Leakage Degradation of Straight Labyrinth Seal Due to Wear of Round Tooth Tip and Acute Trapezoidal Rub-Groove

Abstract

In this paper, labyrinth seal leakage is numerically quantified for an acute trapezoidal rubgroove accompanied with a rounded tooth, as a function of rub-groove sizes and toothgroove axial positions. Analyses parameters include clearance, pressure ratio, number of teeth, and rotor speed. Labyrinth seals wear during engine transients. Radial incursion and axial movement of the rotor-stator pair cause the labyrinth teeth to rub against the unworn stator surface. The labyrinth teeth and/or stator wear depending on their material hardness. Wear damage in the form of material loss or deformation permanently increases seal clearance, and thus, leakage. This leakage is known to be dependent on the shape and geometry of the worn tooth and the stator rub groove. There are two types of reported tooth tip wear. These can be approximated as a mushroom shape and a round shape. The stator rub-groove shapes can be approximately simulated in five forms: rectangle, trapezoid (isosceles and acute), triangle, and ellipse. In this paper, the acute trapezoidal rub-groove shape is specifically chosen, since it is the most similar to the most commonly observed rub-groove form. The tooth tip is considered to be rounded, because the tooth tip wears smoothly and a round shape forms during rub-groove formation. To compare the unworn tooth, the flat stator is also analyzed as a reference case. All analyzed parameters for geometric dimensions (groove width, depth, wall angle, and tooth-groove axial position) and operating conditions (flow direction, clearance, pressure ratio, number of teeth, and rotor speed) are analyzed in their practical ranges. Computational fluid dynamics (CFD) analyses are carried out by employing a compressible turbulent flow solver in a 2D axisymmetrical coordinate system. CFD analyses show that the rounded tooth leaks more than an unworn sharp-edged tooth, due to the formation of a smooth and streamlined flow around the rounded geometry. This smooth flow yields less flow separation, flow disturbance, and less of vena contracta effect. The geometric dimensions of the acute trapezoidal rub-groove (width, depth, wall angle) significantly affect leakage. The effects of clearance, pressure ratio, number of teeth, and rotor speed on the leakage are also quantified. Analyses results are separately evaluated for each parameter.