Contribution to tip leakage loss modeling in radial turbines based on 3D flow analysis and 1D characterization

Abstract

The characterization of tip leakage flow plays an important role for one-dimensional loss modeling and design in radial turbine research. Tip leakage losses can be expressed as function of fluid momentum and mass flow passing through the tip gap. Friction-driven flow and contrariwise oriented pressure gradient-driven flow are highly coupled. However, these numbers are mostly unknown and dependent on tip gap geometry and turbine running condition. Based on a commonly used definition of a non-dimensional tip leakage momentum ratio, a novel correlation has been derived. This allows a consistent characterization for variable tip gap sizes over a wide range of operating conditions. The correlation has been validated by means of CFD data with high variety in reduced speed tip gap geometry and expansion ratios. Results of the novel number show significant improvements of quantitative and qualitative results over a wide range of running conditions in comparison to existing correlations. Furthermore, correlations for tip leakage velocities, that can easily be used in one-dimensional models, have been derived. Finally, it has been demonstrated, that the influence of inlet flow momentum on the tip leakage flow can be analyzed with presented correlations.