Research On Brush Seal And Nonlinear Dynamic Characteristics Of Shaft System In Rotating Machinery

With the implementation of the Two Major Project of Aeroengine and Gas Turbine,the requirement of research and development in gas turbine become higher.Brush seals have superior performance,which could enhance the thrust force and working efficiency.When the gas turbine is working under a high temperature,high pressure,and high velocity condition,there would be some complex dynamic behavior fault arising.It can seriously affect the security and reliability of the system.The seal force and oil-film force are two of the main factors which would cause the instability of rotor system,so it become more important to further study the nonlinear dynamic characteristics of rotor-bearing-seal system.Currently the dynamic behaviors of rotor-brush seal system are mostly studied by CFD simulation and test result for a specific structure.But these researches are still not perfect,the effects of coupling of bristle mechanical behavior and fluid flow characteristics,the contact of bristle and rotor surface,and the fluid flow in the bristle pack to the seal dynamic behavior need further study.In this paper,in order to optimize the seal structure of shafts in a gas turbine(including single-stage brush seal and labyrinth brush seal),the calculation model of rotor system are established considering the effects of geometry parameters of brush seal and operating parameters on the flow characteristics,seal force and nonlinear dynamic behaviors of system,and the selection of key design parameters and the design criterion of nonlinear dynamic characteristics are proposed.The main contents,methodology,and results of this study include the following aspects.The flow fields of brush seal are studied.The method of Reynolds-averaged Navier-Stokes solutions coupling with a non-Darcian porous medium model is adopted,and the reliability of the present numerical method is proved after making comparison with the results from the literature.A computational CFD model is established to analyze the effects of radial clearance,bristle pack thicknesses,fence heights,pressure ratio,rotor velocity and other operating conditions on the leakage flow behaviors of a single-stage brush seal.Then the leakage flow characteristics and distribution of labyrinth brush seal on rotor surface,bristle pack free height and fence height are numerically analyzed by adopting a non-Darcian porous medium model.And a corresponding comparison between labyrinth seal and brush seal is performed.The results show that the leakage increase with the radial clearance,the axial static pressure and turbulent kinetic energy of rotor surface gradually declined with a ladder-like distribution from the inlet to the outlet.The dynamic coefficients of brush seal are analyzed by using CFD method.The CFD computational model is established by using a RANS solutions coupled with a non-Darcian porous medium model.The rotordynamic coefficients of a typical single-stage brush seal for different geometries and operating conditions is numerically analyzed.The results show the cross-coupled stiffness coefficients and direct damping coefficients will increase with the bristle pack thickness,clearance,and pressure ratio.Moreover,the rotordynamic coefficients of labyrinth brush seal under different clearance cases and operating conditions are numerically analyzed.The seal force of rotor–brush seal system is analyzed based on the bending deformation theory considering the interaction of flow force and eccentricity of rotor.The seal force model is established by using Bernoulli-Euler equation and superposition method to analyze the effects of eccentricity,friction coefficient,uniform force of flow,and bristle lay angle on the force of rotor caused by single bristle and bristle pack.And the computational expression of brush seal force is obtained by coefficients fitting.The experimental test rig of rotor-brush seal system is set up,and the effect of seal force on critical speed and vibration characteristics of rotor system are discussed by experimental work.The results show that under the force of flow condition the contact force of rotor is much greater than that without flow condition.And the contact force increase with the increase of force of flow.The theoretical results are in accordance with the experimental results,which validate the seal force model of brush seal.The rotor-brush seal nonlinear dynamic model is established using a new seal force model for brush seal.The effects of geometry parameters and operating parameters including rotor speed,installation spacing of brush seal,damping of rotor,and mass of rotor on the nonlinear dynamic behavior of a rotor-brush seal system are discussed.The result shows that with a seal force the critical speed is greater than that without seal force condition.At low rotor speed the vibration amplitude and power spectrum under without seal force condition is a bit greater than with force condition,but under high velocity conditions the result is opposite.The vibration amplitudes decrease with the increase of installation spacing.When the damping of rotor and the mass of rotor increase,the vibration amplitude decrease significantly,which add stable factor to the rotor.In order to establish the single disk rotor-bearing-brush seal system model and gas turbine rotor-bearing-brush seal system model,a new seal force model for the brush seal is proposed and a nonlinear oil-film force model based on short bearing theory is adopted.The dimensionless equation of motion is solved by time simulation using the fourth order Runge-Kutta method.The effects of key parameters including rotor speed,installation spacing of brush seal,eccentricity of disk,mass of disk,and mass of journal on the vibration response and dynamic behavior of rotor-bearing-brush seal system are discussed.The result shows when the rotor speed increase the system displays various nonlinear motion forms including synchronous periodic-one motion,periodic-two motion,multi-periodic motion,and quasi-periodic motion unregularly.The vibration amplitude decreases with the increase of installation spacing,but increases with the increasing eccentricity of the disk.With increasing of disk mass and journal mass at a lower rotor speed,motions of the system become complicated.Oil whirl and oil whip are more likely to occur at a higher rotor speed or a larger mass of disk condition.