An application of contra-rotating rotors, consisting of front and rear rotors rotating in the opposite direction each other, has been proposed against a demand for developing higher specific speed axial flow pump with more compact structure, higher efficiency and higher cavitation performance. The blade rows interaction between front and rear rotors should be clarified for stable operation and reduction of unsteady losses. In the present paper, static pressure distributions on casing wall are shown at two flow rates by measuring with phase locked sampling method, where double trigger signals from rotation pick-ups equipped on both front and rear rotor shafts are used. From the comparison between experimental results of two types of the rear rotors, one of which has three blades with long chord length and the other has five blades with short length, the difference of the rear blade interaction and the unsteady pressure fluctuation are identified. The pressure fluctuations by rear rotor were observed even upstream of the front rotor and upstream flow field changed due to the pressure fluctuation in the former case. The effect of pressure fluctuation to the flow field will be discussed in more details and the guideline of rear rotor design will be considered.
