Unsteady Flow Interactions Within the Inlet Cavity of a Turbine Rotor Tip Labyrinth Seal

Abstract

This paper focuses on the flow within the inlet cavity of a turbine rotor tip labyrinth seal of a 2 stage axial research turbine. Highly resolved, steady and unsteady 3-dimensional flow data are presented. The probes used here are a miniature 5 hole probe of 0.9mm head diameter and the novel virtual four sensor fast response aerodynamic probe (FRAP) with a head diameter of 0.84mm. The cavity flow itself is not only a loss producing area due to mixing and vortex stretching, it also adversely affects the following rotor passage through the fluid that is spilled into the main flow. The associated fluctuating mass flow has a relatively low total pressure and results in a negative incidence to the rotor tip blade profile section. The dominating kinematic flow feature in the region between cavity and main flow is a toroidal vortex, which is swirling at high circumferential velocity. It is fed by strong shear and end wall fluid from the pressure side of the stator passage. The static pressure field interaction between the moving rotor leading edges and the stator trailing edges is one driving force of the cavity flow. It forces the toroidal vortex to be stretched in space and time. A comprehensive flow model including the drivers of this toroidal vortex is proposed. This labyrinth seal configuration results in about 1.6% turbine efficiency reduction. This is the first in a series of papers focussing on turbine loss mechanisms in shrouded axial turbines. Additional measurements have been made with variations in seal clearance gap. Initial indications show that variation in the gap has a major effect on flow structures and turbine loss.

Investigation of Staggered Labyrinth Tip Shroud on Flow Mechanism of Low-Pressure Turbine

“…Rosic et al (2008a) evaluated the leakage loss in the low-aspect ratio and put forward the structure of turning vanes based on the optimization of intersection angles between leakage jet and main flow, providing evidence for the theory of Denton's (1993). Pfau et al (2005Pfau et al ( , 2007 applied the fast response aerodynamic probe for the unsteady flow field measurements in a two-stage shrouded turbine, infering the model of toroidal vortices influenced by upstream vane wakes at shroud inlet and process of mixing at shroud outlet cavities. Barmpalias et al (2012) provided the experimental and numerical results of a shrouded turbine with the decreasing inlet cavity gap, suggesting the necessity of preventing the toroidal vortices cavity inlet gap from breaking down for reducing the shroud cavity loss.…”

Controlling Mixing at the Interaction Zone Through Systematic Variation of the Inlet Rotor Cavity Volume and Length Scale

“…Flow visualization was also used by Rhode et al [16], [17] to measure the leakage resistance with respect to step shape and height. Pfau et al [18], [19], carried out a detailed study of the dominant kinematic flow feature in the cavity region, the toroidal vortex residing in the cavity and derived design recommendations to make use of the interaction flow.…”

The impact of rotor labyrinth seal leakage flow on the loss generation in an axial turbine

“…The paper of Korschunov and Döhler [7] presents the effect of an artificially produced jet (which simulates leakage flow) entering the main flow at different angles on a following linear cascade. Pfau et al [8] have examined the steady and unsteady interactions within the inlet seal cavity with a three step labyrinth seal on a rotor shroud both experimentally and numerically. Schlienger et al [9] present the effects of labyrinth seal variation on multistage axial turbine flow.…”