Unsteady Simulation and Investigation of Tip Leakage Flow Based on Dissipation Function

Abstract

Further improvements of flow in tip clearance demand a better understanding of its complex structure and this would not be possible if we are not able to provide an interpretative or a more realistic presentation of its main effects, i.e., viscous dissipation and mixing. To do so and to gain further insights into the details and distribution of viscous dissipation, a commercial N-S solver has been employed for simulation and investigation of the unsteady flow field inside the tip clearance of a turbine rotor in first stage. The main objective of this paper is to introduce the direct implementation of dissipation function for viscous dissipation assessment in tip leakage flow. This idea seems to be the simplest and at the same time, the most straightforward approach to simulate and calculate the viscous dissipation caused by viscous effects. It is shown that the dissipation function can be employed as a strong and convenient tool in direct identification and assessment of regions of high viscous dissipation. It has been found that in tip leakage flow, regions of high viscous effects are located near casing rather than blade tip. Near casing, leakage flow creates a source point in pressure side and a sink point in suction side on rotor blade tip projection on the casing. It is shown that the time-averaged viscous dissipation in tip leakage flow is dissimilar for rotor blades. This result, which is caused by flow unsteadiness, is a helpful hint that can be taken by blade designers to design non-uniform rotor blades, that is, to design blades with different geometries and aerodynamic loads, both circumferentially and radially, to minimize the viscous dissipation. The casing passage vortex, the end wall boundary layers, and the wakes from the upstream stator significantly enhance the unsteadiness of the flow to the tip region of rotor blades. Results indicate that there exists a strong interaction between leakage flow and annulus-wall boundary layer.