Slip Factor Prediction for Impellers with Straight, Back-swept Blades

Abstract

Slip factor accounts for the deviation of the flow angle from the trailing edge blade angle at the exit of radial impellers. Accurate values are required to predict impeller torque and energy input. The slip factor prediction method for back-swept radial bladed impellers is based on the so-called single relative eddy (SRE) method, which is an approximation of the classical, two-dimensional analytical solution to the inviscid flow problem. The relatively accurate prediction of the slip factors of 19 impellers found in four data sets published since its formulation, shows the reliability of the SRE method. The characteristics of straight-bladed impeller layouts are explored and incorporated into the SRE method. It turns out that the SRE method as developed for logarithmic spiral blades predicts the inviscid flow slip factor of 42 different straight bladed impeller geometries accurately, when a correction for the critical radius ratio at low blade numbers is introduced. Predicted slip factor values are also compared to new experimental data for five different impellers with straight, back-swept blades, over a range of three blade angles and three blade numbers. Agreement is excellent near the flow coefficient corresponding to the volute design angle.