Thin Film Evaporation Research Articles

Mini-scale pulsating heat pipe cooling systems for high-heat-flux electronic equipment

This paper presents a novel passive two-phase cooling system for efficient thermal management of high-heat-flux electronic equipment, namely a mini-scale, flat-plate pulsating heat pipe, by executing experiments on a test bench. Furthermore, cooling tests on an actual 1-U server are also shown. The pulsating heat pipe used in this study is designed and manufactured with micro-channel technology where passive two-phase flow circulation is promoted by the inherent two-phase instability and the competing effect between nucleate boiling and thin-film evaporation. Pulsating heat pipes properly operate with gravity (in vertical orientations) and without gravity (in horizontal orientations) and they are an optimal solution to dissipate a large amount of heat without the need of active drivers (absence of pumping power). The cooling capabilities of the proposed pulsating heat pipe were investigated using different near zero global warming potential refrigerants at various filling ratios to find the optimal thermal performance up to 800 W (corresponding to a heat flux of 25 W/cm2). The minimum overall thermal resistance was measured to be 0.065 K/W, which includes the thermal interface material and the secondary side. This paper shows that passive two-phase cooling using pulsating heat pipes can be a very promising and reliable technology for achieving high power dissipations of next-generation datacentres (and electronic equipment in general).

Thin film evaporation model for two-phase capillary heat transfer devices: examination of boundary conditions and vapour pressure gradient

Thin film evaporation model has been used by several researchers to study the transport phenomena of two-phase capillary devices, such as heat pipes and capillary pumped loops. The present work is focused on mathematical modeling of the evaporation phenomena in such devices. Liquid-vapour interface in the evaporator is modelled using a thin film model and the lubrication approximation, along with a slip boundary condition at the wall and a shear boundary condition at the interface. Different models are used for the evaporating mass flux and the vapour pressure gradient at the liquid-vapour interface is considered. An attempt is also made to determine the non-evaporating film thickness that satisfies the underlying physics. The film thickness profile and the pressure components are obtained by numerical simulations. It is found that the choice of the evaporating mass flux model has a significant effect on the results, and is very important for heat transfer characterization.

Thermo-fluid dynamic analysis in a micro-textured evaporator based on microscale infrared/visible observations for loop heat pipes

This paper presents a thermo-fluid dynamic analysis in a micro-textured evaporator for loop heat pipes (LHPs) that has a super wetting surface with ethanol on the basis of an experiment and modeling. In the experiment, three kinds of stainless steel plates with various morphologies of heat transfer surfaces were tested: a normal flat plate, a sandblasted plate, and a micro-groove plate. The measured contact angles between the surfaces and ethanol were 8.2° ± 1.3°, 0°, and 0°, respectively. Evaluation of the heat transfer performance and liquid-vapor interface behavior was conducted using microscale infrared and visible observations. As a result, it was revealed that the sandblasted plate and the micro-groove plate showed 7 times and 20 times higher heat transfer coefficient under high heat flux conditions respectively, compared with the normal flat plate. Different liquid-vapor interface behaviors were found between the plate types. In the modeling, the heat transfer coefficient was predicted for each case. It was found that nucleate boiling heat transfer induced by rough surface increased the heat transfer performance in the case of the sandblasted plate. In addition, it was revealed that the thin film evaporation at the long triple-phase contact line significantly enhanced heat transfer performance in the case of the micro-groove plate. The findings indicate a new promising approach to enhance the heat transfer coefficient of LHP evaporators.

Experimental analysis of evaporation heat transfer domains of capillary-assisted thin-film developed on radial parabolic sub-millimeter fin-groove geometries

Evaporation through the capillary-assisted thin-film is an innovative approach to overcome the challenges in heat transfer enhancement and reduce the charge amount of the refrigerant in refrigeration systems. The present study attempts to illustrate the theoretical details of the capillary-assisted evaporation heat transfer domains on radial parabolic sub-millimeter fin-groove geometries that were partially submerged in a refrigerant, i.e., water. A semi-flooded evaporator was used for the evaporation heat transfer experiments under sub-atmospheric pressures of 0.93, 1.00, and 1.07 kPa. The influence of the water filling level on the evaporation heat transfer domains was investigated with three different fin-groove geometries. The highest evaporation heat transfer was obtained when the water filling level was approximately zero to five millimeters with smaller fin-groove dimensions, that is, fin distance, fin pitch, and groove perimeter. Denser fine grooves led to an increase in the number of evaporating thin-film zones, and it was noted that a significant portion of the enhancement in the evaporation heat transfer was contributed majorly by thin-film evaporation developed through the fin-groove structures.

Effects of solid/liquid interface on size-dependent specific heat capacity of nanoscale water films: Insight from molecular simulations

The thermophysical properties of an evaporative thin water film that is near the triple line may change because of the effects of size, geometry, interface, temperature, etc. However, the factor-dependence of the properties was seldom considered for the corresponding nanoscale transport phenomena or processes. In this work, the effects of the interface and temperature on the thickness-dependent specific heat capacities of nanoscale water films are investigated by comparing the values of the freestanding film between vacuum, the sessile film on a copper plate, and the confined film between two copper plates. It is found that the specific heat capacities of the thin water films decrease exponentially with reducing thickness, with the confined film having the highest value, followed by the sessile film and the freestanding film, at the same film thickness. The solid/liquid interface presents a substantial impact on the increase in the specific heat capacity, demonstrated by the analysis of density profile, vibration density of state of hydrogen atoms of water molecules, radial distribution function, and interface energy, compared between the films of different types. The analysis of the temperature effect by comparing the vibration densities of state in different regions of the films at different temperatures shows that the temperature has a negligible influence as compared to the size and interface effects. Therefore, the form of a thin water film will greatly affect its thermophysical properties, including specific heat capacity, which should be considered in applications such as evaporation of thin water film, nanoconfinement transfer in membrane, and interface separation of porous media.

Evaporation from arbitrary nanoporous membrane configurations: An effective evaporation coefficient approach

Thin-film evaporation from nanoporous membranes is a promising cooling technology employed for the thermal management of modern electronic devices. We propose an effective one-dimensional analytical approach that can accurately predict the temperature and density jump relations, and evaporation rates, for arbitrary nanoporous membrane configurations. This is accomplished through the specification of an effective evaporation coefficient that encompasses the influence of different system parameters, such as porosity, meniscus shape, evaporation coefficient, and receding height. Our proposed approach can accurately predict all the typical output evaporation parameters of interest like mass flux, and temperature and density jumps, without the need to carry out computationally demanding numerical simulations. Several exemplar cases comprising of nanoporous configurations with a wide range of parameters have been considered to demonstrate the feasibility and accuracy of this analytic approach. This work thus enables a quick, efficient, and accurate means of aiding the design and engineering analysis of nanoporous membrane-based cooling devices.

Microstructured wettability pattern for enhancing thermal performance in an ultrathin vapor chamber

We investigated the thermal performance of a novel wettability patterned evaporator for an ultrathin vapor chamber. Because the evaporator integrates a wettability patterned substrate underneath the nanostructured mesh wick which can pin the three-phase contact lines on the hydrophilic/hydrophobic boundaries, it enlarges the area of thin-film evaporation. Microstructured wettability pattern is fabricated on the evaporator surface and the wick is pressed onto the evaporator by a micropillar array to make an intimate contact with each other. The micropillar array electroplated on the inner side of the condenser also supports a vapor core as a vapor flow path. The thermal resistance of the ultrathin vapor chamber is experimentally evaluated, and the measurement results show that the wettability pattern underneath the nanostructured mesh wick can greatly reduce the horizontal thermal resistance, giving a better temperature uniformity across the condenser side, though the vertical thermal resistance may be slightly larger than that without a wettability pattern. The highest in-plane effective thermal conductivity of a 200 μm-thick vapor chamber can reach 11914.9 W/(m·K) at 23.91 W/cm2 heat flux, which shows a 210.7% further improvement in comparison with that of the ultrathin vapor chamber with the nanostructured mesh wick only.

Predictive AI platform on thin film evaporation in hierarchical structures

The trend in miniaturization and enhanced functional performance of integrated circuits and power electronics and photonics has amplified the generated thermal energy in these devices making thermal management a bottleneck for further advancement in these fields. A range of geometries of hierarchical structures are developed and examined to address this challenge. However, the numerous form factors and dimension of hierarchical structures in addition to cost and time-consuming synthesis and test procedures make it unfeasible to explore bountiful variations of hierarchical geometries through experimental methods. Here, we introduce a general Artificial Intelligence (AI) platform to address this challenge and guide discovery of hierarchical structures for extreme thermal management of high-performance photonics/electronics. The AI platform is based on Random Forest (RF) algorithm, a robust AI method, and was trained using a large collected experimental data set corresponding to thin film evaporation in various forms of hierarchical structures. Four geometrical dimensions of the hierarchical structures and two dimensionless numbers governing heat transfer and fluid dynamics in these structures were used as independent variables to predict heat flux in these structures. The trained model's performance was analyzed using statistical metrics and showed an excellent prediction of heat flux for all the structures with various working fluids. The performance of predictive AI platform was further validated by two independent studies of different research groups. This predictive platform provides a foundation for rational discovery of hierarchical structures and working fluids to address the ongoing challenge of thermal management in broad spectrum of technologies including electronics, hypersonic aviation and electric vehicles.

Bioactive Apigenin loaded oral nano bilosomes: Formulation optimization to preclinical assessment

AimDiabetic (type-2) is a metabolic disease characterized by increased blood glucose level from the normal level. In the present study, apigenin (AG) loaded lipid vesicles (bilosomes: BIL) was prepared, optimized and evaluated for the oral therapeutic efficacy. ExperimentalAG-BIL was prepared by a thin-film evaporation method using cholesterol, span 60 and sodium deoxycholate. The prepared formulation was optimized by 3-factor and 3-level Box-Behnken design using particle size, entrapment efficiency and drug release as a response. The selected formulation further evaluated forex-vivopermeation,in vivopharmacokinetic and pharmacodynamics study. ResultsThe optimized AG bilosomes (AG-BILopt) has shown the vesicle size 183.25 ± 2.43 nm, entrapment efficiency 81.67 ± 4.87%. TEM image showed a spherical shape vesicle with sharp boundaries. The drug release study revealed a significant enhancement in AG release (79.45 ± 4.18%) from AG-BILopt as compared to free AG-dispersion (25.47 ± 3.64%). The permeation and pharmacokinetic studies result revealed 4.49 times higher flux and 4.67 folds higher AUC0-t than free AG-dispersion. The antidiabetic activity results showed significant (P < 0.05) enhancement in therapeutic efficacy than free AG-dispersion. The results also showed marked improvement in biochemical parameters. ConclusionOur findings suggested, the prepared apigenin loaded bilosomes was found to be an efficient delivery in the therapeutic efficacy in diabetes.

Effect of Marangoni Convection on the Perovskite Thin Liquid Film Deposition.

Motivated by recent advances in the development of thin-film perovskite solar cells, the evaporation and deposition of a perovskite thin liquid film on a hydrophilic substrate, also in some cases subjected to ultrasonic vibrations, are studied in this paper. In practice, in the literature, the complexity of the underlying phenomenon has led to the study of a thick (macroscale) liquid film in the absence of solidification. Here, we investigate evaporation mechanics of a thin (microscale) liquid film of perovskite solution. We demonstrate flow fields within the film and study the reason behind the formation of such flow patterns within a thin liquid film and attribute such flows to surface-tension-induced Marangoni flows and rule out the possibility of the presence of buoyancy-induced Rayleigh-Benard convection. We show that perovskite deposition starts at the film contact lines and propagates toward the film center, creating two regions of the perovskite film with distinct characteristics. Our thermography results (temperature mapping) of the film surface show agreement between the temperature map and the flow patterns on the film surface. Moreover, we show that imposing ultrasonic vibration on an evaporating liquid film results in a more uniformly distributed flow pattern across the film due to micromixing enhancement achieved by ultrasonic vibrations.

High-performance cellulosic filament fibers prepared via dry-jet wet spinning from ionic liquids

We report on a new process for the spinning of high-performance cellulosic fibers. For the first time, cellulose has been dissolved in the ionic liquid (IL) 1-ethyl-3-methylimidazolium octanoate ([C2C1im][Oc]) via a thin film evaporator in a continuous process. Compared to other ILs, [C2C1im][Oc] shows no signs of hydrolysis with water. For dope preparation the degree of polymerization of the pulp was adjusted by electron beam irradiation and determined by viscosimetry. In addition, the quality of the pulp was evaluated by means of alkali resistance. Endless filament fibers have been spun using dry-jet wet spinning and an extruder instead of a spinning pump, which significantly increases productivity. By this approach, more than 1000 m of continuous multifilament fibers have been spun. The novel approach allows for preparing cellulose fibers with high Young's modulus (33 GPa) and unprecedented high tensile strengths up to 45 cN/tex. The high performance of the obtained fibers provides a promising outlook for their application as replacement material for rayon-based tire cord fibers.

Innovative Organic MEH-PPV Heterojunction Device Madeby USP and PVD.

An Al/p-Si/poly[2-methoxy-5-(2-ethylhexoxy)-p-phenylenevinylene] (MEH-PPV)/Ag organic heterojunction has been prepared using homemade ultrasonic spray pyrolysis (USP) equipment for deposition of the organic thin film and physical vapor deposition (PVD) for the metallic contacts. The organic layer produced on glass was analyzed by optical and morphological methods. The bandgap of the organic thin film was found to be ~ 2.03 eV with a thickness of around 140 nm, using ultraviolet–visible (UV–Vis) and scanning electron microscopy (SEM) characterization, respectively. The amorphous nature of the MEH-PPV polymer was confirmed by its x-ray diffraction pattern. To determine the electrical parameters, the heterojunction based on MEH-PPV was characterized by current–voltage (I–V) and capacitance–voltage (C–V) measurements in the dark at room temperature. The ideality factor and barrier height of the organic heterojunction were found to be 3.6 eV and 0.56 eV to 0.59 eV, respectively, with an average series resistance of 94.39 Ω, based on the I–V characteristics. The barrier height was also calculated based on the capacitance–voltage measurements, yielding slightly different results due to the applied frequencies of 10 kHz (phi_{{text{B}}} = 0.50) and 1 MHz (phi_{{text{B}}} = 0.74) , respectively.

Passivation of InP solar cells using large area hexagonal-BN layers

Surface passivation is crucial for many high-performance solid-state devices, especially solar cells. It has been proposed that 2D hexagonal boron nitride (hBN) films can provide near-ideal passivation due to their wide bandgap, lack of dangling bonds, high dielectric constant, and easy transferability to a range of substrates without disturbing their bulk properties. However, so far, the passivation of hBN has been studied for small areas, mainly because of its small sizes. Here, we report the passivation characteristics of wafer-scale, few monolayers thick, hBN grown by metalorganic chemical vapor deposition. Using a recently reported ITO/i-InP/p+-InP solar cell structure, we show a significant improvement in solar cell performance utilizing a few monolayers of hBN as the passivation layer. Interface defect density (at the hBN/i-InP) calculated using C–V measurement was 2 × 1012 eV−1cm−2 and was found comparable to several previously reported passivation layers. Thus, hBN may, in the future, be a possible candidate to achieve high-quality passivation. hBN-based passivation layers can mainly be useful in cases where the growth of lattice-matched passivation layers is complicated, as in the case of thin-film vapor–liquid–solid and close-spaced vapor transport-based III–V semiconductor growth techniques.

Cu-Cu thermo compression wafer bonding techniques for micro-system integration

Copper (Cu) is used as an interconnect material in many applications owing to its high thermal, electrical conductivity and excellent electromigration resistance. Though this material has many advantages, the main drawback is that it gets oxidized on exposure to air. Thermo-compression bonding is a wafer bonding technique that uses metal layers for heaping wafers, which aids in attaining outstanding electrical conductivity without weakening the mechanical properties. The adsorbed oxide layer hurdles the proper bonding to happen between the wafers. In order to enhance the diffusion between the metal layers, the copper oxide layer should be removed which necessitates the requirement of high temperature, pressure, long bonding time and the inert gas atmosphere throughout the Cu-Cu thermo compression wafer bonding process. Simultaneous application of high temperature and pressure for a long time leads to the deterioration of the underlying sensitive components. This paper aims to present several techniques such as surface treatment, chemical pretreatment, surface passivation, crystal orientation modification, stress gradient in the thin film and formic acid vapour treatment which are used in order to avoid the deterioration of underlying sensitive devices and to obtain a proper bonding between the wafers at low temperature and pressure.

Numerical simulation of flow pattern for non-Newtonian flow in agitated thin film evaporator

刮膜蒸发器是通过旋转刮板强制成膜，可实现高粘度非牛顿流体类物料平稳蒸发的新型高效蒸发器。蒸发器内流体的流动、分布与传输机制直接决定了蒸发器的蒸发效率与功耗。不同于现有研究主要基于牛顿流体开展，本文针对不同粘度的非牛顿流体，建立蒸发器三维计算流体动力学模型，系统研究了蒸发器内的流场分布特性和成膜机理。结果表明：低粘非牛顿流体的流场分布特性和牛顿流体类似，物料可在壁面形成均匀且连续的液膜；随着粘度的增加，液膜的均匀性和连续性逐渐变差。通过对流场分布与传输形式的研究，结合液膜分布、速度分布、剪应变率分布，以及粘度分布进行对比分析发现，蒸发器内部结构与运行状态形成的剪切场与粘度分布是蒸发器良好成膜的关键。此外，提出对刮板前缘进行弯折可辅助高粘流体液膜铺展，并对最佳弯折角度进行探索。本研究为刮膜蒸发器的设计和应用提供了理论指导与依据。

From radical to triradical thin film processes: the Blatter radical derivatives

Thermal evaporation of (poly)radicals is possible. More than one radical site in a molecule makes it more reactive, narrowing the windows left for thin film evaporation, and favouring island formation rather than two-dimensional growth.

Experimental study of flow boiling performance of open-ring pin fin microchannels

A type of open-ring pin fin microchannels (ORPFM) was proposed and developed for advanced microchannel heat sinks. They consist of an internal cavity for bubble nucleation and two inner small and outside large rings with separated and converged flow passages. Two arrangements of open-ring pin fins, inline and staggered ones, were prepared and compared for optimization. Flow boiling experiments were performed to assess the two-phase boiling performance of both inline and staggered ORPFM. Tests were conducted using subcooled deionized water with an inlet subcooling of 10°C and mass fluxes of 200 kg/m2·s and 300 kg/m2·s. Two-phase boiling heat transfer performance, pressure drop and two-phase flow instabilities of the ORPFM were systematically explored. Experimental results indicated that both inline and staggered ORPFM samples presented a small wall superheat of 1-2°C for onset of nucleate boiling (ONB). Nucleate boiling, both nucleate boiling and thin film evaporation, and convective boiling in turn dominated with increasing heat flux and vapor quality for ORPFM. The inline ORPFM generally presented a 10% to 80% augmentation in boiling heat transfer at moderate and high heat fluxes, a 2%-20% smaller pressure drop, and more stable flow boiling process with a mitigation of two-phase flow instabilities than the staggered one. Therefore, the inline ORPFM seemed to be more favorable for high heat flux dissipation of advanced microchannel heat sinks.

Flow boiling in microchannels enhanced by parallel microgrooves fabricated on the bottom surfaces

Flow boiling in microchannels is one of the most desirable cooling solutions for high power electronics. However, it is challenging to promote the flow boiling performance, particularly critical heat flux (CHF), due to their unfavorable liquid rewetting. Inlet restrictors (IRs) have been successfully used to manage two-phase instabilities in flow boiling in microchannels. However, IRs would result in extremely high pressure drop. In this study, without using IRs, parallel microgrooves (W=50 µm, H=250 µm, L=10 mm) are fabricated on the bottom surface of five parallel microchannels (W=200 µm, H=250 µm, L=10 mm) to effectively manage two-phase flow instabilities without sacrificing pressure drop. Our visualization study shows that these microgrooves can enable rewetting at highhigh frequencies up to 151 Hz and enhance thin film evaporation with the highest thin film ratio of 0.65. A highly desirable periodic rewetting mechanism can substantially delay CHF conditions and enhance heat transfer rates. Flow boiling in the present microchannel configuration has been systematically characterized with mass flux ranging from 100 kg/m2s to 600 kg/m2s. Compared to plain-wall microchannels, flow boiling heat transfer coefficient (HTC) is enhanced up to ~72% at a mass flux of 210 kg/m2s due to the sustainable thin film evaporation. Moreover, CHF is substantially enhanced by ~155% at a mass flux of 389 kg/m2s due to the rapid and periodic rewetting enabled by these decorated microgrooves. We also observed that the enhancement of HTC fades when mass flux exceeds 346 kg/m2s due to flooding.

Enhanced flow boiling in microchannels by incorporating multiple micro-nozzles and micro-pinfin fences

Phase-change heat transfer is a promising approach for high-power electronics cooling. However, the chaotic two-phase transport and dominant laminar flow in microchannels inhibits flow boiling performance. Thus, the ability to coordinate the two-phase transport in a highly favorable fashion would be highly desired. In this study, a new microchannel configuration by incorporating capillary micro-pinfin fences and multiple micro-nozzles has been proposed to sustain thin liquid film evaporation, promote mixing and global liquid supply simultaneously. A new boundary layer covered with thin liquid film is activated by the capillary micro-pinfin fences along the sidewalls of the channel. Additionally, the sustainable thin liquid film is maintained using capillary effect, which can promote the capillary-driven flow inside the gap between micro-pinfin fences and the sidewalls. As a result, significantly enhanced flow boiling has been achieved on both DI-water and HFE-7100. A critical heat flux (CHF) up to 944 W/cm2 has been demonstrated using DI-water at a mass velocity of 600 kg/m2 s, accounting for a 43% enhancement compared to the configuration of multiple micro-nozzles without capillary micro-pinfin fences. Compared to the base configuration of plain wall microchannels, the enhancement of CHF is over threefold at a mass velocity of 389 kg/m2s. Equally important, this new microchannel configuration works well on dielectric fluids, which is challenging to promote CHF due to their unfavorable thermophysical properties. A CHF of 287 W/cm2 with a noticeable enhancement of 56% has been achieved at a mass velocity of 2772 kg/m2 s on HFE-7100 at room temperature. Moreover, the enhancements of heat transfer coefficient (HTC) on HFE-7100 are more noticeable compared to that of DI-water. For example, 120% higher HTC at a moderate mass velocity of 693 kg/m2 s has been realized. All the enhancements are achieved without the escalating pressure drop.

Direct local heat flux measurement during water flow boiling in a rectangular minichannel using a MEMS heat flux sensor

Heat transfer characteristics of water flow boiling in a minichannel were investigated through the direct measurement of local heat flux using a fabricated MEMS heat flux sensor with a multi-layered structure. Local temperatures and heat fluxes in the water flow boiling were measured at a sampling frequency of 10 kHz in synchronism with the high-speed visualization of the boiling behavior. The measurement results revealed fundamental heat transfer processes, including thin liquid film evaporation, dry-out of the liquid film, transient heat conduction after dry patch rewetting, and single-phase forced convection. The thin liquid film evaporation indicated a high local heat flux that was well over 1 MW/m2 and provided a dominant contribution to the wall heat transfer. Furthermore, the post-rewetting transient heat conduction indicated a high heat flux that was higher than 1 MW/m2. However, it was insignificant in terms of the overall wall heat transfer at all the tested heat fluxes because of its short duration. The contribution of liquid-phase forced convection was also small, except at low vapor qualities.