Numerical Analysis of Tip Clearance Effects in an Unshrouded Impeller for Liquid Rocket Engines

For rocket engine applications an unshrouded impeller is a relatively new technology compared to the traditional shrouded impeller. An unshrouded impeller has advantages in blade optimisation, easier manufacturing, and — moreover — a potential for weight reduction of the turbopump and consequentely of the entire rocket engine. However, it also results in tip clearance during operation which has detrimental effects on the performance. Therefore, a better understanding of the impact of tip clearance on the overall flow would help to develop and adapt new design concepts. In this study a comparison between shrouded and unshrouded impellers has been numerically undertaken. The shrouded impeller L17, designed within the framework of the research project KonRAT at the Technical University of Munich, is chosen as reference geometry for this study. The unshrouded impellers are derived from the L17 design by simply removing the shroud. Three configurations of the unshrouded L17 impeller have been numerically investigated by varying the tip clearance: geometry G4 with a constant tip clearance of 4% and geometry G7 with 7% of the blade passage height at impeller outlet. The third configuration is a fictitious unshrouded impeller without any tip clearance (geometry G0). The intention of the comparison between the shrouded impeller L17 and the geometry G0 is to isolate the effects on the flow caused by the relative movement between casing and blade to allow a quantification of the tip vortex losses. The obtained performance data as well as the mechanisms of the tip vortex inside the blade passage are discussed in detail. The results show that the effects of the shear forces due to the relative movement and the effects of the tip vortex do not undergo a linear coupling and their interaction inhibits a precise quantification of the tip vortex losses. The analysis also shows that the crucial factor for the deterioration of the performance is the change of the inflow due to the influence of the tip clearance on the flow.