A Fundamental Investigation into Brush Seal Fluid Dynamics

密封在燃气轮机中用于控制硬件接口泄漏，对二次空气系统加压，并有助于整体转子动态稳定性。这些密封在空对空和空对油密封位置的性能较差，会增加二次空气系统所需的发动机排气量，从而导致动力输送损失，从而导致特定燃料消耗（SFC）。密封性能在经济和环境方面的重要性得到了证明，全球燃气涡轮发动机所需的排气减少1%，每年可节省近2.8亿加仑的燃料。迷宫密封最常用于燃气轮机，因为它们具有“可靠可靠的运行”和相对较低的成本。然而，随着发展极限的达到，迷宫密封仍然容易受到涡轮机器不稳定性的影响，同时在增加工作压力和转子间隙时产生相对较高的泄漏，必须研究刷式密封等替代方案。电刷密封由一个静压环组成，该静压环由密集排列的细丝刷毛组成，这些刷毛与组件的旋转方向成一定的角度，并对流体提供高阻力以保持压降。这些刷毛弯曲允许装配错位和径向运动期间的操作。衬板支持刷毛，并允许密封在燃气轮机的大压差下运行，而前板有助于刷毛在高涡流条件下的稳定性。与迷宫密封相比，这种密封提供了增强和更稳定的泄漏性能，同时也更好地适应转子偏移和占用更小的轴向空间。然而，过度刚毛尖端磨损，转子表面磨损和局部热产生的不利趋势是常见的。此外，目前对刷式密封流体动力学的理解很狭隘，这限制了它们的成功设计和应用。Cross制造有限公司是燃气轮机制造商的刷式密封供应商，该公司已经为涡轮机械研究中心构建了一个10倍规模的叶栅结构刷式密封模型。该模型在几何上和物理上与实际发动机密封件相似。8 bar的调节压缩空气供应给矩形测试部分，该测试部分由猪鬃包组成；这是由紧密捆绑的皮下注射管，完成使用电火花加工，以确保正确的鬃毛之间的行为。集成了一个窗口以允许光访问。研究人员将通过刷刷密封研究一系列流动条件下的流体动力学，通过整个刷毛包和背板的静压映射，所有的水龙头都安装在部分的中间，以消除末端效应。此外，将进行体积三分量测速（V3V）测试，以详细描述通过刷式密封形成的流动结构。V3V还可以捕捉流体通过密封时的运动轨迹。该实验将允许验证多孔介质模型，这是目前文献中存在的空白，并将提供对刷密封流体动力学的更广泛理解。因此，未来的刷式密封设计将更加明智，以提高燃气轮机二次空气系统的效率。

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.