一个新的核态沸腾汽泡动力学模型

本文在经典汽泡动力学理论基础上，提出了描述汽泡生长过程的综合界面模型．本模型的核心在于汽泡内部的热力学过程的详细分析及汽液界面的传热、传质过程的详细描述．并对汽泡生长过程进行了模拟计算，给出了动力学控制阶段的时间范围．本模型对汽泡生长、汽膜发展的理论分析及数值模拟提供了良好的基础．     

纳米尺度下气泡核化生长的分子动力学研究

采用分子动力学方法模拟纳米尺度下液体在固体壁面上发生核化沸腾的过程,主要研究壁面浸润性对气泡初始核化过程和气泡生长速率的影响以及固-液界面效应在液体核化沸腾的能量传递过程中所起到的作用.研究结果发现：壁面浸润性越强,气泡在固壁处越容易核化.该结果与经典核化理论中“疏水壁面易于产生气泡”的现象产生了明显的区别.其根本原因是在纳米尺度下,固-液界面热阻效应不能被忽略.一方面,在相同的壁温下,通过增强固-液相互作用,可以显著降低界面热阻,使得热量传递效率提高,导致靠近壁面处的流体温度升高,气泡核化等待时间缩短,有利于液体沸腾核化.另一方面,气泡的生长速率随着壁面浸润性的增强而明显升高.当气泡体积生长到一定程度时,会在壁面处形成气膜,从而导致壁面传热性能恶化.因此,通过壁面的热流密度呈现出先增大后减小的规律.     

Mathematical simulation of the compression process of a vapor bubble at a pressure increase in the surrounding liquid

The thermophysical and hydrodynamic processes in a spherical vapor bubble and the surrounding liquid at increasing external pressure are investigated by using a numerical simulation method. The investigation is performed on the basis of a new mathematical model belonging to the class of models of homobaric bubbles (the pressure in the bubble is homogeneous at nonhomogeneous temperature and density). The model takes into account the following main physical effects: the viscosity of the liquid, the heat conductivity of the liquid and vapor, the surface tension, and the phase transitions at the bubble surface. An energy equation taking into account convective heat transfer and viscous dissipation in the liquid is used to calculate the temperature fields in the liquid and vapor. The model also takes into account the dependence of the thermophysical properties on the temperature. A distinctive feature of the proposed model is that the integral conservation law of the system’s total energy (including the kinetic energy of the liquid, the surface energy, and the internal energy of the liquid and vapor) is exactly satisfied (without allowance for the kinetic energy of the vapor). As a result of the numerical simulation of the compression of vapor bubbles in water, we obtained data for the major characteristics of the process at considerable degrees of compression. It is shown that the heat and mass transfer between the vapor in a bubble and the surrounding liquid considerably slow down the temperature increase in the bubble.     

加热探头圆柱上的液体微层厚度

1前言核态沸腾气泡形成时表面存在液体微层。虽然微层很薄,但对加热过程中的传热和气泡的产生具有重要影响。Lay和Dhirl‘］通过更细致的分析微层内的压力分布预言了微层的形状和热流,但考虑的仅是平板上气泡的液体微层。本文考虑了小的加热圆柱探针上气泡层的生长。当?..     

毛细管内薄液膜轮廓和传热特性研究

本文认为毛细管的相变传热机理为液膜的导热和表面蒸发;表面蒸发受蒸汽温度、汽液界面的温度以及汽液压力差的共同控制。汽液流动机理为流动受脱离压力梯度、毛细力梯度支配。汽液相互作用机理为存在由于蒸发导致的动量转移切应力和由于汽液流速不同产生的摩擦切应力。提出的物理模型中较为全面地考虑了毛细管内传热、汽液流动及其相互作用。对毛细管半径和传热功率对薄液膜轮廓和传热特性影响程度的计算结果表明,随着毛细管半径的减小、传热功率的增大,蒸发界面区的长度会有所减小,这是针对微小空间得出的不同于常规情况的结论。     

Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.     

High Speed Visualization and Dynamic Measurement of Heat Driven Pumps

We provide the high speed flow visualization and dynamic measurement results for the U-shaped and the inverted U-shaped heat driven pumps. The U-shaped heat driven pumps at the high heating powers consist of a succession of tiny bubble nucleation, growth and coalescence process. Once the “larger” spherical bubble or the bubble slug forms, it expands quickly in both upstream and downstream directions. The increased pressure leads to the liquid discharge through the outlet check valve. When the advancing vapor/liquid interface reaches a higher position in the vertical discharge branch, the condensation heat transfer in the discharge branch shrinks the bubble slug, leading to the decreased pressure and initiating the open of the inlet check valve. Thus the fresh liquid can be sucked into the system. Heat driven pumps operating at the low heating powers display the similar process. However, two major differences are identified: (1) A full cycle includes a set of positive pressure pulses corresponding to a set of tiny bubble nucleation, growth and coalescence process in each substage. Only at the end of the cycle, an apparent negative pressure pulse is created. (2) For each substage in each cycle, when the newly formed bubble slug is chasing the ahead “old” bubble slug, the deformed liquid bridge is formed due to the gravity force effect. When the two bubble slugs are merging together, a wave vapor/liquid interface occurs along the bottom of the capillary tube. For the inverted U-shaped heat driven pumps, there are fewer positive pressure pulses included, corresponding to lesser number of new bubble nucleation, growth, and coalescence process. The bubble slug in the capillary tube is very standard with the smooth vapor/liquid interface. The cycle periods and the pumping flow rates are given versus the heating powers.     

Critical phenomena at evaporation in a thin liquid layer at reduced pressure

This research is concerned with the problem of heat transfer in a thin liquid layer on a horizontal surface, which evaporates at reduced pressure, when structures shaped as “funnels” and “craters” appear on its surface under the action of vapor recoil force. An approximate model that takes into account the surface tension force, gravity force, vapor recoil force, and disjoining pressure is developed. For the experimentally realized shape of curved surface, in the frames of the model, the distribution of vapor recoil force, temperature, pressure, shear stresses, and local heat fluxes along the interface is found. The density of the heat flux corresponding to appearance of a crater at the place of an array of funnels is estimated. The results are in good agreement with the experimental measurements and the estimates by the Kutateladze formula for the first critical heat flux density.     

Investigation of contact line dynamics under a vapor bubble at boiling on the transparent heater

The paper presents the results of an experimental study of dynamics of vapor bubble growth and departure at pool boiling, obtained with the use of high-speed video recording and IR thermography. The study was carried out at saturated water boiling under the atmospheric pressure in the range of heat fluxes of 30?150 kW/m². To visualize the process and determine the growth rates of the outer bubble diameter, microlayer region and dry spot area, transpa-rent thin film heater with the thickness of 1 μm deposited on sapphire substrate was used in the experiments, and video recording was performed from the bottom side of the heating surface. To study integral heat transfer as well as local non-stationary thermal characteristics, high-speed infrared thermography with a frequency of up to 1000 FPS was used. High-speed video recording showed that after formation of vapor bubble and microlayer region, dry spot appears in a short time (up to 1 ms) under the vapor bubble. Various stages of contact line boundary propagation were ob-served. It was shown that at the initial stage before the development of small-scale perturbations, the dry spot propaga-tion rate is constant. It was also showed that the bubble departure stage begins after complete evaporation of liquid in the microlayer region.     

微重力下凝结和沸腾着的汽泡周围流场

1引言尽管对沸腾换热已有大量的研究，但对沸腾过程中驱动汽泡外流动的机理仍没有透彻理解，通常认为，汽泡外流动是由液体自然对流引起的。但是，近来的微重力实验表明［‘］；在没有自然对流的微重力场中，沸腾换热同样很剧烈。为了弄清正常重力和微重力环境中驱动换热的机理，很有必要作进一步的研究。本文数值分析了微重力环境下驱动壁面汽泡周围液体流动的基本机理，深入分析了Marangoni效应的影响，同时考虑表面凝结和蒸发过程的作用。2理论模型汽泡外流动可由二维层流N－S方程及烙方程来描述，流动是由Marangoni边界条件驱动，该条…     

Microscopic Boiling of Liquid Nitrogen Induced by a Pulsed Laser Irradiation

An experimental investigation was conducted to explore the characteristics of microscopic boiling induced by firing a microsecond pulsed laser beam on a thin platinum (Pt) film that immerged in the liquid nitrogen (LN₂) cryostat. High-speed photography aided by a high-voltage lighting system was employed to visually observe the bubble formation and the dynamical boiling process of LN₂. A rapid transient temperature-measuring system was designed to record the temperature evolution of the heating surface. Explosive boiling, characterized by bubble cluster, was observed within LN₂ at the early stage of laser heating, and conventional boiling followed after a certain time. The transition time, therefore, was introduced for separating these two different boiling modes. The temperature of Pt film rose sharply to its maximum during laser pulse, with a very high rising rate of about 10⁷ K/s, and then dropped rapidly after laser irradiation. A model of bubble cluster was proposed to describe the explosive boiling heat transfer, and the latent heat released by bubble collapse in explosive boiling was explored as an important mechanism considerably influencing the boiling heat transfer.     

过热液体中蒸汽泡上升过程的格子Boltzmann三维数值模拟

应用格子Boltzmann相变模型,在三维空间研究蒸汽泡在过热液体中生长、上升和变形等动力学行为.为研究传热传质对蒸汽泡运动的影响,对比模拟相同条件下气泡在等温环境中上升的物理过程.结果表明：蒸汽泡在过热液体中上升发生的变形程度较小,意味着相变对蒸汽泡的影响和表面张力一样使汽泡保持初始的形状.蒸汽泡在过热液体中的上升速度较小,说明随着汽泡生长拖拽力的影响比浮力大.蒸汽泡生长率在初始阶段达到最大值,随后会趋于一个恒定的值.随着汽泡体积增大和上升速度的增加,其对流场的扰动也越来越剧烈.蒸汽泡生长和上升引起的对流运动对温度场的演化造成很大的影响.     

BUBBLE BEHAVIOR AND HEAT TRANSFER MECHANISM IN FC-72 POOL BOILING

A transparent heater made of a thin synthetic diamond substrate along with a high-speed camera was used to investigate bubble behavior during pool boiling. The heater design, combined with the selected FC-72 liquid, overcame the difficulty of previous thin-film heater experiments where transparency and adequate heat flux could not be simultaneously achieved. It also resulted in an essentially uniform temperature field over the heater surface. The growth and merging of bubbles were visualized and quantitatively documented. The relative contribution from phase change to the overall heat flux was determined at several heat flux levels. At a heat flux level half of the critical heat flux (CHF), surface bubble nucleation was found to contribute to more than 70% of the heat transfer from the heater surface. At a similar heat flux level, the ratio of dry to wetted area was determined to exceed 1/3, significantly higher than that predicted by a recent hydrodynamic model for CHF (approximately 1/16). This result suggests that modifications are needed for the hydrodynamic model when applied to highly wetting fluid on nearly isothermal surfaces. The merging of bubbles to form vapor blankets over the heater surface was observed, as has been assumed in recent hydrodynamic models.     

The effect of magnetic fields on the stability of a cylindrical flow with mass and heat transfer

The effect of axial and radial magnetic fields on the Kelvin-Helmholtz stability of a cylindrical interface between the vapor and liquid phases of a fluid is studied when the vapor is hotter than the liquid and the two phases are enclosed between two cylindrical surfaces coaxial with the interface, and when there is mass and heat transfer across the interface. Both axisymmetric and asymmetric disturbances are considered. The linear dispersion relations are obtained and discussed. It is found that a uniform axial magnetic field has a stabilizing effect on the interface, while the effect of a radial magnetic field depends strongly on the choice of some physical parameters of the system. It is also found that the instability criterion is independent of heat and mass transfer coefficient, but it is different fromthat in the same problem without heat and mass transfer. Finally, the heat and mass transfer has a destabilizing influence on the system.     

Nonlinear stability in magnetic fluids of cylindrical interface with mass and heat transfer

The nonlinear Rayleigh-Taylor stability of the cylindrical interface between the vapor and liquid phases of a magnetic fluid is studied when the phases are enclosed between two cylindrical surfaces coaxial with the interface, and when there is mass and heat transfer across the interface. The method of multiple scale expansion is used for the investigation. The evolution of amplitude is shown be governed by a nonlinear Ginzburg-Landau equation. The various stability criteria is discussed, and the region of stability is displayed graphically. Received 18 January 2002 Published online 13 August 2002     

微槽群内汽泡动力学行为对接触线的影响

针对沸腾情形下毛细微槽群热沉内汽泡对汽液固三相接触线的影响,进行了可视化实验研究。研究结果表明：汽泡动力学行为会直接导致微槽群内三相接触线的形状变化,从而引起蒸发薄液膜的厚度、面积以及汽液界面曲率等对薄液膜的蒸发换热特性有重要影响的物理量的变化。实验证实了在开放式微细尺度槽群结构热沉中,固有弯月面区域里的沸腾与扩展弯月...     

Formation and dissociation of gas hydrate inclusions during migration in water

The paper studies the process of floating a gas hydrate particle in liquid. The typical depths when gas bubble floating is accompanied by gas hydrate formation (or with zero gain of hydrate) were calculated. The low depths were identified when floating occurs with hydrate dissociation. The model assumes that the gas hydrate formation is limited by heat transfer from interface to the surrounding liquid. The model for gas hydrate dissociation assumes the rate governed by thermal conductivity of hydrate particle and by convective heat transfer to surrounding water. The temperature of the gas hydrate surface equals the phase transition temperature at the given water pressure. Comparative analysis of thermal conductivity and convective heat transfer effects on hydrate dissociation rate was performed for different initial radius of the particle.     

A Possible Mechanism for the Formation of Unwettable Dry Spots on the Heated Surface under Nucleate Boiling: Part I. Basic Models and Characteristics of Heterogeneous Nucleate Pool Boiling at a Low Pressure

The problems associated with the physics of heterogeneous pool boiling at a low pressure on a flat horizontal surface are considered. The examples of parametric mismatch between the trends are connected with the thermophysical properties and wall surface microgeometry, with the size of the working area, with the affected zone of a growing bubble, and with the contact angle at the interface between the liquid, solid, and vapor phases. A conclusion is drawn concerning the possible causes of ambiguity in the results of the simulation of nucleate pool boiling modes under calculation of the individual contribution of each of the heat transfer mechanisms (convection, liquid microlayer evaporation, and rearrangement of the thermal boundary layer after bubble detachment). It is emphasized that the problem should be solved in the 3D conjugate formulation.     

液滴撞击液膜的流热耦合界面追踪方法数值模拟

采用界面追踪方法研究蒸馏过程中液滴撞击高温薄液膜的流动和传热特性,将数值结果与解析解和实验进行比较验证模型的正确性,研究气液界面和热流分布的演变过程.同时,分析液滴We数和无量纲液膜厚度对传热的影响.液滴撞击后的热流密度分布显示：液膜可分为撞击区、过渡区和静态区.由于液滴的撞击作用,强制对流是撞击区内主要的传热机制.增大液滴的韦伯数或减小无量纲液膜厚度会加强热量传递.随着液滴韦伯数的增加,冲击引起的扰动增强,在动量和能量共同作用下,平均热流密度明显增大,撞击区冠状水花越明显.无量纲液膜厚度越小,平均热流密度越大,且有更长的时间保持高热流密度换热.     

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.