Thin-liquid-film evaporation at contact line

When a liquid wets a solid wall, the extended meniscus near the contact line may be divided into three regions: a nonevaporating region, where the liquid is adsorbed on the wall; a transition region or thin-film region, where effects of long-range molecular forces (disjoining pressure) are felt; and an intrinsic meniscus region, where capillary forces dominate. The thin liquid film, with thickness from nanometers up to micrometers, covering the transition region and part of intrinsic meniscus, is gaining interest due to its high heat transfer rates. In this paper, a review was made of the researches on thin-liquid-film evaporation. The major characteristics of thin film, thin-film modeling based on continuum theory, simulations based on molecular dynamics, and thin-film profile and temperature measurements were summarized.     

On evaporation of thin liquid films subjected to ultrasonic substrate vibration

Theoretical and experimental studies on evaporation of thin and ultrathin liquid films (all volatile or liquid solutions) are desirable, but scarce. In this context, excitation of thin liquid films by (ultrasonic) vibration is also an interesting theoretical and applied research direction affecting the hydrodynamics, stability, and evaporation of thin liquid films. In this study, the evaporation history of drop-cast stationary and excited thin liquid films subjected to vertical and horizontal ultrasonic vibration is studied, and unprecedented results are obtained and discussed. The evaporation history of two model thin liquid films is captured using video camera and high precision digital balance. Since evaporation of excited thin films by substrate vibration resembles forced convection, the convective heat transfer coefficient and consequently the evaporation rate of the excited thin films are expected to increase compared to those of non-excited thin films. Experimental results substantiate this hypothesis. It is further shown and discussed that the films excited by horizontal ultrasonic substrate vibration evaporate faster than those excited by vertical vibration.     

Experimental investigation of falling film evaporation on horizontal tubes

This paper presents the results of an experimental investigation relating to heat transfer during evaporation of thin liquid films falling over horizontal tubes. Experiments were conducted using 25 mm o.d. copper tubes heated by internal electrical cartridge heaters so that a uniform heat flux was generated on the outside tube surface. Five heated tubes were arrayed on a vertical plane with a pitch of 50 mm. Freon R-11 preheated to the saturation temperature at 0.2 MPa was supplied to the topmost heated tube through feeding tubes. Heat transfer characteristics on each heated tube were clarified in a range of film Reynolds number from 10 to 2000 and the measured data are presented in the form of correlations. Deterioration of heat transfer due to film break down was also considered. © 1999 Scripta Technica, Inc. Heat Trans Jpn Res, 27(8): 609–618, 1998     

椭圆横管外降膜流动和换热特性分析

本研究基于VOF算法编写用户UDF(自定义函数),采用FLUENT软件建立了椭圆横管外降膜流动和换热的计算模型。根据CFD(计算流体力学)模型计算和分析了在不同长短轴比下管外降膜速度分布、压力分布、液膜内温度分布和管外换热分布的变化规律。研究结果表明:长短轴比的变化影响了管外液膜速度分布、压力分布和膜内温度分布;相比圆管,椭圆管的管外膜内液体流速更快。壁面压力沿周向逐渐减少并在X=0.9附近快速回升;随长短轴比e的增加,周向压力最小值位置逐渐向后推移。局部Nu数分布与压力分布在趋势上存在一致性。当e=1.65附近时,椭圆的换热性能最优;最后,通过对管形的研究分析,提出横管的传热分区模型。     

Numerical investigation of the heat and mass transfer in a vertical tube evaporator with the three-zone analysis

A numerical study for the flow, heat and mass transfer characteristics near the inflow region of the vertical evaporating tube with the films flowing down on both the inside and outside tube walls has been carried out. Condensation occurs along the outside wall and evaporation at the free surface of the inside film. The transport equations for momentum and energy are parabolized by the boundary-layer approximation and solved by using the marching technique. In this kind of numerical approach, the accurately predicting the early stage is really important because a small error at the previous step can produce the amplified big error at the next step. To accurately predict the flow at the inflow region of the vertical evaporating tube, the calculation domain of two film flow regions and tube wall is solved simultaneously. The interesting heat transfer characteristics revealed through this three-zone simulation, such as the evaporation delay and the temperature inflection at the very near inflow region are found and discussed along the discrepancy between the inner film inlet temperature and the saturation temperature. The case that the inner film comes in with the saturation temperature shows a good performance. The velocity and temperature fields as well as the amounts of the condensed and evaporated mass in both inner and outer films are predicted for the various conditions.     

Transport from a volatile meniscus inside an open microtube

A generalized model is developed which couples the evaporation at a liquid–air interface with the vapor diffusion processes in air to enable an investigation of the mass transport inside an open microtube. Tube inner diameters ranging from 100 to 1200 μm are considered. Evaporation is strongest at the meniscus junction with the tube wall due to the highest local vapor diffusion flux at this location. A temperature gradient is set up from the axis of the tube to the wall and results in Marangoni convection. The three-dimensional flow structure in the microtube is simulated with the effects of Marangoni convection, buoyancy, and the influx of fluid to the interface being included. For horizontal tubes of diameter 100 μm or larger immersed in a water bath, flow asymmetry due to buoyancy is observed. A large vortex is formed in the lower part of the tube cross-section, while a small vortex forms above. However, the primary cause of asymmetry is found to be the external thermal profile imposed on the microtube, especially when the meniscus is far from the outlet of the tube. The simulated flow patterns are found to be consistent with experimental measurements.     

Analysis of film condensation heat transfer inside a vertical micro tube with consideration of the meniscus draining effect

In this paper we report on a theoretical analysis of film condensation heat transfer in a vertical micro tube with a thin metal wire welded on its inner surface. Both the radial and the axial distributions of condensate liquid along the tube wall and over the meniscus zone, formed by the wire in contact with the tube inner surface, are determined based on the minimum energy principle over the liquid-vapor two-phase flow system. The influences of the contact angle between the condensate liquid and the channel wall as well as the wire diameter on the condensate distributions and the heat transfer characteristics are examined. It is found that an increase in the wire diameter results in significant enhancement of heat transfer in the channel. It is also demonstrated that the wettability between the wire and the condensate has a little influence on the overall heat transfer coefficients, although it affects the condensate liquid distribution. Compared to a round tube with the same inside diameter, significant enhancement of condensation heat transfer is found for the present configured microchannel.     

Comprehensive experimental and theoretical study of fluid flow and heat transfer in a microscopic evaporating meniscus in a miniature heat exchanger

The complex physicochemical phenomena occurring in the contact line region of an evaporating meniscus are described using a unique combination of high-resolution experimental data and three complementary models. The following were used: (1) high-resolution experimental liquid profile data (thickness, slope, curvature and curvature gradient) to obtain the pressure gradient in the evaporating pentane meniscus in a vertical constrained vapor bubble (VCVB); (2) macroscopic outside surface temperature profile data; (3) a finite element model to obtain the two-dimensional heat conduction profile in the solid substrate wall (macro-model) and the solid–liquid interfacial temperature profile in the evaporating meniscus region; (4) a continuum fluid-dynamics model (micro-model) to obtain the liquid–vapor interfacial temperature, mass flow rate, Marangoni stresses, and evaporative heat flux profiles along the length of the evaporating meniscus; and (5) the Kelvin–Clapeyron model to obtain the vapor temperature profile (liquid–vapor interfacial temperature jump) in the evaporating meniscus region.The retarded dispersion constant and high-resolution thickness, slope, curvature and curvature gradient profiles were obtained from the experimental reflectivity profiles. There was a substantial increase in the measured curvature in the transition region, where the evaporation rate and flux are a maximum. To obtain numerical closure between the three complementary models, the continuum fluid dynamics model (micro-model) required slip at the solid–liquid interface to support the observed high mass flow rates in the evaporating pentane meniscus. Mass flow rates due to Marangoni stresses, capillary pressure and disjoining pressure are compared. Depending on the liquid thickness, Marangoni stresses can either enhance or hinder fluid flow towards the contact line for the evaporating pure pentane meniscus. Due to the high heat removal rate by the evaporating pentane meniscus in the transition region, dips in the vapor, liquid–vapor and solid–liquid interface temperature were obtained. The results demonstrate and describe the sensitivity and complexity of the phase change process in micro-regions.     

Measurement of internal wettability of gas diffusion porous media of proton exchange membrane fuel cells

One of the major problems of current proton exchange membrane (PEM) fuel cells is water management. The gas diffusion layer (GDL) of the fuel cell plays an important role in water management since humidification and water removal are both achieved through the GDL. Various numerical models developed to illustrate the multiphase flow and transport in the fuel cell. The accuracy of these models depends on the accurate measurement of the GDL properties such as wettability, surface energy, and porosity. Most of the studies conducted for measuring the wettability of the GDL are based on the external contact angle measurements. However, the external contact angle does not describe adequately capillary forces acting on the water inside the GDL pores. In a recent study, the capillary penetration technique has been used to measure indirectly the wettability of the GDL based on the experimental weight increase due to penetration of the liquid into the porous sample. In essence, the mass penetration technique was used along with the Washburn's equation. The shortcoming of this method is that the external factors such as the mass of the meniscus formed outside the sample as well as evaporation occurring during the experiment were not considered. It was found that these factors affect the wettability measurements of the GDL, especially for a hydrophilic sample. In this paper, the experimental setup of the capillary penetration method has been modified to control the evaporation rate as the liquid is penetrated into the sample. Also, the capillary penetration technique which was initially used based on mass penetration has been modified to the height penetration method to eliminate the effect of the weight of the meniscus formed outside the sample. The experiments were performed for a time period of 10 s. For this time period, it was found that the Washburn's equation is not an accurate model since it does not include the frictional work effects that are significant at the first few seconds of the experiments. Therefore, the Washburn's equation was replaced by a more general form. Using the Levenberg-Marquardt optimization technique, the experimental data obtained from the height penetration technique is fitted to the theoretical curve to find the internal contact angles of a sample GDL. Finally, these contact angle results are used to determine the surface tension of the GDL using two approaches: the Owens-Wendt surface tension components and the equation-of-state models.     

Evaporation heat transfer of steady thin film in micro capillary tubes of micron scale

This paper reports that the heat transfer mechanism of phase change in a capillary tube belongs to liquid film conduction and surface evaporation. The surface evaporation is influenced by vapor temperature, vapor‐liquid interfacial temperature, and vapor‐liquid pressure difference. In the vapor‐liquid flow mechanism, flow is effected by both the gradient of disjoining pressure, and the gradient of capillary pressure. The mechanism of vapor‐liquid interaction consists of the shear stress caused by momentum transfer owing to evaporation, and frictional shear stress due to the velocity difference between vapor and liquid. In the model presented for a capillary tube, the heat transfer, vapor‐liquid flow, and their interaction are more comprehensively considered. The thin film profile and heat transfer characteristics have close relations with a capillary radius and heat transfer power. The results of calculation indicate that the length of the evaporating interfacial region decreases to some extent with decreasing capillary radius and increasing heat transfer power. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 513–523, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ).DOI 10.1002/htj.10050     

Optimization of capillary structures for inverted meniscus evaporators of loop heat pipes and heat switches

Loop heat pipes (LHPs) and other two-phase heat transfer devices are used in the thermal management of electronic devices with high density of heat dissipation. In these two-phase thermal devices, the key component is the capillary structure (CS) that pumps the working fluid using the capillary forces generated by the meniscus, which are formed due to evaporation. The evaporator’s performance depends greatly on the internal structure and external configurations of the CS. However, there is not enough experimental and theoretical data on the optimization of the capillary structures of evaporators. This paper covers some important aspects of the CS design for evaporators working in an “inverted meniscus” scheme and proposes a methodology for analysis and selection of the CS pores size for LHP, flat heat pipes and heat switches, aiming for maximum heat transport capacity. Based on this methodology, two examples of capillary evaporators have been designed and evaluated.     

Visualization and performance measurement of operating mesh-wicked heat pipes

This work presents visualization of the evaporation/boiling process and thermal measurements of operating horizontal transparent heat pipes. The heat pipes consisted of a two-layered copper mesh wick consisting of 100 and/or 200 mesh screens, a glass tube and water as the working fluid. Experimental results indicated that nucleate boiling was prompted for a wick having a fine 200-mesh bottom layer. When the fluid charge approximately equaled the pore volume in the wick, the water–vapor interface receded into more curved menisci with increasing heat load Q. Thus, larger capillary forces and evaporation areas were attained to meet the increasing need of liquid supply and evaporation rate at the water–vapor interface. At Q = 40 and 45 W, the water film became less than 100 μm and the nucleate boiling observed at lower heat loads disappeared. Optimal thermal characteristics with smallest thermal resistances across the evaporator and lowest overall temperature distributions were found for such a wick/charge combination. Under a smaller charge, partial dry-out was observed in the evaporator. Under a larger charge, liquid recession with increasing heat load was limited and bubbles grew and burst violently at high heat loads. The effects of different wicks and fluid charges on the evaporation/boiling characteristics were discussed.     

Simplified numerical study of evaporation processes inside vertical tubes

The paper presents a simplified numerical model of evaporation processes inside vertical tubes. In this model only the temperature fields in the fluid domain （the liquid or two-phase mixture） and solid do- main （a tube wall） are determined. Therefore its performance and efficiency is high. The analytical formulas, which allow calculating the pressure drop and the distribution of heat transfer coefficient along the tube length, are used in this model. The energy equation for the fluid domain is solved with the Control Volume Method and for the solid domain with the Finite Element Method in order to de- termine the temperature field for the fluid and solid domains.     

Characteristics of liquid ethanol diffusion flames from mini tube nozzles

A series of experiments was conducted to explore the combustion characteristics of a diffusion flames from mini tubes fueled by liquid ethanol with visual observations of the flame shape, the dynamic liquid-vapor interface during phase change inside the capillary tubes and the tube outer surface temperature using CCD and IR cameras. As the fuel supply rate increased, the interface location rose to the tube exit and the temperature gradient on the outer tube surface increased, consequently the evaporating became much stronger and the interface tended to be unstable. The combustion characteristics are closely related to the rapid phase change and violent evaporation and interfacial dynamics, with the violent evaporation, actually explosive boiling, inducing an explosive flame. The intensity of the explosive flame became stronger as the flowrate increased with the maximum flame height, interface location movement, and sound intensity all significantly increasing. The periodicity of the explosive flame was directly proportional to the interface moving distance and inversely proportional to the fuel flow rate.     

A study of the influence of initial liquid volume on the capillary flow in an interior corner under microgravity

In this work the influence of initial liquid volume on the capillary flow in an interior corner is studied systematically by microgravity experiments using the drop tower, under three different conditions: the Concus–Finn condition is satisfied, close to and dissatisfied. The capillary flow is studied by discussing the movement of tip of the meniscus in the corner. Experimental results show that with the increase of initial liquid volume the tip location increases for a given microgravity time, the achievable maximum tip velocity increases and the flow reaches its maximum tip velocity earlier. However, the results for the three different conditions show some difference.     

Numerical Study of Boundary-Layer Transition in Flowing Film Evaporation on Horizontal Elliptical Cylinder

ABSTRACT

A numerical study of the onset of transverse and longitudinal transitions between turbulent and laminar regimes during the evaporation of a water film is presented. The water film streams, without interfacial shear stress, along a horizontal elliptical tube under the simultaneous effects of gravity, pressure gradients, and viscous forces. Outside the boundary layer, the vapor-phase velocity is obtained from potential flow. In the analysis, a turbulence model taking into account various pressure gradients is proposed, and the inertia and convection terms are retained. Transfer equations are discretized by using the implicit Keller method. The effects of different turbulence models and the main parameters, such as the initial liquid flow rate per unit of length, the Froude number, the temperature difference between the wall and the liquid–vapor interface, and the ellipticity, on the transition position are evaluated. The transition criterion is given in term of the critical film Reynolds number.     

Effects of radius and heat transfer on the profile of evaporating thin liquid film and meniscus in capillary tubes

The physical and mathematical models are established to account for the formation of evaporating thin liquid film and meniscus in capillary tubes. The core vapor flow is due to gradient of vapor pressure, which is mainly contributed by the shear stress at vapor-liquid interface. The liquid film flow is owing to gradients of capillary pressure and disjoining pressure. The heat transfer is composed of liquid film conduction and evaporation at vapor-liquid interface. The mass balance of vapor flow is considered to obtain the vapor velocity, this can evade directly solving the rarefied gas velocity field.In regard to the capillary tubes of micron scale, the calculation results show that, the bigger the inner radius or the smaller the heat flow, the longer the evaporating interfacial region will be. There only exists meniscus near the wall, and nearby the axial center is flat interface. While as to the capillary tubes of scale about 100 μm, the evaporating interfacial region will increase with heat flux. Compared with capillaries of micron scale, the meniscus region will extend to the center of capillary axis. These can be tentatively explained as strong influence of the thin liquid film.For the capillary tubes of radius about 100 μm, the experimental results indicate that the apparent contact angles and meniscus profiles can almost coincide with those of the theoretical values.     

Experimental optimization of capillary structures for loop heat pipes and heat switches

Loop Heat Pipes (LHP) and Heat Pipes (HP) are used in the thermal management of electronic devices with high-density heat dissipation. In these two-phase thermal devices, the key component is the capillary structure (CS) that pumps the working fluid using the capillary forces generated by the meniscus, which is formed due to evaporation. The performance of the LHP and the HP depends greatly on the internal structure and external configurations of the CS. However, there is not enough experimental and theoretical data on the optimization of the capillary structures of evaporators. The “inverted meniscus” scheme is the usual configuration for Loop Heat Pipes (LHP) but not for Heat Pipes (HP), which use the “classical” scheme. This paper presents the results of extensive experimental investigations on the optimization of the physical and geometrical parameters of the flat CS including thickness, configuration and size of vapor grooves, for different CS materials and working fluids. Additionally, the paper presents a comparative study between the “classical” and the “inverted meniscus” schemes of vaporization. Based on the obtained results, two examples of evaporators have been designed and evaluated.     

Experimental study of spontaneous ignition induced by sudden hydrogen release through tubes with different shaped cross-sections

The shock wave dynamics, spontaneous ignition and flame variation during high-pressure hydrogen release through tubes with different cross-section shapes are experimentally studied. Tubes with square, pentagon and circular cross-section shapes are considered in the experiments. The experimental results show that the cross-section shape of the tube has no great difference on the minimum burst pressure for spontaneous ignition in our tests. In the three tubes with length of 300 mm, spontaneous ignition may occur when overpressure of shock wave is 0.9 MPa. When the spontaneous ignition is induced in a non-circular cross-section tube, the possible turbulent flow in the corner of the tube increases can promote the mixing of hydrogen and air, thus producing more amount of the hydrogen/air mixture. As a result, both the peak light signal and flame duration detected in the non-circular cross-section tubes are more intense than those in the circular tube. The smaller angle of the corner leads to a more intensity flame inside tube. When the hydrogen flame propagates to the tube exit from the circular tube, the ball-like flame developed near tube exit is relatively weak. In addition, second flame separation outside the tube is observed for the cases of non-circular cross-section tubes.     

水平管降膜蒸发器传热传质实验研究

水平管降膜蒸发器因传质传热系数高而被广泛应用于淡化水处理中。搭建了水平管降膜蒸发传热实验台,通过对实验结果的归纳,得到了水平管降膜蒸发器的蒸发量随喷淋密度、蒸发温度、热通量的变化规律及热量利用率随蒸发温度的变化规律。结果表明,热通量范围不同时,蒸发量随喷淋密度的变化规律不同;蒸发量随热通量的增大而增大,随蒸发温度的增大而增大;热量利用率随蒸发温度的增大而增大。