Experiments were conducted on the fluid flow and heat transfer for packed glass beads between plates with longitudinally mini-channeled surface. The measured results were reported for different configuration combinations. Remarkable improvement in performance of pressure drop and heat transfer can be obtained. Optimum aspect ratio-channeled configuration would exist, which was found to be varied with Reynolds number.

The Kelvin-Helmholtz instability of phase-change interface during flow film condensation in vertical mini tube has been studied by means of work or energy analysis. According to the interfacial boundary conditions acquired, the film thinning effect and the phase change area enlarging effect by interfacial waves on heat transfer enhancement are analyzed in different diameter tubes. It is indicated that in mini diameter tube, more obvious heat transfer enhancement due to interfacial waves can be expected than in normal-sized tube, and the interfacial waves enhance the heat transfer mainly by film thinning effect. The revised Nusselt models, which take care the factors neglected by Nusselt, such as variable wall surface temperature and interfacial transport phenomena, are remarkably successful to predict the relevant parameters in traditional industrial fields [1]. It is well-known that, the heat transfer enhancement of condensation affected by phase change interfacial waves is not more than 20% if the shear stress between liquid and vapor phase can be neglected, and the interfacial waves take effect only as Rel>20~30 [2]. However, in many heat transfer conditions, the tube diameter of heat exchangers is reduced to less than 3~5mm, named as mini tube, such as generally appeared in two-phase flow micro system or for conditions of high heat flux. Analysis on flow condensation indicated that, in such mini tubes, the bend effect of condensate film can't be neglected, which void the "plate" approximations in Nusselt theory [3], so the surface tension can take obvious effect on condensation. Also in mini tube, the shear stress on liquid-vapor interface has more important effect than gravity. Surely, the increasing effect of shear stress and surface tension will disturb the condensate film and enhance the instability of the film. It is therefore logical to deduce that the interfacial waves may have more obvious effect on heat transfer in mini tube than in normal scale tube. It is the purpose of the present paper to investigate the effect of interfacial waves on heat transfer during flow film condensation in mini tube. According to literature [4], the present paper assumes the amplitudes of the waves growing due to the Kelvin-Helmholtz instability. Occurence of finite amplitude capillary waves is responsible for such an appearance. The interface waving and the interfacial boundary conditions are treated by work or energy analysis, based on which, the effect of interfacial capillary waves on heat transfer characteristics of flow film condensation in mini tube are analyzed.

微尺度加热表面过冷核态沸腾传热实验的可视化观测、局部细节过程的记录是其机理研究的重要方法.本文利用高速摄像仪分别对30μm、50μm和60 μm铂丝在过冷下汽泡生长及运动情况进行了系统观测.观测到与以往微尺度和常规尺度下加热丝表面汽泡行为不同的“新”特征:微细加热丝上的过热薄液层现象,汽泡在合适热流下的悬浮运动以及汽泡间出现环绕运动等一些实验现象.

Bubble dynamics on a microscale heating source is crucial to the understanding of heat transfer mechanisms in nucleate boiling. In this investigation, the effect of thermocapillary convection, along with other contributors such as viscous force, on the bubble sweeping and circling phenomena observed in the experiment of subcooled nucleate boiling of deionized water over a micro platinum wire was investigated by numerical simulation and theoretical analysis. The simulation indicates that occurrence of sweeping is mainly due to the thermocapillary effect between the sweeping bubble and a neighboring bubble with a temperature drop caused by the bubble-top jet flow. Based on the parabolic interfacial temperature profile, a simplified dimensional analysis was conducted for the characteristic velocities and lengths along the heating wire and along the bubble interface. The theoretical analysis and the simulation show that the thermocapillary force and the viscous force play important roles in the bubble circling phenomenon. This work may be helpful for better understanding the interfacial force effects on the bubble behaviors and hence the boiling heat transfer at the microscale. (C) 2015 Elsevier Ltd. All rights reserved.

A theoretical model is developed for condensation heat transfer of binary refrigerant mixtures in mini-tubes with diameter about 1.0 mm. Condensation heat transfer of R410A and R32/R134a mixtures at different mass fluxes and saturated temperatures are analyzed, assuming that the phase flow pattern is annular flow. The results indicate that there exists a maximum interface temperature at the beginning of condensation process for azeotropic and zeotropic mixtures and the corresponding vapor quality to the maximum value increases with mass flux. The effects of mass flux, heat flux, surface tension and tube diameter are analyzed. As expected, the condensation heat transfer coefficients increase with mass flux and vapor quality, and increase faster in high vapor quality region. It is found that the effects of heat flux and surface tension are not so obvious as that of tube diameter. The characteristics of condensation heat transfer of zeotropic mixtures are consistent to those of azeotropic refrigerant mixtures. The condensation heat transfer coefficients increase with the concentration of the less volatile component in binary mixtures.

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.

Phase change can dramatically alter the interfacial temperature, resulting in surface tension gradients and consequently causing Marangoni convection. The numerical investigation on Marangoni convection of binary fluids in a closed microcavity is accomplished in this paper by using the volume of fluid (VOF) model with source terms added by user defined functions (UDF) due to mass transfer, with detailed velocity and temperature fields. For simple fluid, surface tension decreases with increasing temperature, resulting in thermal Marangoni convection that can drive the liquid leave from hot regions and leading to film dryout. For binary fluids, however, the Marangoni convection could also be caused by concentration gradients, resulting in greatly promoted backflow of the fluid. In particular, for self-rewetting fluids which have unique surface tension characteristics that increase with increasing temperature above a critical value, the Marangoni flow can drive the liquid flow towards hot regions, avoiding film dryout. The influence of non-condensable gas is also considered by providing detailed velocity fields near the contact region and it proves that non-condensable gas can negatively affect the heat and mass transfer.

The flow and heat transfer characteristics of nanofluid were attracting many researchers during the last two decades. Convection heat transfer in it is especially important for its potential applications. Therefore, it is crucial to study the nanoparticle behaviors near the near wall region and the interfacial property, which are dominant in nanofluid convection heat transfer. In this investigation, stable nanofluid was prepared with alumina nanoparticles (30 nm in diameter) and deionized water. The nanoparticle behaviors near the liquid wedge of bubbles were observed by high-speed CCD camera. The effects of nanoparticle behaviors in this region and the interfacial characteristics at the liquid-vapor interface on convection of nanofluid were experimentally investigated. Flocculent nanoparticle clustering was observed swirling near the liquid wedge, when the heat flux is relatively small. The microscopic morphology of the nanoparticle deposition layer at the heated surface was characterized by SEM images. It seems that the deposition layers could modify the morphology, but it also delays the detachment of small bubbles from the heated surface. While n-butanol was included as surfactant which will change the liquid/vapor interfacial property, it intensifies the nanoparticle deposition for low heat flux conditions. The analysis shows that the critical heat flux of nanofluid can be obviously improved when n-butanol was included in the nanofluid. It also shows that the inhibited bubble growth and enhanced nanoparticle clustering in the liquid wedge region are the main reason for the heat transfer deterioration when increasing the amount of surfactant in nanofluid. A comparative experiment indicates that the effect of surfactant on convection heat transfer is greater than the unstable deposition formed by nanoparticles. (C) 2014 Elsevier Inc. All rights reserved.

Jet flow phenomenon is important in enhancing the nucleate boiling heat transfer processes. When heater sizes scale down, jet flow can be observed due to the thermocapillary convection around bubbles attached on microscale heated surface. In this paper, a self-rewetting fluid, aqueous n-butanol solution, was employed for demonstrating the effect of thermocapillary convection on bubble behaviors during subcooled nucleate boiling on thin wire, comparing with deionized water. Bubble-top jet flow for water and multi-jet flows for n-butanol solution were observed around a platinum micro heating wire by high speed CCD camera. Corresponding numerical simulation proved that it is the thermocapillary convection that attracts the subcooled water to flow from the superheated microlayer at the base to the top of a stationary bubble. For n-butanol solution, however, the thermocapillary convection can induce it to flow oppositely, causing the subcooled solution to flow onto the heated surface. The simulation for the solution was in good agreement with the experiment where the subcooled liquid near the bubble top flow towards the bubble base, or the heated surface, and hence the multi-jet flows occur. The multi-jet flows can sustain for a long period and cause bubble emission at the superheated thin liquid layer near the heated surface. The temperature around the bubble presented sharp temperature gradient and the velocity in the nearwall region was almost vertical to the wall. The experimental and numerical studies on the effect of surface tension and thus thermocapillaiy convection are crucial to the mechanisms of subcooled nucleate boiling of fluids. (C) 2014 Elsevier Inc. All rights reserved.

微尺度物体表面过冷沸腾中,汽泡间的相互作用对于传递现象起着重要的作用.本文通过实验观测到微尺度铂金加热丝表面高纯水过冷沸腾过程中汽泡的碰撞,并进行相应的数值模拟.模拟结果表明,汽泡合并过程中表面张力发挥着重要的作用;汽泡的分离是表面张力和热毛细作用的共同结果;小汽泡与大汽泡碰撞后可能发生绕流,此时动量交换很小,界面张力和热毛细力起主要作用,小汽泡在大汽泡顶部停留数毫秒后与大汽泡合并或脱离.模拟结果与实验结果一致.

A model based on the augmented Young–Laplace equation and kinetic theory was developed to describe the nanostructured roughness effects on an extended evaporating meniscus in a microchannel for Wenzel and Cassie–Baxter states. The roughness geometries were analytically related to the disjoining pressure, slip length and thermal resistance across the roughness layer. The results show that the equivalent Hamaker constant and adsorbed film thickness increase with nanopillar height for Wenzel state. Thus, the spreading and wetting properties of the evaporating thin film increase with roughness for Wenzel state, leading to an elongated thin film and enhanced heat transfer rate compared to a flat hydrophilic surface. The equivalent Hamaker constant and disjoining pressure effect decrease with increasing nanopillar height for Cassie–Baxter state. The system wettability, thin film length and heat transfer rate increase with increasing slip length and with decreasing roughness for Cassie–Baxter state. A smaller roughness coexisting with a larger slip length on rough surfaces for Cassie–Baxter state results in a much higher heat transfer rate relative to a flat surface.

The interaction between adjacent bubbles plays an important role in boiling heat transfer on a micro heat source. In this paper, the visualizing bubble behaviors during subcooled pool boiling on microwires were revisited experimentally. Bubble circling phenomena, the circling of small bubble around a larger one for subcooled boiling on platinum microwires submerged in ultrapure water or self-rewetting fluid, were investigated. Different types of bubble circling phenomena, including small bubble sweeping, departing, rotating, returning or chasing at a larger bubble interface, were described in detail. The roles of Marangoni effect and viscous force were analyzed in the process of bubble circling to understand the mechanism of bubble behaviors on micro heated surface. The investigation on the bubble circling phenomenon can help verify the crucial factors that influence the bubble dynamics, bubble interaction and boiling heat transfer. The bubble dynamics in this phenomenon also makes it an interesting choice for many microscale applications.

An inverse model based on the shooting method, Mie theory and the improved Kramers–Kronig (KK) relation was combined with FTIR and Abbe refractometer measurements to calculate the complex refractive indices of various infrared opacifiers. The effects of opacifier sizes, types and shapes were then analyzed based on the Rosseland mean extinction coefficient using Mie theory and anomalous diffraction theory (ADT). This model provides theoretical guidelines for designing materials with optimized parameters, such as size, type and shape of opacifiers, to improve the aerogel thermal insulation at high temperatures. The results show that the optimum diameter of SiC particles to minimize the radiation is 4 μm for T < 400 K and 3 μm for T > 400 K. Carbon black is the optimum opacifier for T < 600 K while SiC is the optimum opacifier to minimize the radiative heat transfer for T > 600 K among the investigated opacifiers of SiC, TiO2, ZrO2, amorphous SiO2 and carbon black. The infrared extinction ability for various shapes is largest for oblate spheroids and decreases for spheres, cubes, cylinders with small length-to-diameter ratios, and then long, thin cylinders.

The parallel proportion factor of solid-gas thermal conductions was presented based on the very realistic aerogel structure. An analytical heat transfer model was then developed with full considerations of the aerogel microstructure and the nanoscale thermal conduction effects. The results show that this model can well predict the total thermal conductivity of aerogels for various pressures, temperatures, densities and microstructures. This model can be used to quickly predict and optimize the relationship between the thermal conductivity and the microstructure. The total thermal conductivity of silica aerogels reaches its minimum at a density of 130 kg·m-3 at ambient conditions. The total thermal conductivity of aerogels decreases with increasing the number of secondary nanoparticles at high temperatures. Thus, the thermal insulating performance of aerogels is mainly affected by the number of secondary nanoparticles at high temperatures.

The silica aerogel is a nanoporous super thermal insulation material with weak mechanical strength. The fibers are usually filled into the aerogel to strengthen the material. A combined conduction and radiation heat transfer model based on the optically thick assumption was developed to investigate the effects of the fiber species effects on the heat transfer through the fiber-loaded aerogel composite. The results show that the silicon glass-b has the largest extinction coefficient for best suppressing the radiation at higher temperatures while the soda lime silica fiber has the smallest extinction coefficient. The selection of proper fiber is important for improving the thermal insulation. The fiber, which has the largest complex refractive index with smallest fluctuations, has the largest extinction coefficient. For the 4 species of fibers in this study, the silicon glass-b is optimum for best reducing the total thermal conductivity for higher temperatures.

The role of temperature gradient induced nanoparticle motion on conduction and convection was investigated. Possible mechanisms for variations resulting from variations in the thermophysical properties are theoretically and experimentally discussed. The effect of the nanoparticle motion on conduction is demonstrated through thermal conductivity measurement of deionized water with suspended CuO nanoparticles (50 nm in diameter) and correlated with the contributions of Brownian diffusion, thermophoresis, etc. The tendencies observed is that the magnitude of and the variation in the thermal conductivity increases with increasing volume fraction for a given temperature, which is due primarily to the Brownian diffusion of the nanoparticles. Using dimensional analysis, the thermal conductivity is correlated and both the interfacial thermal resistance and near-field radiation are found to be essentially negligible. A modification term that incorporates the contributions of Brownian motion and thermophoresis is proposed. The effect of nanoscale convection is illustrated through an experimental investigation that utilized fluorescent polystyrene nanoparticle tracers (200 nm in diameter) and multilayer nanoparticle image velocimetry. The results indicate that both the magnitude and the deviation of the fluid motion increased with increasing heat flux in the near-wall region. Meanwhile, the fluid motion tended to decrease with the off-wall distance for a given heating power. A corresponding numerical study of convection of pure deionized water shows that the velocity along the off-wall direction is several orders of magnitude lower than that of deionized water, which indicates that Brownian motion in the near-wall region is crucial for fluid with suspended nanoparticles in convection.

The internal heat transfer of different gases in microporous media was investigated experimentally and numerically. The experimental test section had a sintered bronze porous media with average particle diameters from 11 μm to 225 μm. The Knudsen numbers at the average inlet and outlet pressures of each test section varied from 0.0006 to 0.13 with porosities from 0.16 to 0.38. The particle-to-fluid heat transfer coefficients of air, CO2 and helium in the microporous media were determined experimentally. The results show that the Nusselt numbers for the internal heat transfer in the microporous media decrease with decreasing the particle diameter, d p, and increasing Knudsen number for the same Reynolds number. For Kn>0.01, the rarefaction affects the internal heat transfer in the microporous media. A Nusselt number correlation was developed that includes the influence of rarefaction. The computational fluid dynamics (CFD) numerical simulation was carried out to do the pore scale simulation of internal heat transfer in the microporous media considering the rarefaction effect. Pore scale three-dimensional numerical simulations were also used to predict the particle-to-fluid heat transfer coefficients. The numerical results without slip-flow and temperature jump effects for Kn<0.01 corresponded well with the experimental data. The numerical results with slip-flow and temperature jump effects for 0.01<Kn<0.13 are lower than the numerical results without rarefaction effects, but closer to the experimental data. The numerical results with rarefaction effects can accurately simulate the unsteady heat transfer in the microporous media.

The surface nanostructure determines the system wettability and thus has significant effects on the thin liquid film spreading and phase change heat transfer. A model based on the augmented Young-Laplace equation and kinetic theory was developed to describe the nanoscale roughness effects on the extended evaporating meniscus in a microchannel. The roughness geometries in the model were theoretically related to the disjoining pressure and the thermal resistance across the roughness layer. The results show that the dispersion constant for the disjoining pressure increases with the nanopillar height when the solid-liquid-vapor system is in the Wenzel state. Thus, the spreading and wetting properties of the evaporating thin liquid film are enhanced due to the higher nanopillar height and larger disjoining pressure. Since the evaporating thin film length increases with the nanoscale roughness due to better surface wettability, the total liquid flow and heat transfer rate of the evaporating thin liquid films in a microchannel can be enhanced by increasing the nanopillar height. The effects of the nanopillar on the thin film evaporation are more significant for higher superheats. Hydrophilic nanotextured solid substrates can be fabricated to enhance the thin film evaporation and thus increase the maximum heat transport capability of the two-phase cooling devices.

An improved analytical model for the total thermal conductivity of fiber-loaded silica aerogels was developed based on the complex refractive index, size, orientation, volume fraction and morphology of the fibers and silica aerogel. A cubic array of spherical porous secondary nanoparticles and a modified parallel-series model were proposed to model the combined solid and gaseous thermal conductivities. An anomalous diffraction theory (ADT) was used to predict the fiber extinction coefficient. Five common fiber types in the composites were studied including amorphous SiO2 glass, silicon glass, common float glass, soda lime silica glass and borosilicate glass. The results show that the total extinction coefficient of the silica aerogel system is largest by loading with the common float glass fiber and lowest by loading with the soda lime silica glass among the five fiber types. The model provides theoretic guidelines for material designs with optimum parameters, such as the type, inclination angle, volume fraction and diameter of the fibers as well as the aerogel nanoparticle and pore sizes. The optimum fiber for improved thermal insulation should have a large spectral complex refractive index throughout the infrared region.

Micro-PIV is an important micro-scale flow velocity measurement technology, capable of acquiring the speed of tracer particles and plotting a flow field of the fluid. On the basis of the currently available microscopic measurement technologies in the laboratory, the authors developed a Micro-PIV three-dimensional measurement image processing technology and conducted an optimization and corrected optimization of the image processing. The key to the precision of the two-dimensional coordinates of the particle image centers lies in the matter how to identify and locate the particles according to the main characteristics of the image centers. A three-dimensional identification of fluorescent particles is based on the diffraction images of the particles, which change with the defocus distance, and a Micro-PIV three-dimensional measurement is realized through a matching of the test images to be measured and the numerical template. The defocus distance of a numerical image is that of the test image. On the basis of an analysis of the test images and the numerical template, the test images were pretreated and the numerical template was corrected respectively, resulting in a better matching effectiveness of the test fluorescent particle images and the numerical template.