A fundamental investigation into brush seal fluid dynamics

Brush seals are one example of a type of shaft-seal used within gas turbine engines. They maintain a pressure drop by virtue of the high resistance to flow provided by a static ring of densely packed, fine wire bristles (usually metallic) that are angled in the direction of rotation of the component. Very low leakage is possible with this type of seal making it a highly attractive option for gas turbine designers.

Although the benefits of using brush seals in gas turbines have long been established, their utilisation in aerospace applications especially has been limited due to the rapid deterioration of in-service performance. Fluid-dynamic mechanisms for this rapid wear have been identified such as bristle blow-down and pressure stiffening. For brush seals to be implemented more readily throughout gas turbines and to allow the potential benefits to be exploited, a greater understanding of these effects is required.

Brush seals are typically modelled theoretically using a porous medium approach where the influence of the seal on the flow is defined by a set of resistance coefficients. These coefficients have to be calibrated using experimental data to enable correct extrapolation to engine conditions. There is currently a distinct lack of published experimental values available in the literature.

This proposal aims to improve the fundamental understanding of brush seals and provide validation data for porous media models by making measurements in large-scale, simplified experiments. The programme will be conducted in close collaboration with Cross Manufacturing Ltd., a world-leading supplier of brush seals to the gas turbine industry. The insight obtained will provide a database which will be used to inform the design of future brush seal configurations, not only by Cross but also the wider gas turbine industry.

The research programme will lead to more detailed design tools that will enable Cross Manufacturing to produce brush seals for locations in the engine where they are currently not able to offer. This will be due to better design processes and access to valuable experimental data. More broadly, the impact of this research will contribute significantly to the company's current level of technology and competitiveness in the gas turbine industry. This work will also lead to longer seal life and hence drop off in sealing efficiency over time will be reduced. For example just one additional aero engine sealing location could increase Cross's revenue by 5% (£0.6M) at their South Site brush seal manufacturing facility in Devizes. This work would provide employment for five staff, which if the location was an additional location this would mean employing five additional staff, or if older locations/ engine sales decline then this would replace these locations and ensure five staff remain in employment for possibly up to 20 years.

There are benefits from an improved understanding of brush seal fluid dynamics to the wider gas turbine industry. The direct impact for engine Original Equipment Manufacturers (OEMs) will be to reduce specific fuel consumption (sfc) and/or increase thrust through the use of brush seals in new applications. The benefits of brush seals over existing sealing technologies have long been established. Ferguson [1] showed a single brush seal implemented in a critical position such as pre-swirl chamber sealing of the high pressure turbine cooling system can yield 0.6% improvement in sfc or 1.5% thrust improvement. The biggest benefit has been shown to come from replacing labyrinth seals at locations in gas turbine where very high pressures drop directly to the ambient pressure - typically main engine and thrust balance seals. A saving of 1 to 2% of the engine flow would directly reduce sfc by 0.7 to 1.4% and operating costs by 0.35 to 0.7% [2]. Hendricks et al. [3] conducted a number of tests with dual element brush seals and a forward facing labyrinth in a GE T-700 engine. They estimated a reduction in sfc of 3% and 5% for both, high and low pressure compressor discharge applications. Steinetz et al. [4] showed for a Rolls-Royce AE3007, a modern 40kN thrust regional engine, a reduction in sfc of 1.9% and and increase in thrust of 4.93% could be achieved by implementing brush seals at two turbine interstage locations.

There are also additional financial benefits to gas turbine suppliers from more reliable shaft-seal designs. As a result of brush seal failure engines occasionally experience in-service issues such as oil leakage or shaft overheating. In the event of an oil-leak either in service or during a pass off test, the OEM is responsible for the cost of the engine strip. The cost of engine rejection, strip and rebuild required to replace the brush seal is in the region of 50 to 100 times the cost of the component.

[1] Ferguson, J. G., 1988, "Brushes as High Performance Gas Turbine Seals", ASME, 88-GT-182.
[2] Research & Technology, NASA 1999, p.122.
[3] Hendricks, R. C., Griffin, T. A., Kline, T. R., Csavina, K.R., Pancholi, A. and Sood, D., 1994, "Relative Performance Comparison between Baseline Labyrinth and Dual-brush Compressor Discharge Seals in a T-700 Engine Test," ASME Paper 94-GT-266.
[4] Steinetz, B.M., Hendricks, R.C. Munson, J., 1998, "Advanced Seal Technology Role in Meeting Next Generation Turbine Engine Goals," NASA/TM-1998-206961.