The interaction of the tip clearance vortex system of tandem blades in a high-speed linear cascade
Final Report Abstract
In this project, the unsteady flow field of the tip clearance endwall of tandem compressor blades is analyzed using a linear high-speed compressor cascade with an inlet Mach number of M a∞ = 0.6. Geometries to be investigated are a tandem configuration and a conventional compressor blade, both at different tip clearance levels. Additionally, so-called differential tip clearances are analyzed for the tandem blade. In these cases, the tip clearance level at tandem front and rear blade are dissimilar, allowing an insight into the question which tip clearance vortex is dominant. The results show that by application of statistical means of the pressure data the development of tip clearance vortex break down (VBD) can be divided into two stages. In the first stage, the area of negative skewness expands continuously until the pressure side of the neighbouring blade is reached. As the incidence increases further, the negative skewness moves upstream until suddenly a distinct area of intense negative skewness is formed in the front part of the passage. These two stages can be identified for both conventional and tandem blades. The investigations of differential gaps show that the front blade tip clearance is decisive for the development of the flow topology within the passage and the entry into the first stage of VBD. However, the clearance on the rear blade influences the starting point of the second stage. The results of the wall shear stress probe show reduced losses for a configuration with small clearance on the front blade and a larger clearance on the rear blade for flows with an intact tip clearance vortex. The effect is superimposed by the tip clearance VBD. Thus, additionally to the geometrical circumstances, the stage of tip clearance VBD has to be taken into account.
