A Fundamental Investigation into Brush Seal Fluid Mechanics

Abstract

Gas turbines are an energy-dense solution for aircraft propulsion and power generation, with a wide range of power outputs and fuel versatility that offer advantages over other technologies in the aviation and energy sectors. The secondary air system in a gas turbine diverts air from the main gas path to cool high-temperature turbine hardware and seal bearing and disc cavities. Sealing is crucial in maintaining the efficiency of the system by regulating leakage flows between the interface clearances that exist between rotating and stationary components. Seals therefore have a significant impact on the overall performance of gas turbines. Improving seal technology is hence critical to achieving the targets set by the International Air Transport Association Fly Net Zero by 2050 and the International Energy Association Net Zero by 2050. Sufficient reduction of global greenhouse gas emissions to limit temperature increases will require enhanced efficiency of turbine technologies. Improved effectiveness of the secondary air system, achieved in part through enhanced sealing performance, facilitates higher turbine temperatures and reduced specific fuel consumption.

Brush seals consist of a static ring of densely packed, flexible, fine wire bristles that provide resistance to the flow. A superior performance is achieved compared to other sealing solutions commonplace in turbomachinery. However, the application of brush seals is principally limited by issues surrounding high seal stiffness and an incomplete understanding of their fluid dynamic behaviour. Experimental and theoretical analyses of large- and aero-engine scaled brush seals were conducted to study the inter-bristle pressure field and bristle deflection behaviour to a high degree of detail and fidelity. This investigation was carried out in co-operation with Cross Manufacturing (1938) Ltd.

In the first phase of the study, a large brush seal of ten-times scale was constructed and commissioned to examine the leakage characteristics in direct relation to the pressure field within and surrounding the bristle pack. Multiple seal clearance conditions were considered for a conventional brush seal design. The governing parameter controlling leakage behaviour transitioned from pressure ratio for a large clearance, to pressure load for a line-on-line configuration. In all cases, leakage flow converged to an asymptotic value once maximum levels of bristle blow-down and pack compaction were attained. For both clearance configurations, this occurred at a pressure ratio corresponding to that at which axial distributions of pressure converged; equivalent behaviour was noted for the line-on-line configuration with pressure drop. Comparatively small changes were experienced in leakage behaviour and to the inter-bristle pressure field with increasing pressure drop for the line-on-line brush seal. This indicated that brush seal performance is more influenced by changes in bristle blow-down than bristle pack compaction.

The fluid dynamic behaviour of brush seals with back plate designs that incorporate pressure relieving pockets, which are employed to overcome issues such as hysteresis, was studied in comparison to conventional brush seals. This investigation followed a concomitant methodology that exploited the benefits of both the large- and aero-engine scale testing facilities. Leakage data were fitted using a porous medium model found in the literature to quantify viscous and inertial resistance coefficients. Shaft rotation was shown to cause a reduction in seal leakage and an increase in static pressure on the back plate surface. The pressure relieving back plates also resulted in increased static pressures at this location, causing a reduction in flow resistance that increased leakage through the porous bristle pack. Interrogation of the large-scale inter-bristle pressure field for the two back plate designs revealed the distributions of axial pressure diverged towards the rear of the bristle pack. The detail gathered using the large-scale facility was shown to be representative, hence the insight is generically applicable to brush seals.

The final phase of the study considered the deflection behaviour of the bristles. When subjected to a pressure load, the bristles – which are fitted at a lay angle to the radial plane – compact to resist the oncoming flow and deflect towards the rotor in a process known as blow-down. Digital Image Correlation was employed to track individual bristle tips in three spatial axes throughout the large-scale brush seal test facility. This is the first time direct measurements of blow-down throughout the bristle pack have been presented, providing a unique insight into the mechanical behaviour of brush seals.

Increased magnitudes of blow-down and axial bristle deflection were demonstrated in upstream bristle rows and at larger clearances. Analysis of these results in conjunction with the interrogation of the inter-bristle pressure field proved that blow-down is more prevalent for pressure relieving brush seals in comparison to conventional configurations. The reduction in the through-flow clearance area resulted in a significant enhancement in sealing performance for a clearance seal, highlighting a key advantage of the pressure relieving back plate design.

Date of Award	13 Nov 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	James Scobie (Supervisor), Andrew Cookson (Supervisor) & Carl Sangan (Supervisor)