Development of Wedge Type Impellers for Low Specific Speed Centrifugal Compressors

Abstract

Low specific speed, namely low flow coefficient, Wedge type impellers with high performance are developed, which impellers are applied to high-pressure multistage compressors. Conventional low specific speed impellers because of their low volume flow rates have thin blades and their blade heights are small. Therefore, efficiency of low specific speed impellers turns out to be lower than that of high specific speed impellers, because the large wetted perimeter causes large skin friction loss. Wedge type impellers with thick blades and large blade heights are examined to reduce the skin friction loss. Blade geometry of Wedge type impellers is determined by the distribution of blade angles and thicknesses that are defined in terms of a polynomial of meridional distance. The determination of impeller geometry also takes into account the inlet and the outlet blade heights. Adiabatic efficiency of many Wedge type impellers, designed by studying various combinations of these distributions and blade heights, is estimated by the one-dimensional prediction method. This procedure leads to the determination of optimized impeller geometry with higher efficiency and wider operating flow range than available otherwise. In Wedge type impeller optimization by their design, the relative velocity ratios from impeller “inlet to throat” and from impeller “inlet to outlet”, and flow incidence at impeller inlet are restricted to certain specific values. This is done in order to ascertain wide operating flow range. Estimated efficiency of optimized Wedge type impeller with designed suction flow coefficient of 0.021 is by some percent higher than that of the conventional two-dimensional impeller with thin blades, as was determined by both the one-dimensional prediction method and by commercial CFD code. Designed Wedge type impellers were fabricated and tested for their aerodynamic performance. Results show that in case of Wedge type impeller the normalized stage efficiency increases by 6% and operating flow range expands significantly as compared with those of the conventional 2D impeller stage. The performance estimation by CFD also shows a qualitative agreement with the experimental results regarding the increase in efficiency and operating flow range. Moreover, rotating stall which often exhibits sub-synchronous low frequency shaft vibration in high-pressure centrifugal compressors was not observed in Wedge type impeller stages.