# Experimental and Numerical Research on End-Wall Flow Mechanism of High-Loading Tandem Stators
## 4 Loss Mechanism of Tandem Stators
### Development and Evolution Mechanism of RB Corner Separation  
Another feature in the variation law of RB loss was that the total pressure loss and the proportion of loss in the root region gradually decreased. This trend was caused by the development and evolution of RB corner separation. In the tandem stator, the RB was influenced by the wake and corner region of the FB and exhibited little sensitivity to changes in incidence. Consequently, the flow mechanism of the RB differed significantly from that of the FB.  
The most significant flow characteristic in the corner region was that corner separation occurred at higher mass flow rates. In Figure 21, the red box highlights the distribution of hub limit streamlines of the RB inlet at NC. It could be seen that the limit streamlines sharply turn toward the suction surface of the RB at the FB outlet, creating a pronounced overturning effect in the flow pattern. In fact, the configuration of tandem blades determined that it was easy for the fluid in the boundary layer at the RB end-wall to migrate to the suction surface of the RB.  
Figure 22 further illustrates this behavior by presenting the pressure distributions across the blade surfaces for both tandem and conventional blades. Different from the continuous pressure variation observed in conventional blades, the pressure redeveloped in the RB of tandem blades, and the peak pressure on the suction surface was also in the front half of the blades (indicated by the green dotted line). The substantial circumferential pressure difference, as depicted in the figures, was a significant factor affecting the flow dynamics within the RB channel. When fluid first entered the RB channel, the considerable local circumferential pressure difference of the RB effectively forced the fluid near the hub to migrate towards the suction surface, compounded by the inflow of low dynamic pressure head. This particular flow migration pattern made it more susceptible for corner separation to occur on the RB, especially at higher mass flow rates.  
Figure 21: Limit streamlines of the suction surface and the hub for RB at NC.  
As mass flow rates decreased from higher to lower values, the wake and corner separation of the FB gradually intensified. This intensification led to a gradual increase in blockage within the expansion channel of the RB. Consequently, both the circumferential pressure difference across the RB and its pressure rise diminished over this range. The limit streamline on the RB suction surface, as shown in Figure 19, indicated that FB corner separation developed rapidly at NS, leading to significant blockage of the RB. As a result, the load on the RB decreased, which in turn reduced the severity of RB corner separation.  
Combining the development of FB corner separation within the RB channel and the evolution of RB corner separation, the change mechanism of loss and loss proportion for the RB, as indicated by Figure 15, could be revealed: the corner separation of the FB developed rapidly with the increase in load, and the corner separation of the FB entered the RB channel. Subsequently, it migrated to the mid-span region of the RB under the influence of secondary flow dynamics, which made the loss proportion of the RB increase sharply. On the other hand, the corner separation of the RB did not deteriorate with the decrease in flow coefficient; rather, it remained relatively stable. This led to a corresponding decrease in the loss proportion attributed to the end-wall region of the RB.