Secondary Flows and Vortex Motion in a High-Efficiency Backswept Impeller at Design and Off-Design Conditions

The three-dimensional viscous flow field of a 4.7:1 pressure ratio backswept impeller was studied experimentally and numerically by using laser velocimetry and an advanced three-dimensional viscous code. The impeller was designed by a CAD method, and a maximum rotor efficiency of 94 percent was achieved. Both the experimental and the theoretical approach revealed comparatively smooth impeller discharge velocity profiles at all three operating conditions (design, choke, and near surge) differing widely from the well-known jet/wake-type flow pattern. The three-dimensional viscous code was used for detailed flowfield studies, i.e., secondary flows; vortex motion and tip-clearance effects were analyzed at design and off-design conditions. The comparison of experimental and numerical results indicates that the tip-clearance effect should be properly modeled to predict the impeller flow pattern properly and that optimum shape of rotor exit flow pattern can be obtained by controlling the swirling vortex motion.