Study Of Heat And Mass Transfer Mechanism In Cryogenic Mixed Fluid Of Oxygen And Nitrogen Under Magnetic Field

Industrial gases are the 'blood' of modern industry,among which oxygen,nitrogen and argon are mainly produced from air separation.Cryogenic distillation is the most widely used method of air separation,with the ability to provide large quantity and high purity products.The cost of cryogenic air separation unit takes a large part in the investment and operation fees of petrochemical and steel industries,so it's of great significance to study the enhancement and energy conservation of cryogenic distillation.Currently,these studies focus on the advancement of process design,manufacture technologies and control techniques.Innovation from the aspect of distillation mechanism is quite limited,which could bring about many possible changes in the whole system.In this paper,method of cryogenic distillation coupled with non-uniform magnetic field for air separation is firstly proposed to increase the separation efficiency,by comprehensively utilizing the boiling point differences and the magnetism differences between oxygen and nitrogen.Up to now,the mechanism behind this multiphysically coupled proces's is still unrevealed thus cannot provide references for industrial experiments and applications.Under this circumstance,we have conducted numerical and experimental studies on the cryogenic distillation coupled with magnetic field from aspects of fluid flow,heat transfer and mass transfer.Main works are listed as follows:1)Free-surface flow of liquid oxygen under non-uniform magnetic field is studied using finite element analysis,and the possibility of using magnetic field to control the gas-liquid oxygen interface and flow pattern is verified.Free-surface flow is a kind of two-phase flow widely existed in the air separation unit.In this chapter,the free-surface flow of gas-liquid oxygen in a rectangular channel is numerically studied using two-dimensional phase field method,and is compared with air-water flow at room temperature.The liquid level near the center of high-magnetic channel is lifted upward by the inhomogeneous magnetic field thus forms a low-flow-velocity area under the liquid main stream.Meanwhile,as the magnetic flux density increases,the pressure drop decreases due to the expansion of cross-sectional area of the main stream.For all the inlet velocities,the pressure drop under 0.25 T is reduced by 7-9%due to the expanded local cross-sectional area,compared to that without magnetic field.This work demonstrates the effectiveness of employing non-uniform magnetic field to control the free-surface flow of liquid oxygen.2)Convective heat transfer process of liquid oxygen in a rectangular channel under alternating magnetic field is studied numerically based on a multiphyiscal model,results showing that heat transfer enhancement of oxygen enriched fluid driven by magnetic field has the advantage of high efficiency and low resistance.Compact heat exchangers mainly distributed in evaporators,main hear exchangers and subcoolers,which has paramagnetic liquid oxygen as part of the working substances,play a major role in the cryogenic air separation units.Convective heat transfer process of liquid oxygen in a rectangular channel under alternating magnetic field is studied numerically based on a multiphyiscal model.The results show that the periodic generation and shedding of vortexes caused by alternating magnetic field can effectively break the stable thermal boundary layers,thus accelerate the mixing between cold and heat fluid and improve the convective heat transfer efficiency of liquid oxygen.An 86.1%increase of overall Nusselt number is observed with the magnetic field of a single steel bar,compared with the free-flow case without any enhancement.Furthermore,heat transfer evaluation index PEC for magnetically heat transfer enhancement is 1.18-1.45,which is higher than the PEC results of traditional heat transfer enhancement with fins(0.67-1.02).All these results show that the enhancement method driven by magnetic field can effectively increase the convective heat transfer coefficient without an significant increase in pressure drop.3)Influences of non-uniform magnetic field and high gradient magnetic medium on the heat and mass transfer between liquid oxygen and liquid nitrogen are revealed through laser interferometry.The core objective for cryogenic distillation enhancement is to:increase the efficiency and to decrease the energy consumption during the two-phase mass transfer.Laser interferometry system,which is used to visualize the cryogenic mass transfer process,is designed and installed to conduct experiments on the mass transfer process under permanent magnetic field.One-dimensional continuous wavelet transform is used to extract the two-dimensional concentration distribution from the interference patterns.Results have revealed 4 stages during the mass transfer process,including liquid oxygen filling stage,stratified diffusion stage,stable evaporation stage and unstable bubbling stage.The time for the stratified diffusion under magnetic field is maintained longer than that without magnetic field,due to the magnetic force acting on the liquid oxygen.The high gradient magnetic medium,which fill between the magnet poles,can further enhance the mass transfer efficiency.With 0.5g magnetic medium,the mole fraction of oxygen decreases by 30%at 120 min from the beginning of mass transfer compared to that without medium,and decreases by 38%compared with that without magnetic field.The stratified concentration diffusion phenomenon and the bubbling phenomenon in the experiments will provide a basis for the further research and the development of the cryogenic distillation apparatus.