Abstract
Reducing losses in the secondary air system of gas and steam turbines can significantly increase the efficiency of such machines. Meanwhile, brush seals are a widely used alternative to labyrinth seals. Their most valuable advantage over other sealing concepts is the very small gap between the sealing package and the rotor and thus reduced leakage mass flow. This small gap can be achieved due to the great radial flexibility without running the risk of severe detrimental deterioration in case of rubbing. Rubbing between rotor and seal during operation might occur as a result of e.g., an unequal thermal expansion of the rotor and stator or a rotor elongation due to centrifugal forces or manoeuvre forces. Thanks to the flexible structure of the brush seal, the contact forces during a rubbing event are reduced; however, the frictional heat input can still be considerable. Particularly in aircraft engines with their thin and lightweight rotor structures, the permissible material stresses can easily be exceeded by an increased heat input and thus harm the engine’s integrity. The geometry of the seal has a decisive influence on the resulting contact forces and consequently the heat input. This paper is a contribution to further understand the influence of the geometrical parameters of the brush seal on the heat input and the leakage during the rubbing of the seal on the rotor. In this paper, a total of three seals with varied back plate inner diameter are examined in more detail. The experimental tests were carried out on the brush seal test rig of the Institute of Thermal Turbomachinery (ITS) under machine-relevant conditions. These are represented by pressure differences of 1 to 7 bar, surface speeds of 30 to 180 m/s and radial interferences of 0.1 to 0.4 mm. For a better interpretation, the results were compared with those obtained at the static test rig of the Institute of Jet Propulsion and Turbomachinery (IFAS) at the Technical University of Braunschweig. The stiffness, the blow-down and the axial behaviour of the seals as a function of the differential pressure can be examined at this test rig. It could be shown that the back plate inner diameter has a decisive influence on the overall operating behaviour of a brush seal.
Highlights
Radial adaptive seals like brush seals offer the possibility to reduce the internal leakage in turbomachines considerably, in particular in comparison with labyrinth seals
The stiffness, the blow-down and the axial behaviour of the seals as a function of the differential pressure can be examined at this test rig
It could be shown that the back plate inner diameter has a decisive influence on the overall operating behaviour of a brush seal
Summary
Radial adaptive seals like brush seals offer the possibility to reduce the internal leakage in turbomachines considerably, in particular in comparison with labyrinth seals. It should be more flexible at lower pressure drops due to a higher number of start cycles in the case of stationary gas turbines or jet engines With this background, research results were published, where the relationship between the geometrical parameters and the sealing behaviour of brush seals had been investigated [1,2,3]. As shown by Crudgington et al [1], a larger front plate diameter results in a higher leakage and a reduced compression of the bristle pack, which, on the other hand, reduces the stiffness of the bristle package The latter can be beneficial with regard to rubbing of the seal. When the fence height is too large, increased leakage and gap opening, referred to as blow-down, can be observed
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