Performance and laser Doppler anemometry experimental investigation of squirrel cage fans with half-cone rotors

Abstract

Squirrel cage fans with non-cylindrical rotors, shaped as frustums of right circular cones, are compared with a similar fan but with a cylindrical rotor. Performance measurements for cone angles ranging from -10° to +10° showed that for positive angles, which means that a larger diameter at the rotor inlet decreases towards the back plate, it was possible to have higher fan efficiencies for a similar head. Alternatively, at negative cone angles, the head coefficient could be superior for a similar efficiency. The optimum angles were different for each case. These statements are true for the working part of the performance curves, which is located to the right of the maximum head coefficient. The velocity profiles at selected circumferential positions at the rotor exit showed that the flow did not spread evenly round the rotor when fan geometry was changed or the fan was throttled. If air flowed out of the rotor at a position closer to the fan exit, this flow then travelled a shorter distance inside the volute. A smaller loss and, therefore, a higher efficiency ensued. This pattern was true for both cylindrical and conical rotors and, therefore, the better efficiency for the positive half-cone rotor was a result of the way the rotor and volute combination guided the flow. The negative cone rotor had a higher head as the rotor diameter was larger where the flow passed through the blades.