Abstract: In order to investigate in detail the velocity field near the tongue of a forward curved multi-blades centrifugal fan in various working conditions, a performance test is firstly conducted which insures the rig’s reliable repeatability. Then the particle image velocimetry (PIV) is used to measure the velocity field near the tongue at different operating conditions and different axial location. The analysis shows that the flow pattern near the tongue mainly has relation with the influence of the stagnation area around the tongue as well as the direction of velocity vector bearing down on the tongue. When small flow rate state of working conditions, the influence of the stagnation region chiefly focuses near the passage inlet of the tongue clearance so that the clearance flow downstream is prone to separate from the tongue wall and deteriorates. When flow rate increasing, however, the influence of the stagnation area leans towards the passage of the volute outlet and the flow in tongue clearance does not form a distinct separating region. Furthermore, the flow rate returning into the volute through the tongue clearance varies from impeller shroud to hub by working difference of the blade at different segment, near the inlet end of the fan is low. The knowledge is very useful in improving fan performance.

Key words: Centrifugal fan, Multiblade, PIV, Tongue, Velocity field