Shear-driven flows of locally heated liquid films

This paper considers the flow of a liquid film sheared by gas flow in a channel with a heater placed at the bottom wall. A one-sided 2D model is considered for weakly heated films. The heat and mass transfer problem is also investigated in the framework of a two-sided model. The exact solution to the problem of heat transfer is obtained for a linear velocity profile. The double effect of Marangoni forces is demonstrated by the formation of a liquid bump in the vicinity of the heater’s upper edge and film thinning in the vicinity of the lower edge. The criterion determining the occurrence of “ripples” on the film surface upstream from the bump is found. Numerical analysis reveals that evaporation dramatically changes the temperature distribution, and hence, thermocapillary forces on the gas–liquid interface. All transport phenomena (convection to liquid and gas, evaporation) are found to be important for relatively thin films, and the thermal entry length is a determining factor for heaters of finite length. The thermal entry length depends on film thickness, which can be regulated by gas flow rate or channel height. The influence of the convective heat transfer mechanism is much more prominent for relatively high values of the liquid Reynolds number. The liquid–gas interface Biot number is shown to be a sectional-hyperbolic function of a longitudinal axis variable. Some qualitative and quantitative comparisons with experimental results are presented.     

STRUCTURAL AND MICROSTRUCTURAL EFFECTS ON THE THERMAL CONDUCTIVITY OF ZIRCONIA THIN FILMS

Thermal properties of thin films are involved in a large number of applications such as electronic and electrooptical devices, or thermal barrier coating. It is known that the structure and the microstructure of thin solid films have a strong influence on the thermal conductivity, which may be considerably lower than for bulk materials. The aim of this study is to highlight the role of structure and microstructure on the thermal conductivity of ZrO 2 thin films (stabilized with Y 2 O 3 or not). Investigations have focused on the influence of film thickness, substrate roughness, and crystallites size on the thermal properties of dielectric thin film samples. For this purpose, a new photothermal method has been developed to measure the thermal conductivity of such films on various kind of substrates with an accuracy better than 10%. It has been observed that a decrease in the dielectric thickness leads to a drastic drop of the apparent thermal conductivity, k a , whatever the ZrO 2 phase is. k a is affected by an additional thermal resistance, R fs , especially between ZrO 2 and alumina substrate. This resistance R fs varies linearly with the substrate roughness. Finally, the influence of crystallite size and grain boundaries on k a have been shown.     

生物基底材料上纳米金薄膜导电、传热性能研究

利用瞬态电热技术测量了不同生物基底材料的热扩散率。通过增加镀层数目,采用微分测量法进行实验,根据实验结果计算了相应基底材料上厚度为5.00～20.00nm金薄膜的洛伦兹数、导热系数以及导电系数。研究结果表明,由于晶格边界散射大大减小了薄膜的导电和导热系数,所有基底上金薄膜的导热和导电系数均小于体材料金的导热和导电系数。由于蚕丝中的蛋白晶体可促进电子跳跃并产生隧道效应,蚕丝基底上金薄膜的导电系数大于其他几种基底上金薄膜的导电系数,因此蚕丝更适合作为柔性电子器件中的基底材料。     

Mixed Thermocapillary and Forced Convection Heat Transfer Around a Hemispherical Bubble in a Miniature Channel—A 3D Numerical Study

It has been established that for certain conditions, such as microgravity boiling, thermocapillary Marangoni flow has associated with it a significant enhancement of heat transfer. Typically, this phenomenon was investigated for the idealized case of an isolated and stationary bubble resting atop a heated solid that is immersed in a semi-infinite quiescent fluid or within a two-dimensional cavity. This article presents a three-dimensional numerical study that investigates the influence of thermal Marangoni convection on the fluid dynamics and heat transfer around a bubble during laminar flow of water in a minichannel. This mixed thermocapillary and forced convection problem is investigated for channel liquid inlet velocity of 0.01 m/s to 0.03 m/s and Marangoni numbers in the range of 10 to 300 under microgravity conditions. Three-dimensional effects become particularly important on the side and rear regions of the bubble. The thermocapillary forces accelerate the flow along almost the entire bubble interface. The hot core fluid from the heated bottom wall region is forced inward and propelled upward into the thermocapillary jet above the bubble. It can be quantified that the influence of thermocapillary flow on heat transfer enhancement shows an average increase by 40% at the downstream of the bubble and by 60% at the front and rear regions. This heat transfer enhancement depends mainly on the temperature differential as the driving potential for thermocapillary flow and bulk liquid velocity.     

Mixed Convective Heat Transfer Due to Forced and Thermocapillary Flow Around Bubbles in a Miniature Channel: A 2D Numerical Study

Marangoni thermocapillary convection and its contribution to heat transfer during boiling has been the subject of some debate in the literature. Currently, for certain conditions, such as microgravity boiling, it has been shown that Marangoni thermocapillary convection has a significant contribution to heat transfer. Typically, this phenomenon is investigated for the idealized case of an isolated and stationary bubble resting on a heated surface, which is immersed in a semi-infinite quiescent fluid or within a two-dimensional cavity. However, little information is available with regard to Marangoni heat transfer in miniature confined channels in the presence of a cross flow. As a result, this article presents a two-dimensional (2D) numerical study that investigates the influence of steady thermal Marangoni convection on the fluid dynamics and heat transfer around a bubble during laminar flow of water in a miniature channel with the view of developing a refined understanding of boiling heat transfer for such a configuration. This mixed convection problem is investigated under microgravity conditions for channel Reynolds numbers in the range of 0 to 500 at liquid inlet velocities between 0.01 m/s and 0.0 5m/s and Marangoni numbers in the range of 0 to 17,114. It is concluded that thermocapillary flow may have a significant impact on heat transfer enhancement. The simulations predict an average increase of 35% in heat flux at the downstream region of the bubble, while an average 60% increase is obtained at the front region of the bubble where mixed convective heat transfer takes place due to forced and thermocapillary flow.     

Thermal and solutal effects on convection inside a polymer solution droplet on a substrate

The convective flow inside polymer solution droplets drying on a lyophobic substrate is numerically studied. The evaporating droplet is presumed as a hemisphere shrinking with time at the constant contact angle. The thermal and solutal effects are simultaneously considered in the computation. The thermal Marangoni convection is induced due to the quick thermal diffusion, and this convection transports the solute resulting in the solutal Marangoni flow. The solutal dependence corresponds to our previous experimental work, but the flow pattern does not. Consideration of the pseudo evaporation rate distribution depending on the contact angle yields to the flow pattern correspondence.     

An experimental study of rupture dynamics of evaporating liquid films on different heater surfaces

Experimental data were obtained to reveal the complex dynamics of thin liquid films evaporating on heated horizontal surfaces, including formation and expansion of dry spots that occur after the liquid films decreased below critical thicknesses. The critical thickness of water film evaporating on various material surfaces is measured in the range of 60–150 μm, increasing with contact angle and heat flux while decreasing with thermal conductivity of the heater material. In the case of hexane evaporating on a titanium surface, the liquid film is found resilient to rupture, but starts oscillating as the averaged film thickness decreases below 15 μm.     

Numerical investigation of heat and mass transfer from an evaporating meniscus in a heated open groove

The process of evaporation from a meniscus into air is more complicated than in enclosed chambers filled with pure vapor. The vapor pressure at the liquid–gas interface depends on both of the evaporation and the vapor transport in the gas environment. Heat and mass transport from an evaporating meniscus in an open heated V-groove is numerically investigated and the results are compared to experiments. The evaporation is coupled to the vapor transport in the gas domain. Conjugate heat transfer is considered in the solid walls, and the liquid and gas domains. The flow induced in the liquid due to Marangoni effects, as well as natural convection in the gas due to thermal expansivity and vapor concentration gradients are simulated. The calculated evaporation rates are found to agree reasonably well with experimentally measured values. The convection in the gas domain has a significant influence on the overall heat transfer and the wall temperature distribution. The evaporation rate near the contact lines on either end of the meniscus is high. Heat transfer through the thin liquid film near the heated wall is found to be very efficient. A small temperature valley is obtained at the contact line which is consistent with the experimental observation.     

重力场对液池内热-溶质毛细对流影响的数值研究

数值研究了不同重力场下液池内耦合热-溶质毛细对流流动特性,模型中考虑了热毛细效应和溶质毛细效应相当这一特殊情况。计算结果显示,当重力加速度较大时液池内存在周期性迁移的对流涡,而当重力加速度较小时,液池内的对流涡迁移消失,因而重力加速度能够促使热-溶质毛细对流失稳。随着重力加速的减小,监测点的温度和浓度振荡幅度减小。常重力条件下自由表面速度分布受浮力对流控制;微重力条件下,自由表面的速度分布基本一致,随着重力加速度减小自由表面速度略微减小。     

Chemical and electronic interface structure of spray pyrolysis deposited undoped and Al-doped ZnO thin films on a commercial Cz-Si solar cell substrate

We have studied differences in the interface between undoped and Al-doped ZnO thin films deposited on commercial Si solar cell substrates. The undoped ZnO film is significantly thicker than the Al-doped film for the same deposition time. An extended silicate-like interface is present in both samples. Transmission electron microscopy (TEM) and photoelectron spectroscopy (PES) probe the presence of a zinc silicate and several Si oxides in both cases. Although Al doping improves the conductivity of ZnO, we present evidence for Al segregation at the interface during deposition on the Si substrate and suggest the presence of considerable fixed charge near the oxidized Si interface layer. The induced distortion in the valence band, compared to that of undoped ZnO, could be responsible for considerable reduction in the solar cell performance.     

Enhanced heat transfer by room temperature deposition of AlN film on aluminum for a light emitting diode package

Heat transfer is crucial to the fabrication of high efficiency light emitting diode (LED) packages. The effectiveness of the heat transfer depends on the package materials and design. This paper presents an application of high thermal conductivity aluminum nitride (AlN) films to replace low thermal conductivity epoxy resin or alumina substrates. The AlN film was directly deposited on an aluminum plate which enabled the removal of thermal interface materials (TIM) such as the adhesive thermal bonding sheets that are used in conventional metal printed circuit board (PCB)-based LED packaging process. A fully dense AlN ceramic film was successfully deposited at room temperature using the aerosol deposition method. The thermal resistance, a parameter of the heat transfer characteristic of an LED package, was measured using a thermal transient tester. The results showed that the thermal resistance of the LED package mounted on the AlN thick film was 28.5 K/W, while an LED package mounted on a conventional epoxy-based metal PCB and a PCB with thermal vias were 47.2 K/W and 36.5 K/W, respectively. This indicates that an aerosol-deposited AlN-based LED package exhibits greatly enhanced heat transfer compared to the conventional metal PCB.     

MOLECULAR DYNAMICS SIMULATION OF THERMAL CONDUCTION IN NANOPOROUS THIN FILMS

Molecular dynamics simulations of thermal conduction in nanoporous thin films are performed. Thermal conductivity displays an inverse temperature dependence for films with small pores and a much less pronounced dependence for larger pores. Increasing porosity reduces thermal conductivity, while pore shape has little effect except in the most anisotropic cases. The pores separate the film into local regions with distinctly different temperature profiles and thermal conductivities, and the effective film thermal conductivity is lowest when the pores are positioned in the center of the film. Such tunability by pore placement highlights new possibilities for engineering nanoscale thermal transport.     

Flow and thermal field in sessile droplet evaporation at various environmental conditions

Sessile droplets' evaporation is a complex process that involves fluid flow coupled with heat and mass transfer. In this study, mathematical modelling of sessile droplet evaporation on hydrophobic substrates is developed and simulations are carried out on COMSOL. The model results are validated with the data available in the literature. Postvalidation, the simulation of droplet evaporation is carried out on the various substrate hydrophobicities and various environmental conditions. For these conditions, contours are plotted for temperature, velocity, and mass concentration for the droplet and moist air domain. The result shows that Marangoni convection plays a very important role in droplet evaporation. A high rate of evaporation is observed at the droplet interface at low relative humidity and a large degree of subheating. The effect of air velocity on the evaporation rate is studied, however, its effect is very marginal as compared to relative humidity and degree of subheating. The heat flux at the three-phase contact line is large for a smaller Prandtl number fluid. Overall, the evaporation rate increases with increasing the Prandtl number because it has a large value of Marangoni convection.     

Pulsed laser deposition of yttria stabilized zirconia for solid oxide fuel cell applications

Yttria stabilized zirconia (YSZ) films were grown by pulsed laser deposition (PLD) from a 8YSZ target using a KrF excimer laser source (248 nm). The films have been deposited under oxygen atmospheres on porous NiO/YSZ substrates heated from room temperature to 600 °C. YSZ films were obtained in the range of 1–2 μm thickness. The films have been investigated with respect to surface morphology, microstructure and film–substrate interface interaction. The film morphology varied from columnar to an irregular crystalline structure depending on the oxygen pressure and the substrate temperature. In all cases the films consisted of YSZ with the cubic fluorite structure. The formation of oxide layers under low oxygen pressures on the NiO/YSZ substrates is due to a film–substrate redox interaction. The NiO grains close to the coating interface are partially reduced and serve as an oxygen source for the oxidation of the film. The measured He leakage rates to analyze the gas tightness of the YSZ films have so far shown no improvements as compared with uncoated substrates.     

A numerical model for transport in flat heat pipes considering wick microstructure effects

A transient, three-dimensional model for thermal transport in heat pipes and vapor chambers is developed. The Navier–Stokes equations along with the energy equation are solved numerically for the liquid and vapor flows. A porous medium formulation is used for the wick region. Evaporation and condensation at the liquid–vapor interface are modeled using kinetic theory. The influence of the wick microstructure on evaporation and condensation mass fluxes at the liquid–vapor interface is accounted for by integrating a microstructure-level evaporation model (micromodel) with the device-level model (macromodel). Meniscus curvature at every location along the wick is calculated as a result of this coupling. The model accounts for the change in interfacial area in the wick pore, thin-film evaporation, and Marangoni convection effects during phase change at the liquid–vapor interface. The coupled model is used to predict the performance of a heat pipe with a screen-mesh wick, and the implications of the coupling employed are discussed.     

Influence of free surface curvature of a liquid layer on the critical Marangoni convection

The Pearson instability was suggested to discuss the onset of Marangoni convection in a liquid layer of large Prandtl number under an applied temperature difference perpendicular to the free surface in the microgravity environment. In this case, the temperature distribution on the curved free surface is non-uniform, and the thermocapillary convection is induced and coupled with the Marangoni convection. In the present paper the effect of volume ratio of the liquid layer on the critical Marangoni convection and the corresponding spatial variation of the convection structure in zero-gravity condition were numerically investigated by two-dimensional model.     

MOLECULAR DYNAMICS SIMULATION OF THERMAL CONDUCTIVITY OF NANOSCALE THIN SILICON FILMS

Molecular dynamics simulations are performed to explore the thermal conductivity in the cross-plane direction of single-crystal thin silicon films. The silicon crystal has diamond structure, and the Stillinger-Weber potential is adopted. The inhomogeneous nonequilibrium molecular dynamics (NEMD) scheme is applied to model heat conduction in thin films. At average temperature T = 500 K, which is lower than the Debye temperature Θ_D = 645 K, the results show that in a film thickness range of about 2–32 nm, the calculated thermal conductivity decreases almost linearly as the film thickness is reduced, exhibiting a remarkable reduction as compared with the bulk experimental data. The phonon mean free path is estimated and the size effect on thermal conductivity is attributed to the reduction of phonon mean free path according to the kinetic theory.     

A sharp-interface level-set method for analysis of Marangoni effect on microdroplet evaporation

A level-set method is presented for computation of microdroplet evaporation including not only the effects of heat and mass transfer, phase change and contact line dynamics but also the Marangoni effect, which is a key parameter affecting the internal flow of the droplet and the particle deposition pattern. A sharp-interface formulation of the Marangoni force is derived and tested for two-phase Marangoni convection in a cavity. The computed results show good convergence in both the liquid and gas regions and are in excellent agreement with the analytical solutions. The level-set formulation is applied to microdroplet evaporation on a solid surface to investigate the Marangoni effect.     

Predicting Marangoni convection caused by transient gas diffusion in liquids

The onset of convection driven by surface tension during gas diffusion in a liquid is investigated. Gas diffusion at the gas-liquid interface results in the variation of concentration of the solute that may cause an increase in surface tension leading to Marangoni convection. The onset of convection for unsteady-state gas desorption can be predicted from the maximum transient Ma_t, which is here derived by analogy with its equivalent in thermal convection. It is a function of the transient Biot number (Bi_D) for interfacial gas diffusion, which depends strongly on the state of vapour-liquid equilibrium at the interface. The transient Marangoni numbers, critical times for stable mass diffusion and the critical sizes of convection cells have been formulated. The desorption of ethyl-ether from chloro-benzene in L.M. Blair’s [The onset of cellular convection in a fluid layer with time-dependent density gradients, PhD thesis, University of Illinois, Urbana, 1968] experiments is liquid phase-controlled, hence, the highly soluble system is characterized by Bi_D = 0. Therefore, his experiments that were initiated with a step-change in pressure cannot be analyzed by a step-function boundary that is characterized by Bi_D = ∞. The surface concentration may change very slowly, it has been approximated to be about 0.1% of the initial pressure change at the point of onset of convection. The average critical Marangoni number for this condition was estimated to be 53.3, which is fairly close to the theoretical value of 67 for an interface with a Biot number of 0. Therefore, the high value of 3100 calculated by I.F. Davenport and C.J. King [The initiation of natural convection caused by time-dependent profiles, Lawrence Berkeley Lab, Report NBR LBL-600, 1972] is wrong, who wrongly assumed a fixed surface-concentration boundary that is applicable only to a sparingly soluble solute. The critical sizes of convection cells predicted by theory are generally less than 1 mm for reported critical times of less than 20 s, they would be difficult to measure.     

Formation of semiconducting iron pyrite by spray pyrolysis

Conditions for the production of thin pyrite layers by spray pyrolysis have been investigated for potential use as a solar cell material. Best results were obtained from an aqueous molar ferric chloride to thiourea ratio of 0.03M:0.072M. The films were deposited on glass substrates at 350°C in presence of gaseous sulfur and were sprayed with nitrogen as carrier gas. A simple hydrolysis reaction mechanism is proposed where thiourea, iron chloride and sulfur react on the hot substrate to form CO₂, NH₃, HCl and FeS₂. The crystallinity and phase of the films was confirmed as pyrite by X-ray diffractometry. Steady state conductivity measurements showed the films to be extrinsic (self compensated) semiconductors with an activation energy of 0.03 eV. Steady state photoconductivity was negligible, although greater photoconductivity was found in Ru doped layers. Optical transmission measurements indicated a soft band edge due to grain boundaries. Non-contract time resolved microwave conductivity measurements were conducted to study the lifetime of photo-excited carriers. Only at high excitation intensities were reasonable carrier lifetimes detected. The film-substrate interface on the films showed a much higher recombination rate than the film-air interface. This effect can be explained by strain at the substrate-film interface. The films on glass substrates exhibited cracking pinholes that are believed to be due to the cooling action of the spray droplets and the differences in thermal expansion between pyrite and glass materials.