【作者】 陈治良；

【导师】 朱家骅；

【作者基本信息】 四川大学 ， 化学工程， 2012， 博士

【摘要】 气溶胶颗粒在温度场中的运动与传热传质研究,对了解大气中水份蒸发、凝结和PM2.5迁移机理以及多相化学反应过程均有重要意义,国内外对此进行了大量研究,对温度梯度作用于颗粒产生的热泳力、温度梯度对传热传质的影响提出了多种模型。本文在前人工作基础上,采用理论分析、计算及数值模拟方法,分析研究了温度梯度条件下气溶胶颗粒与气相介质的传热传质及运动受力。首先研究了温度场中水蒸汽的扩散,表明温度变化和组分扩散对气体性质产生影响。温度梯度又影响气体组分扩散,对气体性质产生影响。重力场中,热浮力与soret效应产生的浮力的相对大小决定了体系的稳定性。当热浮力小于soret效应产生的浮力,即soret效应使较轻的分子富积在下部,而这种因素又占主导的话,上部空气较重而沉积,原有的温度梯度就会被破坏,原来的热泳力转变成湍流作用对颗粒产生的作用力。这种效果能增强热量传递和使体系温度均匀当下部冷端的水蒸汽浓度超过饱和水蒸汽浓度时,水蒸汽开始凝结,凝结的结果使水蒸汽不断由高温向低温扩散,从而加强了水蒸汽扩散与热流的传递。温度梯度不但影响各类气体分子组分浓度的重新分布,还影响气体分子的扩散,即温度梯度下的扩散系数与均匀温度条件下的扩散系数是有差别的。采用化学势为推动力,推导出了在对流存在情况下,温度梯度对于水蒸汽扩散影响作用的公式：方程式中,xA为水汽的摩尔分数,D为扩散系数,c为水蒸汽的浓度,S为水汽的摩尔熵,Cp为摩尔热容,R为气体常数。该方程式计算表明温度梯度能在较大程度上减缓水汽由高温向低温的扩散,但能增强由低温向高温的扩散。采用直接对扩散通量求导的方法,推导出的温度梯度对于水蒸汽扩散影响作用的公式：以上方程式中D0为298K时的扩散系数,T为温度,c为浓度,△I为温度梯度对扩散通量的影响。以上两个温度梯度对扩散影响的公式计算结果一致。水蒸汽在空气中,质量较轻,还会产生浮力,这种浮力既影响气体浓度分布,又影响组分扩散。不少研究者都指出了这种影响,但尚未有定量研究的报道。本文采用Nikolai Kocherginsky方法,并将各种影响因素归结到化学势中,定量表达水汽比空气轻的浮力因素对水蒸汽浓度分布及其扩散的影响。当温度恒定,除浮力(重力)外其它因素都相同的情况下,考虑浮力(重力)因素对扩散的影响时,推导出的重力因素对扩散的影响：D为扩散系数,c为混合气体摩尔密度,Mig为空气对水汽的浮力。当浮力方向与扩散方向一致时,浮力的效果是增强扩散；反之则反。在本研究中,热浮力、soret效应及水汽重力因素产生的浮力效应,都使气体上部密度低而下部密度高,阻止了上部空气沉降,从而形成稳定的浓度梯度。对各种颗粒与介质的传热传质模型进行了比较研究,从中选择了合适的颗粒传热传质方程,并表达为努塞尔数Nu和舍伍德数Sh与雷诺数Re准数关联式,Re由颗粒与空气相对运动速度计算。水滴(液态颗粒)与气相介质的传热包括水汽蒸发或凝结所吸收或释放的热量、颗粒与介质间的导热、介质温度与颗粒温度差异所携带的热量。计算这些不同形式的热量,颗粒表面与环境介质的温度差及水汽含量差有重要影响。当颗粒与环境温度相差不大,而水汽含量差较大时,水滴蒸发或凝结所释放热量占主导地位；当颗粒与环境温度相差较大,而水汽含量差较小时,颗粒与介质间的对流传热或导热占主导地位。通常情况下,水蒸汽由于介质温度与颗粒温度差异所携带的热量相对于其它两种热量较小,但当颗粒表面与环境气体温度相差较大时,水蒸汽由于介质温度与颗粒温度差异所携带的热量变得较大,忽略这种热量会造成误差。在饱和情况下,颗粒与介质物质能量交换达到动态平衡,物质和能量净的交换量为0。对于不饱和空气,水滴在空气中蒸发使温度降低。随着空气相对湿度降低,颗粒吸收热量与释放热量相等、与介质达到平衡时温度相差越大,平衡时交换的热量越多。在颗粒与气体没有相对速度的情况下,达到平衡的温度几乎与颗粒大小无关。除非是直径小到10-8m左右的颗粒,颗粒大小对饱和蒸汽压的影响一般不太明显。当颗粒相对于气体运动时,颗粒吸收和释放的热量相等时的平衡温度随颗粒大小的变化,主要是由于颗粒大小影响Nu与Sh值(影响其计算式中Re值)。即使颗粒与空气有相对运动速度,如果为饱和空气,颗粒与环境气体达到平衡时仍然没有净的物质和能量传递。不饱和空气在与颗粒有相对运动速度时,相对运动速度能增加颗粒与气体的物质和能量传递,但对达到平衡的温度影响很小,因为这种相对速度促进了颗粒蒸发、吸收较多能量,也能促进空气能量向颗粒的传递。当相对速度较大时,蒸汽扩散量显著增加,忽略扩散气体由于颗粒与介质气体间温差传递的热量可能会导致较大的误差,这种形式的能量在颗粒与气体没有相对速度时,相对于其它能量传递形式往往是非常小的,可以忽略。对不均匀温度场中惰性颗粒（与周围气体没有物质交换）及活性颗粒（颗粒蒸发或凝结）的受力运动进行了研究。静止在气体中的颗粒（或相对运动速度非常小的情况下）的受力主要包括重力、热泳力、saffman升力等。热泳力是温度梯度环境中才存在的力，是由于温度梯度造成颗粒受到不均匀碰撞动量引起。研究表明，对极小颗粒，布郎力起决定性作用；对较大颗粒，重力起决定性作用。大颗粒不必考虑布郎力和热泳力，对于10-9m以下的细小水珠，重力和热泳力也不必考虑。研究表明，不均匀温度条件下运动的水滴,由于水滴的蒸发或凝结,颗粒温度变化是使各处颗粒温度趋于相互接近。高温处颗粒要么蒸发较多，吸收较多热量，要么凝聚较少，释放较少热量。低温处颗粒要么蒸发较少，吸收较少热量，要么凝聚较多，释放较多热量。更多还原

【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.更多还原