Analysis of Labyrinth Seal Flow Patterns to Improve Bulk Flow Code Predictions

Abstract

Non-contacting annular seals are frequently used in turbomachinery to reduce leakage of a fluid through a section with a large pressure differential. A typical type of non-contacting seal is the labyrinth seal, where circumferential grooves are cut into the rotor, stator or both. Using a tortuous path, labyrinth seals reduce leakage by dissipating the fluid’s kinetic energy through viscous forces caused by the formation of vortices in each seal groove. Due to a lower cost when compared to experimental measurements, bulk flow codes are frequently used for predicting seal contributions to rotordynamic performance. Existing seal codes use constant or linear values for the fluid film thickness at different seal sections and display inaccuracies in their prediction of velocity and pressure profiles and rotordynamic coefficients for labyrinth seals when compared to experimental data. The primary objective of this study is to determine the effect of implementing an effective film thickness into the governing bulk flow equations on the code prediction of axial velocity and pressure profiles. Simulations were run using ANSYS CFX with cross-sectional models of individual seal grooves. Seal parameters, including inlet circumferential velocity and rotor speed, were varied to better understand the behavior of the film thickness under various operating conditions. Streamlines were used to determine the maximum film thickness and an effective film thickness profile that can be used in the modified bulk flow code. Modified governing equations were developed, and predictions for the axial profiles resulting from the modified code solutions for the zeroth order governing equations are compared to CFD results and previous code predictions for improved accuracy. Preliminary results for a set of cases indicate far higher accuracy when an effective film thickness is used and represent the first results from a seal bulk flow code that implements a nonlinear effective film thickness. Improvement in code prediction of flow behavior across the seal, and subsequently in the seal codes accurate prediction of rotordynamic coefficients, allows for the design of more efficient and effective seals and machine systems.