Discussion: “A Unified Procedure for the Calculation of Off-Design Performance of Radial Turbomachinery” (Davis, R. C., and Dussourd, J. L., 1971, ASME J. Eng. Power, 93, pp. 133–145)

This paper represents an impressive prediction of off-design performance of centrifugal compressor rotors and diffusers. I think the authors have produced admirable results with a very simple description of the blockage and loss characteristics of centrifugal machines. I note that although the shape of the static pressure ratio characteristics of the rotors is well predicted, the levels are overestimated at higher speeds. On the plot of measured vs. calculated rotor total pressure ratio there is a deficit of calculated pressure ratio to the order of 5-7 percent, which is about 4 to 5 points in efficiency at 3:1 pressure ratio. This discrepancy in impeller efficiency is considerable. In addition, no stage pressure ratio performance comparison is shown. I am interested in the correlation of overall stage efficiency by this technique. Possibly this impeller efficiency discrepancy is related to the secondary and leakage losses not accounted for. At stage pressure ratio of the order of 6:1 at UACL we find that our analysis predicts a major portion of the rotor loss to be secondary loss. I would expect that the method of the authors might be most effective at low pressure ratios in its present form.