Investigation of the Dynamic Characteristics of Brush Seals Using a Transient Fluid-Structure Interaction Method

Abstract Brush seals operate within harsh environments within rotating machinery. They are typically exposed to high pressures and temperatures, rotor–stator relative movements, and high shaft rotational speeds leading to highly swirling inlet flow. The compliant nature of the bristle elements makes them susceptible to flow-induced vibrations, particularly associated with high levels of inlet swirl velocity. This can lead to fatigue fracture of the bristles and may also impact seal leakage. This paper establishes a two-way fluid-structure interaction (FSI) method that combines a three-dimensional transient flow model of brush seals with an analytical mechanical model of bristle deflections. This method can quickly obtain the flow field characteristics and bristle deflection of brush seals with satisfactory accuracy. Based on this method, bristle deflections under typical aerodynamic loads in operation, up to 0.4 MPa differential pressure and 200 m/s inlet swirl velocity, were investigated to understand bristle response under steady-state and transient conditions. In the absence of inlet swirl, simulated bristle axial deflections and leakage were validated against experimental measurements using an in-house seal test rig. The results for axial deflection without inlet swirl show that the bristles undergo oscillation whose amplitude quickly diminishes over time, as expected, reaching an equilibrium for the compressed bristle pack. An increase in differential pressure results in increased bristle tip oscillation displacements and longer settling times, however in all cases the settling time is of the order of milliseconds. The bristle pack center region exhibits a stratification, under conditions of fluctuating pressure and due to the inertia of the bristles themselves. This may have implications for pack stability and sealing performance, particularly when an inlet swirl is also imposed. This study shows, for the first time, that under the influence of inlet swirl, and particularly at inlet swirl velocities of 200 m/s where bristle slip is known to occur for similar pack geometries, the upstream row of bristles exhibits a circumferential displacement oscillation due to interaction between the swirling flow and bristle structure. This occurs with a characteristic frequency of 441 Hz which is of the order of the bristle natural frequency. Results indicate that a consistent oscillation amplitude is established over the simulation timescales, suggesting a typical forced vibration response. The amplitude of the bristle axial oscillation in the first row is observed to decrease when inlet swirl is applied, but the axial oscillation is sustained, occurring at a similar frequency to the circumferential oscillations but with a slight phase shift. The results also indicate that there is a substantial leakage penalty which is associated with the oscillation of the bristle upstream row, to more than double that of the stable pack. Under these conditions, bristle fretting wear and fatigue failure may be a concern, which could have significant adverse consequences for seal performance and life. Initial results show that bristle circumferential displacements do not propagate into the downstream rows. However, in the case of fatigue failure and detachment of the upstream bristle row, a progressive failure mechanism might be possible.