Numerical Investigation on Radial Turbines Aerodynamics Aimed at the Definition of Design Rules for Industrial Applications

The design of turbomachines is a long iterative process, which involves several teams and disciplines. Hence, a lot of efforts are put on making it as smooth and accurate as possible in order to reduce the time-to-market. On this premise, this paper introduces a procedure aimed at improving the aerodynamic design process of radial-inflow turbines for industrial applications. The procedure is based on a Baker Hughes proprietary tool for the generation of the Turbo-Expander stage geometry. An extensive CFD campaign is used both to investigate the design space of the stage and analyze the performance while varying the geometry of the rotor. Three main geometrical parameters of the wheel are identified as the ones having the biggest impact on the stage performance: the stiffness (wheel hub outlet diameter-to-outer diameter ratio, D_2h/D₁), the inlet metal angle and the outlet metal angle. An automatic procedure is developed in order to generate and calculate all the different stages, with the goal of finding the values for stiffness, wheel inlet and outlet metal angles which maximize the total-to-static stage efficiency in a wide range of flow coefficients. More than 700 different geometries are ultimately generated and simulated with the TRAF code, a 3D flow solver developed at the University of Florence. The results of all these simulations describe response surfaces, which not only allow to define the optimum geometry, but also to understand the influence of the main input parameters on the stage performance. Finally, the procedure was tested on selected case studies, confirming the validity of the design rules in a wide range of operating conditions.