A Numerical Investigation on the Effects of Vaned Diffusers on the Aerodynamic Performance of a Low Pressure-Ratio Methane Centrifugal Compressor

Vaned diffusers are widely used in centrifugal compressors due to their higher pressure-recovery coefficients compared to vaneless diffusers. In this study, the effects of the diffuser vanes’ wrap angle and number of vanes on the aerodynamic performance of an industrial Methane centrifugal compressor with a pressure-ratio of 1.288 are studied using high fidelity steady-RANS numerical simulations. Three wrap angles (WA = 19.3o, 22.3o and 25.3o) and three number of vanes (NDiff. = 16, 20 and 24) are examined, while all the other geometrical and operational parameters are kept constant. Results showed that decreasing the wrap angle can enhance the choke flow rate of the compressor, with slight reduction in pressure ratio at low flow rates. However, increasing the diffuser wrap angle, intensifies the flow separation over the diffuser vanes. On average, the best aerodynamic performance of the compressor occurred at WA=22.3o. Results also showed that reducing the number of diffuser vanes enlarges the operating range of the compressor, however, the pressure ratio will be lower at the flow rates less than the design point. Conversely, higher pressure ratios will be achieved at the flow rates greater than the design flow rate. The optimal aerodynamic efficiency of the diffuser, considering both the pressure ratio and the total-to-total efficiency, was achieved when the NDiff value was set to 20.