An application of contra-rotating rotors, consisting of front and rear rotors rotating in the opposite direction from each other, has been proposed against a demand for developing a higher specific speed axial flow pump with a more compact structure, higher efficiency and higher cavitation performance. As axial flow pumps are used for standby operations of air-lock and air/water mixing discharge to prevent floods, air/water two-phase flow performance of the contra-rotating pump has to be also investigated. In the present paper, therefore, experimental results on air/water two-phase flow performance of a test pump with contra-rotating rotors are shown and compared with those of a conventional axial flow pump, consisting of a front rotor and a rear stator. Even under two-phase flow conditions head characteristic curve of the contra-rotating type has a more strongly negative slope than that of the conventional type. The contra-rotating type maintains higher head and higher efficiency even in the low flow rate range and vice versa in the high flow rate range. This result will be discussed by considering the change of outlet flow from front rotor due to two-phase flow with the help of observed air behavior in the rotors. Then effects of changes of rear rotor rotational speed different from front rotor speed, which is an advantage of the contra-rotating axial flow pump, on two-phase flow performance are examined. Under the condition of constant ratio of air to water flow rates, the head rise of the rear rotor linearly increases with rear rotor rotational speed. Air/water two-phase flow performance of the contra-rotating axial flow pump can be improved by this control procedure for the rear rotor rotational speed.

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