Effect of Impeller Outlet Flow Affected by Casing Treatment on Rotating Stall in Vane-Less Diffuser in Centrifugal Turbomachinery

Abstract

The effect of impeller outlet flow affected by a recirculating casing treatment on rotating stall in a vane-less diffuser in centrifugal turbomachinery was investigated experimentally. Centrifugal compressors require higher efficiency and wider operating range. At lower flow rates, rotating stall sometimes occurs in a vane-less diffuser and generates significant subsynchronous vibrations, which often mark the limit of the operating curve of actual compressors. For this reason, the vibration and load on thrust and radial bearings should be reduced when rotating stall occurs. Many studies have investigated rotating stalls in vane-less diffusers in centrifugal turbomachinery. On the other hand, few studies have investigated how stall characteristics change when a casing treatment which is an effective means of expanding the operating range of the centrifugal compressor, is implemented. From this aspect, this research experimentally investigated the effect of the impeller outlet flow on rotating stall in a vane-less diffuser and evaluated the possibility of decreasing shaft vibration. Two different centrifugal impellers with the same recirculating casing treatment were investigated to examine the effect of the impeller outlet flow field. Impeller-A had a smaller exit blade angle with a higher deceleration ratio of impeller relative velocity from inlet to outlet. Impeller-B had a larger exit flow angle with a lower deceleration ratio. Both impellers had the same meridional geometry. Experimental results showed that both impellers obtained map width expansions for lower flow rate regions of more than 10% from the casing treatment. In the case of Impeller-A, the static pressure rise increased as a result of the casing treatment. However, the pressure fluctuations at the diffuser due to the rotating stall increased. On the other hand, in the case of Impeller-B, the static pressure rise increased at the vane-less diffuser instead of the impeller. In Impeller-B, the casing treatment decreased the pressure fluctuation at the diffuser due to the rotating stall and kept the static pressure rise with a negative slope. The results indicate that improving the outlet flow in cases such as Impeller-B may decrease the shaft oscillations due to rotating stall.