A sirocco fan using contra-rotating rotors in which an inner rotor is settled inside the sirocco fan rotor and each rotor rotates in an opposite direction was proposed for the purpose of getting the higher pressure and making the structure of a sirocco fan compact more. If the high discharge pressure is obtained with the adoption of the contra-rotating rotors, it could be used for various purposes. Pressure coefficient of a sirocco fan with contra-rotating rotors is 2.4 times as high as the conventional sirocco fan and the maximum efficiency point of contra-rotating rotors shifts larger flow rate than a conventional sirocco fan. On the other hand, it was clarified from the flow measurement results that the larger back flow region than conventional one occurred at the shroud of the casing in the case of contra-rotating type and this would induce decrease of the efficiency. In the present paper, the performance of a conventional sirocco fan and a sirocco fan with contra-rotating rotors are shown and the internal flow field at the outlet of the outer rotor of both cases is clarified. Then, the effect of casing width on the performance and internal flow field is investigated and the casing shape which would be suitable for the contra-rotating type would be discussed.

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