Basic Study on High Efficiency Operation and Surge Control of Axial Flow Pump with Contra-Rotating Rotors

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Design concepts and an experimental validation of the contra-rotating axial flow pump, consisting of front and rear rotors instead of a rear stator of conventional type and rotating in opposite directions each other, were discussed for improving of pump performances in the range of higher specific speed. Experimental results showed that using contra-rotating type was an efficient means of recovering the swirl kinetic energy without the use of stator and a means with wider range of high efficiency. It was also demonstrated that the reverse flow and cavitation inception could be postponed in the contra-rotating axial flow pump as compared with the conventional type. As the specified pump head and maximum efficiency of designed rotors, however, were not satisfied at design point, the rear rotor was redesigned. Experimental data for two types of rear rotors under the same front rotor were compared and appropriate design parameters were discussed to obtain higher performance of rear rotor. Results were as follows: The stagger angle should be selected with considering the output flow of front rotor. Adopting thinner blade with low solidity for rear rotor yielded higher efficiency at design point but at the off design-points the efficiency and cavitation occurrence were deteriorated. By the control of rear rotor rotational speed the wider operating range with high efficiency and cavitation performance was achieved especially in the case of rear rotor with higher efficiency at the design point.