Theoretical study of stable dropwise condensation on an inclined micro/nano-structured tube

In this paper, a numerical model for dropwise condensation on a micro/nano-structured inclined tube is developed based on previous theoretical and experimental studies for flat plates. Thermal resistances consisting of conduction through a droplet, vapor–liquid interfacial resistance, droplet curvature, promoter layer, and micro/nano-pillars are included into the model and incorporated in calculating the heat transfer rate across an individual droplet. Droplet morphologies (Wenzel, Cassie, and partially wetting) are considered in the model. Effects of various parameters are investigated on single droplets behaviors and total heat transfer. Results show that average droplets radii on a horizontal tube are higher than on a vertical tube; consequently, vertical tubes have better heat transfer rate. For example, vertical tubes have five times higher overall heat flux than horizontal tubes in some cases. In addition, partially wetting droplets have higher heat transfer rates in comparison with Wenzel or Cassie state in all configurations.     

表面超疏水性能的数值模拟研究进展

表面超疏水是自然界中一种常见的极端润湿现象。一般认为,表面超疏水是由表面微观结构和化学成分共同决定的。超疏水表面由于独特的非润湿性能,在润湿性变化、减阻、液滴弹跳、传热、蒸发、冷凝等方面具有广泛的应用前景。总结了近年来研究人员在超疏水表面润湿性变化、减阻、液滴弹跳、传热、蒸发、冷凝等方面的数值模拟研究情况,归纳了在超疏水性能研究中常用的数值模拟方法,并对其进行了评价,指出了这些方法存在的优点和缺点,提出了超疏水性能数值模拟研究应该关注的重点和研究方向。     

Analysis of the influence of drop diameters on the intensity of heat transfer in dropwise condensation

An analysis of the contribution of drops of different sizes to the intensity of heat transfer is performed for dropwise condensation of water vapor on a surface that is stimulated by fluorine-containing disulfide. The fraction of the heat-exchange surface and the lifetime of different classes of drops for specified values of the temperature difference are determined. A comparison with the results of other investigators is made.     

Can Metal Matrix‐Hydrophobic Nanoparticle Composites Enhance Water Condensation by Promoting the Dropwise Mode?

Incorporation of condensers treated with hydrophobic modifiers that promote the efficient dropwise condensation mode can significantly enhance the performance of power generation and desalination plants. However, this approach is not used in industry because of low thermal conductivity and limited durability of the thin hydrophobic coatings. Here, it is argued that metal matrix hydrophobic nanoparticle composites can be a robust and high thermal conductivity alternative to hydrophobic polymeric and ceramic thin films. By characterizing condensation on a wide range of mimicked composite surfaces, it is demonstrated that to promote dropwise condensation the filler nanoparticles must have size and spacing significantly smaller than the few micrometer average center‐to‐center separation distance between closest neighboring droplets prior to onset of the coalescence dominated growth stage. Furthermore, the nanoparticle density does not have to be high enough to make the surface hydrophobic, but only sufficiently high to reduce the contact angle hysteresis, and with that pinning of droplets during coalescence and gravity assisted shedding. Using experimental results in conjunction with condensation models, it is estimated that a substantial heat transfer enhancement can be achieved via promotion of sustained dropwise mode even by using copper and aluminum matrix composites fully loaded with polytetrafluoroethylene nanoparticles if the film thickness is below ≈0.5 mm.     

Condensation heat transfer and pressure drop characteristics of CO2 in a microchannel

The condensation heat transfer coefficient and pressure drop of CO₂ in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm⁻²s⁻¹ and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.     

Sessile Drop Wettability in Normal and Reduced Gravity

This paper presents initial work performed to develop a database of contact angles of sessile drops in reduced gravity. Currently, there is no database of wettability of sessile drops in reduced gravity. The creation of such a database is imperative for continued investigations of heat and/or mass transfer in reduced gravity and future engineering designs. In this research, liquid drops of water and ethanol were created on aluminum and PTFE substrates. The formed drops were characterized by their dimensions including contact angle, wetted perimeter and droplet shape in both normal gravity and reduced gravity. The droplets were recorded during testing with high definition video and the images obtained digitally analyzed, post-test, to determine their characteristics as a function of the experimental parameters. The Queensland University of Technology (QUT) Drop Tower Facility was utilized for the reduced gravity experimentation. For droplets with diameters above their capillary length, the changes in drop dimensions and/or wettability was observed. The Young-Laplace equation was validated to accurately predict the contact angle in reduced gravity for small droplets, however it was not adequate to describe the contact angle for larger drops (above the drops associated capillary length).     

Interception of two spherical drops in linear Stokes flow

A boundary-integral method is developed for computing the interception of two spherical drops with arbitrary radii and viscosities in infinite linear Stokes flow. At any instant, the flow is computed in a frame of reference with origin at the center of one drop, using a cylindrical polar coordinate system whose axis of revolution passes through the center of the second drop. Taking advantage of the axial symmetry of the interfaces in the drop coordinates, the problem is formulated as a system of integral equations for the zeroth, first, and second Fourier coefficients of the normal component of the jump in the interfacial traction and for the meridional and azimuthal components of the interfacial velocity with respect to the meridional angle. The integral equations are solved with high accuracy using a boundary-element method featuring adaptive boundary-element distribution and automatic time-step adjustment according to the interfacial gap. Simulations of two drops intercepting in uniaxial straining flow provide accurate data on the drop collision velocity and particle stress tensor for gaps as small as 10⁻⁴ times the drop radius. Simulations of two drops intercepting in simple shear flow confirm that slightly offset drops collide during the interception. Accurate data are presented for Batchelor’s relative mobility functions in linear Stokes flow used to describe the relative droplet motion.     

Condensation heat transfer and pressure drop in flattened microfin tubes having different aspect ratios

In this study, condensation heat transfer coefficients and pressure drops of R-410A are obtained in flattened microfin tubes made from 7.0 mm O.D. round microfin tubes. The test range covers saturation temperature 45 °C, mass flux 100–400 kg m⁻² s⁻¹ and quality 0.2–0.8. Results show that the effect of aspect ratio on condensation heat transfer coefficient is dependent on the flow pattern. For annular flow, the heat transfer coefficient increases as aspect ratio increases. For stratified flow, however, the heat transfer coefficient decreases as aspect ratio increases. The pressure drop always increases as aspect ratio increases. Possible reasoning is provided based on the estimated flow pattern in flat microfin tubes. Comparison with existing round microfin tube correlations is made.     

An experimental study of the fluid flow and heat transfer characteristics in micro-condensers with slug-bubbly flow

Experiments were conducted to study the condensation flow pattern in silicon micro-condensers using water as the medium. Slug-bubbly flow was found to be one of the dominant flows in the micro-condenser and it was a major factor in determining the heat transfer and pressure drop properties of the fluid inside the micro-condenser. The transition from the slug-bubbly flow to a mixed flow pattern was studied. A correlation was obtained to predict when the transition of the flow pattern would occur. Only slug-bubbly flow existed under low steam mass flow rate and high heat transfer rate conditions. As the steam mass flow rate increased or the heat transfer rate dropped, the mixed flow pattern would then appear. In the slug-bubbly flow regime, the heat transfer coefficient and pressure drop in the micro-condensers were investigated in detail. It was found that micro-condensers with smaller channels could exhibit higher heat transfer coefficients with the same Reynolds number. The condensation heat transfer coefficient was higher than that in the tubes with the diameter of centimeter. Pressure drops in the micro-condensers with smaller channels were higher due to the increased transition loss. At the same time, the pressure drop in the micro-condenser was found to be lower than what could be predicted using the macro-scale correlation. Increasing the heat flux would create a longer bubble–film region and fewer unit cells in the micro-condenser resulting in an increased heat transfer coefficient and a decreased pressure drop.     

Research on Marangoni Condensation Modes for Water–ethanol Mixture Vapors

In this paper the condensation experiments for water–ethanol vapors were carried out at different vapor pressures over a wide range of ethanol concentration. The condensation modes were observed and quantitatively analyzed in order to clarify the condensation phenomenon and dropwise condensation mechanisms. The cycle time of dropwise condensation, affected by vapor-to-surface temperature difference, ethanol concentration and vapor pressure, was approximate 0.2 s to 2 s. The quantity proportion of drops with the diameter less than 1 mm was more than 70% in all drops for all mixture vapors. The peak values of the maximum departing diameters increased with the ethanol vapor concentration, and were weakly affected by the vapor pressure, and the values were about 1.5 mm to 5 mm. The rivulet condensation mode was usually observed as a transition state appeared when the drop mode changed to film mode. The maximum distance between rivulets was sensitive to the ethanol vapor concentrations and little dependent on the vapor-to-surface temperature difference.     

蒸汽流量对凝结换热可视化过程的影响

本文通过对实验数值的分析来研究冷凝传热传质问题,以套管式冷凝器为研究对象,设计出一种对冷凝段凝结换热过程进行可视化研究的实验台。研究的目的在于,寻求蒸汽管内换热由珠状凝结渐变为膜状凝结时的管长与热力参数之间的的特征关系,在相变发生的管长位置设法将凝结液提前排出,以维持较高的对流换热系数。通过研究表明：凝结状态转折点所对应的管长与蒸汽入口速度密切相关,并给出了具有指导意义的关联式,且实验值与理论计算值的误差值小于10％。     

冷金属过渡条件下AZ61镁合金在两种钢板上的润湿行为

采用动态座滴法研究冷金属过渡条件下,AZ61镁合金分别在Q235钢板和镀锌钢板表面的润湿行为及其界面微观结构。结果表明:润湿行为与焊接工艺参数中的送丝速率密切相关;无论基板采用镀锌钢还是Q235钢在界面处均观察到Al-Fe金属间化合物层,其形成符合热力学形成条件;在Q235钢表面润湿时,送丝速率增加,界面反应变得剧烈,因而润湿性变好,在镀锌钢表面润湿时,送丝速率增加,加剧锌的挥发,使裸露的表面显金属性,因而润湿性变好;当送丝速率≤10.5m·min~(-1)时,镁在Q235钢板上的润湿性要好于镀锌钢板,且后者锌的挥发将导致工艺不稳定。     

汽-气液滴形管与圆管外凝结换热的研究

为了研究管型对汽-气凝结换热的影响和强化汽-气凝结换热过程,建立了汽-气在液滴形管与圆管外凝结换热所形成的气液膜的厚度及换热系数沿管壁分布的综合数学模型。通过有限差分的方法,以天然气燃烧产生的烟气为例对两种管型进行了计算比较。在有效换热面积相等的情况下,与圆管相比,液滴形管上半部分管径小,压力梯度大,有利于排液,下半部分表面曲率大,亦有利于排液;液滴形管表面形成的气膜薄,液膜亦薄,珠状凝结区域大,液珠尺寸小,凝结换热系数大;液滴形管对显热传递亦有一定的强化作用。通过实验对计算结果进行了比较验证,结果表明该模型亦适用于椭圆管和其它汽-气混合流体种类。     

Experimental study of condensation heat transfer of R-404A in helically coiled tubes

In this research, the heat transfer coefficients of R-404A vapor condensation inside helically coiled tubes are studied, experimentally. The effects of different coil pitches and curvature radii at different vapor qualities and mass velocities are considered. The vapor is condensed inside the helically coiled tubes by transferring heat to the cooling water flowing in annulus. Results show that the coil diameter has significant effect on condensation heat transfer coefficient. By decreasing the coil diameter or increasing the Dean number, the heat transfer coefficient is increased as the highest value is obtained at curvature radius of 4.35 cm which is 45% greater than the corresponding figure of curvature radius of 7.65 cm at mass velocity of 125 kg m⁻² s⁻¹. Also, at low vapor qualities, the coil pitch effect is more pronounced. Finally, based on the results, a new correlation is developed to evaluate the condensation heat transfer coefficient of R-404A inside helically coiled tubes.     

Theoretical investigations of hydrogen,nitrogen, and oxygen homogenous and annular wick heat pipes

In any heat pipe, the capillary pressure developed at the liquid-vapour interface balances the sum of the various pressure drops throughout the pipe. This study analyses the different contributions to pressure drop for both homogenous wick and annular wick heat pipes operating at low temperatures. The pressure drop in the wick structure is of primary importance for a homogenous wick heat pipe. The heat transfer capacity of an annular-wick heat pipe is strongly affected by the interphase pressure drop due to non-zero evaporation and condensation rates at the liquid—vapour interfaces. Theoretical heat transfer rates as functions of the vapour temperature have been computed for both homogenous and annular wick structures of heat pipes using hydrogen, nitrogen, and oxygen as working fluids. The heat transfer capacity of the annular wick design is more than an order of magnitude higher than that of the corresponding homogenous wick design.     

一种新型高效传热铜管的冷凝传热性能实验研究

建立无润滑油实验台，以R22。R134a和R410A为工质。测试新型铜管Turbo-DWT和常规内螺纹铜管Turbo—A的冷凝传热性能，并进行比较。从实验数据可知。新管型Turbo-DWT的冷凝传热系数高于Turbo-A约42％，且压降低于Turbo-A约65％（R134a）。三种制冷荆相比，R22的传热系数最高，R410A的压降最小。Turbo-DWT是一种更高效的冷凝传热管，且适用于各种冷媒。     

The inclusion of the effect of the evaporation coefficient on the rate of variation of droplet radius

The paper deals with the investigation of the effect of the evaporation (condensation) coefficient of droplet substance on the rate of unsteady variation of the radius of a spherical aerosol droplet in the cases of direct and indirect inclusion of this coefficient. It is found in both cases that the effect of evaporation coefficient is most significant at the initial instant of unsteady-state process of evaporation and of condensation growth of the droplet. At this instant, the size of spherical droplet has hardly any impact on the rate of variation of its radius. As the unsteady-state process continues, the effect of the evaporation coefficient on the rate of variation of the droplet radius depends significantly on the droplet size. The larger the droplet under consideration, the lower the effect of the evaporation coefficient on the rate of variation of its radius. The rates of variation, calculated for the same values of the evaporation coefficient but for different ways of inclusion of this coefficient, differ less for larger aerosol droplets. These methods of inclusion of the evaporation coefficient are considered for the process of slow evaporation of a droplet.     

微圆管内R1234ze(E)流动沸腾的环状流特性模拟研究

本文建立了制冷剂R1234ze(E)在微圆管内流动沸腾过程中的环状流模型,对传热和气液两相流动压降进行了模拟研究。综合考虑重力、表面张力及气液界面剪切力的影响,模拟分析了周向液膜不均匀分布特性及该特性对流动与换热的影响,经验证,计算结果与已有实验结果吻合较好。本文还研究了不同因素对环状流区域表面传热系数与压降的影响。模拟结果表明:在流动起始区域,截面液膜厚度的分布受重力作用影响,随着流动沸腾过程的进行,该影响作用开始减弱,且有重力作用时的环状流平均表面传热系数高于无重力作用时的环状流平均表面传热系数,随着重力加速度的增加,环状流的平均表面传热系数不断增大;随着质量流速的增大,表面传热系数与压降均随之增大;随着管径增大,表面传热系数与压降均随之减小。     

Ice-slurry production using direct contact heat transfer

An ice slurry generation system was developed using direct contact heat transfer between water and the coolant, Fluroinert FC-84. The location of the coolant nozzle is an important design consideration to avoid clogging due to freezing of water. An ice fraction of up to about 40 percent was obtained with the nozzle located at the bottom of the ice slurry tank and the jet directed upwards into the water. Two simplified model were developed to extract the heat transfer coefficient between the coolant drops and the water. The first model requires as input the average drop diameter and the residence time while the second model uses the measured drop diameter distribution. The estimated heat transfer coefficients are much smaller than those computed using single-sphere correlations.     

分液冷凝器的管程理论设计及热力性能评价

根据分液冷凝器强化换热思想对其管程理论设计方法进行了研究。依据质量流速和干度来判断每一流程中制冷剂的流型,并依此选取Cavallini换热模型公式的方法求其平均换热系数,同时采用Cavallini两相压降模型和Darcy-Weisbach单相压降模型分别确定冷凝区和过冷段的压降。针对一个案例计算了三种管程设计方案下冷凝器管内冷凝换热系数和端压值,并用惩罚因子PF对其综合热力性能进行了评价。计算结果表明:不同的管程设计方案中管内制冷剂的流量分配均匀性存在较大的差异,均匀性越好,其综合热力性能越优。在质量流速为1200~1500 kg/(m2.s)范围内,与同等换热面积的蛇形管冷凝器相比,其中最好的分液冷凝器的PF值减小了48.5%~54.1%,可见设计优良的分液冷凝器的综合热力性能明显优于蛇形管冷凝器。