The Effect of Inlet Clearance Geometry on the Performance of a Centrifugal Fan

A commercially manufactured, medium specific speed centrifugal fan, having an impeller with ten serofoil blades and a conical shroud was used to determine the effect of changing the geometry of the: inlet a clearance on performance. This was achieved by moving the “impeller axially, thus altering the intrusion of a fixed and stationary inlet cone into the rotating impeller.
Full fan characteristics were determined for various settings of the intrusion. Yawmeter traversing was carried out at impeller outlet and thus velocity distributions were determined. This led to the calculation of a "distribution factor", which was introduced as a measure of uniformity of outlet velocities in the meridional plane.
Velocities of air issuing from the inlet clearance into the impeller were measured using a hot wire anemometer. This enabled the volumetric efficiencies to be calculated for various settings. of the inlet clearance and at different points on fan characteristic.
It was established that the volumetric efficiency increased with the decreasing clearance area and it also increaséd with the: increasing fan flowrate. The geometry of the clearance was found to be of secondary importance. It was however primarily responsible for the changes in the distribution factor. Thus, an increasing intrusion resulted in an increasing value of ‘the distribution factor and simultaneously caused the volumetric efficiency to decrease. Optimum performance and the best fan total efficiency were achieved when: the product of the distribution factor and the volumetric efficiency attained a maximum.
In addition a new ‘surface flow visualisation technique was developed. Its application to the shroud demonstrated flow separation, downstream of the impeller lip, mixing between the re-circulatory and the main flows and their reattachment to the shroud. These findings were in good agreement with the values of the distribution factor:
low values correspond to large separation areas and vice versa.
The existence of a passage vortex near the shroud was also demonstrated.
Results of a two-dimensional investigation carried out initially provide some supporting evidence.
Finally, a tentative exploration of the flow phenomena resulting from the interaction between the re-circulatory flow through the clearance and the main flow is offered on the basis of the results of this investigation.