Investigation on the Flow and Loss Mechanism of Supersonic Through-Flow Variable-Pitch Tandem Cascade Under Extremely Wide-Speed Range Conditions

Abstract

Abstract. The supersonic through-flow variable-pitch tandem cascade has a significant potential for widening the acceptable inflow range of fans, but its internal flow mechanisms are not yet fully understood. This study employed numerical simulations to investigate the flow-field characteristics and loss mechanisms of the variable-pitch tandem cascade under transonic, through flow, and high-speed windmilling modes, with inflow Mach numbers ranging from 1.5 to 4. The results indicate that the strong shock within the passage induced separation on the suction side of the rear blade and produced a wider wake, which are the primary causes of losses in the transonic mode. Additionally, this mode exhibits a dual “unique incidence angle” under low pressure ratio conditions. A “re-laminarization” phenomenon occurs on the suction side of the rear blade in the supersonic through-flow mode, which is conducive to reducing boundary layer losses caused by the first shock. As the Mach number increases, the second shock moves downstream. This reduces the accumulation of downstream boundary layer losses, thereby decreasing the proportion of boundary layer losses on the suction side of the rear blade. In high-speed windmilling mode, the proportion of shock losses increases sharply as the Mach number rises. The influence of the shock at the leading edge of the front blade on the airflow angle at the leading edge of the rear blade, as well as the interaction between the fishtail shock at the trailing edge of the front blade and the oblique shock at the leading edge of the rear blade with the boundary layer, determine the magnitude of losses in the boundary layer on the pressure side of the rear blade. Additionally, at high Mach numbers, the shocks are more likely to interact with the separated flow region at the trailing edge of the rear blade, further exacerbating trailing-edge separation and increasing wake losses.