US Patent 8251642 Centrifugal fan

A centrifugal fan includes a top cover plate, a bottom plate, a sidewall and an impeller. The sidewall is defined with an air outlet. The impeller includes a hub and a plurality of blades extending radially outwardly from the hub. An air passage channel is defined between outmost free ends of the blades and an inner surface of the sidewall. A tongue is provided near the air outlet and protrudes inwardly and laterally from the inner surface of the sidewall towards the outmost free ends of the blades. The tongue has an outer surface facing the outmost free ends of the blades. A plurality of grooves is defined in the outer surface along a lengthwise direction of the tongue.