In order to reduce the energy losses associated due to diffusion in the centrifugal compressor stage, several radial diffuser designs have been tried and rotating vaneless diffuser is one among them. Forced rotating vaneless diffuser is formed by extending the impeller disks alone beyond the blade tip without affecting the blade tip geometry. Studies on completely replacing a stationary vaneless diffuser with rotating vaneless diffuser are not available in open domain yet. This paper reports the effect of rotating vaneless diffuser based on shroud extension concept on flow diffusion, performance and flow parameters in a centrifugal compressor stage at design and off-design flow conditions. A backward curved impeller with diffuser diameter ratio 1.40 is chosen for the present study. Rotating vaneless diffuser is formed by extending the impeller disks by 40% above the impeller exit diameter. The comparative studies are done with the same impeller having a stationary vaneless diffuser of equivalent diffuser diameter ratio in the downstream. Static pressure rise in RVD-ES configuration is higher compared to SVD and the energy coefficient improved by around 57.14% for RVD-ES over the entire flow range. The efficiency of RVD-ES and SVD are almost identical at Φ = 0.156. At design and other off-design flow coefficients, the efficiencies of RVD-ES are slightly lesser compared to SVD by 1.0 to 1.52%. The static pressure recovery coefficient of RVD-ES is higher than SVD. The stagnation pressure losses reduced drastically for RVD-ES. From the velocity vectors and contours of stagnation pressure distribution, it seen that additional energy is added to the fluid by the rotating walls of the vaneless diffuser. This results in increased kinetic energy of the fluid. Due to better diffusion process in RVD-ES, this results with a gain in increased static pressure rise with acceptable efficiency.

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