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Research on Aerosol Particle Moving and Heat/Mass Transferring under Temperature Gradients

Author: ChenZhiLiang
Tutor: ZhuJiaZuo
School: Sichuan University
Course: Chemical Engineering
Keywords: aerosol particle non-uniform temperature field thermophoresis force buoyancy force diffusion liquid droplet heat/mass transfering
CLC: O648.18
Type: PhD thesis
Year: 2012
Downloads: 38
Quote: 0
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Abstract


Research on the movement, heat transfer and mass transfer of water particle in gas, has important rolenot only on understanding the movement, evaporation,condensation,PM2.5movement mechanism andmultiphase chemical reaction process in the atmosphere. And there are a lot of researches on this field athome and abroad.Specially, many models about the effect of temperature gradient on the thermophoreticforce of particle and on the mass and heat transfer between particle and environment are put forward on.Based on the work by predecessor, this paper adopted theory analysis, caculation and numericalsimulation method, studied the mass and energy transfer under temperature gradient,the mass and heatbetween particle and gas,the moving force of particle in different temperature.Firstly, the water vapour diffusion under temperature gradient was investigated, indicating that bothtemperature changing and water vapour concentration influenced the gas properties. Temperaturegradient also affected on the water vapour diffusion and gas properties.In the field of gravity,the stability of system depends on the relative buoyancy value from the thermalbuoyancy and the buoyancy by sorer effect. When the thermal buoyancy is less than the buoyancy causedfrom soret effect,lighter water vapour distributes at bottom place and the soret effect is dominant and thegas density at the top place is greater.The gas at the top sinks and the temperature gradient andthermophoretic force does no longer exist, then the turbulent flow affected on the particle. The turbulentflow makes temperature uniform and mass and energy transfer between gas region enhancing.When the water vapour concentration at bottom exceeds the saturated water vapor concentration, thewater vapour begins to condense on the wall, resulting that the water vapour continously diffuses fromhigher temperature region to lower temperature region,and increase mass and energy transfer. Thetemperature gradient influence on not only the water vapour concentration redistributed but also thewater vapour diffusion. The diffusion coefficient at uniform temperature condition is different from thatat temperature gradient condition. In this paper, by the chemical potential driving force,a diffusionformula at convective condition is derived under temperature gradient as followed. X is the distance of temperature gradient,xA is water vapour mole fraction, D is diffusion coefficient, c is water vapour concentration,S is molar entropy of water vapour, Cp is mole heat capacity, R is gas constant,This formula indicates the enhancement of diffusion by temperature gradient from low temperature to high temperature and the weakness from high temperature to low temperature.By the method of derivation of diffusion flux,deduce a new formula that temperature gradient influences water vapour diffusion:In this formula, DO is diffusion coefficient at298K,T is temperature, c is concentration,ΔI is changing water vapour diffusion flux by temperature gradient. The result by former two formula that temperature gradient influences water vapour diffusion is same.The water vapour is lighter so that the buoyancy is produce. This buoyancy effects the water vapour concentration redistribution and the water vapour diffusion.Many reseachers report the buoyancy effect, but no quantitative result is reported till now.In this paper, various factors are summed up to chemical potential using Nikolai Kocherginsky, then the buoyancy of lighter water vapour and the influence on the water vapour concentration redistribution and water vapour diffusion are analyzed by the chemical potential. When temperature is constant and all the other parameters are the same except the buoyancy(gravity), the deduced formula that diffusion influenced by buoyancy (gravity) is given:D is diffusion coefficient, c is the mole density of mixed gas. Mig is water vapour buoyancy by air. Mi molar mass, is the mole mass difference between water vapour and ambient gas.When the buoyancy direction h and the diffusion direction are same, the diffusion of water vapour is enhanced. When buoyancy direction h and the diffusion direction are opposite, the diffusion of water vapour diffusion is weakened.In this study, the comprehensive effect of soret effect, thermal buoyancy and water gravitational potential energy makes the concentration gradient stable with the lower density at top and higher density at bottom to keeping top air from depositing.Also we compare the variable heat and mass transfer model between particle and environmental gas and choose the correct model, and express with a equation correlated with Nu,Sh and Re, Re is caculated with relative velocity between particle and ambient air. There are three kind of heat transfer between particle and ambient gas including the heat by watervaper condensing or water particle evaporation, the thermal conductivity and the heat by the transfervapour caused from temperature difference between particle and environmental gas.The temperature difference between particle and ambient gas and water vapoure content is importantfor the calculation of various kind of heat. When the temperature of particle and environment is close andthe water vapour concentration is greatly different, the heat released by water evaporationandcondensation is dominatant. When the temperature of particle and environment is greatly different and hewater vapour concentration is close, the thermal conductivity or the heat by convection between particleand medium is dominatant. Generally, the heat brought by water vapor due to medium temperature andparticle temperature differences is very small relative to the other two kinds heat transfer, but when theparticle surface and the ambient gas temperature difference is larger, the heat brought by water vapor dueto medium temperature and particle temperature differences becomes larger, the ignorance of this heatcan cause mistake.At the saturated condition, when a dynamic equilibrium is achieved, the net heat transfer is zero. Forthe unsaturated air, the evaporation of water droplets in the air lowers the temperature. With thedecreasing of relative humidity, the temperature difference is bigger and the heat transfer amountbetween particle and environmental gas is increased when achieving a balance. When there is no relativevelocity between particle and gas, the balance temperature almost can not be influenced by particlediameter. Unless particle diameter is small to10-8m, the size of particle has no obvious influence on thesaturated vapor pressure. When there is relative velocity between particle and gas, particle equilibriumtemperature variation is main factor by particle diameter influence on Nu and Sh value(Re value infomula).Although there is relative velocity between particle and the saturated gas, net mass and heat transferbetween particle and gas is zero when achieving a balance. When there is relative velocity betweenparticle and the unsaturated gas, this kind of relative velocity can increase mass and energy transferbetween particle and gas, but has very small influence on equilibrium temperature. Because this velocitynot only increases particle evaporation and water vapour condensation, but also increases thermalconductivity or convective heat transfer. When relative velocity increases, the mass transfer betweenparticle and gas becomes great, the heat by temperature difference between particle and environmentalgas of transfer vapour also become great at this condition. If this heat transfer is neglected, greatermistake may appear, but when there is no relative velocity, this heat transfer is very small and can beneglected.Research on the inert particle movement(without mass exchanging between particle and ambient air)and active particle movement(with particle evapouring or condensing).The forces acted on static particle (or with very low relative velocity)in air include gravity,thermophoretic force,saffman liftforce,brownian force.Thermophoretic force is only existed in the temperature gradient condition,and iscaused by unuiform impact momentum. To the very tiny particles, brownian force has decisive role;tothe larger particles, gravity has decisive role. There is no necessary to count brownian force andthermophoretic force for larger particle,to the tiny particle whose diameter below10-9m,there is nonecessary to count gravity and thermophoretic force.Thermophoretic force varies with temperature gradient value, thermophoretic force works whenparticle diameter between10-5-10-8m under most temperature gradient conditions. Thermophoretic forcevaries with particle diameter, temperature gradient value and the air condition. the diameter of particles issmaller than10-9m,For the moving water drop under temperature gradient, the water particle temperature at different parttends to be equal because of vapouration and condensation of the water drop. Particle at highertemperature area maybe evaporate and absorb more heat or condenses less and releases less heat. Particleat lower temperature area maybe evaporates less and absorb less heat or condenses more and releasesmore heat.

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CLC: > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Colloid chemistry ( physical and chemical dispersion system ) > Colloid > Aerosols ( smoke,fog )
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