Comparative Analysis of Helically Grooved and Circumferentially Grooved Labyrinth Seal

Abstract

Abstract-Labyrinth seals are used in turbo-machinery for reducing the leakage of working fluid. The energy lost by the fluid passing through the gap between labyrinth seal and rotor causes the reduction in leakage rate. But the fluid in between the labyrinth seal and rotor induce various forces (radial and tangential forces). These forces can be represented by certain numbers of coefficients, such as stiffness and damping coefficients. In this work, a comparative analysis has been carried out between a helicallygrooved and a circumferentially-grooved labyrinth seal using a CFD program Fluent. Parameters such as fluid forces, leakage and rotordynamic coefficients for both the labyrinth seals are calculated. Modelling and analysis have been performed with seal-rotor configurations having different eccentricity ratios (0.1, 0.2, 0.34, 0.4, 0.5 and 0.68). Leakage and rotordynamic coefficients have been calculated and compared for the two seals under different inlet pressures, rotor speed, etc. The calculated leakage was same for both the labyrinth seals, but the calculated rotordynamic coefficients were different. Keywords -Helically-grooved, circumferentially-grooved, stiffness coefficient, damping coefficient, labyrinth seal I. INTRODUCTIONLabyrinth seals are one type of non-contacting seals which are used in turbomachinery components such as turbines, compressors and pumps. The main purpose of using the labyrinth seals is to reduce the leakage of working fluid through the gap between the rotor and the stator. Labyrinth seals are classified into two types, straight-through labyrinth seal and stepped labyrinth seal. In order to reduce the leakage the labyrinth seals are carefully designed. The leakage may be occurring less at a particular time, but when used for longer period of time it may ruin the efficiency of the whole system. Hence accurate prediction of leakage through the labyrinth seal becomes important.On the other hand, labyrinth seal being a non-contact seal causes some instability problems due to the presence of eccentricity. The instability is due to the fluid forces which are created due to the eccentricity of shaft from the center of stator. These forces tend to move the rotor away from the centre of stator which eventually creates the instability. Hence it is essential to investigate on the instability caused due to labyrinth seals.Many researchers have worked on leakage analysis through seals. Works of few researchers have been discussed here. The authors in [1] experimentally recorded the variation in the discharge because of pressure ratio and Reynolds number over the labyrinth seal used in jet engine. The authors in [2] conducted leakage tests experimentally on labyrinth seal for different tooth profiles, tooth thickness and eccentricities. They concluded that increasing the tooth thickness by two times the leakage reduced by 26%. The authors in [3] have analysed two different labyrinth seals of different configuration and clearance to record the variation in leakage. The authors in [4] have inv...