Data center has been built with spread of cloud computing. Further, electric power consumption of it is growing rapidly. High pressure and large flow rates small-sized cooling fans are used for servers in the data center and there is a strong demand to increase pressure and flow rate of it because of increase of quantity of heat from the servers. Contra-rotating rotors have been adopted for some of the high pressure and large flow rate cooling fans to meet the demand of it. There is limitation of space for servers and geometrical restriction for cooling fan because spokes to support the fan motor, electrical power cables and so on should be installed in the cooling fan. It is important to clarify performance of each front and rear rotor of contra-rotating small-sized cooling fan and internal flow condition to achieve the high performance cooling fans.

In the present paper, detailed performance and the flow condition around the rotor of the high pressure and large flow rate contra-rotating small-sized axial fan with 40mm square casing are investigated. Pressure, shaft power and efficiency curves of each front and rear rotor are clarified by the experimental and the numerical results. Furthermore, the internal flow condition of the rear rotor is clarified by the numerical analysis results and design guideline to improve the performance is discussed by these results.

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