竖直管外气液逆流环状降膜速度与温度分布

建立了竖直管外环状降膜气液逆流传热传质条件下稳态层流降膜一维速度分布和二维温度分布模型,以及膜厚和降膜表面热通量的数值计算方法。表面热通量的模型计算值与实验值在气体Reynolds数Re_g<1200的范围内吻合较好,表明基于界面摩擦因子求解模型的方法在两相均为层流条件下是可靠的。模型显示了降膜速度分布和温度分布的非线性特征,降膜表面附近陡降的温度梯度表明,减小膜厚是强化降膜传热传质过程的有效途径。     

自润湿液体过冷核态沸腾射液流现象分析

对比去离子水和应用具有自润湿特性的正丁醇水溶液,进行Marangoni效应对微加热面汽泡行为影响的研究。通过高速摄像技术观测到了去离子水中的泡顶射液流和正丁醇溶液中的多射液流现象。相应的数值模拟表明,Marangoni效应引起过冷去离子水从过热表面向泡顶流动,而引起正丁醇溶液向相反方向流动,即过冷溶液流向过热表面。多射液流现象持续时间比去离子水的长,还可引起泡底微液层的瞬时紊乱现象。多射液流现象发生时,汽泡附近的温度梯度较大,而速度梯度则接近垂直于加热表面。正丁醇溶液的模拟结果与实验观测一致,表明表面张力及所引起的Marangoni效应对分析不同流体的过冷核态沸腾机理至关重要。     

竖直窄矩形通道内环状流的流动传热特性

为了研究竖直窄矩形通道内环状流的流动传热特性,建立了窄矩形通道内环状流的数学物理模型,并进行了实验验证。通过数值求解环状流的数学物理模型得到了环状流区域的压降梯度、沸腾传热系数和液膜内的速度分布。结果表明窄矩形通道内的环状流模型能够很好地预测环状流区域的压降梯度和沸腾传热系数,而且环状流液膜内速度在法向的分布是非线性的,在层流边界层区速度梯度较大。热通量和窄矩形通道的尺寸对液膜的流速有很大影响,随热通量的增加和窄矩形通道尺寸的减小液膜的流速逐渐增加,然而质量流速对液膜流速的影响较小,而且随质量流速的增加液膜的速度逐渐减小。     

Detachment of particles from surfaces by thermocapillary flows induced by a moving laser beam

The thermocapillary flows produced by heating with the moving laser beam in a thin liquid layer on a light-absorbing solid surface have recently been recognized to be effective for the removal of particulate impurities from the surface. We performed the comparative analysis of adhesion and thermocapillary removal forces acting on particles attached to the solid surface. A simplified hydrodynamic model was used to evaluate the velocity field of the thermocapillary flow in the liquid layer covering particles. Hydrodynamic forces such as the drag force, the lifting force induced by the shear flow far away from the receding contact line and the maximal lifting force caused by the transversal velocity in the layer near the receding contact line were identified as the cleaning forces, which are determined by the temperature gradient along the surface. It was found out that the lifting force is not enough to overcome the adhesion force and detach particles from the solid surface. The drag and the maximal lifting forces were shown to be responsible for removal of particles by using the proposed method. A reasonable qualitative agreement between experimental results and the quantitative force analysis was observed.     

Flame spread over fuel films in opposed gas flow

The effect of the velocity of forced oxidizer flow on the pattern and velocity of flame spread over a fuel film was experimentally studied, and the limiting conditions of steady-state flame propagation were determined. New experimental evidence was obtained for the validity of the previously proposed model of flame propagation in a thermally thin system. It was found that, in a thermally thin system at a certain value of the gas flow velocity, laminar flame propagation is followed by spin flame propagation in a narrow range of gas flow velocities, and then by quenching. In the laminar layer-by-layer propagation regime, the flame velocity does not depend on the average velocity of the opposed gas flow. The proposed model for the laminar layerby-layer flame propagation agrees with experiment taking into account the fuel film flow under the action of the Marangoni effect due to the condensed-phase temperature gradient.     

多排平直翅片管换热器表面气液降膜流动特性的三维数值模拟

基于VOF模型建立了考虑重力、表面张力及界面摩擦力源项的多排平直翅片管换热器表面气液两相降膜流动三维瞬态CFD模型。不同气流速度下液膜厚度模拟结果与文献中试验值吻合较好,最大偏差小于5%,表明所建立CFD模型是可靠的。通过研究壁面接触角为30°时不同气液Reynolds数下液膜流动特性,结果表明：翅片管表面满膜流的临界Reynolds数Re_l为239,临界喷淋密度为0.06 kg/(m·s);在239 ≤ Re_l ≤ 995内,其平均液膜厚度较Nusselt理论解高16.8%～35.1%;气液逆流和顺流时气相Reynolds数Re_g应分别小于2190.7和3286.0,其主要原因在于过高的Re_g会导致气液界面摩擦力快速增大,从而引发液膜破裂和液滴脱落等现象恶化设备性能。总之,气液顺流更有利于在较高气相Reynolds数下实现翅片管表面的较薄满膜流动。     

Experimental investigation on evaporation interface temperature and evaporation rate of water in its own vapor at low pressures

Evaporative phase transitions are widely present in industrial production and daily life such as thin film processes and crystal growth. The evaporation of the liquid layer and the thermocapillary convection affect each other and restrict each other, making the energy transfer mechanism of the evaporation interface very complicated. To understand the evaporation characteristics of water in its low-pressure pure vapor environment, a series of experimental studies were carried out on the temperature distributions and evaporating rate of water evaporation in the annular pool. The cylinder temperature of the annular liquid pool is controlled between 3℃ and 15℃, and the evaporation environment pressure ranges from 394 Pa to 1467 Pa, when the temperature measurement starts, the depth of water is 10 mm. The results show that the temperature of the vapor side on the liquid-vapor interface is higher than that of the liquid side and there is an obvious temperature jump across the vapor-liquid interface. With the decrease of the pressure ratio, the evaporation rate increases, and the interface temperature jump is enlarged. Meanwhile, with the increase of the distance from the cylinder, the local evaporation rate decreases, thus, the temperature jump decreases. At the same pressure ratio, as the cylinder temperature increases, the heat flux from vapor side decreases, the temperature jump decreases at all measurement points. Within the experimental controlled parameters, the maximum temperature jump obtained in the measurements is 2.56℃. Due to the coupling effect of evaporation cooling and thermocapillary convection, there is a uniform temperature layer with a thickness of about 2 mm under the evaporation interface. The thickness of the uniform temperature layer near the cylinder is always larger than that in the middle of the evaporation interface. In the uniform temperature layer, the thermocapillary convection induced by radial temperature gradient transfers heat from the cylinder to the liquid-vapor interface to compensate for the latent heat of evaporation. Below the uniform temperature layer, the temperature rises rapidly due to heat conduction and buoyancy convection.     

低压蒸汽环境中水蒸发界面温度和蒸发速率的实验研究

蒸发相变广泛存在于薄膜过程及晶体生长等工业生产和日常生活中,液层表面蒸发和热毛细对流相互影响、相互制约,使得蒸发界面能量传递机制变得非常复杂。为了深入了解水在低压纯蒸汽环境中的蒸发特性,对环形液池内水蒸发时的温度分布和蒸发速率进行了一系列实验研究。环形液池壁温控制在3~15℃之间,蒸发环境压力在394~1467 Pa之间变化,开始测量时液层深度为10 mm。结果表明,蒸发界面气相侧温度总是高于液相侧,气液界面存在明显的温度跳跃。随着压比减小,蒸发速率增加,界面温度跳跃随之增大;随着距壁面距离增加,局部蒸发速率降低,温度跳跃值减小;相同压比下,随着壁面温度的升高,气相侧热通量减小,蒸发界面温度跳跃值整体降低;在实验范围内测得的最大温度跳跃值为2.56℃。由于蒸发冷却效应和热毛细对流的耦合作用,蒸发界面下液相侧存在一个厚度为2 mm左右的温度均匀层,且壁面附近温度均匀层厚度大于中间区域厚度。在温度均匀层内,径向温度梯度诱导的热毛细对流将热量从壁面传输至气液界面以补偿蒸发所需汽化潜热;在温度均匀层以下,浮力对流和导热共同作用使得液相温度迅速升高。     

竖直多孔平板上液膜流动特性的研究

孔结构被广泛应用于传质塔填料中,对填料上的液膜流动和传质行为影响较大。对竖直光板和多孔板上的液膜流动进行了三维模拟,并通过实验验证了模拟的准确性。通过模拟研究了孔结构对液膜流动特性的影响。结果表明,干燥孔会阻碍液膜的铺展,而润湿孔促进液膜的铺展。与光板相比,多孔板上的液膜具有起伏波,这将影响液膜的厚度分布和速度分布。液膜厚度波动和水平方向的速度波动随着孔径的增加而增加,而竖直流动方向的速度随着孔径的增加而降低。当孔径增加到一定值时,毛细波将出现在孔中的液膜中,这大大增加液膜水平方向上的波动速度,而降低流动方向上的速度。当孔径继续增加到临界值时,液膜将破裂。多孔板上孔内和气侧区域存在涡旋,能够促进内部液体交换和增大气侧扰动,从而增强传质能力。     

Three-dimensional CFD model of the temperature field for a pilot-plant tubular loop polymerization reactor

A three-dimensional (3D) computational fluid dynamics (CFD) model, using an Eulerian-Eulerian two-fluid model which incorporates the kinetic theory of granular flow, the energy balance and heat transfer equations, was developed to describe the steady-state liquid-solid two-phase flow in a loop propylene polymerization reactor composing of loop and axial flow pump. The entire temperature field in the reactor was calculated by the model. The predicted pressure gradient data were found to agree well with the classical calculated data. Furthermore, the model was used to investigate the influences of the circulation flow velocity, the slurry concentration, the solid particle size and the cool water temperature on the temperature field in the reactor. The simulation results showed that the whole loop can be divided into four sections. In addition, the simulation results also showed that the continuous stirred-tank reactor (CSTR) assumption is invalid for the entire field in the loop reactor.     

A numerical study of flow boiling in a microchannel using the local front reconstruction method

The rapid advances in performance and miniaturization of electronic devices require a cooling technology that can remove the produced heat at a high rate with small temperature variations, as is obtained in flow boiling. To obtain insight in flow boiling, we performed numerical simulations in a 200 μm square microchannel using the local front reconstruction method. Besides validation with literature results, a parametric study shows an increasing heat removal rate and bubble growth rate with increasing wall temperature, liquid mass density, and liquid heat capacity and decreasing inlet velocity indicating the importance of phase change compared to convective transport. Finally, the heat transfer in the liquid film is studied using a Nusselt number defined with the film thickness, which is comparable to Nusselt number for falling films on hot surfaces. It is observed that convective effects are more pronounced at the bubble rear compared to the bubble front.     

Flow-induced stripe pattern formation in phase-separating fluids

We demonstrated that the introduction of a temperature gradient along the free surface induces a particular stripe pattern in phase-separating fluids. The horizontal temperature gradient drove lateral-periodic spiral liquid motion flowing from warmer to cooler places due to thermocapillarity. Properly chosen polymer compositions and initial film thicknesses in ternary solutions allowed us to promote a phase separation in the presence of spiral flow, which assembled the demixed polymer droplets along the flow-stagnation lines. The resulting assembled phases aligned in the temperature gradient direction and eventually formed periodic polymer stripes involving the same spacing as that of the flow axis. The critical condition for the stripe pattern formation was given by the ratio of two relevant film thicknesses, i.e. the thickness for the onset of the phase separation and that for the cessation of liquid motion.     

撞击速度对连续液滴撞击热圆柱壁面局部传热特性影响的实验

借助高速摄像机捕获连续液滴撞击热圆柱壁面后的动力学行为,通过直接测试与数值计算方法相结合,获得了不同撞击速度下沿周向和轴向的局部对流传热特性。结果表明,当液滴撞击速度较小,液膜未发生飞溅时,由于圆柱面的各向异性,沿轴向的对流传热系数单调减小,而沿周向,对流传热系数先减小后略有增大;根据对流传热系数沿周向的变化,将圆周划分为撞击区域、热扩散区域和尾部脱离区域;增大液滴撞击速度主要提高撞击区域和热扩散区域的对流传热系数,而对尾部脱离区域对流传热系数的影响并不明显。当液滴撞击速度超过某一临界值(在本文的实验条件下约为1.53 m/s)时,液膜发生飞溅,此时继续增大撞击速度,壁温的降低不再明显。     

Flow in the nose region and annular film around a Taylor bubble rising through vertical columns of stagnant and flowing Newtonian liquids

The flow in the nose region and in the annular film around individual Taylor bubbles rising through stagnant and co-current vertical columns of liquid were studied, employing particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST) at the same time. The combined techniques enabled simultaneous determination of the bubble shape and the velocity profiles in the liquid film. Experiments were performed with water and aqueous glycerol solutions in a wide range of viscosities , in an acrylic column of 32 mm ID.Values for the distance ahead of the nose in which the flow is disturbed by the presence of the bubble are presented for the conditions studied. The bubble shapes in the nose region are compared with Dumitrescu's shape for potential flow. The velocity profiles show that after the nose region the liquid begins to accelerate downwards, and at a certain distance from the bubble nose the velocity profile and the liquid film thickness stabilise. The liquid film acquires characteristics of a free-falling film. Values of the developing length and film thickness are reported for the experimental conditions studied. Average velocity profiles in the fully developed film are also presented. A critical Reynolds number of around 80 (based on the mean absolute velocity in the liquid film and on the film thickness) is reported for the transition from laminar to turbulent regime. Shear stress profiles (in the fully developed film) are also provided.The data reported are relevant for the validation of numerical codes in slug flow.     

Computational Fluid Dynamics Modeling of Gas‐Liquid Two‐Phase Flow around a Spherical Particle

Microscale studies, which can provide basic information for meso‐ and macroscale studies, are essential for the realization of flow characteristics of a packed bed. In the present study, the effects of gas velocity, liquid velocity, liquid‐solid contact angle, and liquid viscosity on the flow behavior were parametrically investigated for gas‐liquid two‐phase flow around a spherical particle, using computational fluid dynamics (CFD) methodology in combination with the volume‐of‐fluid (VOF) model. The VOF model was first validated and proved to be in good agreement with the experimental data. The simulation results show that the film thickness decreases with increasing gas velocity. This trend is more obvious with increasing operating pressure. With increasing liquid velocity, the film thickness tends to be uniform on the particle surface. The flow regime can change from film flow to transition flow to bubble flow with increasing contact angle. In addition, only at relatively high values does the liquid viscosity affect the residence time of the liquid on the particle surface.     

Experimental investigation of fast-mode liquid modulation in a trickle-bed reactor

Unsteady-state operation of trickle-bed reactors (TBRs) is a promising technique to improve reactor performances especially when mass transfer phenomena are rate controlling. Among the different techniques, fast-mode modulation of the liquid flow rate seems to be one of the most successful. In fact cycling the liquid flow rate at very low frequencies can induce the reactor to work at the high-interaction regime where mass and heat transfer phenomena are strongly enhanced. Fast-mode periodic operation, then, can be considered an extension of the natural high-interaction regime at a mean range of gas and liquid flow rate normally associated with trickling regime in steady-state conditions.Experimental tests have been performed in a TBR employing α-methyl styrene hydrogenation on Pd/C catalyst in unsteady-state conditions by “on-off” fast-mode liquid modulation. Results have been compared with the steady-state experiments at the corresponding average liquid flow rate, revealing a conversion rate improvement up to 60%. All experiments have been performed in isothermal conditions, so conversion improvement can be ascribed only to mass transfer increase and not to thermal effects. The variation of gas and liquid flow rates and liquid cycle parameters presented several important implications about the optimal working conditions.     

Structure of trickle-to-pulse flow regime transition and pulse dynamics at elevated temperature in slow-mode cyclic operation

The effects of temperature and pressure on the structure of the trickle-to-pulse flow regime transition in slow-mode cyclic operation in trickle-bed reactors were reported. The relationship between liquid holdup and liquid velocities at the trickle-to-pulse flow transition in cyclic operation, the shock wave behavior as a function of bed depth, as well as the pulsing flow regime properties were investigated for Newtonian (air-water) and non-Newtonian (air-0.25% carboxymethylcellulose (CMC)) liquids. At a given temperature, the breakthrough, plateau and decay times of the shock wave were found to decrease with bed depth. The pulse velocity and pulse frequency for pulsing flow regime both in cyclic operation and in natural pulsing (constant-throughput operation) were observed to increase with temperature. However, increasing the reactor pressure led to increased pulse frequency and decreased pulse velocity. Analysis of the transition liquid holdups for natural pulse flow and cyclic operation revealed that the liquid holdup decreased with temperature and pressure. The transition liquid holdups and superficial liquid pulse velocities in symmetric peak-base cyclic operation surpassed those in constant-throughput operation for given temperature, pressure and gas velocity, giving rise to wider trickle flow regime area in cyclic operation. The behavior of both Newtonian and power-law non-Newtonian fluids was similar regarding the effect of temperature, pressure and gas velocity.     

蒸汽在扁平管内逆流流动凝结实验与理论分析

针对火电机组直接空冷凝汽器扁平管内蒸汽凝结换热过程,设计并搭建了水冷扁平管内可视化流动凝结实验台,开展管内凝结换热研究。建立涵盖壁面薄液膜和圆弧段液池的流动凝结分析模型,得到凝结液膜发展相关方程。特别对于壁面液膜双曲型发展方程,考虑其对流特征和物理守恒特性,提出共轭梯度法-牛顿法-模拟退火相结合的稳态求解算法,避免传统非稳态方法的复杂性。可视化实验结果表明,管内汽液两相分层流动,扁平管圆弧底部形成了液池。对应的理论分析结果更加细致地表明,管内凝结换热为分层流型,液池液膜厚度为1 mm量级,壁面液膜相比液池液膜薄很多,因而具有较高的凝结传热系数。分析结果为判断冬季低温环境运行时管内冻结起始位置提供了依据。     

An NMR Imaging Study of Steady-State and Periodic Operation Modes of a Trickle Bed Reactor

NMR imaging has been applied to study the steady-state and the periodic operations of a functioning trickle bed reactor. It has been revealed that under conditions of the continuous supply of a liquid reagent to the catalyst bed, the bed was mostly filled with the liquid phase and was characterized by the uniform and stationary distribution of a liquid phase, whereas under conditions of the periodic supply of a liquid reagent to the catalyst bed with the same liquid flow velocity the bed was mostly dry and was characterized by a non-stationary distribution of the liquid phase. The oscillations of the liquid phase content within the bed, corresponding to the modulated liquid flow, have been observed. It has been shown that performing the hydrogenation reaction in a trickle bed reactor under conditions of the periodic supply of a liquid reagent to the catalyst bed leads to the intensification of the hydrogenation process. It becomes apparent in the significant increase of the temperature of the catalyst bed as well as in the increase of the conversion degree in the regimes under forced time-varying liquid flow rates in comparison to the steady-state regime of the reactor operation.     

A Comprehensive Computational Fluid Dynamics Study of Droplet‐Film Impact and Heat Transfer

The Eulerian multiphase model and continuum surface force (CSF) are employed to simulate the liquid droplet impinging onto a solid wall with a pre‐existing thin film of the same liquid. The numerical results are compared with the experimental data reported in the literature, indicating a reasonable matching. The flow field and splashing behavior of a droplet impinging onto a liquid film are analyzed. The reason for the edge of the crown to eject into secondary drops is found. The splashing behavior can be influenced by the impacting velocity and fluid properties. The effects of impact velocity, droplet diameter, depth of film, liquid property, and droplet and wall temperature on the heat removal are investigated. Numerical results demonstrate that an increase in impact velocity, droplet diameter, film depth, cooling droplet, and wall temperature enhances the dissipated heat. These results can provide a reference for designing spray‐cooling systems.