Supercritical compressed air energy storage system requires high turbine efficiency over a wide working range at both the design-point and off-design point. The operating range of the turbine is often limited by the occurrence of flow instability, such as distinct vortex and load deterioration. In specific situations, the development of aerodynamic instabilities at low inlet pressure operating conditions can lead to lower turbine efficiency, even lead to turbine choke, limiting the wide operating range of the turbine. Specifically, the high-pressure stage turbine of the expander, namely the first stage turbine, will not only bears a sharp change of inlet pressure but also experiences the maximum and variable pressure drop. In this paper, Real Gas Property (RGP) file for supercritical air is used for simulations. The CFD method and RGP file are validated by the turbine in the NASA report. A detailed 3D CFD analysis is performed for the preliminary designed first stage turbine under variable inlet pressure. With the purpose of improving the aerodynamic performance of the first stage turbine under extreme working conditions, a series of simulations are conducted to examine the effects of the IGV parameters (i.e. different types of IGVs, the nozzle-impeller radial gap distance and the nozzle blade installation angle) and blade thickness, which led to the optimization in overall turbine stability. The results show that IGV TC-2P have good aerodynamic performance, and the matched rotor can achieve an isentropic efficiency of 80% under off-design working conditions. The optimal blade installation angle blade is 35°. And there exists an optimal nozzle-impeller radial gap distance Δr. The turbine efficiency obtains a maximum value at Δr = 5 mm. In all cases for blade thickness, the different configurations have a significant change in inlet total pressure of 4.5 MPa compared to the design pressure of 7 MPa. The maximum efficiency is reached at the ellipse ratio is 1:1 for Case-2. The results of the optimal IGV TC-2P performance optimization is essential to improve the isentropic efficiency at low inlet pressure.

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