Research On Axial Clearance Leakage And Tooth Crest Seal Of Oil-free Scroll Compressor

Scroll compressors are widely used in industries such as refrigeration,air conditioning industries because of such characteristics as high efficiency,low noise and energy-saving materials.With the rapid development of oil-free scroll compressors,their application also can be found in food machinery,medical machinery,new energy and other fields.Since the scroll compressor realizes the gas compression by the the change of volume formed by the engagement of two scroll plates,there must be axial and radial clearance between the two plates.The clearance results in the internal leakage which influences the efficiency and triggers the technical problems in the application of scroll compressor under high power condition.Especially,it is the leakage of axial gap that plays the key bottleneck role in the development of scroll compressors.This paper took the variable section oil-free scroll compressor as the research object.Based on the theory of geometric design of scroll tooth profile,the influence of scroll tooth deformation on axial clearance was analyzed.The leakage mechanism and control method of axial clearance were studied by means of fluid mechanics and sealing technology.The design of axial clearance seal structure of scroll plate and the development of high performance seal material were also studied to meet the engineering application demands.The study provides a theoretical reference for the development of an efficient oil-free scroll compressor with core competitiveness.The specific research work and achievements are as follows:(1)Study on the calculation model of gap leakage line based on the theory of variable section profile generation.According to the meshing theory of profile,the equation of composite profile and the calculation method of instantaneous volume of each working chamber were established.Based on the theory of instantaneous type line composition in working cavity,the function of the length of the radial leakage line on the rotation angle of the spindle was established and the general calculation model of the radial leakage line was developed.The influence of profile parameters on the length of leakage line was also analyzed.This part lays a foundation for the accurate calculation of axial clearance leakage,the optimal parameters of selected type lines,and the stress and deformation analysis of scroll teeth.(2)Flow field calculation in scroll compression cavity based on dynamic grid technique.In order to study the influence of stress and deformation of scroll teeth with variable section on axial clearance,the temperature and pressure loads of the scroll teeth must be calculated accurately.The 3-D transient simulation of scroll compressor was completed byusing the method of network pattern reconstruction to encrypt the leakage clearance.The calculated results revealed the flow field distribution of each axial section in scroll compressor.The instantaneous changing rules of temperature,pressure and velocity could be obtained.The gas pressure is the highest at the moment of exhausting.When the central cavity was connected with the vent,the pressure of the central cavity was rapidly equal to that of the exhausting cavity.As heat flows along the air flow along the vent,the temperature near the vent was the highest.And the gas pressure difference before and after exhaust was large,the airflow speed was the fastest.(3)Study of the law of deformation and axial gap of scroll tooth with variable section based on coupling field.The study established the thermodynamic calculation model of scroll teeth with variable section considering the actual axial clearance.The results of flow field analysis were applied as boundary load to the stress analysis model.The stress and deformation of scroll tooth under the action of single field and multi-field coupling were analyzed,the stress distribution and axial deformation law of scroll tooth under the action of multi-field coupling were obtained,and the influence of scroll tooth deformation on axial clearance was also obtained.The results showed that the influence of temperature on the deformation of scroll tooth was greater than that of air force.As the wall thickness of the scroll tooth varied with the angle of expansion,the anti-deformation ability of the scroll tooth with variable section was obviously enhanced compared with that of with the constant section.After the assembly of dynamic and static scroll disks,the analysis under the action of coupling field showed that there was interference in the deformation between the orbiting and fixed scroll,because the axial deformation influenced the size of axial gap directly.It was found that it was more reasonable to control the installation clearance of scroll teeth within0.025 mm,and the variation of axial clearance with the Angle of vortex profile was obtained by studying the transformation mechanism of scroll disks.(4)Study on the mechanism of axial clearance gap of scroll teeth considering variable clearance and tooth thickness.Based on the variation law of axial clearance and tooth thickness,the formula for calculating the axial clearance of scroll teeth with variable section considering the overturning moment was derived.Combining with the deformation of scroll tooth,the expression of the tooth thickness with the angle of expansion of scroll tooth with variable section was obtained.Based on the theory of Fanno flow and turbulence,the axial clearance leakage model of scroll compressor with variable section was established by comprehensively considering the parameters such as rotation speed,viscosity,axial clearance value and tooth thickness.The influences of base circle radius,axial clearance and rotating speed on axial clearance leakage were analyzed.The theoretical model was verified by thesimulated leakage test.The results showed that the turbulent flow model could accurately predict the axial clearance leakage of scroll compressor at high rotational speed.The calculation model based on Fanno flow theory was more suitable for medium and low speed.The axial clearance should be controlled reasonably to reduce the clearance leakage and improve the working efficiency of scroll compressor.The calculated model could be used to predict the leakage of axial clearance which provided a basis for the design of axial clearance seal structure.(5)Study on the axial clearance sealing structure of scroll teeth with variable section based on leakage control method.According to the deformation mechanism of scroll tooth and the calculation model of axial clearance leakage,a test platform based on the actual working condition of scroll compressor was built.The leakage mechanism of labyrinth seal,smooth seal and seal groove seal structure was studied,and the sealing performance of the three sealing structures were compared by test.By comparing the measured value of axial leakage with the calculated value,it showed that the variation trend of measured and calculated values was similar,and the accuracy of the model was proved.The measured value of labyrinth seal leakage was about 80% of the measured value of smooth seal,and the measured value of groove seal leakage was about 63% of the measured value of smooth seal.The advantages of seal groove seal were proved.However,when the test conditions were changed,the friction performance defects of seal groove structure and seal strip material became prominent,resulting in the failure of seal ahead of time.Based on the above research results,a new type of seal structure with axial clearance seal groove was designed to improve the sealing performance and reliability.The high pressure chamber gas was introduced into the bottom of the seal strip by opening at the bottom of the groove to compensate the axial clearance,and tested the leakage of the seal structure was more than 50% less than that of the original structure by a leakage testing platform.(6)Research on the clearance sealing materials based on the practical working condition of scroll compressor with variable cross section.The reliability of the sealing structure mainly depended on the performance of the sealing material.At present,the poor tribological performance and short life of expectancy the sealing strip material can not meet the current requirements for the use of scroll compressor seals.In this paper,polyimide and graphite were mixed and filled with PTFE to make composite materials based on the boundary condition of the actual working condition of the scroll compressor with variable section,PTFE and other materials were mixed and filled with PTFE in different proportions.Through the simulation test,the optimum ratio was determined,and the sealing material with low friction factor,low wear rate and high temperature resistance was developed.That is,25% polyimide,5% graphite and trace copper powder were mixed with PTFE to make the composite material.In order to further verify the application of the mixed material in the tooth crest seal of scroll compressor,a seal strip made of the material was tested on the simulation test bench and compared with the existing seal material.The results showed that the friction factor of PTFE decreased by nearly one time,the wear rate decreased by nearly 30 times,and the bearing temperature limit increased by nearly two times.