Analysis And Experimental Research On Dynamic Pressure Lubrication Characteristics Of Compliant Foil Gas Film Seal

Advanced sealing technology is the key to the long-term and efficient operation of aero-engines,and therefore has drawn wide research attention.While serving in large airspace,aero-engine seals usually face harsh environments with uncertain interference caused by complex airflow and changing attitudes.Various sealing structures have been proposed worldwide,but the requirements for aero-engine seals have been higher along with the introduction of hypervelocity vehicle design and the rapid advancement of ground testing under full working conditions.For instance,high-speed stability and sealing capability are needed to ensure the reliability of the seals under the coupling influence of multiple environmental factors.The compliant foil gas film seal is an advanced sealing technology proposed for the application requirements of aero-engines.Its main design concept is to replace the traditional rigid floating ring seal end face with a new type of flexible foil seal face.Through numerical modeling and experimental testing,this work evaluated the sealing performance for such a sealing structure at both macroscopic and microscopic design levels.Based on the general Reynolds equation and Iordanoff’s foil stiffness model,wave foil fixations with one free end and two free ends are used for the compliant foil gas seal structure,respectively.The hydrodynamic pressure lubrication equation for the simultaneous circumferential motion of the gas film,the gas film thickness control equation,and the foil seal surface deformation equation are established;the key parameters for evaluating the static seal performance are obtained by coupled solution.The research results show that the working parameters have a significant influence on the compliant foil gas film sealing,and the comprehensive static sealing performance of the free fixing method at one end is better than that of the free fixing at both ends.A dynamic Reynolds equation including a time term is established based on the static Reynolds equation.Taking into account the balance relationship between the gas film pressure,the foil deformation gas and the film thickness,this work derived the calculation differential equation of the dynamic characteristic coefficient of the compliant foil gas film seal using the small perturbation method.The perturbation equation is solved by the finite difference method,and the influence law of the parameter variables on the dynamic characteristic coefficient is discussed.It is found that the cross-stiffness and the cross-damping coefficients always maintain an approximately symmetrical relationship,and the compliant foil gas film seal has good stability under high-speed operation.A simple method based on image recognition of interface micro-shapes is proposed,and the recognition results are compared with other literature data to verify the method’s effectiveness.Two continuously distributed dynamic pressure grooves and six discontinuously distributed dynamic pressure cavities are designed on the surface of the flat foil,and the interface shape is optimized by analyzing the static and dynamic characteristics.The research results show that the stepped slope groove is unfavorable to reducing the leakage,but is beneficial to improving the gas film buoyancy.The dynamic pressure cavity shows better stability when the depth is lower than 9 μm,with the inverted triangle showing the best overall sealing performance.The theoretical models for compliant foil gas film sealing considering each of the three effects,i.e.,turbulent flow effect,ultra-thin gas slip flow effect,and real topography effect,are established respectively;the applicable conditions of the revised theoretical model are determined;and the influence degree of three special effects on sealing characteristics is analyzed.The research results show that the buoyancy force of the gas film increases when the turbulent effect is considered,but it decreases when the migration effect is considered;increasing the characteristic scale factor or reducing the fractal dimension within a certain range can effectively enhance the gas film buoyancy and control the mass leakage rate,but they are detrimental to reducing the viscous friction and improving the stability of the sealing system.The processing flat foil and wave foils are designed.A test-bed equipped with systems for gas supply,driving,sealing,testing and anti-interference is built to evaluate the performance of compliant foil gas film seal.Two test programs for constant pressure/speed to variable speed/pressure were developed,and detailed test methods and procedures were clarified for different fixation methods of wave foils.The test results show that controlling the initial gas film gap directly affects the speed limit of the compliant foil gas film seal,and when the rotating speed rises to a certain range,there will be obvious scratches at the near-fixed end of the flat foil.This work analyzed the distribution characteristics of the flow field in the microscale sealing gap of the compliant foil gas film seal,revealing the variation laws of the static and dynamic characteristics.An interface modeling identification method with a wide range of applications is proposed;the geometric shapes and structural parameters of two dynamic pressure micro models,i.e.,groove type and cavity type,are optimized.The dynamic pressure lubrication mechanism under different special effects is elaborated.The installation and debugging problems of the compliant foil gas film sealing test bed are solved.Currently,research on compliant foil gas film sealing is still in the initial stage,and there remains much room for improvement and perfection in the theoretical model and structural design.The research results of this thesis provide a reference for the future structural design of compliant foil gas film seals and lay the foundation for possible future engineering applications.