Unsteady Flow Condition of Contra-Rotating Small-Sized Axial Fan

Small-sized axial fans are used as air cooler for electric equipments. But there is a strong demand for higher power of fans according to the increase of quantity of heat from electric devices. Therefore, higher rotational speed design is conducted, although, it causes the deterioration of efficiency and the increase of noise. Then, the adoption of contra-rotating rotors for the small-sized axial fan is proposed for the improvement of performance. In the case of contra-rotating rotors, it is necessary to design the rotor considering the unsteady flow condition of each front and rear rotor. In the present paper, the fan performance of the contra-rotating small-sized axial fan with 100mm diameter at a designed and a partial flow rates is shown, and the unsteady flow conditions at the inlet and the outlet of each front and rear rotor are clarified with unsteady numerical results. Furthermore, the relation between the performance and the unsteady flow condition of the contra-rotating small-sized axial fan is discussed and the methods to improve the performance are considered.