Numerical Studies on the Effect of Diffuser Rotational Speeds on Low Pressure Ratio Centrifugal Compressor Performance

Numerical investigations are conducted to analyze the effect of rotational speeds of rotating vaneless diffuser on performance of a low-pressure ratio centrifugal compressor stage at off-design and design flow coefficients. Four different rotational speeds are selected for the rotating vaneless diffuser. Free type rotating vaneless diffusers are rotated at speed ratios SR0.25, SR0.50 and SR0.75. Total temperature of the fluid at stage exit increases with increase in rotational speeds of the free rotating vaneless diffuser. This affects the efficiency of free rotating vaneless diffusers. Its flow angles are smaller resulting in a shorter flow path length thereby reducing the frictional losses substantially. Gain in stagnation pressure is observed for all free rotating vaneless diffuser configurations. The rotational speeds determine the extent of net gain in energy level of the fluid and drop in total pressure losses. Based on static and stagnation pressure distributions at stage exit, the flow in FreeRVDSR0.75 undergoes a comparatively better diffusion process. In general, efficacy of diffusion process in a compressor stage with free rotating vaneless diffuser is better at speed ratio above 0.50. Subsequently, the speed ratio is increased to SR1.0 thereby the rotating vaneless diffuser behave like a forced type. Various free rotating vaneless diffuser configurations are compared with the forced type of rotating vaneless diffuser. Based on this study, it is understood that an optimum rotational speed of the rotating vaneless diffuser plays an important role in facilitating the effective diffusion process within the diffuser passage.