# Experimental and Numerical Research on End-Wall Flow Mechanism of High-Loading Tandem Stators
## 1 Introduction  
Producing lighter weight and higher efficiency have always been the goals of aero-engine compressor design, and increasing compressor stage load is the key method to achieve these goals. High-loading compressor design means that the blades do more work and the flow turning angle is larger. Conventional compressor blades often face the problems of narrow incidence and insufficient stall margin. As an advanced aerodynamic configuration, tandem blades divide blades into front blades (FBs) and rear blades (RBs), which makes the boundary layer redevelop in the RB and could improve the pressure rise limit of compressor blades. With the increasing demand of compressor load, using tandem blades to achieve high-loading design has been widely concerned. The research on tandem blades could be traced back to the 1970s [1]. The flow mechanism of two-dimensional tandem cascades, three-dimensional tandem blades, and the comparison between tandem blades and conventional blades were the main focus. In terms of two-dimensional tandem cascades, [PERSON] [2], [PERSON] et al. [3], [PERSON] et al. [4], [PERSON] and [PERSON] et al. [5, 6, 7, 8] compared the performance of tandem blades with that of conventional blades by theoretical and experimental methods, which basically showed the lower loss advantage of tandem blades at off-design incidence. In recent years, [PERSON] [9], [PERSON] [10],and [PERSON] [11] used numerical simulation and experiment to compare the performance of tandem blades with conventional blades under subsonic and transonic inflows. The research results confirmed the performance superiority of tandem blades at large positive incidence and pointed out that a large gap between FB and RB should be maintained to obtain better performance at negative incidence. [PERSON], [PERSON] et al. [12; 13] and [PERSON] et al. [14] used numerical methods to study the effects of camber ratio of FB and RB, solidity ratio of FB and RB, axial relative position, and circumferential relative position on the performance of tandem blades, and the results showed that tandem blades were able to broaden the design space and reduce the loss by 20% when diffusion factor (DF) was 0.62. [PERSON] et al. [15] and [PERSON] et al. [16] used particle image velocimetry (PIV) to visualize the velocity field of tandem cascades. PIV results showed that the acceleration of airflow in the gap stabilized the boundary layer at the trailing edge of the FB, reduced the risk of boundary layer separation, and improved the pressure rise. [PERSON] et al. [17] analyzed the matching characteristics of transonic tandem stator blades, and the research showed that the mixing effect produced by the wake of the FB and the mainstream of the RB was beneficial to reduce the loss. The study results of two-dimensional tandem cascades showed that tandem configurations were able to broaden the range of incidence and improve the loading limit of cascades. However, in the three-dimensional tandem blades, the interaction between the FB and RB was affected by the end-wall, which made the three-dimensional flow more complex. In recent years, researchers have made a preliminary study on the complex three-dimensional flow of tandem stators. [PERSON] et al. [18] studied the flow characteristics of tandem stators at the exit stage by numerical simulation. The results showed that the matching of the FB and RB in tandem stators was an important factor affecting the three-dimensional flow performance of tandem blades. [PERSON] et al. [10] measured the three-dimensional flow of tandem cascade in the cascade wind tunnel designed by [PERSON] et al. [9]. The results showed that the topology of the FB secondary flow of the tandem stator changed greatly with the increase in incidence, while the topology of the RB remained basically unchanged. Compared with the conventional blade of the same camber, the tandem blade had better control of the corner separation, but corner separation also developed sharply to the same level near the stall point. [PERSON], [PERSON], and [PERSON] et al.