Computational extrapolation of turbine sealing effectiveness from test rig to engine conditions

Abstract

The commercial computational fluid dynamics code ANSYS CFX 12.1 has been employed to carry out unsteady Reynolds-averaged Navier-Stokes computations to investigate the fluid mechanics of two different rim-seal geometries in a three-dimensional model of a turbine stage. The mainstream annulus, seal and wheel-space geometries are based on an experimental test rig used at the University of Bath. The calculated peak-to-trough pressure difference in the annulus, which is the main driving mechanism for ingestion, is in good agreement with experimental measurements. There is also a good agreement between the computed and measured swirl ratios in the wheel-space. Computed values of concentration-based sealing effectiveness are obtained over a range of sealing flow rates for both an axial clearance and a radial clearance rim seal. A good agreement with gas concentration measurements is found for the axial clearance seal over a certain range of sealing flow rates. Some under-prediction of the amount of ingestion for the radial clearance seal is obtained. The computed mainstream pressure coefficient increases progressively with mainstream Mach number in moving from quasi-incompressible experimental rig conditions to the compressible flow conditions encountered in engines. It is shown that the minimum sealing flow rate required to prevent ingestion increases as mainstream Mach number increases. A scaling method is proposed to allow sealing flow rates to prevent ingestion obtained from low Mach number experiments to be extrapolated to engine-representative conditions.

Effects of Purge Jet Momentum on Sealing Effectiveness

“…In addition to rotational Reynolds number [3][4][5], the Mach number at the upstream airfoil exit has been shown to be important in accurately predicting ingestion. Using simplified but relevant axial and radial overlap rim seals, Teuber et al [13] showed that the minimum flow rate required to fully seal the rim cavity increased with airfoil exit Mach number. Experiments were only performed up to Ma ¼ 0.44, but CFD solutions were generated up to Ma ¼ 0.86 showing an increase in the vane exit nondimensional pressure difference, which has been shown to significantly affect ingestion [4,14].…”

Effects of Purge Flow Configuration on Sealing Effectiveness in a Rotor–Stator Cavity

“…Many of these studies, however, have been performed at simplified conditions with simplified geometries. Teuber et al [14] showed that higher external Mach numbers resulted in more ingestion. In regard to the topic of hot gas ingestion, Green and Turner [15] stated that "oversimplified experimental rigs operated far from engine conditions may often only serve to confuse the issue….…”

Use of Pressure Measurements to Determine Effectiveness of Turbine Rim Seals

“…The Mach number effect was considered by Teuber et al [23] who showed, theoretically and computationally for the Bath rig, that the magnitude of ACp increases as the Mach number increases. They showed that, by correcting ACp and assuming that the discharge coefficients are unaffected by Mach number, the sealing effectiveness, sc> determined by concentration measure ments in an experimental rig at one Mach number, could be used to compute the effectiveness in an engine at another Mach number.…”