A design strategy for a 6:1 supersonic mixed-flow compressor stage and its viscous flow-based performance analysis

Open Access
Author:
Sadagopan, Aravinth
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 17, 2019
Committee Members:
  • Cengiz Camci, Thesis Advisor
Keywords:
  • mixed-flow compressor
  • mean-line design
  • turbomachinery
  • computational fluid dynamics
  • shock stability
  • aerospace propulsion
  • Bezier curves
  • NASA Rotor 37
  • Supersonic Diffuser
  • 6:1
  • pressure-ratio
  • blade optimization
Abstract:
A current surge in the small jet engine market requires compact and robust high-performance compressors. This thesis presents the design of a single-stage high-pressure ratio mixed-flow compressor with a prescribed maximum diameter in the (1-10) kg.s-1 mass flow segment. Its compactness and reliability demonstrate its suitability in replacing a multistage axial compressor design in the small aero-engine segment with a high-performance envelope. A comprehensive background of mixed-flow stage design is provided based on historical developments since the 1940s. A high-pressure ratio demand necessitates supersonic rotor exit flow. The high-pressure ratio and small diameter requirements push the compressor toward a "highly-loaded" supersonic “shock-in rotor” design with a supersonic stator/diffuser. Hence, tandem stator configurations with two blade rows were investigated in the past to reduce blade loading for efficient diffusion. Even so, most of the previous stage designs were inefficient, due to the inability of stators to efficiently diffuse supersonic flow. Thus, this thesis implements a tandem design based on Quishi et al. A unique mean-line design procedure is presented based on the isentropic equations defined for a mixed-flow stage. Mass flow rate, stage total pressure ratio, and maximum diameter were chosen as the main design constraints, and a geometry construction technique based on Bezier curves was used. The advancement of multi-dimensional and viscous computational tools has improved accessibility to and reduced overall effort in the thorough analysis of complex turbomachinery designs. Therefore, the aim of this thesis is to include all three dimensionality effects of the stage, viscous flow, and compressibility including the shock wave systems. The computational model employed was thoroughly assessed for its ability to predict compressor performance, as compared to existing well-established experimental data. The results from a RANS-based computational fluid dynamics model were compared to the experimental results of NASA Rotor 37 and the RWTH Aachen supersonic tandem diffuser. The computational approach shed light upon the mixed-rotor and supersonic-stator 3D shock structures, as well as the viscous/secondary flow. Furthermore, a rotor design evaluation study was conducted for a 3.5 kg/s mass flow based on the current mean-line code and additional computational analysis. A relatively high single-stage pressure ratio of 6.0 was targeted. The performance map for the mixed-flow stage was obtained to better understanding the viscous flow details and shock systems of this high-pressure ratio mixed-flow compressor. Areas of potential design optimization were highlighted to further improve the stage’s performance. The in-house mean-line design code predicted a pressure ratio and efficiency of ΠTT = 6.0 and 75.5%, respectively for a mass flow rate of 3.5 kg/s. The mean-line design code obviously lacked the ability to fully capture three-dimensionality, viscous flow, and compressible flow effects due to its inherent over-simplifying assumptions. The inclusion of the RANS-based computations improved the fidelity of the mixed-flow compressor design performance calculations significantly. Comprehensive computational analysis in the current stage showed that the design goal was met with a stage total pressure ratio of ΠTT = 5.83 and an efficiency of η_IS = 77% for a mass flow rate of m ̇ = 3.03 kg/s. A total pressure ratio of 6.12 was achieved at a slightly higher rotational speed of Ω/Ωo = 1.035 for an efficiency of 75.5 %.