Loss Analysis of a Transonic Rotor with a Differential Approach to Entropy Generation

Abstract

The loss mechanism of transonic axial compressors is a long-standing problem that involves almost all types of entropy generation in fluid flows, such as skin friction, shock waves, shear flows, corner separation, and tip vortices. Primarily, sources need to be identified and quantitative comparisons of their contributions need to be made. For such determination, we propose herein a differential approach to entropy generation, called the “differential approach”. Two case studies are analyzed to determine the applicability of this approach: (1) laminar and turbulent incompressible flows in straight circular ducts and (2) turbulent compressible flows in convergent-and-divergent nozzles with shock waves. The results lead to the following conclusions: (a) Qualitatively, the differential approach works well, and the quantified measure is reliable if it is calculated with quality meshes and a suitable turbulence model. This means that the differential approach can be a good tool for predesign optimization. (b) When shocks occur within flow fields, the shock-induced boundary-layer separation can generate more loss than the shock loss alone. Subsequently, the differential approach is applied to complex flows in the NASA Rotor 67, which is a well-known bench-test transonic rotor. The results show that the differential approach not only determines the local losses and associates the source of losses with the flow structures but also qualitatively compares and identifies the largest contributors. These results provide a theoretical foundation for optimizing rotor design and enhancing stability.