A rocket engine should be small and low weight, a turbopump for a rocket engine must be smaller and have higher rotation speed than the conventional pumps. However, to achieve high thrust, pump discharge pressure must be high enough. As a result, a low specific speed impeller is often chosen for a rocket engine impeller. Generally speaking, efficiency of such a low specific speed impeller is lower since blade loading becomes high and large scale secondary flow will likely occur especially around the trailing edge. Therefore, to clarify the high efficiency shape, multi objective optimization of low specific speed impeller was carried out in the present study. The optimized result showed that there is a strong tradeoff between head and efficiency, and this tendency is not influenced by the flow rate. This means that performance dependency by a flow rate may be small by such a low specific speed impeller. Shape comparison between efficiency and head optimum results showed that not only outlet blade angle but also inlet blade angle are important for high efficiency impeller. By modifying these two blade angles, blade loading distribution is changed and blockage by secondary flow region is changed. As a result, for the high head impeller, large scales blockage occurs at the trailing edge, however, for the efficiency optimum result, blockage near the trailing edge becomes smaller.

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