Analysis Of Conductance Of The Thermal Transpiration Vacuum Pumpand Such Pumps'Application In Vacuum Precooler

There is a kind of vacuum pump which works by thermal transpiration effect.Different from common vacuum pump,it has the advantages of simple structure,no moving parts and useing low-grade thermal energy.At present,it is drawing people's attention increasingly.This kind of vacuum pump is made up of multiple single-stage Knudsen pumps in series.The single-stage Knudsen pump is only composed of microchannels,cold cavity and hot cavity.So they can also be seen as part of the vacuum system pipeline.As the pipeline of the vacuum system,its conductance can be studied in detail,which is also an important parameter of vacuum system.Therefore,studying its conductance will be helpful to have a deeper understanding of the operation mechanism of the thermal transpiration vacuum pump,and provide guidance for its optimal design and application.In this paper,firstly,several models of long microchannels and short microchannels with typical cross-sections shape were established,and then the regular pattern of the variation of conductance with Knudsen numbers,temperature difference,microchannels characteristic size and length and different working fluids?H₂?He?N₂?O₂?air?Ar?CO₂?CH₄?is analyzed.The results show that the conductance of the microchannels in the transition flow region is 2-4 orders of magnitude larger than that in the free molecular flow region.Under the same conditions,the short microchannels have excellent conductance performance than the long microchannels'.In most cases,the greater the symmetry of the cross-section shapes have the better the conductance.In the free molecular region,the cross-section shapes have little influence on the conductance.In the transition flow region,it is different,the conductance of different cross-section shapes can be several tens of times different.The conductance of H₂is always the best in any case,and under different conditions,the temperature difference can best reflect the superiority of H₂.It can be seen that the thermal transpiration vacuum pump should be designed to work in the transitional flow region as much as possible,and the short microchannels'structure with strong section shape symmetry should be used,but the mutual coordination and restriction among Knudsen number,temperature difference,microchannels'characteristic size and length should be considered comprehensively.Then the microchannels'conductance was simulated by Monte Carlo method.The results show that when length-width ratio L/W is determined,the rectangular microchannel's conductance increases with the increase of height-width ratio H/W,and almost linearly.Under the same cross-sectional area,the microchannel's conductance decreases with the increase of microchannel length L.When microchannel's length L is small and the same,the microchannel's conductance decreases with the increase of height-width ratio H/W.Among the three kinds of microchannels,the circular microchannel has the best conductance.Finally,it is verified that the trend of the simulation results of microchannel conductance is the same as that of the formula calculation results,and the error between them is relatively small.Monte Carlo simulation method also has high research value in microchannel conductance.It provides a new method for the calculation of the conductance of the microchannel and solves the problem that the general formula can not accurately calculate the conductance of the irregular microchannel.Based on the mathematical model of microchannel conductance,it was extended to the calculation of conductance of the whole vacuum pump.The results show that the conductance of 8 working fluids?H₂,He,N₂,O₂,air,Ar,CO₂,CH₄?in the thermal transpiration vacuum pump is closely related to length of cold and hot cavity L₃and microchannel characteristic size L_r.Considering from the above two aspects,length of cold and hot cavity L₃can be reduced or microchannel characteristic size L_rcan be increased to improve the conductance of the thermal transpiration vacuum pump,but the thermal transpiration effect can be is closely related to microchannel characteristic size L_r,so microchannel characteristic size L_rcannot be increased blindly.The conductance of the 8kinds of working fluids in the thermal transpiration vacuum pump increases with the increase of diameter-length ratio k_L,but the growth rate of the 8 kinds of working fluids in the small diameter-length ratio k _Ltends to be gentle slowly,and the H₂working fluid has obvious advantages in the conductance,but its conductance has not been greatly improved with the increase of k _L.Based on the model of thermal transpiration vacuum pump,the model of thermal transpiration vacuum precooler was further established.The results show that the heat consumption Q_tand heat efficiency?of the thermal transpiration vacuum precooler increase with the increase of temperature difference?T,but we certainly hope to increase heat efficiency?with little heat consumption Q_t,so the determination of temperature difference?T should be considered from the two aspects.The equilibrium water content EMC of the food in the precooling box increases with the increase of pressure P,and the rate of change decreases gradually.Air humidity h has a great influence on the water content of the food,the higher the air humidity is,the higher the water content of the food.The food preservation must be controlled at a certain water content.Therefore,the vacuum precooling preservation technology should be based on the actual situation to determine the air humidity h and pressure P.When it is in the free molecular flow region,the pressure ratio of only 30 stages in series can reach 11.59.When it is in the transition flow region,the pressure ratio of only 30 stages in series can reach 5.39.To achieve a larger pressure ratio,only the series number needs to be increased.Compared with general vacuum precooler,thermal transpiration vacuum precooler has obvious advantages,such as low cost depletion and low power consumption.It can save a lot of money and a large part of energy,which provides a new solution to reduce the high energy consumption of traditional precooling technology.