Computational Evaluation of Labyrinth Seal Configurations

摘要

The secondary air system in gas turbines is responsible for distribution of cooling air to hot sections and sealing in roller bearings and gaps between rotating and stationary components. They usually contain labyrinth seals to provide controlled pressured drop along a series of cavities separated by sharp fins. The secondary air system for cooling and sealing can account for more than 20% of the mass flow in the engine, having a significant impact on the thermodynamic efficiency. The precise control of cooling mass flow is mandatory in order to avoid overheating or excessive cooling mass flow. There are a number of empirical correlations relating mass flow to pressure drop as a function of labyrinth seal geometry. Manufacturing cost, maintenance and reliability are other relevant parameters to be considered in seal design. The present analysis aims at the evaluation of a proposed labyrinth seal configuration in terms of fin width to pitch length ratio and cavity geometry. The investigated labyrinth seal is used on the last stage of a five stage axial compressor. A less demanding configuration in terms of manufacturing cost is compared to a more usual configuration to access the sealing quality of the proposed labyrinth. Mass flow versus pressure drop are investigated and correlations for discharge coefficient are compared based on numerical simulations. The flow streamlines in both configurations are compared for the actual topology consisting of a radial inlet followed by a 90 degree bend just before the first fin. Numerical simulations are performed using an open source control volume based CFD code (OpenFoam) to solve the two-dimensional compressible Reynolds averaged Navier-Stokes equations. The results show that the least expensive manufacturing configuration results in flow parameters that are different from the standard configuration by no more than about 5%. Adding grooves to the stationary wall change flow parameters by another 3%. Comparisons between the original design and the modified design reveals that the same discharge coefficient as the original design may be obtained with only three fins after the modifications.