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The efficiency is reduced in very small centrifugal compressors due to low Reynolds numbers. In the past, the effect of the Reynolds number on centrifugal compressor performance has been studied experimentally, and empirical correction equations for the efficiency have been derived based on those results. There is a lack of numerical investigations into the effect of the Reynolds number on centrifugal compressor performance and losses. This paper aims to compare the numerical results to the efficiencies predicted by the correction equations found in the literature. The loss generation in the impeller blade passages is also studied in order to find out which loss production mechanism has the most potential to be reduced or eliminated. The effect of the Reynolds number on compressor performance is investigated in the chord Reynolds number range varying from 0.8 · 105 to 17 · 105 by simulating numerically the original compressors and downscaled ones. The numerical results are validated against experimental data and the results are compared with the efficiency correction equations used in the literature. The results indicate that the performance of the downscaled compressors follow quite precisely the most recently published correction equation. The results also show that the increased losses in low-Reynolds-number compressors are caused both by the relatively increased boundary layer thickness and by the shear stress resulting from the increased vorticity.

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Three vaneless diffuser designs, varying in diffuser width, for centrifugal compressor were studied experimentally. Along with the diffuser width, the tip clearance was altered. The compressor overall performance and diffuser flow fields were studied for each of the three diffusers at four different tip clearances. For the diffuser flow fields, the total pressures were measured with probes traversed over the diffuser width both at the diffuser inlet and outlet. Along with the total pressures, the static pressures were measured adjacent to the probes. This enabled the axial flow angle and velocity distributions to be studied. The pinches tested improved the stage efficiency mainly by suppressing the secondary flow region present near the shroud at the impeller outlet. This leads to a lower strain rate, resulting in lower losses. The efficiency decrease due to the increased tip clearance was similar with and without pinch present. This indicates that in the diffuser, the main source of the tip clearance associated losses is the tip jet, and the tip clearance vortices mix out already in the impeller.

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