A Fundamental Investigation into Brush Seal Fluid Dynamics

Seals are used in gas turbines to control hardware interface leakages, pressurise secondary air systems and to contribute to overall rotordynamic stability. Poor performance of these seals in both air-to-air and air-to-oil sealing locations increases the quantity of engine bleed required for the secondary air system which results in a loss of power delivery and hence specific fuel consumption (SFC). The importance of seal performance both financially and environmentally is demonstrated by a 1% reduction in bleed required in gas turbine engines worldwide translating to a fuel saving of nearly 280 million gallons annually.

Labyrinth seals are most commonly used in gas turbines due to their "proven reliability with robust operation" combined with their relatively low cost. However, with development limits reached and labyrinth seals remaining vulnerable to turbomachine instabilities while engendering comparatively high leakage at increasing operational pressures and rotor clearances, alternatives such as brush seals must be studied.

A brush seal consists of a static ring of densely packed fine wire bristles that are angled in the direction of rotation of the component and which provide high resistance to the flow to maintain a pressure drop. These bristles flex to allow for assembly misalignments and radial movements during operation. A backing plate supports the bristles and allows the seal to operate under the large pressure differentials experienced in gas turbines, while a front plate aides bristle stability in high swirl conditions. This seal provides an enhanced and more stable leakage performance compared to labyrinth seals whilst also better accommodating rotor excursions and occupying a smaller axial space. However, adverse tendencies of excessive bristle tip wear, rotor surface wear and localised heat generation are common. Furthermore, a narrow understanding of brush seal fluid dynamics currently exists, limiting their successful design and application.

Cross Manufacturing Ltd, a brush seal supplier to gas turbine manufacturers, have constructed a brush seal model at 10 times scale in a cascade configuration for the Turbomachinery Research Centre. This model is geometrically and physically similar to an operational engine seal. Regulated compressed air at 8 bar is supplied to the rectangular test section which consists of the bristle pack; this is made up of tightly bundled hypodermic tubing which is finished using electrical discharge machining to ensure correct inter-bristle behaviour. A window is integrated to allow for optical access.

The fluid dynamics will be studied through the brush seal for an array of flow conditions through static pressure mapping throughout the bristle pack and along the back plate, where all taps are installed in the middle of the section to eliminate end effects. Additionally, volumetric 3-component velocimetry (V3V) testing will be performed to detail the flow structures forming through the brush seal. V3V will also capture the trajectory of the flow as it propagates through the seal. This experimentation will allow for the validation of porous medium models, for which a vacancy currently exists in literature and will provide a greater general understanding of brush seal fluid dynamics. Therefore, future brush seal design will be more informed allowing for greater efficiency of gas turbine secondary air systems.