The experimental and numerical investigation of the influence of shaft rotation on leakage rate of non-contacting seals found in turbine applications

Abstract

This project was initiated by ESKOM power generation. ESKOM loses up to 22% of their steam energy in the HP turbines due to leakage at the turbine labyrinth seals. Therefore the need was expressed their need to implement improved sealing configurations. The aim of this study is to investigate the effect that shaft rotation has on the leakage rate of labyrinth and brush seals. This is done by means of experimental and numerical methods. For many decades it was assumed that the shaft rotation has no or little effect on seal performance and therefore it was neglected in seal design. It was decided to investigate this subject, in order to assist and improve in future seal design and operation. Both labyrinth and brush seals were investigated experimentally on a test rig. A real life application of the labyrinth or brush seals can be found in the power generation industry where a turbine shaft has a diameter of 300 mm and rotates at 3 000 rpm. The test rig was designed to assist in this application. Therefor the test rig had a shaft diameter of 150 mm with shaft speeds ranging between 0-10 000 rpm and with five different upstream pressures ranging from 1-5 bar. The same seals were then simulated using the commercial Computational Fluid Dynamics (CFD) package STAR-CCM+ with the bristle pack of the brush seal modelled as a porous medium. The coefficients of resistance for the porous medium were experimentally obtained. The two investigation methods are compared for the labyrinth and brush seals. The labyrinth and brush seals are also compared against each other. The results show that the experimental leakage rates have a good correlation with those predicted by CFD. The CFD simulation provided detailed leakage flow fields and pressure distributions of both seals. It was found that shaft rotation has an influence on the leakage rate of both seals. The leakage rate increased at higher shaft speeds, with the brush seal performing better than the labyrinth seal. An increase of up to 1.7% was found at 10 000 rpm for the labyrinth seal and 1.45% for the brush seal at 10 000 rpm. Guidelines were created based on these results to assist with advanced seal design. It is recommended that these guidelines are used in future seal design and other research aspects of non-contacting seals in turbo machinery.