Influence of Large Relative Tip Clearances for a Micro Radial Fan and Design Guidelines for Increased Efficiency

A radial fan with a blade tip of 19.2 mm and a blade height of 1.82 mm was tested both with a tip clearance of 150 μm and 50 μm. This corresponds to a relative clearance (blade tip clearance divided by channel width) of 0.0761 and 0.0267, respectively. With respect to the nominal point (rotational speed of 168 krpm, air mass flow rate of 4.9 kg/h, and total inlet temperature of 200 °C) the measured fan total-to-total pressure rise could be increased by 20 mbar to 75 mbar. The isentropic total-to-total efficiency increased thus from 55 % to 65 %, suggesting a significant impact of the blade tip clearance for such a micro radial fan. The widely-used Pfleiderer correlation with an experience coefficient of 2.3 fits both the simulation and the experiments within ± 5 efficiency points. The authors suggest two measures to omit the blade tip clearance losses: (1) gas-bearing supported rotors (here: herringbone-grooved journal bearings and a single-sided spiral-grooved thrust bearing) to realize low blade tip clearances of down to 40 μm, due to the tight bearing tolerances and (2) unloading the fan leading edge, and thus reduce the blade tip clearance vortex in the fan passage, as well as the interaction of the vortex with the splitter blade leading edge.