To investigate the design strategy of highly loaded tandem cascades at both the midspan and endwall, the overall performance and flow mechanisms of four typical tandem cascades based on the optimization were analyzed from multiple perspectives numerically. The results show that the interference effects on the Front Blade (FB) and Rear Blade (RB) should not be overlooked during the design phase, and the design strategies at the midspan and endwall are completely different. At the midspan, the optimization aims to increase the interference effects and the strength of the gap jet while maintaining the same load on the FB and RB. However, the endwall optimal airfoil exhibits weakening interference effects, advancement of the gap jet location, and load transfer from the FB to RB. Through further analysis of flow characteristics, the midspan optimal airfoil is beneficial for inhibiting the low-energy fluid from interacting with the suction surface of RB under the design condition, but results in earlier occurrence of corner stall. The endwall optimal airfoil helps suppress the development of the secondary flow and delay the onset of corner stall. Furthermore, by combining the benefits of these two design approaches, additional forward sweep effects are achieved, further enhancing the performance of the tandem cascade.

To overcome the limitations posed by three-dimensional corner separation, this paper proposes a novel flow control technology known as passive End-Wall (EW) self-adaptive jet. Two single EW slotted schemes (EWS1 and EWS2), alongside a combined (COM) scheme featuring double EW slots, were investigated. The results reveal that the EW slot, driven by pressure differentials between the pressure and suction sides, can generate an adaptive jet with escalating velocity as the operational load increases. This high-speed jet effectively re-excites the local low-energy fluid, thereby mitigating the corner separation. Notably, the EWS1 slot, positioned near the blade leading edge, exhibits relatively low jet velocities at negative incidence angles, causing jet separation and exacerbating the corner separation. Besides, the EWS2 slot is close to the blade trailing edge, resulting in massive low-energy fluid accumulating and separating before the slot outlet at positive incidence angles. In contrast, the COM scheme emerges as the most effective solution for comprehensive corner separation control. It can significantly reduce the total pressure loss and improve the static pressure coefficient for the ORI blade at 0°-4° incidence angles, while causing minimal negative impact on the aerodynamic performance at negative incidence angles. Therefore, the corner stall is delayed, and the available incidence angle range is broadened from −10°–−2° to −10°–4°.This holds substantial promise for advancing the aerodynamic performance, operational stability, and load capacity of future highly loaded compressors.

The complex flow characteristics in the tip region of a tandem cascade with tip clearance have been calculated and analyzed using Delayed Detached Eddy Simulation (DDES). The coherent mechanism of the vortex structures near the blade tip was discussed, and the unsteady behaviors and features in the tip flow field were analyzed. Additionally, the interaction between the tip leakage flow and the gap jet was revealed. The results show that, compared to the datum cascade, the blade tip load of the rear blade increases while that of the front blade decreases. Unsteady fluctuations of the tandem cascade are mainly caused by the interaction between the tip leakage flow and gap jet, and by the mixing of the vortex structures, but there is no essential change in the spectrum feature of the tip leakage flow. Finally, a detailed analysis of the development of vortices in the tip region is conducted by the topological structures of the flow field. Combined with the three-dimensional vortex structures, the schematic diagram of the vortex system of the datum single-row cascade and tandem cascade is summarized.