Experimental and Computational Investigation of Flow Instabilities in Turbine Rim Seals

Abstract

In high-pressure turbines, cool air is purged through rim seals at the periphery of wheel-spaces between the stator and rotor disks. The purge suppresses the ingress of hot gas from the annulus but superfluous use is inefficient. In this paper, the interaction between the ingress, purge, and mainstream flow is studied through comparisons of newly acquired experimental results alongside unsteady numerical simulations based on the DLR TRACE solver. New experimental measurements were taken from a one-and-a-half stage axial-turbine rig operating with engine-representative blade and vane geometries, and overlapping rim seals. Radial traverses using a miniature CO2 concentration probe quantified the penetration of ingress into the rim seal and the outer portion of the wheel-space. Unsteady pressure measurements from circumferentially positioned transducers on the stator disk identified distinct frequencies in the wheel-space, and the computations reveal these are associated with large-scale flow structures near the outer periphery rotating at just less than the disk speed. It is hypothesized that the physical origin of such phenomenon is driven by Kelvin–Helmholtz instabilities caused by the tangential shear between the annulus and egress flows, as also postulated by previous authors. The presence and intensity of these rotating structures are strongly dependent on the purge flow rate. While there is general qualitative agreement between experiment and computation, it is speculated that the underprediction by the computations of the measured levels of ingress is caused by deficiencies in the turbulence modeling.

Inertial waves in turbine rim seal flows

“…[11], report large-scale unsteady flow modes with characteristic frequencies unrelated to those of the rotating blade in the main annulus. In the presence of the intrinsic unsteady flow modes empirical correlations and Reynolds-averaged Navier-Stokes (RANS) models give solutions for sealing effectiveness that depart from the experimental measurements [14,15].…”

“…The first published experimental observation of these large-scale unsteady flow modes was reported by Cao et al [16] the T-C instability include Boudet et al [17], O'Mahoney [18] and Gao et al [19], and those proposing the K-H instability include Rabs et al [20], Chilla et al [21], Savov et al [22] and Horwood et al [15]. Another possible source of unsteadiness, suggested by Berg et al [23], are the Helmholtz and shallow cavity modes.…”

“…Increasing the difference in tangential velocity at the rim seal leads to greater unsteadiness. Horwood et al [15] studied the unsteady flow modes in a rim seal geometry through experimental and numerical results. In agreement with Rabs et al [20] and Chilla et al [21], they showed the flow unsteadiness amplifying with the increase of the velocity deficit in the rim seal.…”

Sensitivity Analysis on the Impact of Geometrical and Operational Variations on Turbine Hub Cavity Modes and Practical Methods to Control them

“…More recently, [12] investigated the flow instabilities at the turbine rim seal using both experimental and computational methods. Their computations showed 16 modes with a rotational speed of 85% of the rotor while the experiments showed 23<N<26 modes rotating at 95% of the rotor speed.…”

Unsteady Flow Phenomena in Turbine Shroud Cavities

“…Further examination suggests that the number of nodes around the annulus varies over time. Horwood et al (2018) conducted a combined experimental and computational investigation. The study also identified structures, which rotate at an angular velocity of ω s/Ω = 0.81 .…”