Performance and flow fields of a supersonic axial turbine at off-design conditions

The increasing demand of energy efficiency and the utilisation of small-scale energy sources require efficient, small and versatile turbines. Supersonic turbines have a high power density and therefore small size and fewer stages than the subsonic ones. However, the performance of a supersonic turbine can decrease rapidly when operating at off-design conditions. This raises a need for the improvement of the turbine off-design performance. In this article, a supersonic axial flow turbine is studied numerically to find the causes of efficiency decrement. This article presents the most thorough study so far about the reasons that lead to the decreased off-design performance with supersonic axial flow turbines and explains the loss sources individually for the stator and the rotor. Three operating conditions are studied, and it is suggested that at the lower than design pressure ratios, the shock losses of the stator decrease while simultaneously the stator secondary losses increase. The high positive incidence at the lowest modelled pressure ratio, mass flow and rotational speed caused a significant decrease in the rotor and stage performance. This highlights the importance of incidence even in shock-driven supersonic turbine flows.