The volute casing of a radial flow impeller is replaced by an “equivalent” logarithmic spiral which, at the design flow rate of the pump, does not exert a force on the fluid discharged by the impeller. At other flow rates there is a nonzero angle of incidence and the spiral, which may be regarded as the single blade of a radial flow cascade, exerts a “lift” force. This force is calculated by the methods of perfect fluid theory, first mapping the spiral onto a flat-plate cascade, then establishing the pressure distribution. Incidence angle and the magnitude of the absolute exit velocity from the impeller govern the lift forces and by a consideration of the velocity triangles at impeller exit some qualitative features of the behavior of radial forces on the impeller are deduced. Quantitative comparison with the experimental results of Iversen, et al., [4] also shows good agreement.

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