The Research On Hydrodynamic Lubrication And Sealing Performance Of Supercritical Carbon Dioxide Dry Gas Seal

Currently,the Brayton cycle system with supercritical carbon dioxide（Supercritical-CO₂,S-CO₂）as the working medium has a good prospect in waste heat recovery,Gen IV nuclear reactors,concentrated solar power and other fields due to its many advantages.As a closed cycle operating at high temperature,high pressure,and high speed,the efficiency of its turbomachinery is severely affected by the shaft end seal.Therefore,high-performance sealing technology must be used to achieve a near-zero leakage sealing effect.The dry gas seal is recommended for S-CO₂turbomachinery because of its excellent sealing performance compared to any other seal applied in this condition.Although dry gas seals have been widely used in the petrochemical and energy power industries,there are no dry gas seal design guidelines that can be applied to S-CO₂ turbomachinery.Therefore,investigating and developing dry gas seals for S-CO₂ turbomachinery and forming a set of S-CO₂ dry gas seal design theories and methods have great engineering value and strategic significance to enhance the independent development level of advanced dry gas seals for our country,as well as to promote the application of S-CO₂Brayton cycle system in our country’s energy field.In this regard,this dissertation employs a combination of theoretical modeling and experimental approaches to conduct an intensive study of S-CO₂ dry gas seals.The Reynolds equation for laminar flow considering the centrifugal inertia force effect is adopted as the pressure control equation.The real gas effect and the real viscosity of carbon dioxide are described by the Virial equation and Lucas equation,respectively.Then,based on the N-S equation and the law of energy conservation,the energy control equation considering the centrifugal inertia force effect under the adiabatic condition is established.Thus,the laminar flow lubrication theory is refined.The laminar hydrodynamic lubrication model is solved numerically by the finite difference method.The effects of boundary conditions and centrifugal inertia force effect on the flow field distribution are illustrated,and the phase-distribution regularity and its main influencing factors in the gas film under the effect of multiple complex effects are preliminarily revealed,as well as the change regularity of steady-state seal performance is obtained.To investigate the performance of the S-CO₂ dry gas seal under high operating parameters,the turbulent Reynolds equation considering the centrifugal inertia force effect is employed,and the Ng-Pan turbulence coefficient expression and Constantinescu’s inertia coefficient expression are selected.Then,the turbulence and inertia coefficients are corrected for discontinuities in the transition region.The real physical properties of CO₂ are obtained by calling REFPROP,a physical property software developed by NIST.According to the general energy equation,the simplified energy equation for compressible fluid in the adiabatic condition is established by introducing the mean velocity containing the turbulence effect and centrifugal inertia force effect.By conducting the coupled solution of the modified and refined turbulent hydrodynamic lubrication model,the mechanism of the turbulence effect on the flow field is clarified,and the influence law of the turbulence effect and the centrifugal inertia force effect on the steady-state sealing performance is revealed.A two-dimensional fluid-thermal-solid coupled deformation model of the S-CO₂dry gas seal ring is developed with full consideration of the generalized load consisting of mechanical load,thermal load,and other loads.The turbulent hydrodynamic lubrication model is used as the fundamental governing equation for the fluid domain to solve the gas film.Then,the solid mechanic’s equations under spatial axisymmetry are used as the theoretical foundation,and the solid domain is calculated by APDL parameterized programming in ANSYS.By coupled iterative solution of the fluid domain and solid domain under different loads,the spatial distribution regularity of equilibrium film thickness is obtained,the key loads affecting the seal end face deformation are clarified,and the stationary ring that needs to focus on deformation control is identified.Meanwhile,the order of leakage rate for no deformation,force deformation,thermal deformation,and fluid-thermal-solid coupling deformation is given.Under the premise of considering multiple complex effects,the unsteady Reynolds equation and the energy equation,which consider the centrifugal inertia force effect and turbulence effect,are used as the primary control equations.Then,a complete variable perturbation model under the adiabatic flow process is established by using the frequency perturbation method to fully perturb the film thickness,pressure,temperature,physical parameters,Reynolds number,turbulence coefficient,and inertia coefficient.By studying the dynamic characteristic coefficients of gas film in three degrees of freedom（one axial direction and two angular directions）under different frequency ratios and calculation conditions,the distribution law of dynamic characteristic coefficients of S-CO₂ dry gas seal gas film is revealed,the disturbance strength of various perturbation factors is identified,and the influence law of working condition parameters on the dynamic characteristic coefficients of the gas film is obtained at the same time.With an S-CO₂ dry gas seal test cycle system,a set of S-CO₂ dry gas seal prototypes is tested for leakage rates under different operating conditions.Then,a comparative analysis between the theoretical leakage rates and the experimental test values is performed.It is shown that variations in both rotational speed and pressure can cause changes in the flow state within the gas film.Under low rotational speed and low-pressure conditions,the flow in the gas film follows the laminar flow.As the rotational speed and pressure increase,the flow in the gas film gradually transforms into turbulent flow,and the turbulence effect has a significant effect on the leakage rate.Compared with the isothermal flow,the flow inside the gas film is closer to the adiabatic flow process.In addition,the experimental results better validate the relevant model established in this dissertation.This dissertation systematically illustrates the flow field distribution,steady-state performance,and dynamic characteristic coefficients of the S-CO₂ dry gas seal under multiple complex effects.It solves the problem of insufficient theoretical methods in designing S-CO₂ dry gas seals.These research results provide a solid foundation for S-CO₂ dry gas seal in groove design,seal performance analysis,engineering application,etc.,in the future.