Viscous flow and performance issues in a 6:1 supersonic mixed-flow compressor with a tandem diffuser

The advancement of multi-dimensional and viscous computational tools has eased the accessibility and overall effort for thorough analysis of complex turbomachinery designs. In this paper, we computationally evaluate a high-pressure ratio supersonic mixed-flow compressor stage designed using an in-house mean-line code. Objective is to include three dimensionalities, viscous flow and compressibility effects including the shock wave systems into account. As mixed-flow compressors are advantageous especially for small jet engine applications we choose mass flow rate, stage total pressure ratio and maximum diameter as the main design constraints. This computational analysis is the second paper of a two-part series explaining strategy for designing a high-pressure ratio mixed-flow compressor stage. The high-pressure ratio and small diameter requirements push this compressor for a highly-loaded supersonic ‘shock-in rotor’ design with supersonic stator/diffuser. The used RANS based computational fluid dynamics model is thoroughly assessed for its ability to predict compressor performance using existing well-established experimental data. NASA Rotor 37 and RWTH Aachen supersonic tandem stator are chosen as the test cases for exhibiting similar flow characteristics to present design. The computational approach helps to shed light upon the mixed-rotor and supersonic-stator 3D shock structures and viscous/secondary flow. Stage performance map, pressure and velocity distribution of this high-pressure ratio mixed-flow compressor is obtained. Areas of design optimization are highlighted to further improve performance and efficiency. The in-house mean-line design code predicted a pressure ratio of 6.0 with 75.5% efficiency for a mass flow rate of 3.5 kg/s. The mean-line code obviously lacked to fully represent three-dimensionality effects due to its inherent over-simplifying assumptions thus, inclusion of RANS based computations improves the fidelity of mixed-flow compressor design performance calculations at a great rate. Comprehensive computational analysis of the stage shows that our design goal is met with a stage total pressure ratio of Π _TT = 5.83 with an efficiency of η _IS = 77% for a mass flow rate of m˙ = 3.03 kg/s. A total pressure ratio of 6.12 at 75.5% efficiency is reached with a 3.5% increase in design rotational speed.