In an effort to improve the performance of centrifugal compressors, detailed studies were carried out on three different impeller-diffuser configurations covering a wide range of design flow coefficients. These were two-dimensional impellers chosen from a multi-stage compressor series with identical blade geometry. The impeller exit widths covered were 24.5, 18.5, and 11.5 mm, respectively. Extensive experimental studies have been carried out on the chosen stage configurations with parallel walled vaneless diffusers (VLDs) as well as low solidity vaned diffuser (LSD). For all the tested configurations, the aerofoil shape of the diffuser vanes remained same while the diffuser width, setting angle, and solidity were chosen as variables. The performance for each of the stages with LSDs was compared with that of the corresponding VLD. Significant improvement in performance in terms of polytropic efficiency, head coefficient, and static pressure recovery coefficient was observed with LSD vanes as compared with the VLD. Significant improvement in performance was noticed from low flow till design point for all the tested cases. The study revealed that among the tested configurations, there is an optimum width at impeller exit and diffuser for which the performance of the stage was the best. Performance in terms of static pressure recovery coefficient was also evaluated for all the tested configurations and compared. All the studies were conducted for solidity ratio of 0.81. The effect of setting angle on wall static pressure coefficient was also studied at 24° and 28° for the flow coefficients corresponding to 80, 100, and 120 per cent of design flow. Flow surveys at impeller exit for the three cases covering impellers/parallel walled diffusers of different widths have also been carried out to know the flow behaviour in the passage between the impeller and the diffuser.

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The results of experimental studies on a centrifugal compressor stage using uncambered constant thickness low solidity diffuser vanes (LSD) are reported in this paper. In the present work, efforts are concentrated for study of stage performance with LSD vanes for a typical compressor stage at a tip mach number of 0.35. The effect of solidity and setting angle on overall stage performance is evaluated in terms of flow coefficient, head coefficient and efficiency. The results were normalised with the data available at design flow using vaneless diffuser. Improvement in performance as well as static pressure recovery was observed with LSD vanes of the chosen configuration as compared to vaneless diffuser configuration. Variation of blade loading is studied from the measurements of static pressure on pressure and suction surfaces of LSD vane.