Vacuum Pumping Characteristics And Vacuum Cooling Application Of Thermal Transpiration Effect

Thermal transpiration effect of gas is a micro-flow phenomenon by temperature gradient driven,which can transport gases and create pressure differential in the micro/nanochannels,and that can be used to pumping vacuum.Based on this effect,people developes a new type of vacuum pumping device-thermal vacuum pump（also known as Knudsen pump）,which is used in MEMS.Each stage of the cascaded device is generally composed of a cold chamber,a micro-channel group,a heat chamber,and a connector channel group.The device not only has simple structure,no moving parts,but also can utilize low-grade thermal energy adequately.If it is applied to vacuum system,which will break a new path to solve these problems such as over-dependent on high-grade electric or mechanical energy and application of micro vacuum pump in the vacuum field.The paper establishes a mathematical model that describing thermal transpiration effect and it’s vacuum pumping characteristics.Firstly,taking one micro-channel as an thermal vacuum pump’s simplest example,and using air as working medium,within the two flow regimes（the free molecular flow and the transitional flow）,some factors such as characteristic size,cross-sectional shape,diameter to length ratio of the micro-channel,temperature difference,pressure differential coefficients and gas characteristics that how to influence pumping static characteristics（pressure ratio,net mass flow,etc.）and dynamic characteristics（pumping speed,dimensionless pumping time,etc.）has been discussed.The results show that pressure ratio,net mass flow rate,pumping speed and dimensionless pumping time all increase with temperature difference increasing,but it isn’t obvious to increase net mass flow rate and will relay pumping process that reaching steady state for excessive temperature difference.So appropriate temperature difference should be determined based on actual conditions.In the transitional,the net mass flow rate or pumping speed are also influenced by diameter to length ratio,the former decreases with the later increasing,moreover,pressure ratio and dimensionless pumping time θ0 increases with Kn increasing.The free molecular flow has nothing to do with Kn.In the two flow regimes,θ0 ranges from 4.5 to 5.031 approximately,and θ0 of the transitional is always smaller than that in the free molecular.Owing to pressure ratio and flow rate（or pumping speed）are coupled with each other,in the practical applications,which can be weighed by adjusting pressure differential coefficients to determine a suitable generating conditions of thermal transpiration so as to achieve a better pumping characteristic.Whether it is in the free molecular or the transitional,micro-channel’s conductance could achieve to the largest when it is in the circular cross-section,furthermore,it also shows that properties of the circular are superior.Secondly,in the two flow regimes,the static and dynamic characteristics of common eight kinds of gases:H₂,He,O₂,N₂,air,Ar,CO₂,CH4 that influenced by these factors such as temperature difference,and diameter to length ratio are analyzed.The results show that,for the static characteristics,the net mass flow rate of H2 and rare gases are higher in the free molecular,but the situation in the transitional is more complicated.for the dynamic characteristics,pumping speed of H₂ and He is all higher than other gases under the same pressure ratio in the free molecular,while the situation in the transitional is more complicated.Whether in the free molecular or the transitional,the dimensionless time constants θ0 are similar basically.In order to increase pressure ratio and flow rate（or pumping speed）further,based on the the micro-channel pumping model,a thermal vacuum pump cascading mathematical model was established.Numerically,the influences of temperature differences,flow regimes,cascaded stages,and different gases on the dimensionless mass flow and net mass flow are analyzed.The results indicates that the static performance is better when the connector channel is in the smaller Kn ranges of the transitional,and larger temperature difference can reduce the cascaded numbers of thermal vacuum pumps,but the effect of heat consumption needs to be considered.In addition,based on the conclusions above,and combined with actual vacuum pumping system,the paper designs a vacuum cooling systems that using thermal vacuum pump as the core component and water as the refrigerant.For a 100L vacuum container,when its internal vacuum pressure reaches 1000 Pa,the 305 K,79.76 kg working fluid（water）can be converted to 290.7 K,77.84 kg chilled water.And design parameters of the thermal vacuum pump matched to that are:cascaded stages are 49 series,its external volume is approximately 0.014m3,and it needs to consume energy 3.07 kW（Kn_m =15,Knc=0.3,the temperature differential is 60 K）.