Analysis of leakage flow and dynamic coefficients of gas labyrinth seals using a three control volume method.

摘要

Labyrinth seals are widely used in compressors in the rotating machinery industry to prevent leakage. When flow induced vibration excitation properties in labyrinth seals was recognized, it became important to predict seal rotordynamic stiffness and damping coefficients besides the prediction of leakage. Researchers have developed some mathematical models which some obtained results for the seal direct stiffness coefficients. However, the calculation of the cross-coupled stiffness and direct damping coefficients are still not properly evaluated by existing methods. ROMAC has an existing analysis method using a single control volume method.; This research developed a three control volume analysis method, based upon Hirs equations for turbulent flow, to improve the analytical prediction rotordynamic coefficients for gas labyrinth seals. Additional momentum terms are added to the simplified Navier-Stokes equation to model the momentum exchange between flow over the cavity and flow in the cavity. The dissertation develops a better analytical model by taking advantage of the use of commercial CFD packages, comparing the experimental results, CFD results and simple model results.; A CFD package is used to validate the code by comparing the flow leakage predicted by CFD simulation and experiments. A series of CFX simulations were carried out for a generalized formula for entrance loss coefficient, land inlet loss coefficients, exit pressure recovery coefficient, which will be used as part of the boundary conditions in the developed three-control-volume model. 3D, full geometry simulation for flow in gas labyrinth seals was carried out at first on a 4 teeth case. A procedure was then set up to obtained the correct inlet axial velocity, which will serve as inlet boundary conditions, by given upstream pressure and inlet swirl velocity. Rotordynamic coefficients were obtained and compared with experimental results. Then a grid independent study is carried out for a 16 teeth labyrinth seal. Parallel computing is necessary to run such a case.; A computer code has been developed to predict seal leakage and rotordynamic coefficients. A comparison of leakage flow rate and rotordynamic coefficients is presented between experiments, CFD simulation, the new code prediction and previous analysis.; Leakage results predicted by the new code are within 4% compared with experimental data, tested, which is a better agreement with simulation solutions compared with the previous results, which prediction usually lies in 20-30% catalogue. Rotordynamic coefficients are given even for some cases where the previous analysis failed. Direct stiffness agree to within 2% for a case and cross coupling with a minimum of about 5% and a maximum of 70% for an experiment compared.