dimensional (conical diffuser), two-dimensional (linear blade cascade), and three-dimensional (annular blade cascade) flows. This article concludes that the consequence of decreasing the flow path length is to decrease total pressure loss, provided that the adverse streamwise pressure gradient remains subcritical. The loss calculation technique and measurement data published by Lieblein were post-processed in this article. On this basis, this article elaborates a method for estimating the total pressure loss as well as its modification due to change of flow path length in blade cascade flows. This method can contribute to preliminary axial flow rotor blade design. It was pointed out that, in comparison to free vortex design, controlled vortex design that is applied to straight axial flow rotor blades tends to represent an additional source of total pressure loss farther from the endwalls. This is due to elongation of the length of flow paths on the blade suction side, due to intensified radial outward flow. This adverse effect can be counterbalanced by incorporation of forward sweep in controlled vortex design, via shortening the flow paths on the suction side, without the beneficial features of the controlled vortex design being lost. This article presents a literature-based comparative case study to illustrate such loss-reducing effect of incorporating forward sweep in controlled vortex design, away from the endwalls.