Experimental investigation of the influence of the casing geometry upon the flow conditions and the noise of a centrifugal fan

Beginning with a casingless impeller with six backward curved blades, the influence of a small logarithmic volute casing with a cut-off clearance of delta r/R = 0.014 (R = impeller radius) upon the flow through the rotating impeller ducts was studied. It is found that there is a blockage of the flow in each impeller duct when passing the cut-off due to the small cut-off clearance. An optimization of the casing geometry regarding low noise and high fan efficiency lead to a logarithmic spiral with a large out-off clearance of delta r/R = 0.3. The noise was reduced by 22 dB without loss in fan efficiency.