Investigation Of The Pressurized Gas Diffusion Properties Using Large Lateral Shearing Interferometry

Gas diffusion under the high pressure plays a significant role in chemical engineering,industrial production,combustion science,pollution control and many other fields.For chemical reactions,diffusion can be a rate control step in the reaction process.Since the coal combustion/gasification reaction under the high temperature and high pressure is already in the diffusion-controlled zone,conversion rate will not increase with reaction temperature.on the contrary,the refractory material of the reaction chamber will be ablated.Therefore,by studying the gas diffusion properties under high pressure conditions,we can find the critical temperature of the diffusion-controlled zone.The gas diffusion coefficient in the layer at the surface of the coal particles during the coal pressurized combustion/gasification is an important parameter for predicting the critical temperature of the diffusion-controlled zone.Theoretically predicted data of the pressurized binary gas diffusion coefficients differ from the experimental value by more than 20%.Therefore,it is of great significance to study the gas diffusion under high pressure.With the advantages of non-destructive and high-precision,optical interferometryhas been exhaustively utilized in liquid diffusion,only a few applications for gaseous systems has been reported.It's because the change of the interference fringe caused by the gaseous diffusion is very small,which increases the difficulty of the processing of the images.Large lateral shearing interferometry has advantages such as simple structure and anti-vibration capability,and its fringe pattern is similar to the simple one of double exposure holographic interferometry.The inherent background fringe pattern can provide the"basic fringe"for the interference image of the gas diffusion process with small refractive index change.The analysis of the fringe pattern image becomes easy.Therefore,it is very suitable for the research of high-pressure gas diffusion.In this paper,large lateral shear interferometry is used for the first time to study the pressurized binary gas diffusion.An experimental setup which can be used under high pressures?pressure?10 MPa?has been built up.Experiments of one dimensional binary gas diffusion under the atmospheric and pressurized environment have been carried out.The directions of the diffusion are horizontal and vertical.Large lateral shear interferometry improves the diffusion coefficient measurement accuracy.The binary gas diffusion coefficient at atmospheric pressure and the concentration distribution of horizontal diffusion and vertical diffusion of pressurized binary gas diffusion have been obtained.The interference fringe patterns of the experiments of CO₂-O₂ and N₂-O₂ horizontal diffusion under atmospheric pressure have been analysed by the fixed-point method and the peak method to analyse the fringe patterns.Diffusion coefficients of CO₂-O₂ and N₂-O₂ under atmospheric pressures have been obtained and the maximum average absolute deviation between the experimental results and the reference values is 1.94%.The fixed-point method has less error than the peak method.In the experimental conditions of each gas pair,error of the condition with the highest pressure is the largest.Furthermore,the experimental results of CO₂-O₂ diffusion are more deviated from the literature values than that of the N₂-O₂diffusion.That is because the fact that the flow caused by the difference in density of the two components accelerates the diffusion process.The effect of density difference increases with pressure.It can be seen that the horizontal diffusion experiment is more suitable for gas pairs with similar molecular weight.Experiments of pressurized one dimensional binary gas?N₂-O₂ and CH₄-O₂?diffusion in the horizontal and vertical directions have been carried out,respectively.Changes of the large shear interference fringe patterns and gas concentration field indicate that the influence of the difference in the density of the component gases on the diffusion include:1)after the diffusion begins,gas with higher density sinks to the lower part of the diffusion cell,while the gas with lower density floats to the upper part,forming a concentration gradient in the vertical direction.Interference fringe patterns and concentration distributions show distinct two-dimensional diffusion characteristics.The diffusion process cannot be considered as one-dimensional diffusion.;2)in the horizontal direction,the diffusion rate is greatly accelerated,and the diffusion process is no longer considered as free diffusion.Peak method and fixed-point method based on one-dimensional free diffusion theoretical model are not suitable for the pressurized experiments.Effects of the difference in density of the component gases on the diffusion increase with pressure.The greater the difference in density,the stronger the convection effect.In addition,the horizontal diffusion experiments of the two gas pairs show the adsorption of gas near the upper and lower walls of the diffusion cell.The interference fringe pattern and the concentration distribution indicate the phenomenon of gas residue caused by the wall adsorption.This phenomenon is more evident in CO₂-O₂ diffusion experiments.The experimental results of N₂-O₂ and CH₄-O₂ vertical diffusion show that the interference fringe pattern and the gas concentration field change regularly.The diffusion direction is generally vertical.Interference fringe patterns of CH₄-O₂ diffusion show local oscillations occur in the horizontal direction.This may be due to the unbalanced potential caused by the difference in density of the component gases.Furthermore,when the component gas with lower density is arranged in the upper diffusion cell,the diffusion time is prolonged much,and it cannot be explained by the existing diffusion theory.In the coal/char pressurized combustion reaction,combustion reaction is controlled both by gas diffusion and chemical kinetics,and three temperature zones?chemical kinetics zone,chemical-diffusion zone,diffusion controlled zone?are formed.The critical temperatures of three zones are very important for the research on the mechanism of the coal/char combustion reaction.In this paper,based on the diffusion flux equation of oxygen molecules in coal/char under normal pressure,considering the real gas effect,the average free paths of oxygen under the high pressures are determined.Then the diffusion rates of oxygen molecules in the pores of char under the high pressures are calculated.Based on the data of the combustion rates calculated from the reaction kinetics model of the simple collision theory and reported in the other literatures,the critical temperatures of three zones under normal pressure and high pressure are determined.The results show that the critical temperatures of the three control zones are 1360 K and 1600 K under normal pressure,while the critical temperatures are 1200K and 1400 K under pressurized condition?1.0 MPa?.It is observed that the increase in the pressure favors the formation of diffusion-controlled zone.