A Fundamental Investigation into Brush Seal Fluid Dynamics

密封件用于燃气涡轮机中，以控制硬件接口泄漏，对二次空气系统加压，并有助于整体转子动力学稳定性。这些密封件在空气-空气和空气-油密封位置处的不良性能增加了二次空气系统所需的发动机放气量，这导致功率输送损失，从而导致比燃料消耗（SFC）损失。密封性能在经济和环境方面的重要性通过全球燃气涡轮机发动机所需的放气减少1%来证明，这意味着每年可节省近2.8亿加仑的燃料。由于其“经过验证的可靠性和稳健的运行”以及相对较低的成本，密封件最常用于燃气涡轮机。然而，随着发展的限制，迷宫式密封仍然容易受到发动机不稳定性的影响，同时在增加操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品。刷式密封由密集排列的细金属丝刷毛构成的静态环组成，这些细金属丝刷毛与部件的旋转方向成一定角度，并对流体提供高阻力以保持压降。这些刷毛弯曲以允许在操作期间的组装不对准和径向移动。背板支撑刷毛并允许密封件在燃气涡轮机中经历的大压差下操作，而前板在高旋流条件下辅助刷毛稳定性。与迷宫式密封相比，该密封提供了增强的和更稳定的泄漏性能，同时还更好地适应转子偏移并占据更小的轴向空间。然而，过度刷毛尖端磨损、转子表面磨损和局部发热的不利趋势是常见的。此外，目前存在对刷式密封流体动力学的狭隘理解，这限制了它们的成功设计和应用。Cross Manufacturing Ltd是燃气涡轮机制造商的刷式密封供应商，已为涡轮机研究中心构建了一个10倍比例的叶栅结构刷式密封模型。该模型在几何和物理上类似于运行中的发动机密封件。将8 bar的调节压缩空气供应到由刷毛组组成的矩形测试部分;这是由紧密捆绑的皮下注射管组成，使用放电加工完成，以确保正确的刷毛间行为。集成了一个窗口以允许光学访问。将通过刷式密封对一系列流动条件进行流体动力学研究，方法是通过绘制整个刷丝束和沿着背板的静压图，其中所有分接头都安装在截面中间，以消除端部效应。此外，还将进行体积三分量测速（V3 V）试验，以详细说明通过刷式密封形成的流动结构。V3 V还将捕获通过密封传播的流体轨迹。该实验将允许多孔介质模型的验证，目前文献中存在空缺，并将提供对刷式密封流体动力学的更全面的理解。因此，未来的刷式密封设计将更加明智，以提高燃气涡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.