A zero inlet swirl turbine rotor (ZISTR) is originally presented as the first stage in a multistage vaneless counter-rotating turbine (MVCT), which only consists of 4 rotors without any vanes. The vanes upstream of a ZISTR are removed to reduce the turbine weight and length, as well as the viscous losses and coolants associated with vanes. However, due to the lack of inlet swirl the stagger angles for ZISTR blade profiles are high and the blade deflections are very small, resulting in almost straight cambers and very thin airfoils. The motivation of this paper is to reveal the overall performance and key loss sources of a ZISTR associated with its special blade profile, and provide corresponding optimization approaches for its practical usages. The 3D viscous numerical results show that the wake, the suction side trailing edge shock and the tip leakage flow have substantial influence on the rotor performance. To optimize the performance of a ZISTR, reducing blade solidity is proposed to decrease the viscous and shock losses by increasing the portion of the inviscid mainstream. Leaned blade is also presented to restrict the tip leakage flow by adjusting the axial position of stagnation points on the blade profile, obtaining an increase in efficiency of 0.9%. The off-design performance of the optimized rotor is also presented to show the effect of the blade lean on efficiency at various rotating speeds and back pressures.

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