This paper considers three-dimensional (3D) computational fluid dynamics (CFD) and structural modeling of brush seals, and investigates the effects of inlet swirl on the bristle pack. The model couples aerodynamic forces generated by CFD to a structural model that includes interaction between bristles. At a critical value of inlet swirl, aerodynamic forces cause circumferential slip of the upstream bristle row. In practice, this may lead to instability of the bristle pack and is consistent with anecdotal reports of seal behavior. The critical swirl velocity was reduced when the downstream pressure level was raised, keeping the same upstream total to downstream static pressure difference. This is caused by the increased dynamic head associated with the inlet swirl. Inclusion of a front plate in the seal design does not offer the intended protection to the bristle pack in highly swirling environments. This is associated with highly swirling flow impinging on the bristle tips. Fitting of roughness elements on the upstream face of the front plate could improve stability by reducing swirl of the flow impacting on the bristles. Increasing the bristle diameter and bristle stiffness does not necessarily prevent slip at higher inlet swirl velocities, but reduces the magnitude of slip of the upstream bristles.

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In a cover-plate system, rotating receiver hole is an important component, because its structure and characteristics directly influence the aerodynamic loss and cooling performance in the preswirl system. A new type of vane shaped (VS) receiver hole was designed and presented in this paper. Numerical simulations were carried out to compare the performances among high-radius direct transfer system (model-A), low-radius cover-plate system with simple drilled (SD) receiver holes (model-B), and low-radius cover-plate system with VS receiver holes (model-C). Results indicate that for the operating conditions simulated here, temperature drop effectiveness of the high-radius preswirl system is much better compared to the low-radius system with SD receiver hole. With VS receiver hole, the aerodynamic loss in model-C is the lowest. The nondimensional static pressure at preswirl nozzle exit is only 0.93, around 10% lower than model-B. Moreover, it has a more remarkable cooling performance. The temperature drop effectiveness of model-C can be as high as 0.52, around 67.7% higher compared to model-A. The system with VS receiver hole could not only realize the advantage of low leakage flow as a low-radius system, but also could achieve higher temperature drop compared to high-radius system.

Theoretical analysis shows that the temperature drop of a pre-swirl system directly relates to the pre-swirl effectiveness of the pre-swirl nozzle. Besides, increasing the discharge coefficient and reducing the actual flow angle are the main ways to increase the pre-swirl effectiveness. A new design of pre-swirl nozzle called vane-shaped hole nozzle was introduced and analyzed in this paper. By keeping the throat area, the radial location and the pre-swirl angle (15), numerical comparisons were carried out between the vane-shaped hole nozzle and other three typical nozzles, widely used in industry: simple drilled nozzle, aerodynamic nozzle and cascade vane nozzle. In order to involve the mixing and rotating effects in the pre-swirl cavity downstream of the nozzles, the rotating pre-swirl cavity and receiver hole were included in the computational models. Numerical results show that aerodynamic nozzle could alleviate the rapid acceleration and deflection at nozzle inlet, which results in 13.7% higher discharge coefficient than the simple drilled nozzle. The discharge coefficient of the cascade vane nozzle can be as high as 0.97 due to the aerodynamical design; however, the pre-swirl effectiveness is only 0.86 because of a large deviation angle (2.4). For the vane-shaped hole nozzle, the vane height/pitch ratio could be flexibly adjusted to an appropriate value, which makes its performance better than that of the traditional ones. Higher vane height/pitch ratio and close to zero trailing edge radius lead to a small deviation angle (0.56). Consequently, the pre-swirl effectiveness of the vane-shaped hole nozzle is about 8% higher than that of the cascade vane nozzle, though the discharge coefficient is about 5% lower. Additionally, the volume of the solid block in the vaneshaped hole nozzle is larger than that in the cascade vane nozzle, which could reduce the difficulty in manufacturing but increase the total weight.