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.     

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。     

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     

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.     

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.     

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]     

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     

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     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：PartI－Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

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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。     

An investigation of vapor concentration during boiling of a liquid mixture: Measurement of temperature and concentration variations of vapor in bubbles

Variations of the vapor concentration of a water‐ethanol mixture during saturated nucleate pool boiling were studied. The temperature changes of the vapor in bubbles produced by the boiling were measured at several different positions near the boiling surface by using a thin thermocouple having high responsivity. The superheat of the bubbles was initially higher in the superheated liquid layer near the heat transfer surface and then approached the bulk temperature along the upward direction. It was clarified that the temperature and concentration differences of the bubbles from the bulk almost disappeared at a height of 3 to 5 mm from the heat transfer surface and within 20 ms after the appearance of the bubbles. Thus, the former result of the liquid‐vapor equilibrium with the bulk condition was confirmed. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(6): 475–485, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10047     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures ：Part I—Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

A mechanism is proposed for nucleate pool boiling heat transfer along with a general model for both pure liquids and binary mixtures. A combined physical model of bubble growth is also proposed along with a corresponding bubble growth model for pure liquids on smooth tubes. Using the general model and the bubble growth model for pure liquids, an analytical model for nucleate pool boiling heat transfer of pure liquids on smooth tubes is developed.     

Simultaneous visualization of dry spots and bubbles for pool boiling of R-113 on a horizontal heater

A new experimental attempt was made to simultaneously observe the dynamic behaviors of bubbles and dry spots in the vicinity of boiling surface. Also, the two-dimensional bubble structures were obtained separately. From the visualization results, the formation of bubbles and dry spots occurs simultaneously. At critical heat flux (CHF), the surface rewetting is repeated by the local nucleate boiling around the large vapor film. At just after CHF, nucleate boiling at the locally wetted region is extinguished, resulting in the dryout of the whole heater surface. Therefore, we conclude that CHF is initiated from the locally limited nucleate boiling activity rather than any hydrodynamic instability.     

An experimental investigation on the confined and elongated bubbles in subcooled flow boiling in a single microchannel

The characteristics of the confined bubble and elongated bubble in subcooled flow boiling in a single horizontal rectangular microchannel with hydraulic diameter Dh =1mm are studied experimentally.The channel with 1×1mm cross section is fabricated in a thin copper plate whose confinement number is Co=2.8 and the degassed deionized water is used as the working fluid.Visualization on the confined and elongated bubbles inside the microchannel is carried out by employing a high-speed CCD camera with a microlens.The recorded images are carefully analyzed to illustrate the behaviors of the confinement and elongation processes of the bubble.The boiling number is used as an adjustable parameter to regulate the operating conditions which is eventually found to take a vital role in the bubble elongation process.Two formation patterns of the confined and elongated bubble are identified and the interactions between the neighboring confined and elongated bubbles are elucidated.     

Oscillation characteristics of droplets on solid surfaces with air flow

Visual experiments were conducted to observe droplet oscillation on horizontal surfaces with air flow. The different Cu test surfaces were polished with different grit sandpapers. Two liquid drop oscillation modes, forward‐backward and upward‐downward, were visually observed in the experiments. Additionally these two modes were observed to transition from one to another under some conditions. The results indicate that the oscillating characteristics were closely dependent on surface roundness, droplet size, and air flow velocity. A larger radius and higher air speed would decrease the oscillation frequency, while the frequency would initially decrease and then increase as the roughness changed from the smooth to rough plates. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 13–19, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20098     

An experimental investigation of the breakup characteristics of secondary atomization of emulsified fuel droplet

In this study, the breakup characteristics of secondary atomization of an emulsified fuel droplet were investigated with a single droplet experiment. In the single droplet experiment, the emulsified fuel droplet suspended from a fine wire was inserted into an electric furnace, and then secondary atomization behavior was observed using a high-speed video camera. Moreover, a mathematical model to calculate the generated water vapor at micro-explosion was proposed by using the mass and energy conservation equations under some assumptions. In the proposed model, that can be calculated by using the inner droplet temperature history at micro-explosion. As a result, puffing and micro-explosion occurred even when the fine ceramics fiber was used. The proposed model showed that about 50–70 wt% of water in the emulsified fuel changed to water vapor instantaneously at the occurrence of micro-explosion. The mass of water necessary for micro-explosion was shown. The breakup time was closely related to the superheat temperature just before the occurrence of micro-explosion.