沿垂直壁面气-液降膜流动传质过程的数值研究

在考虑气-液两相间质量源项和能量源项的条件下,基于VOF算法,建立了水和空气沿竖直平板壁面两相降膜流动传热传质的CFD模型。利用该模型研究了水和空气两相间的传质特性,分析了液膜波动、进气进液速度以及温度对传质的影响,计算结果表明:一定程度的液膜波动、进气速度和进液速度的提高、气-液之间的温差的增加,都能强化气-液之间的传质过程。     

燃料液滴超临界蒸发综述

从常压液滴蒸发的传热传质过程出发,建立了常压液滴蒸发的数学模型。通过比较液滴超临界蒸发和常压蒸发的差异,研究液滴超临界蒸发所涉及的关键问题。以二甲基醚(DME)在氮气介质中超临界蒸发为例,介绍了状态方程法计算DME-N2体系的高压气液相平衡,在相关文献的基础上,总结了流体热物性和输运参数的变化特性。     

高温高压下机油液滴蒸发特性的数值模拟

建立了高温高压环境下双组分单液滴的一维非稳态蒸发模型.该模型可描述气/液两相质量及能量平衡、液相传热传质和相变过程.使用所建立的液滴蒸发模型,以正二十四烷(C_(24)H_(50))和正三十烷(C_(30)H_(62))作为机油的表征组分,分析了双组分机油液滴蒸发过程中液滴温度和组分摩尔分数分布的变化趋势,并对比了相同环境条件下机油与异辛烷液滴的不同蒸发特性.在此基础上,研究了环境压力、环境温度和液滴初始半径对机油液滴寿命、液滴蒸发百分数、液滴温度和组分摩尔分数等的影响.结果表明:在高温高压环境下,机油液滴能够留存较长时间,形成缸内高温早燃源的可能性较大.     

重力热管内部相变及传热传质过程的数值模拟

重力热管内部包含复杂的两相流动以及相变传热过程,传统理论分析及实验手段不能直观给出其内部流动、相变、热质传递的详细信息。采用VOF(volume of fluid)多相流模型对重力热管内气液两相流动及传热进行模拟,捕捉到蒸发段气泡产生、合并、长大、上升,以及冷凝段壁面附近液滴形成、合并、下滑、汇集到液池的全过程,得到的壁温分布与实验测量值对比体现良好一致性,表明数值模拟的正确性。同时,以热阻、传热量和热效率为评价标准,研究不同充液率和倾斜角度下对重力热管运行性能的影响。结果表明:在所研究的参数范围内,随着充液率的增加,热阻逐渐减小,冷凝段传热量逐渐增大。且工质初始充注量充满蒸发段时热管性能较好;倾角对热阻的影响不明显,冷凝段传热量和热效率均随倾角增加而增长。     

分子动力学方法研究纳米尺度下液滴蒸发的物理规律

采用分子动力学方法对纳米尺度下氩液滴在氩蒸气中蒸发过程进行了模拟,其中液相分子采用球形截断的Lennard-Jones势能函数描述。模拟过程首先在三维模拟空间产生准稳态平衡的液滴和周围气相环境,随后控制液滴的外界物理条件形成蒸发现象,同步记录气液两相分子坐标和动量变化,从微观信息中统计计算出相应的宏观物理信息。研究了蒸发初始液滴半径的不同研究其对液滴蒸发过程的影响,结果表明纳米尺度下液滴蒸发现象与微米以上尺度液滴蒸发现象存在差异;引入等效辐射能的概念在分子动力学方法中实现了对辐射能传递过程的模拟,证实了辐射传递能量会对纳米尺度液滴蒸发过程产生很大的影响。     

高温气流中双液滴蒸发过程实验研究

建立了液滴蒸发的实验系统,采用悬挂液滴法对高温气流中单、双液滴的蒸发特性进行研究.实验结果表明：双液滴实验时的液滴蒸发过程与单液滴蒸发过程类似;液滴间相互作用使液滴周围蒸汽的浓度增大,气液传质浓度差减小,液滴与周围环境的传质速度降低,使蒸发速率减小;在纯辐射环境中液滴间相互作用对蒸发过程的影响较强,在辐射对流环境中液滴间相互作用对蒸发过程的影响较弱.     

竖直螺旋槽管壁面液膜在蒸发/冷凝时的传热特性的研究

研究竖直螺旋槽管壁面液膜在传热条件下的液膜形成及流动特性，建立了单组分流体的物理和数学模型并得出解析解，且分析了壁面液膜在蒸发，冷凝及无热传输时的液膜厚度分布及速度分布，结果表明，液膜的形状主要受表面张力影响，在表面内弯处流膜较厚，而在槽道起始部液膜较薄，相对于光滑直管，竖直螺旋槽管壁面液膜具有均匀的厚度分布和更好的传热传质性质，特别在冷凝时壁面液膜更薄且分布更加均匀。     

海水淡化中水平管降膜蒸发器传热传质的研究现状

水平管降膜蒸发器广泛应用于制冷、食品和海水淡化领域,其传热传质过程直接影响到整个蒸发器的性能,因而受到各国研究者的重视。介绍了水平管降膜蒸发器的传热传质过程及影响因素,通过对传热温差、蒸发温度、热通量、喷淋密度、管束布置方式等对液膜厚度、液膜流动状态、热阻、液体黏度、表面张力等的影响分析进而得出其对传热传质过程的影响。     

加热板上液滴沸腾实验研究

采用高速摄像技术研究了不同加热表面上液滴蒸发和沸腾的相变特性和壁面温度变化特性,讨论了局部相变行为对壁面温度变化的影响.同时定量的研究了三种不同表面特性的加热板对沸腾和传热的影响,以及液滴初始体积对相变的影响.结果表明,表面特性和液滴尺寸对沸腾传热有较大影响.     

环形布膜器垂直管内R113降膜蒸发换热特性数值研究

通过建立垂直管内降膜蒸发物理数学模型,对环形插头型布膜器管内R113的气液两相逆流降膜蒸发换热特性进行二维非稳态数值研究,分析了管内液膜流动分布以及壁面温度和液膜表面温度分布,对比了加热前后液膜厚度的变化.结果表明:随着降膜蒸发过程的进行,液膜下端开始出现液滴飞溅,且不断向上端发展;R113在管内降膜蒸发过程中壁面温度和液膜表面温度沿流动方向逐渐升高,气相温度变化趋势则相反;从壁面到管中心,温度沿径向逐步降低,在近壁面1mm前后其分布趋势相反;加热后液膜厚度明显减小,且下游液膜厚度变得相对均匀.     

正丁醇水溶液液滴的蒸发特性实验研究

自湿润流体是一种具有特殊的表面张力特性的二元流体,了解其蒸发传热特性对于揭示其强化传热机理十分重要.为了探究添加自湿润流体液滴的蒸发特性,采用液滴形状分析仪(DSA100)研究了不同温度(30、40、50、60℃)下铜底板上去离子水、正丁醇水溶液(质量分数为0.5％)液滴的蒸发特性.结果 表明:加入少量正丁醇溶液并不影...     

Interacting effects of uniform flow, plane shear, and near-wall proximity on the heat and mass transfer of respiratory aerosols

Individual and interacting effects of uniform flow, plane shear, and near-wall proximity on spherical droplet heat and mass transfer have been assessed for low Reynolds number conditions beyond the creeping flow regime. Validated resolved volume simulations were used to compute heat and mass transfer surface gradients of two-dimensional axisymmetric droplets and three-dimensional spherical droplets near planar wall boundaries for conditions consistent with inhalable aerosols (5 ? d ? 300 μm) in the upper respiratory tract. Results indicate that planar shear significantly impacts droplet heat and mass transfer for shear-based Reynolds numbers greater than 1, which occur for near-wall respiratory aerosols with diameters in excess of 50 μm. Wall proximity is shown to significantly enhance heat and mass transfer due to conduction and diffusion at separation distances less than five particle diameters and for small Reynolds numbers. For the Reynolds number conditions of interest, significant non-linear effects arise due to the concurrent interaction of uniform flow and shear such that linear superposition of Sherwood or Nusselt number terms is not allowable. Based on the validated numeric simulations, multivariable Sherwood and Nusselt number correlations are provided to account for individual flow characteristics and concurrent non-linear interactions of uniform flow, planar shear, and near-wall proximity. These heat and mass transfer correlations can be applied to effectively compute condensation and evaporation rates of potentially toxic or therapeutic aerosols in the upper respiratory tract, where non-uniform flow and wall proximity are expected to significantly affect droplet transport, deposition, and vapor formation.     

Numerical investigation on the evaporation of droplets depositing on heated surfaces at low Weber numbers

The evaporation of water droplets, impinging with low Weber number and gently depositing on heated surfaces of stainless steel is studied numerically using a combination of fluid flow and heat transfer models. The coupled problem of heat transfer between the surrounding air, the droplet and the wall together with the liquid vaporisation from the droplet’s free surface is predicted using a modified VOF methodology accounting for phase-change and variable liquid properties. The surface cooling during droplet’s evaporation is predicted by solving simultaneously with the fluid flow and heat transfer equations, the heat conduction equation within the solid wall. The droplet’s evaporation rate is predicted using a model from the kinetic theory of gases coupled with the Spalding mass transfer model, for different initial contact angles and substrate’s temperatures, which have been varied between 20–90° and 60–100 °C, respectively. Additionally, results from a simplified and computationally less demanding simulation methodology, accounting only for the heat transfer and vaporisation processes using a time-dependent but pre-described droplet shape while neglecting fluid flow are compared with those from the full solution. The numerical results are compared against experiments for the droplet volume regression, life time and droplet shape change, showing a good agreement.     

Interaction of transfer processes during unsteady evaporation of water droplets

The change of water droplets state is modelled numerically under various heat and mass transfer conditions during their unsteady evaporation. The modelling is performed using the method of combined analytic–numeric research of heat and mass transfer in a two-phase “droplets–gas” flow. The algorithm of an iterative research is constructed for the analytically obtained system of integral equations. Regularities of heat transfer process interaction are examined. The dependence of the droplet state change on its heating manner is determined. Unsteadiness and interaction of transfer processes, as well as selectivity of radiant absorption in water droplets are evaluated. It is indicated that cognition of the droplet state change regularities in the case of conductive heating is very important in determination of two-phase flow and in construction of an engineering research method.     

Modeling gas oil spray coalescence and vaporization in gas solid riser reactor

The sprayed feed droplet behavior, including coalescence and vaporization into gas–solid flow, is complex especially near the atomizer region in fluid catalytic cracking (FCC) riser reactor. A three dimensional CFD model of the riser reactor has been developed, which takes into the account three phase hydrodynamics, heat transfer and evaporation of the liquid droplets into a gas–solid flow as well as phase interactions. A hybrid Eulerian–Lagrangian approach was applied to numerically simulate the collision and vaporization of gas oil droplets in the gas–solid fluidized bed. This numerical simulation accounts the possibility of coalescence of feed spray droplets in computing the trajectories and its impact on droplet penetration in the reactor. The modeling result shows that droplet coalescence mainly occurs at the initial part of the atomizing region and where three phase flow hits the reactor wall and bounces back. The model has the ability of inspecting the effects of feed injector geometry on the overall reactor hydrodynamic and heat transfer. The CFD simulation results showed that the evaporated droplet gas caused higher local velocities of the gas and solid particles and gas–solid flow temperature reduction.     

不同润湿性表面液滴铺展及蒸发过程的LBM数值模拟

基于格子玻尔兹曼方法（Lattice Boltzmann Method, LBM）对固着在加热基板上的液滴铺展及蒸发过程进行模拟,主要研究重力场、基板润湿性以及初始环境温度对液滴铺展及蒸发过程的影响。通过预测蒸发过程中液滴与基板的接触直径变化和液滴剩余质量变化,分析液滴形状及体积变化。研究结果发现,液滴形貌及蒸发过程受重力影响较大,重力作用下液滴铺展现象明显且蒸发加快。基板的接触角越小,液滴铺展现象越明显,其接触直径越大,蒸发越快。当环境温度与基板温度相差较大时,液滴内部出现涡流,强化换热使蒸发过程加快。     

An experimental investigation of sessile droplets evaporation on hydrophilic and hydrophobic heating surface with constant heat flux

Droplet evaporation widely exists in the daily life and industrial production. In most of previous experimental studies, the evaporation of sessile droplets was conducted under a constant substrate temperature condition. However, drops often evaporating on a heating surface under a constant heat flux condition in many practical applications. In this paper, we have carried out an experiment on sessile 3 μl DI water droplets evaporated on hydrophilic and hydrophobic heating surfaces under constant heat flux in the range from 1153 W/m² to 6919 W/m². A high-speed camera was used to record the changing shapes of two sessile droplets on a hydrophilic and a hydrophobic heating surface placed side by side. The droplet height, dynamic contact angle, droplet contact diameter, evaporation mode and evaporation rate are presented.     

Interaction of the transfer processes in semitransparent liquid droplets

The study presents the mathematical model of unsteady heat transfer in evaporating semitransparent droplets of non-isothermal initial state and the numerical research method, evaluating selective radiation absorption and its influence on the interaction of transfer processes. The relation of the transfer processes inside droplets and in their surroundings and the necessity of thorough research of these processes are substantiated. When modeling the combined energy transfer in water droplets, the evaluation of thermoconvective stability in evaporating semitransparent liquid droplets is presented; the influence of the droplet initial state on its heating and evaporation process is investigated. The influence of heat transfer peculiarities on the change of the evaporating droplet state is indicated. Main parameters, which decide the peculiarities of the interaction of unsteady transfer processes in droplets and their surroundings, are discussed. The results of the numerical research are compared to the known results of the experimental studies of water droplet temperature and evaporation rate.     

Comparative Investigation on Droplet Evaporation Models for Modeling Spray in Cross-Flow

The coupling model of flow and heat and mass transfer for gas-spray droplet two-phase flow has been developed to simulate the evaporating spray in cross-flow. The correlations used for describing the droplet evaporation and motion in convective flow have been compared. The comparisons of calculated results show that the different correlations for determining Nusselt number and Sherwood number impose a significant influence on the lifetime of droplet. The modification of Nusselt number and Sherwood number with regard to the heat and mass boundary around the droplet is of great importance, while different mixing laws for mixture properties and different drag coefficient equations only demonstrate a slight effect on the evaporation characteristics of droplet. The characteristics of spray droplets and cross-flow in terms of both evaporation and motion are obtained. The secondary flow phenomenon is observed in the simulation results and contributes to achieving a more even distribution of temperature and an improved mixing effect of the vapor and cross-flow.     

Flow and thermal field in sessile droplet evaporation at various environmental conditions

Sessile droplets' evaporation is a complex process that involves fluid flow coupled with heat and mass transfer. In this study, mathematical modelling of sessile droplet evaporation on hydrophobic substrates is developed and simulations are carried out on COMSOL. The model results are validated with the data available in the literature. Postvalidation, the simulation of droplet evaporation is carried out on the various substrate hydrophobicities and various environmental conditions. For these conditions, contours are plotted for temperature, velocity, and mass concentration for the droplet and moist air domain. The result shows that Marangoni convection plays a very important role in droplet evaporation. A high rate of evaporation is observed at the droplet interface at low relative humidity and a large degree of subheating. The effect of air velocity on the evaporation rate is studied, however, its effect is very marginal as compared to relative humidity and degree of subheating. The heat flux at the three-phase contact line is large for a smaller Prandtl number fluid. Overall, the evaporation rate increases with increasing the Prandtl number because it has a large value of Marangoni convection.