# Experimental and Numerical Research on End-Wall Flow Mechanism of High-Loading Tandem Stators
## 4 Loss Mechanism of Tandem Stators
### Evolution Mechanism of FB Corner Separation Within the RB Channel  
The main reason for the difference in loss between the RB and the conventional stator was that the RB was significantly influenced by the flow of the upstream FB. Specifically, the loss proportion of the mid-span region reaching more than 60% at NS was mainly caused by the development of FB corner separation in RB channel.  
The experimental flow field at the exit of the tandem stator, as shown in Figure 15, revealed that the flow within the RB channel at DE was characterized by the interaction  
Figure 19: Limit streamline of the suction surface and the hub for the tandem FB and single FB at DE and NS: (**a**) DE; (**b**) NS.  
between the FB wake and the RB mainstream. This interaction represented a typical \"wake-dominated type\" flow pattern. As the flow coefficient decreased and the front blade load increased, corner separation in the root and tip regions of the FB gradually intensified. Consequently, the flow within the RB channel evolved into a mixed type, combining wake effects with corner separation, referred to as the \"corner-dominated type\". The \"corner-dominated type\" had a greater influence on the tandem stator flow compared to the \"wake-dominated type\", and it caused higher losses. Therefore, this section primarily focused on the development and characteristics of the \"corner-dominated type\".  
Figure 20 illustrates the progression of FB root corner separation development within the RB channel at NS. The figure presented a series of cross-sections, each taken at 20% intervals along the axial direction from the RB inlet to the outlet. In this depiction, the red arrow highlighted the migration path of the secondary flow and the black arrow indicated velocity vectors. At the RB inlet, the FB root corner separation was initially concentrated and attached to the wake. As the flow progressed toward the RB outlet, a strong secondary flow developed in the root region. By the time it reached 60% of the axial distance, it became clear that the vortex formed by the secondary flow transported low-energy fluid from the corner region and the wake towards the blade surface. As this migration continued across the spanwise direction, the momentum component parallel to the blade decreased, leading to an accumulation of low-energy fluid between approximately 20% and 50% of the blade height at the RB outlet. This accumulation corresponds to the \"A\" low-energy fluid mass measured experimentally, as shown in Figure 15.  
The limit streamlines of the FB, depicted in Figure 19, demonstrated the migration of fluid from the tip of the suction surface to the mid-span region. However, there was no obvious corner accumulation at the RB exit, indicating that the development of corner separation in the tip region differed from that in the root region. In the tip region, due to the radial equilibrium established within the RB channel, a contraction effect occurred, causing  
Figure 20: Development and evolution of the root corner flow of FB in RB channel at NS.  
the mainstream fluid to migrate towards the tip. This counteracted the migration of the tip secondary flow to the mid-span region. Concurrently, within the RB channel, high-energy fluid from the mainstream supplemented the energy of the FB corner separation and wake, reducing the likelihood of low-energy fluid accumulation due to the decreased dynamic pressure head. Nevertheless, this process also increased mixing losses, which was the primary reason for the increase in tip loss for the RB.