Performance Enhancement of a Low-Pressure Ratio Centrifugal Compressor Stage with a Rotating Vaneless Diffuser by Impeller Disk Extended Shrouds

Numerical simulations have been carried out to examine the performance and flow parameters of forced rotating vaneless diffuser obtained by the extension of impeller disks of a low-pressure ratio shrouded type centrifugal compressor stage with diffuser diameter ratio 1.40. Four different levels of shroud extensions (i.e., impeller disks alone) forming the rotating vaneless diffusers are analyzed at four different flow coefficients. The extension of impeller disks alone by 40% of impeller exit diameter leads to a fully forced rotating vaneless diffuser thereby replacing the existing stationary vaneless diffuser. The comparative studies are performed using the same impeller with a stationary vaneless diffuser also having a diffuser diameter ratio of 1.40. Static pressure rise in ES40 is found to be higher than SVD by around 9.84% at design flow coefficient and also at above off-design flow rates. Energy coefficient is highest for ES40, followed by ES30 compared to SVD. For ES40, the static pressure recovery coefficient also is higher compared to SVD. The efficiency of ES40 is lesser by around 5.40% to 3.43% compared to SVD, at design as well as at above off-design flow coefficients. The stagnation pressure losses for ES40 drastically reduced compared to SVD. The comparison of stagnation pressure contours and absolute velocity contours near the hub and shroud walls of ES40 and SVD configurations shows that the rotating diffuser walls as in ES40 causes further addition of energy to the fluid. This adds up the kinetic energy level of the fluid which due to better diffusion, results in gain of static pressure rise. Moreover, there is a net increase in stagnation pressure distribution at the exit of diffuser due to rotating vaneless diffuser. Also, the presence of a fully rotating vaneless diffuser (ES40) smooth out the distorted entry flow profiles, thereby improving the performance of the centrifugal compressor stage.