Study On Gas Expansion Process Of Fast Dynamic Vacuum Calibration In Millisecond Range

Many industrial fast processes operating under vacuum environment require system pressure vary rapidly by several orders of magnitude relative to ambient pressure.Such processes are often monitored and controlled by readings of vacuum gauge.Therefore,the manufactures and researchers of vacuum gauge are interested to know how fast their gauges can follow such rapid pressure change.In order to study the dynamic response characteristics of vacuum gauge to the rapid pressure change,a rapid dynamic vacuum calibration apparatus was developed in the millisecond(ms)range by Lanzhou Institute of Physics(LIP).It's aim is to change the pressure from atmospheric conditions to below 100 Pa within 1 s or less in a predicted manner.This can be accomplished by expansion of a test gas at 100 kPa in a small chamber(upstream chamber)via a current limit element into a large evacuated chamber(downstream chamber)at 10~-66 Pa magnitudes,which is similar to the rapid pressure change processes encountered in industrial productions.The expansion is initiated by a fast opening ultra-high vacuum gate valve(hereinafter called fast valve).However,during the gas expansion,the upstream and downstream flow regimes are different at the same time and evolve with time.And,they may exhibit various flow regimes,such as turbulent flow,turbulent-viscous transitional flow,viscous flow,viscous-molecular transitional flow and molecular flow,et al.Under different gas flow regimes,the duct conductance needs to be calculated in different methods and often has different values,which poses significant challenges in the calculation for it.Therefore,it is of great theoretical significance and engineering application value to carry out rarefied gas flow mechanism research of the current limit components and theoretical analysis.Some main work has been carried out as following based on the dynamic vacuum calibration system developed by LIP.(1)A theoretical model of dynamic vacuum standard pressure corrected by real gas characteristics and temperature variation was established.The establishment process of fast dynamic vacuum standard pressure should reflect the dynamic characteristics of fast(in millisecond magnitude ranges)step changes(pressure increase or decrease of 1000 times).This is achieved by expansion of a test gas at 100 kPa in a small chamber via a current limit element and a fast valve into an evacuated large chamber.During the expansion,differential equations of gas mass conservation were established and solved under initial conditions,then a theoretical model of dynamic vacuum standard pressure was obtained.In addition,considering effects of real gas characteristics and temperature variation to it,the correction factors of them were introduced.The analysis of standard pressure theoretical model shows that the upstream chamber pressure decays exponentially,and the conductance of current limit element and the opening time of fast valve are the significant factors that affect the establishment time of standard pressure.(2)The gas expansion process was analyzed theoretically and the choked flow regime approximation was adopted.The gas rarefaction parameters in the period of expansion were calculated.According to the Knudsen criteria,during the gas expansion,the flow regime in upstream changes from viscous flow to slip flow,and the viscous regime is dominant,while correspondingly the downstream transfers from molecular flow to viscous flow.Since commercially available computational fluid dynamic(CFD)software can't solve the whole expansion process(from 100 kPa to 100 Pa)in all areas(including upstream and downstream areas),the choked flow approximation was adopted in this research.Also,the mathematical expression of dynamic vacuum theoretical standard pressure was obtained in the condition that ensures choked flow prevails.(3)The numerical simulation study was performed for the gas expansion process.Based on the flow regime identification and the choked flow approximation,the geometric structure of the dynamic vacuum expansion system was simplified,and correspondingly the finite element model was established by commercial CFD software,COMSOL Multiphysics.The flow filed and temperature filed distributions in upstream chamber during the gas expansion were obtained by numerical simulation.Also,according to numerical simulation results,the current limit element conductance and the corrected factors of real gas characteristics and temperature variation were calculated,respectively.(4)Experiments were performed to verify the results of theoretical analysis and numerical simulation.Two capacitance diaphragm gauges(model for CDG045Dhs)by INFICON with fullscale of 133 kPa and 133 Pa were used for the fast measurement of the pressure.The results of theoretical analysis,numerical simulation and measurement all show that gas pressure in the upstream decays exponentially throughout expansion.The maximum deviation between theoretical and measured results is about 10 ms in time and is 10%in pressure value,and correspondingly between simulated and measured results is 5 ms in time and 4.65%in pressure,respectively.The present study is helpful for a better understanding of the establishment process of dynamic vacuum standard pressure and has some reference value for the research of dynamic vacuum calibration technique and rarefied gas unsteady flow.