Thin Film Evaporation Research Articles

Orientation effects on liquid-vapor phase change heat transfer on nanoporous membranes

Liquid-vapor phase change heat transfer on nanoporous membranes in facing horizontal upwards, downwards and vertically was investigated. Anodic aluminum oxide (AAO) membranes were as the nanoporous substrates. Water was supplied through nanoporous membranes by pressure difference. Heat transfer characteristics and temperature instability were analyzed and compared among three orientations. Results showed that negative slope and “special points” appeared on boiling curves. The interaction between liquid film thickness and heat transfer coefficient (HTC) was proposed and verified to be the main mechanism that leading to this phenomenon. Because the rapid replenishment of liquid and removal of heated liquid under gravity, horizontal downward surface always had the best heat transfer performance which exhibited maximal enhancement of HTC was 170% and 70% at heat flux of 39 W/cm2 over horizontal upward and vertical surfaces. Temperature instability in horizontal downward and vertical surfaces had the similar trend and became more pronounced at higher heat fluxes. In horizontal upward surface, temperature was stable at the heat flux range of 40– 55 W/cm2 and “thin film evaporation” was realized. In addition, the maximum heat flux was calculated by liquid transport limit and kinetic theory limit for evaporation and a comprehensive understanding was established.

Capillary-fed, thin film evaporation devices

Evaporation plays a critical role in a range of technologies that power and sustain our society. Wicks are widely used as passive, capillary-fed evaporators, attracting much interest since these devices are highly efficient, compact, and thermally stable. While wick-based evaporators can be further improved with advanced materials and fabrication techniques, modeling of heat and mass transport at the device level is vital for guiding these innovations. In this perspective, we present the design and optimization of capillary-fed, thin film evaporation devices through a heat and mass transfer lens. This modeling framework can guide future research into materials innovations, fabrication of novel architectures, and systems design/optimization for next generation, high-performance wick-based evaporators. Furthermore, we describe specific challenges and opportunities for the fundamental understanding of evaporation physics. Finally, we apply our modeling framework to the analysis of two important applications—solar vapor generation and electronics cooling devices.

Self-powered photodetector based on SnSe2/MoSe2 heterostructure

This article presents the influence of anisotropy of MoSe2 crystal on self-powered photodetection of SnSe2/MoSe2 heterojunction. The atomic structure and polymorph confirmation of thermally evaporated SnSe2 thin film and direct vapor transport grown MoSe2 crystals were accomplished by powder X-ray diffraction, electron diffraction, and Raman spectroscopy. The heterojunction device exhibited obvious rectifying nature of n-n junction. The device exhibited type-II junction with high on/off ratio of 105–106 which reflects its applicability in power generation without bias. The modulation of photoresponse is realized because of anisotropic charge transport in bulk MoSe2 crystal. The high-performance heterojunction exhibits photoresponsivity of 7.09 A/W, detectivity of 6.44 × 1012 Jones, and external quantum efficiency of 1313% for top-top contact geometry, whereas these parameters are significantly reduced for top-bottom contact geometry. The influence of anisotropy and defect states on device performance is explained in detail. The SnSe2/MoSe2 heterojunction with extremely good environmental stability and excellent self-driven photoswitching advocates huge development in optoelectronics.

Enhanced flow boiling performance in high-aspect-ratio groove-wall microchannels

The dry-out phenomenon easily occurs on the side walls in high aspect ratio (channel depth to width ratio, AR) microchannel due to the decreasing of liquid film thickness. It is desirable to design better microchannels to address this issue. In this study, we proposed a new high-aspect-ratio groove-wall microchannel (Height=200 μm, Width=80 μm, Length=10 mm, AR=2.5), which features a series of rectangular grooves etched on the plain side walls. Flow boiling experiments were conducted in the novel groove-wall microchannel and conventional plain-wall microchannel at two inlet subcoolings of 40 and 70 °C, three mass fluxes (G) of 446, 630 and 815 kg/m2·s and wall heat fluxes of 3.8–139.5 W/cm2, using deionized water as the working fluid. The outlet of the channel is connected to the atmosphere, maintaining a saturation temperature of 100 °C. Flow visualization was captured by a high-speed camera to better understand the flow and heat transfer characteristics. Compared with plain-wall microchannel, the groove-wall microchannel demonstrates earlier boiling incipience, higher heat transfer coefficient and lower pressure drop in the two-phase flow region. It was found that the groove-wall microchannel obtains better heat transfer coefficient (HTC) at higher inlet subcooling. In comparison with plain-wall microchannel, the HTC enhancements of 25.4%, 73.0% and 55.0% were obtained respectively for the mass fluxes of 446, 630 and 815 kg/m2·s at ΔTsub=70 °C, while the corresponding HTC enhancements are 25.0%, 36.5% and 56.7% at ΔTsub=40 °C, respectively. Since the groove walls characterize the thin film evaporation especially at high heat flux, an enhanced critical heat flux (CHF) was obtained. At G = 446, 630 and 815 kg/m2·s, the groove-wall microchannel enhances the CHF respectively by 26.5%, 31.1% and 45.0% at ΔTsub= 40 °C, and the corresponding enhancements are 5.3%, 10.3% and 10.0% at ΔTsub=70 °C, respectively. Furthermore, flow reversal and boiling instability (pressure drop and wall temperature fluctuations) were drastically suppressed. Boiling instabilities are more severe at high inlet subcooling. The maximum pressure drop reductions of 27.4%, 42.0% were achieved in groove-wall microchannels for the mass fluxes of 630 and 815 kg/m2·s respectively at ΔTsub= 40 °C, and the corresponding maximum reductions are 9.07%, 23.6% at ΔTsub=70 °C, respectively. Almost no difference of the pressure drop was found in two microchannels at G = 446 kg/m2·s.

Thin liquid films formation and evaporation mechanisms around elongated bubbles in rectangular cross-section microchannels

Thin film evaporation is the dominant mechanism of heat transfer in the microchannel flow boiling process. Hence, its understanding is paramount to development of predictive models and more efficient microchannel heat exchangers. Although microchannels mostly have a rectangular cross-section, the existing models are developed for circular cross-section microchannels. The non-uniformity of the liquid film thickness in microchannels with a rectangular cross-section has made prediction of the onset of dryout and heat transfer coefficient greatly challenging. Here, a test device capable of measuring the wall heat flux is utilized to fully characterize the thin film formation and evaporation process. The effects of flow capillary number (Ca) and channel aspect ratio on the liquid film thickness are determined in microchannels with 300 μm width and 300, 150, and 75 μm heights, representing aspect ratios of 1, 2, and 4, respectively. It is shown that the only existing mechanistic model for the elongated bubble regime in microchannels fails to predict the experimental heat transfer coefficient (HTC) both quantitively, by a 50–120% margin, and qualitatively, due to a fundamental difference in evaporation and dryout of a variable versus a uniform film assumed in the model. The results presented here pave the way for development of a more accurate mechanistic model for the thin film evaporation heat transfer mechanism in microchannels with a rectangular cross-section.

Quasi-steady and transient study of heat transfer during sub-cooled flow boiling in a small aspect ratio microchannel

Owing to the transient nature of microchannel flow-boiling, gaining an in-depth understanding of the associated trends in time-averaged heat transfer variables as well as mechanisms of heat transfer is quite challenging. This work studies sub-cooled flow-boiling of de-ionized water in a microchannel. The micro-channel heights studied are 0.14, 0.28 and 0.42 mm while the tested mass fluxes range from 200 to 1000 kg/(m2s). Slug flow regime is observed under most of the tested conditions. Transient wetted surface heat flux and temperature as well as heat transfer coefficient are evaluated from the wall temperature data captured at a high data-logging frequency (synchronously with flow-visualizations). Heat transfer mechanisms as well as the trends observed in plots associated with time-averaged heat transfer variables are explained from the perspective of transient data correlated to the observed flow-boiling phenomena. Thin-film evaporation emerges as the prominent heat transfer mechanism. Two-way coupling between transient heat transfer variables and the boiling phenomena is evident.

Capillary-Assisted Evaporation/Boiling in PDMS Microchannel Integrated with Wicking Microstructures.

To enable rapid development of flexible microelectronic systems, effective thermal management is needed. Flexible polydimethylsiloxane (PDMS)-based microchannel flow boiling may provide a desirable solution. However, the heat transfer performance of PDMS-based microchannels is diminished by its poor thermophysical properties. The development of PDMS wick is proposed to address this dilemma. Herein, a new PDMS wick structure is designed and integrated in the microfluidic device to significantly enhance its thermal performance by promoting capillary-driven flow. Furthermore, to achieve highly efficient vapor removal, a dedicated vapor pathway is designed in the microfluidic device. Experiments have been conducted to investigate the capillary-assisted evaporation/boiling for mass flux ranging from 70 to 245 kg/m2 s on dielectric fluid HFE-7100. The capillary-assisted sustainable and stable thin film evaporation and efficient vapor removal have been demonstrated through the visualization studies. With this new wick design, the liquid rewetting at global and local levels is facilitated through the capillary-driven flow and the efficient vapor removal. This hybrid liquid rewetting mechanism is experimentally demonstrated to significantly enhance the capillary-assisted evaporation/boiling in PDMS-based microchannel. A critical heat flux (CHF) of 14.7 W/cm2 is achieved at a mass flux of 245 kg/m2 s. The heat transfer coefficients (HTC) range from 2000 to 9800 W/m2 K. These values are comparable to that in copper/silicon microchannels, with the aforementioned benefits of flexibility. Equally importantly, stable two-phase transport is achieved as well.

Mass exchange at liquid-solid interface: a molecular simulation scheme applied to evaporation phenomena

A numerical technique is presented that enables mass exchange at the liquid-solid interface region in a molecular simulation. Particles can be inserted and deleted in the solid region where interaction between the fluid and solid atoms is temporarily inactivated during the process. A simple momentum-increase scheme drives the inserted particles against the unfavorable free energy. The technique is efficient and stable for insertion of particles into dense and inhomogeneous regions. The thin film and sessile-drop evaporation phenomena are then investigated using the proposed technique that allows steady-state simulations. The evaporation coefficients for the nanoscale thin film and contact line were accurately and reliably measured. The evaporation coefficient near the contact line shows a discrepancy compared to that far from the contact line.

Local Meniscus Curvature During Steady-State Evaporation from Micropillar Arrays.

Micropillar arrays are an ideal model system for capillary-aided thin film evaporation that can be fabricated with precise geometric control using microfabrication methods. The capillary limit leading to dryout is a critical performance metric for capillary-aided thin film evaporation and is proportional to the product of the permeability and capillary pressure. Capillary flow models for steady-state thin-film evaporation typically employ capillary pressure and permeability as separate parameters; however, it is difficult to separate the two from experimental hemiwicking characterizations or dryout observations. Furthermore, for micropillar arrays, local permeability depends on meniscus curvature varying spatially and with the evaporation rate. In this work, we use thin-film interference microscopy to profile local meniscus curvature during steady-state evaporation of water in a pure vapor environment. Local capillary pressure is calculated from curvature without requiring knowledge of contact angle or permeability. Results are compared against a Darcian semianalytical model for flow through micropillar wicks incorporating local permeability due to meniscus curvature. Although traditionally a slip boundary condition has been assumed at the liquid-vapor interface, we find much better agreement using a no-slip condition. The consequence of no-slip behavior is larger pressure gradients for a given evaporation flux and a lower dryout heat flux relative to a full slip condition. Heat transfer coefficient data are also presented and discussed in terms of curvature and sample geometry.

A simple figure of merit for devices utilizing thin film evaporation

We introduce a new figure of merit to judge the performance of two-phase capillary cooling devices working on thin film evaporation. This quantity is the heat transfer coefficient of the film thickness where the liquid pressure is zero, and represents its maximum attainable value in the thin film region. It shows good agreement with four different sources of data: data from the literature, maximum heat flux limit, interline heat flow parameter and heat pipe figure of merit, for different working fluids and a wide range of saturation temperatures. The proposed quantity is very simple, requires minimal computation, and can be used to compare the performance of different working fluids for such devices.

Molecular dynamics study on the effect of hydrophilicity on thin film evaporation over biphilic surface

The objective of this study is to understand the effect of surface wettability and how nano-scale heat transfer is affected by different surface wetting conditions over biphilic surface by molecular dynamics simulations. The system of 3 nm liquid Argon film over Platinum surface is equilibrated at 90 K and then wall temperature is raised to higher value to study the effect of temperature. According to this study at low wall temperature and low surface wetting conditions, liquid temperature increases proportionally with the increment of hydrophilic portion but at higher wall temperature and better surface wetting conditions, maximum value of liquid Argon temperature increases with increasing hydrophobic portion. Evaporation number, evaporative mass flux and heat flux increase significantly with slight increment of hydrophilic portion. Inception time decreases with increasing hydrophilic portion and surface wettability. Also, for better surface wetting conditions, average evaporative mass flux is more sensitive to wall temperature and increases significantly at higher wall temperature.

Experimental Investigation of a Flat-Plate Oscillating Heat Pipe With Groove-Enhanced Minichannels

Abstract The thermal and capillary performance of a groove-enhanced, or “microchannel-embedded,” flat-plate oscillating heat pipe (MC FP-OHP) was experimentally investigated while varying heating width, orientation, working fluid and operating temperature. The copper MC FP-OHP possessed two layers of 1.02 × 1.02 mm2 square channels, with the center 14 channels possessing two embedded microchannels (0.25 × 0.13 mm2) aligned coaxially with the primary minichannels. A FP-OHP without embedded microchannels, but with deeper minichannels (DC FP-OHP), was also tested for comparison. The FP-OHPs were filled with Novec 7200 or water (both at 80% ± 2% by volume), and the heating widths were varied between full-width and localized configurations: 38.71 cm2 and 14.52 cm2, respectively. Experimental results demonstrate that the MC FP-OHP is significantly less sensitive to operating orientation and can perform with less detriment as heat flux increases. The MC FP-OHP has a lower startup heating requirement and provides more fluid wetting along the FP-OHP structure—which is advantageous for pumping liquid from the evaporator to the condenser. The MC FP-OHP has enhanced convective heat transfer during operation, as it was observed to have similar or lower thermal resistances to that of the DC FP-OHP for a wide range of operating conditions. The groove-enhanced minichannel within the MC FP-OHP also provides for enhanced heat transfer because there being more thin-film evaporation sites and vapor–liquid mixing between the minichannel and microchannels.

A new approach to simulate the fluid dynamics in a wiped film evaporator using Modelica

Wiped film evaporators (WFE) or agitated thin film evaporators are efficient equipment for separating highly thermosensitive and viscous mixtures. There are different approaches to tailor residence time distribution (RTD) for specific applications and to maximise heat and mass transfer by equipment design or process parameter adjustment, like liquid load or wiper speed. However, the fundamental principles of fluid dynamics are only known to a limited extent. Although flow patterns and mass and heat transfer are highly interdependent, the effects of different wipers on liquid flow and, finally, on residence time behaviour are poorly understood. In this work, a new model describing fluid dynamics and its impact on RTD is presented. The model is implemented in the equation-based modelling language Modelica. Several system variables – e.g. film thickness, hold-up or velocity profiles within the liquid film – can be estimated. The effect of varying process parameters, like liquid load and fluid temperature, can also be evaluated. The model has been calibrated with results from RTD experiments in a WFE using a roller wiper. It is capable of describing the residence time behaviour of the roller wiper system with good accuracy. Mean residence times can be reproduced within approx. ±20%. Further results, such as film thickness or hold-up, provide additional insight into the fluid dynamics of characteristic regions – like bow wave, wiping and thin film zone – and the effects of several process parameters.

System for ammonia removal from anaerobic digestion and associated ammonium sulfate production: Simulation and design considerations

A side-stream ammonia stripping system coupled with a full-scale anaerobic digestion plant is considered. The system is based on an ammonia stripping unit (thin film evaporator, TFE), a Venturi scrubber and a reactive absorption column. A portion of the sludge treated in the digesters is continuously withdrawn and transferred to the TFE, where ammonia is stripped through a biogas stream. The ammonia rich biogas is then treated with a sulfuric acid/water solution in the Venturi scrubber and in the absorption column, with production of an ammonium sulfate solution, which is stored in a tank until utilization as a fertilizer. The aim of this paper is to provide a comprehensive engineering framework supporting the design of a full-scale ammonia stripping system with associated ammonium sulfate production, to be coupled with pre-existing anaerobic digestion plants. On the one hand, an analytical equation is proposed for the simulation of ammonia desorption and the design of the ammonia stripping unit as a function of the selected operating conditions of flowrate, temperature and pH. On the other hand, corrosion issues affecting the storage tank are studied. Corrosion tests, carried out on a selected paint system, demonstrate no development of coating degradation or disbonding, even if local damage occurs in the film. Thus, guidelines to avoid corrosion are given.

Pool boiling enhancement on biphilic micropillar arrays: Control on the thin film evaporation and rewetting flow

In the present article, a new surface treatment is proposed for pool boiling enhancement. For this purpose, a combination of hydrophilic–hydrophobic micropillar arrays with hydrophilic insulation strips between them is used to control and manage the thin film evaporation, surface rewetting, and bubble dynamics. The distance between hydrophobic sections of micropillars acts as permanent active nucleation site for pool boiling, which liquid thin films are established at the hydrophilic–hydrophobic interface. The hydrophilic section of micropillars is used for surface rewetting based on the wicking mechanism. The bubble dynamics is controlled by insulation strips to avoid the merging of departure bubbles and also vapor jets and columns. The effect of micropillar array dimensions on the pool boiling is investigated for the case of water on the hybrid silicon micropillar array. The maximum heat flux is 377.5 W/cm2 at 10 K wall superheat. Moreover, the variation of boiling heat flux is almost linear with respect to the wall superheat due to the permanent active nucleation sites. It leads to significant pool boiling enhancement at low superheat temperatures.

Thin-Film Evaporation Heat Transfer of Liquid Nitrogen and Its Application in Cell Vitrification

Abstract Cell vitrification has been an important method of cell cryopreservation. The faster the cooling rate is, the higher the cell survival rate is. However, in conventional cell vitrification methods, film boiling forms a vapor-blanket on the surface, which hinders further improvement of the cooling rate. To eliminate the problem, this article attempted to replace film boiling with thin-film evaporation (TFE) of liquid nitrogen. The experimental system was developed to investigate the TFE heat transfer characteristics of liquid nitrogen. Then, prostate cancer cells were cryopreserved by TFE vitrification method, open pulled straw vitrification method, and equilibrium freezing method. The results showed that the vitrification method of TFE obtained a higher cooling rate and better cell survival rate than the two other cell cryopreservation methods. Thus, the feasibility of this method was preliminarily proved viable when applied to the cell vitrification process. In addition, both the cooling rate and the cell survival rate are affected by the concentration of the cryoprotectant in the cell suspension. The cooling rate decreases as the concentration of the cryoprotectant increases, but the cell survival rate increases first and decrease afterward with an increase in the cryoprotectant concentration, in which an optimum value exists. This study demonstrates the practicality of the new ultrafast cell vitrification method.

Assessing the impact of disjoining pressure on thin-film evaporation with atomistic simulation and kinetic theory

Molecular dynamics (MD) simulations are applied to validate the Hertz–Knudsen–Schrage (HKS) relation for the evaporation mass flux in the presence of disjoining pressure. A non-equilibrium MD simulation system is designed to directly extract the evaporation mass flux for a Lennard-Jones fluid. The temperatures, pressures, properties, and disjoining pressure required to evaluate the HKS relation are obtained from the non-equilibrium MD simulation and complementary equilibrium MD simulations. The direct MD evaporation mass flux and that from the HKS relation agree within 10%. We define an evaporation Kapitza length that estimates where the liquid conduction and evaporation thermal resistances are equal. We propose that surface structures that promote film thicknesses comparable to or smaller than the evaporation Kapitza length, but larger than the adsorbed film thickness, will maximize the evaporation rate.

Saturated flow boiling of isolated seed bubble across a heated square cylinder in two-dimensional microchannel

Two-phase flow boiling within pin-fin microchannel heat sinks has gain increasing attentions over the last decades owing to the demands for high efficiency cooling applications, while the detailed interactions between a vapor bubble and heated pillar structures are still not fully understood. In this paper, the saturated-interface-volume phase change model is coupled with VOF method to investigate the saturated flow boiling of an isolated vapor bubble traveling across a heated square cylinder in a microchannel (with Reynolds number Re ranging from 60 to 360 and dimensionless bubble diameter db* from 0.2 to 0.6). The numerical model is first validated against literatures, and a mesh-independent study is conducted to ensure the accuracy of the present simulations. Then the bubble dynamics and its influences on flow structures are presented, where the symmetrical vortices attached behind the cylinder at small Re (60 and 120) are found replaced with the vortex-shedding flow and the deformed bubble (db* = 0.6) squeezes the cylinder and forms evaporating thin film not only on the adjacent side but also the windward and leeward sides. The local wall superheat and Nusselt number are presented along all the four sides of the heated cylinder. Two competing mechanisms governing heat transfer enhancement, i.e. phase change heat transfer improved by the thin film evaporation and convective heat transfer suppressed by the wrap of the bubble, are revealed. Both the dynamic evolution and time-averaged heat transfer performance are obtained for each side of the cylinder and their sums. The global heat transfer enhancement is found decreases monotonically with increasing Re and decreasing db*.

Formulation, Optimization and Evaluation of Novel Ultra-deformable Vesicular Drug Delivery System for an Anti-fungal Drug.

The present study is aimed at enhancing the skin penetration of ketoconazole by formulating it as transethosome. Ketoconazole-loaded transethosome formulations were prepared by conventional thin film evaporation and hydration method and were optimized using concentration of edge activator (span 80), ethanol and sonication time as factors and particle size, polydispersity index and entrapment efficiency as responses. The optimized formulation was further evaluated for in vitro diffusion, anti-fungal activity, ex vivo penetration and in vivo pharmacodynamic activity. The results of in vitro drug diffusion and ex vivo skin penetration studies demonstrated that the amount of drug diffused and penetrated through the skin was increased. Optimized transethosomes showed enhanced in vitro antifungal and in vivo pharmacodynamic activities against Candida albicans in Wistar albino rats when compared to conventional liposomes. Therefore, the developed ketoconazole encapsulated transethosome formulation is capable of enhancing the skin penetration of the drug by overcoming the stratum corneum barrier function and acting as an effective drug delivery system for ketoconazole through the skin for its anti-fungal activity.

Experimental study on two-phase boiling in wavy copper microchannels fabricated with ultrafast laser micromachining

In this study, the sinusoidal wavy (SW) copper microchannels with triangular cross section were fabricated with the ultrafast laser micromachining approach. The flow boiling of de-ionized water in SW microchannels were experimentally studied under mass fluxes of 43.43–217.15 kg (m2s)−1 and heat fluxes of 0–590 kW m−2. The flow patterns in the SW microchannels were visually observed and the boiling heat transfer characteristics were analyzed based on the obtained data. It shows the SW microchannel has a maximum enhancement of 127.7% in the local heat transfer coefficient with an acceptable increase of pressure loss compared to the straight one in the present work. In SW microchannels, a low degree of superheat can trigger the onset of nucleate boiling. A slow decreasing tendency of local heat transfer coefficient under a high effective heat flux or a high vapor quality was observed. The heat transfer mechanism of flow boiling in SW microchannels was analyzed based on the experimental visualization. Nucleate boiling and thin film evaporation are the dominant modes under different thermal boundary conditions. The flow boiling instability phenomenon in microchannels can be diminished using the wavy structures.