InfoScience (Czechia)

A radial fan with a tip diameter of 19.2 mm and a constant blade height of 1.82 mm was tested with a running tip clearance of 0.14 mm and 0.04 mm.This corresponds to a relative clearance (blade tip clearance divided by blade height) of 0.077 and 0.022, respectively.With respect to the baseline operating conditions (rotational speed of 168 krpm, air mass flow rate of 5.3 kg h -1 , and total inlet temperature of 200 • C) the fan totalto-total pressure rise could be increased by 20 mbar to 68 mbar, by reducing the tip clearance.The isentropic total-to-total efficiency increased thus from 53 % to 64 %, suggesting a significant impact of the blade tip clearance for such a micro radial fan.A single passage CFD simulation correlates well to these observations.The widely-used Pfleiderer correlation with an empirical coefficient of 2.8 fits the simulation and experiments within ±2 efficiency points.The authors suggest three measures to mitigate the blade tip clearance losses for small-scale fans: (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 0.040 mm, due to the tight bearing tolerances, (2) a mid-loaded blade design, and (3) unloading the fan leading edge, and thus reducing the blade tip clearance vortex in the fan passage.