Heat transfer at evaporation of falling films of Freon mixture on the smooth and structured surfaces

The paper presents the results of experimental investigation of heat transfer and hydrodynamics of falling films of binary mixtures of R21 and R114 freons on the surfaces with complex configuration. The vertical tubes of 50-mm diameter with the smooth and structured surfaces, made of D16T alloy, were used as the working sections. The range of film Reynolds number at the inlet to the working section was Re =10÷155. The image of wave surface of the falling liquid film was visualized and recorded by a high-speed digital video camera. At evaporation the heat transfer coefficients on the smooth and structured surfaces are determined by the liquid flow rate and weakly depend on the heat flux. At low liquid flows, the heat transfer coefficients on the structured surface decrease in comparison with the smooth surface because of liquid accumulation and enlargement of efficient thickness in microtexture channels. At high liquid flows, a change in the structure of the wave film surface leads to an increase in heat transfer coefficients in comparison with the smooth surface.     

Heat transfer and crisis phenomena at boiling in Freon mixture films falling down the structured tube

Investigation results on hydrodynamics, heat transfer, and crisis phenomena in laminar-wave liquid films falling down the surfaces of different geometry are presented in this study. Freon mixture R21/R114 with initial concentration of low-boiling component of 4?C16.6 % was used as the working liquid. The film Reynolds number at the inlet to the experimental section varied from 60 to 700. The heat flux density was changed within 0?C5 W/cm². The images of wave surface of the falling liquid film and formation of dry spots were visualized and recorded by the high-speed video camera. Results of investigation of the wave structure of the film surface, measurements of heat transfer coefficients under the conditions of boiling, and critical heat fluxes in the film flow over the smooth and structured surfaces are presented.     

Spreading of a viscous liquid film over the corrugated surface and the heat and mass transfer calculations

There are a lot of industrial applications of structured packing. Distillation columns are one of the examples where the liquid flows over the corrugated surface as a thin film to provide a good mass-transfer surface between the liquid and vapor phase. The purpose of the present paper is to study the hydrodynamics and the heat-mass transfer of the liquid film spreading down the corrugated surfaces when the corrugation amplitude is comparable with Nusselt’s film thickness (the amplitude corresponds to a small texture of the structured packing). As a result, a nonlinear type diffusion equation is obtained to describe the evolution of the film thickness profile. The nonlinear diffusion coefficient is obtained for three cases: a smooth inclined plate, a corrugated plate with large ribs, and an inclined corrugated plate with small ribs. The equations are solved numerically. As a result, it has been obtained that the small texture significantly increases the rate of the film thickness evolution in comparison with a smooth plate. To obtain the nonlinear diffusion coefficient in the case of a small texture, the hydrodynamics of the film flow over an inclined corrugated surface are studied. The viscosity, inertia, and surface tension forces are taken into account. The calculations were carried out on the basis of the Navier-Stokes equations. The influence of the microcorrugations on both the heat transfer from the wall and the mass transfer through the free surface was investigated.     

液滴撞击液膜过程的格子Boltzmann方法模拟

本文采用格子Boltzmann方法对液滴撞击液膜过程进行了研究, 主要考察了雷诺数(Re)、韦伯数(We)、相对液膜厚度 (h) 以及表面张力 (σ) 等物理参数对界面运动过程的影响. 首先, 随着Re数和We数的增加, 可以明显观察到液滴撞击液膜过程中形成的皇冠状水花以及卷吸现象; 当Re数较大时, 液体会发生飞溅, 由液体飞溅形成的小液滴则会继续下落, 并与液膜再次发生碰撞. 其次, 当相对液膜厚度较小时, 液滴撞击液膜并最终导致液膜断裂; 然而随着相对液膜厚度的增大, 尽管撞击过程溅起的液体会越来越多, 但是液膜并不会发生断裂. 再次, 随着表面张力的增大, 界面变形阻力增大, 撞击过程中产生的界面形变也逐渐减弱. 最后还发现皇冠(由液滴溅起形成)半径r 随时间满足r/(2R) ≈ α√Ut/(2R), 这一结果与已有结论是一致的.     

表面弹性和分离压耦合作用下的垂直液膜排液过程

针对含不溶性活性剂的垂直液膜排液过程,在考虑表面弹性和分离压耦合作用的基础上,采用润滑理论建立了液膜厚度、表面速度和活性剂浓度的演化方程组,通过数值计算分析了表面弹性和分离压单独作用和耦合作用下的液膜演化特征.结果表明:表面弹性与分离压均对垂直液膜排液过程有显著影响.表面弹性单独作用时,液膜初始厚度随弹性增大,黑膜仅在液膜顶部形成,长度较短且不能稳定存在;分离压单独作用时,活性剂随流体不断汇集在底端,液膜表面无法形成表面张力梯度,不发生逆流现象;当二者耦合作用时,可得到较稳定的液膜,排液前期增加表面弹性可提高液膜的厚度、降低表面速度和促使液体逆流,从而减缓排液过程;后期出现黑膜后,分离压中的静电斥力起主要作用,延缓液膜"老化".     

INTERFACIAL WAVES WITH CONDENSATION OF A VAPOR FLOWING INSIDE A HORIZONTAL,RECTANGULAR DUCT

Interfacial waves on condensate films inside ducts increase heat transfer rates and lead to changes inflow regimes. We report the results of an experimental study of interfacial waves with condensation of a vapor on the cooled bottom surface of horizontal duct and with air-glycerine-water mixture flow in an identical duct. R-I13 and FC-72 (developed by the 3M Company) were used in the condensation experiments. Wavy flow patterns with two- and three-dimensional waves were observed. Interfacial waves with condensation appeared when the vapor velocity was in the range 1.24-1.56 m/sfor R-I13 and 0.81-1.13 m/sfor FC-72, corresponding to vapor Reynolds numbers in the range 20,000-30,000—the value depends on the difference in the temperatures of the condensing surface and saturated vapor. The ratio of the distance from the leading edge (at which the waves appeared) to the liquid film thickness was found to be a Junction of the liquid Reynolds number. Condensation stabilizes the interface. None of the several available criteria for the incipience of the waves was found to be satisfactory. A modification to the criterion employed by Jensen and Yuen ill is suggested. Other details such as wave speed and wave separation distance are included.     

Numerical simulation of two droplets impacting upon a dynamic liquid film

The impact of droplets on the liquid film is widely involved in industrial and agricultural fields. In recent years, plenty of works are limited to dry walls or stationary liquid films, and the research of multi-droplet impact dynamic films is not sufficient. Based on this, this paper employs a coupled level set and volume of fluid (CLSVOF) method to numerically simulate two-droplet impingement on a dynamic liquid film. In our work, the dynamic film thickness, horizontal central distance between the droplets, droplets' initial impact speed, and simultaneously the flow velocity of the moving film are analyzed. The evolution phenomenon and mechanism caused by the collision are analyzed in detail. We find that within a certain period of time, the droplet spacing does not affect the peripheral crown height; when the droplet spacing decreases or the initial impact velocity increases, the height of the peripheral crown increases at the beginning, and then, because the crown splashed under Rayleigh-Plateau instability, this results in the reduction of the crown height. At the same time, it is found that when the initial impact velocity increases, the angle between the upstream peripheral jet and the dynamic film becomes larger. The more obvious the horizontal movement characteristics, the more restrained the crown height; the spread length increases with the increase of the dynamic film speed, droplet spacing and the initial impact velocity. When the liquid film is thicker, more fluid enters the crown, due to the crown being unstable, the surface tension is not enough to overcome the weight of the rim at the end of the crown, resulting in droplets falling off.     

WATER VAPOR ABSORPTION ENHANCEMENT IN LiBr/H2O FILMS FALLING ON HORIZONTAL TUBES

An experimental investigation was conducted into the enhancement of water vapor absorption in aqueous lithium bromide solution films falling on horizontal tubes. The variable parameters included the solution flow rate, the inlet temperature of the cooling media, the solution inlet temperature, and the effective extended surface. The experimental results, which were correlated with the film Reynolds number, showed that the cooling effect of the tube wall was the governing factor in the absorption process for small solution flow rates, but the inlet solution subcooling dominated the absorption performance for large flow rates. Also, the R tube, which has a unique ridged fin shape, exhibited the best absorption performance among the tested tubes.     

高雷诺数下降液膜流动换热分析

本文分析了降液膜高雷诺数区域换热系数随液膜长度变化趋势,引出临界长度的概念。在较高雷诺数的湍流区,回流区的存在以及表面波的影响能有效地减少局部薄膜厚度并增加对流换热,流速的增加进一步强化换热,增加降膜整体传热系数。在高雷诺数区域,厚度形成的热阻超过回流的增强作用,而使换热削弱。液膜小于临界长度L换热系数随长度增长而增加,大于L换热系数随液膜增长而减小。     

Lubricant film flow and depletion characteristics at head/disk storage interface

The characteristics of lubricant film at head/disk interface(HDI) are essential to the stability of hard disk drives. In this study, the theoretical models of the lubricant flow and depletion are deduced based on Navier–Stokes(NS) and continuity equations. The air bearing pressure on the surface of the lubrication film is solved by the modified Reynolds equation based on Fukui and Kaneko(FK) model. Then the lubricant film deformations for a plane slider and double-track slider are obtained. The equation of lubricant film thickness is deduced with the consideration of van der Waals force, the air bearing pressure, the surface tension, and the external stresses. The lubricant depletion under heat source is simulated and the effects of different working conditions including initial thickness, flying height and the speed of the disk on lubricant depletion are discussed. The main factors that cause the lubricant flow and depletion are analyzed and the ways to reduce the film thickness deformation are proposed. The simulation results indicate that the shearing stress is the most important factor that causes the thickness deformation and other terms listed in the equation have little influence. The thickness deformation is dependent on the working parameter, and the thermal condition evaporation is the most important factor.     

水平螺旋槽管壁面升膜形成机理的研究

对驱动升膜形成的润湿力进行分析,建立单组分流体的数学模型,得出壁面液膜蒸发时的速度和厚度分布。基于此得到蒸发过程中水平螺旋槽管管外壁升膜的形成机理和流动特性,并给出液膜润湿整个管壁面的临界条件。     

蒸汽爆炸粗混合过程中的蒸发阻力模型

在蒸汽爆炸的粗混合过程中,由于液体的快速蒸发,高温金属液滴的周围会产生一层很薄的蒸汽膜,此时液滴周围的边界层流动与没有液体蒸发时有很大的不同,因此,常温情况下的小球在连续液体中运动时的通用阻力模型在这种情况下是不适用的．本文通过受力分析,考虑了高温小球受力的分布和表面蒸发对小球周围力的影响,从阻力的基本机理上分析了蒸发状态下小球的运动阻力,分别提出了高温颗粒穿过自由表面时与其在液体中运动时的蒸发阻力模型．分析表明,当小球温度高于2500 K,特别是在靠近自由表面的区域,由于小球表面液体蒸发而产生的蒸发阻力作用非常明显．分析指出,小球的入水初速、小球表面的液体蒸发速率以及汽膜厚度都是影响小球运动阻力大小的重要因素．     

Dynamics of dewetting at the nanoscale

We use large-scale molecular dynamics simulations to model the dewetting of solid surfaces by partially wetting thin liquid films. As observed experimentally and in previous simulations, the films recede at an initially constant speed, creating a growing rim of liquid with a constant receding dynamic contact angle. Film recession is faster on the more poorly wetted surface to an extent that cannot be explained solely by the increase in the surface tension driving force. Furthermore, the rates of recession of the thinnest films are found to increase with decreasing film thickness. These results suggest not only that the mobility of the liquid molecules adjacent to the solid increases with decreasing solid-liquid interactions, but also that the mobility adjacent to the free surface of the film is higher than in the bulk, so that the average viscosity of the film decreases with thickness. Recent simulations of films with a wide range of solid-liquid interactions lend support to this view.     

液态镓在石墨烯表面的润湿性及形貌特征

液态Ga及其合金的熔点低、毒副作用小、导电率高,使得这类液态金属能像石墨烯一样被广泛应用于微流器件、柔性电子器件中,制备这些器件的关键在于有效控制各生产环节中液态金属在固体界面上的润湿性及形貌特征.基于Lennard-Jones(L-J)势函数,利用分子动力学模拟方法研究了金属Ga在石墨烯表面的润湿性,根据模拟结果拟合的L-J势参数能正确描述Ga原子与衬底之间的相互作用并得到了与实验值极为接近的润湿角,发现衬底与液膜间相互作用的微小改变都会对最终润湿形态产生极大影响,平衡态的润湿角和脱离衬底速度随着Ga-C间势能的减小而增大,并成功获得了不同厚度的Ga液膜在石墨烯表面的形态演变规律,极为符合液态Ga的基本特性.利用所得L-J势函数参数模拟了液态Ga在粗糙度相同但纳米柱尖端形貌不同的C材料表面的润湿演变,发现纳米柱尖端形貌对液态Ga的润湿过程及状态影响极大.     

Numerical Simulation of Vapor Bubble Growth and Heat Transfer in a Thin Liquid Film

A mathematical model is developed to investigate the dynamics of vapor bubble growth in a thin fiquid film, movement of the interface between two fluids and the surface heat transfer characteristics. The model takes into account the effects of phase change between the vapor and liquid, gravity, surface tension and viscosity. The details of the multiphase flow and heat transfer are discussed for two cases： （1） when a water micro-droplet impacts a thin liquid film with a vapor bubble growing and （2） when the vapor bubble grows and merges with the vapor layer above the liquid film without the droplet impacting. The development trend of the interface between the vapor and liquid is coincident qualitatively with the available literature, mostly at the first stage. We also provide an important method to better understand the mechanism of nucleate spray cooling.     

波纹竖壁上低雷诺数饱和蒸发液膜流动的研究

对饱和蒸发状态下的低雷诺数的液膜在小波幅正弦型波纹壁面上的自由降落进行了理论分析.通过无量纲化、引入流函数、采用摄动展开对数学模型进行处理,得到了这种情况下液膜流动的分析模型,计入了饱和蒸发压力的影响,得到了近似分析解.讨论了壁面波纹的波幅、波数、液膜表面张力和蒸发压力对液膜波动的影响.     

工质对微通道沸腾液膜瞬态变化的影响

本文采用去离子水和无水乙醇两种工质,利用微通道流动沸腾同步测量实验系统,研究了液膜厚度的瞬态变化规律,实验发现流动沸腾形成的初始液膜厚度在毛细数Ca很宽的范围内都遵循Taylor流动原理;液膜形成后,在蒸发和蒸汽流动携带的耦合作用下,厚度迅速减薄直至蒸干;由于水的汽液黏度比小,速度梯度小,剪切作用带来的液膜厚度减少量小,且水的汽化潜热大,吸收相同热量时蒸发量小,导致水的液膜厚度变化斜率较小,通过理论分析提出了沸腾液膜厚度变化的计算模型,计算结果与实验结果的误差小于20%。     

双液滴撞击平面液膜的流动与传热特性

采用耦合的水平集-体积分数法(CLSVOF)对双液滴连续撞击恒定壁温壁面上的热液膜的流动和换热特性进行了数值模拟及分析,得到了双液滴撞击热液膜后形态演变的过程.分析了液滴垂直间距、撞击速度、液膜厚度以及液滴直径对双液滴撞击液膜后的流动与传热特性的影响,结果显示,壁面平均热流密度随液滴撞击速度的增大而增大,液滴垂直间距、液膜厚度和液滴直径对平均热流密度的影响较小,但会对热流密度在撞击区域和交界区的分布产生重要影响.     

液滴撞击液膜的射流与水花形成机理分析

建立了单液滴撞击平面液膜的物理与数学模型,采用Coupled Level Set and Volume of Fluid方法对这种现象进行了数值模拟,探讨了黏度和表面张力对冠状水花形态的影响.通过分析撞击后液体内部的压力和速度分布,揭示了液滴颈部射流的产生机理,验证了Yarin和Weiss提出的运动间断理论.研究显示,表面张力对冠状水花形态的影响远大于黏度的影响.颈部射流的产生主要是由于撞击后颈部区域局部较大压差造成的,随着撞击过程的继续,压差作用减弱;液膜内流体的径向运动对射流发展成冠状水花具有推动作用.     

微液膜动力学特性与稳定性实验研究

气-液两相流设备的性能受限于临界热流密度,开展流动微液膜动力学特性及其稳定性的相关研究是深入理解沸腾危机及临界热流密度机理的关键。采用光学玻璃制成的矩形通道作为实验段,使用微流量齿轮泵驱动去离子水,使其在实验通道入口处与在其上部流动的压缩空气接触形成同向流动的分层流。利用共轭光学探测器对流动微液膜的厚度进行了测量,利用高速摄像机对气-液两相分层流波动特性进行了可视化观测。研究表明,在绝热情况下,当液速一定时,液膜的平均厚度随着气速增加而减小,当气速增加到某一阈值时会导致液膜破裂。