Inertial waves in turbine rim seal flows

Abstract

Rotating fluids are well-known to be susceptible to waves. This has received much attention from the geophysics, oceanographic and atmospheric research communities. Inertial waves, which are driven by restoring forces, for example the Coriolis force, have been detected in the research fields mentioned above. This paper investigates inertial waves in turbine rim seal flows in turbomachinery. These are associated with the large-scale unsteady flow structures having distinct frequencies, unrelated to the main annulus blading, identified in many experimental and numerical studies. These unsteady flow structures have been shown in some cases to reduce sealing effectiveness and are difficult to predict with conventional steady Reynolds-averaged Navier-Stokes (RANS) approaches. Improved understanding of the underlying flow mechanisms and how these could be controlled is needed to improve the efficiency and stability of gas turbines. This study presents large-eddy simulations for three rim seal configurations-chute, axial and radial rim seals-representative of those used in gas turbines. Evidence of inertial waves is shown in the axial and chute seals, with characteristic wave frequencies limited within the threshold for inertial waves given by classic linear theory (i.e. |f * /f rel | ≤ 2), and instantaneous flow fields showing helical characteristics. The radial seal, which limits the radial fluid motion with the seal geometry, restricts the Coriolis force and suppresses the inertial wave.

Performance of a Turbine Rim Seal Subject to Rotationally-Driven and Pressure-Driven Ingestion

“…The experimental set up reproduced rotationally-driven ingestion without vanes and conditions of pressure-driven ingestion with vanes. The maximum rotor speed was 9000 rpm corresponding to a rotational Reynolds number of 3.3x10 6 with a flow coefficient of 0.45. Measurements of mean pressures in the annulus and the disc rim cavity as well as values of sealing effectiveness deduced from gas concentration data are presented.…”

“…An experimental study by Beard et al [5] confirmed the presence of rotating flow modes in a chute seal subject to rotationally-driven ingestion through detailed analysis of extensive fast response pressure measurements. Subsequently, Gao et al [6] explored the physics behind these inertial waves as the third major driver for hot gas ingestion, Figure 1. This has raised new questions regarding the effects of rotation and this study aims to investigate the interaction and combined effect of the disc pumping and mainstream pressure asymmetries.…”

“…This has raised new questions regarding the effects of rotation and this study aims to investigate the interaction and combined effect of the disc pumping and mainstream pressure asymmetries. Figure 1: Driving mechanisms of rim seal ingestion [6].…”

Sensitivity analysis on the impact of geometrical and operational variations on turbine hub cavity modes and practical methods to control them

“…Their finding confirms the reduction in the intensity of the cavity modes by increasing the purge flow rate. But (Gao et al, 2020) relates the cavity modes to the inertial waves attributed with Coriolis forces. By a computational study of three typical rim seal geometries (chute, axial, and radial), they observed large scale fluctuations rotating at an angular speed close to the core flow.…”

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

“…Their finding confirms the reduction in the intensity of the cavity modes by increasing the purge flow rate. But [13] relates the cavity to inertial waves due to Coriolis forces. By a computational study of three typical rim seal geometries (chute, axial, and radial), they observed large scale fluctuations rotating at an angular speed close to the core flow.…”