A Numerical Study on Flow Characteristics of Super Sonic Diffuser for the Position and Nose Cone Angles of Center Body

The center body diffuser is one of supersonic diffuser that can simulate high-altitude environment. There is center-body structure inside the diffuser, and a complex fluid flow is occurred inside the diffuser because of the interaction of the CB structure with gas exhausted from the nozzle outlet. In this study, starting point and flow characteristics of diffuser were investigated according to changing the CB nose cone angle and the length of distance between nozzle and CB structure. The differences of the supersonic flow were compared through each parameter of CB distance and CB nose angle. First changed parameter was length between nozzle and CB. According to the length of distance between nozzle and CB, axial momentum was developed and oblique shock wave moved front of CB from end of CB nose cone. Also, when CB position was located on a certain length, starting point of CBD decreased. Next change parameter was angle of CB nose cone. According to the angle raised, angle of oblique shock wave was raised and radial momentum of supersonic diffuser developed. But, according to radial momentum of supersonic flow over certain angle, the starting pressure of CBD increased. Because axial momentum which isolated vacuum chamber from atmospheric pressure. Through these CFD analysis results, it was shown that angle and length of distance between nozzle and CB influent performance of CBD.

Performance Investigation Into Supersonic Diffuser for a High Pressure Centrifugal Compressor

In high pressure centrifugal compressors, the overall stage performance is greatly influenced by its diffuser performance. Extremely complicated non-uniform and unsteady flow exists in the region between the impeller exit and the diffuser inlet. Furthermore, in the case of supersonic diffuser, shock waves can be observed near the diffuser inlet. These can cause aerodynamic losses. Therefore, it is essential to recognize such complicated flow to realize an appropriate diffuser design. An investigation into the performance of supersonic diffuser was carried out using a high pressure compressor test rig for a small industrial gas turbine with a high back swept impeller and a quasi pipe-shaped channel diffuser. In addition, 3D quasi-unsteady flow analyses of the entire compressor by a RANS code with Non Linear Harmonic method at several operating conditions between surge and choke were conducted to investigate the details of unsteady flow between the impeller exit and the diffuser exit. The results of the performance test and that of the 3D unsteady flow analyses have shown good agreement in the pressure rise and the isentropic efficiency at several operating conditions. These support high accuracy of the flow analyses and the performance measurements.