Three-Dimensional Motion in Axial-Flow Impellers

Abstract

Detailed surveys have been made of the flow through a low-pressure-rise axial-flow impeller (hub/tip diameter ratio 0·33), which did not produce ‘free vortex’ whirl distribution. It was found that (1) at all flows, centrifugal forces caused large variations in axial velocity component through the rotor; (2) the angular direction of the flow varied considerably within a short distance of the rotor, and the flow did not stabilize until about two blade chords downstream; (3) as the tip clearance was increased, secondary motion was induced opposite to the motion caused by centrifugal effects and, for clearances greater than 1 per cent of the blade height, seriously restricted the use of radial equilibrium theory. The measured head-flow characteristics for various sections along the impeller blades did not agree well with those calculated by the following two-dimensional design methods: (1) ‘slip’ theory, (2) ‘cascade’ theory, (3) ‘aerofoil’ theory. Better results were given by the use of a new method which consisted of two steps: (1) calculating the change in axial velocity by the radial equilibrium theory, involving the use of cascade data; and (2) calculating the head-flow characteristics, using the aerofoil theory based on the mean axial velocity obtained from (1). By this compromise the calculated local performance over most of the blade agreed within 4 per cent of experiment for a wide range of flows between design point and the stall. The new method may be generally applicable to low-pressure-rise fans and pumps, but further experimental evidence will be required to confirm this.