Conventional vane designs come with the disadvantage of flow detachment for high reflection. For axial machines, often tandem vanes are prevalent which combines two vanes for redirection of the flow and thus effectively prevents from detachment. However, their application for radial machines has hardly been investigated so far. This study investigates the influence of tandem vanes on the performance in terms of head and efficiency of a centrifugal pump. First, different tandem designs were numerically investigated based on Computational Fluid Dynamics (CFD). For a selected tandem variant, the numerical results were validated by experimental investigations. The preceding numerical investigations were done stationary by using a single passage model to compare the performance of different tandem impellers with a conventional impeller (unsplit vane). Therefore, a method was developed to design tandem vanes from given undivided vanes. In this way, the influence of the offset in circumferential direction and the angle of attack of the rear blade profile were systematically investigated. Transient CFD simulations have been one for a complete single-stage volute pump. In the experiments the performance curves for both the conventional and tandem impeller were determined. The results of this study show that tandem blades can increase the pressure rise of a centrifugal pump while maintaining almost the same efficiency if a suitable combination of blade angle and offset in circumferential direction of the rear blade has been selected. Furthermore, the CFD results show that the detachment can be prevented inside the impeller passage.
Radial fluid machines with conventional impeller often lead to flow separation in the blade passage in case of high flow deflection. Tandem blades can be used to counteract this problem. Due to their properties, a new thin boundary layer forms at the rear blade profile, which benefits the deflection of the flow. The present investigation focuses on a comparison between numerical and experimental streamline patterns on the impellers blades suction side. For this purpose, a conventional impeller is compared with a tandem impeller. Furthermore, a method for creating oil painting patterns in a rotating system is presented. The numerical investigations were performed using a full model (including inlet section, impeller, volute, outlet section and impeller side gaps). A total of four operating points were examined in the range from high partial load to overload. The same operating points were run in a pump loop during the experiment. The results of the present work show that the numerically and experimentally generated streamline patterns agree well with exceptions. It could be shown that a flow separation of the blades suction side can be prevented using tandem blades.
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