核化沸腾液滴的铺展实验观察

采用高速摄像技术可视化观察10μL液滴在不同固体表面上的蒸发及核化过程,壁面温度110~190℃。通过测量小液滴铺展面积、铺展速率,确认液滴在蒸发沸腾情况下受迫铺展与其内部气泡动态演化关系密切,也受加热板表面特性影响。130℃时液滴最容易铺展,铺展趋势随表面粗糙度增大而加强。160℃时液滴各板面液滴铺展的速度和面积非常相近。160℃以后液滴的铺展趋势与温度较低时相反,金属板越光滑越容易铺展。     

Effect of contact angle on wetting limit temperature

The effect of surface wettability on evaporation of a water drop has been examined experimentally using surfaces with various contact angles. To greatly change the surface wettability, TiO₂ superhydrophilicity, plasma irradiation, and super‐water‐repellent surface are adopted as the heating surface. The range in contact angle achieved by these methods was between 0^° and 170^°. The relationship between the contact angle and the wetting limit temperature was obtained and it was found that the lifetime of a water drop dramatically decreases with contact angle in the lower temperature region, and that the wetting limit temperature increases with the contact angle. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(7): 513–526, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20128     

固体表面振荡液滴接触角演化

固体表面液滴振荡存在于很多实际生产当中,了解和掌握其中的界面特性对于实际的生产具有指导意义。利用可视化实验台对吹风条件下固体表面液滴的振荡现象进行了观察。实验在不同的条件下进行,着重风速、液滴尺寸和表面粗糙度等因素对实验现象的影响。实验观察到液滴与固体壁面接触角在振荡过程中有一定的变化规律,而接触线在振荡过程中始终保持不动。通过对实验结果的分类整理和对比,发现固体表面振荡液滴接触角变化和风速、液滴尺寸及表面粗糙度存在一定的关系。最后,通过接触线力平衡条件初步分析解释了接触角的演化规律。     

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。     

An experimental investigation of bubble nucleation of a refrigerant in pressurized boiling flows

An experimental investigation is performed to determine the effect of system pressure and heat flux on flow boiling and associated bubble characteristics of a refrigerant in a narrow vertical duct. A high-pressure flow boiling test loop was built and TLC (thermo-chromic liquid crystal) was applied to the back of the heater foil for high resolution and accurate measurement of heater surface temperature. Refrigerant R-134a is used as the test fluid at different pressures ranging from 690 to 827 kPa and different heat fluxes to quantify their influence in bubble characteristics such as bubble nucleation, growth, departure, and coalescence. Two synchronized high resolution and high-speed cameras are used to simultaneously capture TLC images as well as bubbling activities at high frame rates. By varying flow rate and system pressure, TLC and bubble images were captured and analyzed. Results show that the bubble generation frequency and size increase with heat flux. An increase in pressure from 690 to 827 kPa increased the bubble frequency and size by about 32 Hz and 20 μm, respectively. Bubble coalescence was also observed after departure from the nucleation site.     

高频感应等离子场中液滴运动蒸发过程模拟

建立在高频感应热等离子体环境下单个溶液液滴的运动蒸发模型,采用数值计算的方法模拟了液滴在等离子体射流中的运动和传热过程,分析了不同操作参数对液滴运动蒸发过程的影响.结果表明:液滴初始入射尺寸越小,表面溶质质量分数达到饱和状态所用时间越短;初始入射速度越快,表面溶剂蒸发速度越快,溶质结晶析出时间越短;入射角较大时,液滴会被反向涡流卷吸,表面浓度达到饱和状态的时间较长.     

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.     

An experimental study on solar evaporation of sugarcane juice

In this study, an attempt is made to obtain concentrated sugarcane juice and distilled water using solar radiations that would save energy as well as environment. In this study work, the experimental performance of a sugarcane juice solar evaporation system (SJSES) is investigated for the evaporation of water from sugarcane juice. To check the influence of flow rate on the performance of SJSES units, experiments are carried out at 20, 25, 30, and 35 ml/min. The flow of sugarcane juice at 25 ml/min in SJSES unit is found optimum that resulted in highest energy saving, distilled water, and relatively more brix content of final product. The performance of SJSES unit loaded with paraffin wax at optimum flow rate (unit IV) resulted in maximum distilled water (605.1 g) which is 82.4%, 14.8%, and 39.4% higher than other units. Paraffin wax loading in SJSES increased its distillate output, brix of juice, and working time by 78.3 g, 7.9%, and 1 h, respectively. Thermal and exergy efficiencies are found maximum for unit IV having values 58.56% and 4.53%, respectively. The amount of energy saved is observed maximum for this unit along with least distillate cost and payback period.     

水平均质表面上非等直径水滴聚合的可视化实验

对水平均质表面上非等直径水滴的聚合过程及特性进行可视化实验研究,获得了其聚合过程中水滴液桥半径、接触角和接触线的变化特性和规律,并与等直径水滴聚合进行了比较。结果表明,非等直径水滴聚合后的液桥半径的振幅小,接触角小,接触线的收缩与扩张幅度较大。     

加热板液滴蒸发的理论分析及实验研究

液滴蒸发是由气-液浓度差驱动的一种常见而复杂的扩散现象.通过实验与理论相结合对去离子水在玻璃表面和有机硅油表面的蒸发特性进行研究,测量了液滴接触角和接触直径随时间的动态演变过程.结果 发现:玻璃表面的液滴蒸发为典型的定底半径模式和混合模式;而液滴在有机硅油表面较为特殊,除了定底半径模式和混合模式还有周期性的黏滑模式.出...     

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     

Effect of solid surface properties on dynamic contact angles

An experimental investigation was conducted to understand the effect of solid properties on dynamic wetting. Using a liquid tank method, the wetting behavior of silicone oil over glass, aluminum, and stainless steel surfaces was measured. For all three surfaces, the dynamic contact angles disagreed with the universal function proposed by Hoffman. However, if a dimensionless parameter is introduced to describe the nature of the solid surfaces, the experimental results can be in good agreement with the model. Furthermore, the present experiments indicated that the Hoffman–Voinov–Tanner law should be valid for a wider range of dynamic contact angles or capillary numbers. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 1–12, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20099     

An investigation into the heat transfer characteristics of spiral wall with internal rib in a supercritical sliding-pressure operation once-through boiler

Within the pressure range of 9–28 MPa, mass velocity range of 600–1 200 kg/(m²·s), and heat flux range of 200–500 kW/m², experiments were performed to investigate the heat transfer to water in the inclned upward internally ribbed tube with an inclined angle of 19.5 degrees, a maximum outer diameter of 38.1 mm, and a thickness of 7.5 mm. Based on the experiments, it was found that heat transfer enhancement of the internally ribbed tube could postpone departure from nucleate boiling at the sub-critical pressure. However, the heat transfer enhancement decreased near the critical pressure. At supercritical pressure, the temperature difference between the wall and the fluid increased near the pseudo-critical temperature, but the increase of wall temperature was less than that of departure from nucleate boiling at sub-critical pressure. When pressure is closer to the critical pressure, the temperature difference between the wall and the fluid increased greatly near the pseudo-critical temperature. Heat transfer to supercritical water in the inclined upward internally ribbed tube was enhanced or deteriorated near the pseudo-critical temperature with the variety of ratio between the mass velocity and the heat flux. Because the rotational flow of the internal groove reduced the effect of natural convection, the internal wall temperature of internally ribbed tube uniformly distributed along the circumference. The maximum internal wall temperature difference of the tube along the circumference was only 10 degrees when the fluid enthalpy exceeded 2 000 J/g. Considering the effect of acute variety of the fluid property on heat transfer, the coreelation of heat transfer coefficient on the top of the internally ribbed tube was provided. Translated from Proceedings of CSEE, 2005, 25(16): 90–95 [译自: 中国电机工程学报]     

稳定塔釜温控失效的原因分析及对策

用显热加热重沸器已被广泛应用于石油化工装置,但如果设计不当,也会造成重沸器温度控制的失败.某催化汽油加氢脱硫装置,其稳定塔重沸器采用高温工艺流体作为加热介质,塔釜温度由设在加热介质主管线上的主调节阀和旁路线上的压差调节阀共同控制.但开工初期塔釜温度难以控制,其原因主要有两个:一是重沸器加热介质流量小,且存不稳定的过渡沸腾区操作;二是旁路调节阀两侧的管路压差过大.改进措施主要有:适当降低稳定塔的操作压力,并增加主管线上加热介质的流量,减少旁通线流量,使重沸器完全在稳定的泡核沸腾区操作;将稳定塔温度控制改为三通调节阀控制,以避免旁路调节阀两侧的压差测量值不能准确反应主调节阀的压差变化,从而导致控制的紊乱.上述措施实施后,稳定塔釜温控得到明显改善,且产品质量合格.     

Wetting on a plate with three‐dimensional random roughness and heterogeneity

A theoretical study was conducted to investigate the wetting behavior of liquid meniscus on a vertical plate with three‐dimensional random characteristics of heterogeneity and roughness. The thermodynamic stable condition was derived by considering the minimum of system free energy. The local stable condition leads to a result similar to that obtained for a plate with two‐dimensional characteristics, that is, the system has many meta‐stable states. For the stable condition of the whole system, a relation was derived between the macroscopically observed contact angle and the surface characteristics. The product of cosine of the contact angle and liquid surface tension is equal to the energy difference for the liquid to wet the plate by apparent unit area. If the liquid wets the solid surface reversibly, there is only one contact angle observed macroscopically. This fact suggests that the contact angle hysteresis is caused by the irreversible motion when the liquid advances or recedes on the solid surface. The well‐known Cassie's and Wenzel's contact angles are explained as those corresponding to a thermodynamically stable condition when the liquid wets the solid reversibly. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 371–382, 2001     

Theoretical Aspects of Nucleate Pool Boiling with Dielectric Liquids

Direct cooling with inert,dielectric liquids may well become the technique of choice for the thermal manage-ment of future electronic systems.Due to the efficiency of phase-change processes and the simplicity of naturalcirculation,nucleate pool boiling is of great interest for this application.This paper examines the characteristicsof vapor bubbles and nucleate pool boiling of the dielectric liquids.The results provide a theoretical foundationfor understanding and interpreting the often complex empirical results reported in the literature.     

基于混合热格子玻尔兹曼模型的液滴蒸发数值模拟

采用格子玻尔兹曼方法(LBM)的单组分伪势模型与有限差分耦合的混合热格子玻尔兹曼模型(TLBM)对液滴蒸发过程进行了研究。首先,通过对液滴在方腔内蒸发过程进行模拟,验证了所采用计算方法及程序的有效性。随后,模拟了液滴撞击高温壁面后的蒸发过程,研究了壁面温度、液滴邦德数和液滴雷诺数对蒸发过程的影响。结果表明,壁面温度、液滴邦德数和液滴雷诺数的增加均会造成液滴撞击高温壁面后蒸发速率的增大。     

An experimental investigation of the feasibility of Pb based bipolar plate material for unitized regenerative fuel cells system

The unitized regenerative fuel cell (URFC) system has attracted significant attention and interests because of its round-trip energy conversion with high energy density. However, the identification of low-cost bipolar (BP) plate with higher corrosion resistance is required to produce a sustainable system. In this work, we investigated the possibilities of using cost-effective lead (Pb) metal-based plate as a BP plate material for URFC system. To further enhance the advantageous properties of Pb plate, silver (Ag) was coated onto the Pb plate. Different types of structural and microstructural analyses (XRD, XPS, SEM, EDAX, and mapping) were conducted to characterize the properties of Pb-based plate and the presence of Ag-coated layer on the surface of Pb plate. When compared with the Pb plate, the Ag-coated Pb plate exhibited a higher water contact angle. The obtained water contact angles of Pb and Ag-coated Pb plates were 77.83° and 92.21°, respectively. The obtained water contact angle of the Ag-coated Pb plate is quite acceptable in the URFC system because it offers exceptional advantages during the URFC operation. Furthermore, the interaction between Pb plate and Ag coated layer resulted in an efficient electrochemical performance. Based on these results, we could conclude that the Pb-based BP plate can be possibly considered for the URFC applications.     

Motion of droplets induced by the Marangoni force on a wall with a temperature gradient

Motion of silicone oil and water droplets induced by the Marangoni force was numerically simulated by using two‐ and three‐dimensional second‐order finite difference methods with the CIP and the level set methods. The surface tension was introduced by the continuum surface force (CSF) method. The results clearly showed the flow induced by the Marangoni force and the dependence of droplet velocity on droplet size, contact angle, temperature gradient, and fluid properties. The Marangoni force balanced with the viscous force in the small contact angle case; on the other hand, in the large contact angle case, it balanced with the normal component of surface tension. As for the effect of fluid properties on droplet motion, the temperature coefficient of surface tension had a much larger effect than did viscosity, thermal diffusivity, or surface tension. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 81–93, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20004     

功能表面降膜蒸发传热特性的实验研究

研究了处理表面镀铬铝管、PTFE铜管和纯铝氧化管水平管降膜蒸发传热，研究了喷淋密度、热流密度、管内蒸汽速度和管表面处理对降膜蒸发传热特性的影响。实验结果表明：在表面蒸发区，水平管降膜蒸发传热系数随热流密度的增加而提高，随喷淋密度增大先降低后升高，冷凝例传热系数基本保持不变。总传热系数对操作条件变化很不明显，表面阳极氧化膜使传热系数略有下降，但由于其优良的抗垢时蚀性能，非常有必要再进行深入地研究。