Enhancement Of Evaporation Rate Research Articles

Introducing ultrasonic falling film evaporator for moderate temperature evaporation enhancement

In the present study, Ultrasonic Falling Film (USFF), as a novel technique has been proposed to increase the evaporation rate of moderate temperature liquid film. It is a proper method for some applications which cannot be performed at high temperature, such as foodstuff industry, due to their sensitivity to high temperatures. Evaporation rate of sodium chloride solution from an USFF on an inclined flat plate compared to that for Falling Film without ultrasonic irradiation (FF) at various temperatures was investigated. The results revealed that produced cavitation bubbles have different effects on evaporation rate at different temperatures. At lower temperatures, size fluctuation and collapse of bubbles and in consequence induced physical effects of cavitation bubbles resulted in more turbulency and evaporation rate enhancement. At higher temperatures, the behavior was different. Numerous created bubbles joined together and cover the plate surface, so not only decreased the ultrasound vibrations but also reduced the evaporation rate in comparison with FF. The highest evaporation rate enhancement of 353% was obtained at 40 °C at the lowest Reynolds number of 250. In addition, the results reveal that at temperature of 40 °C, USFF has the highest efficiency compared to FF.

Optical thermogeneration induced enhanced evaporation kinetics in pendant nanofluid droplets

Optofluidic manipulation using lasers and colloidal systems have immense applications in microscale thermofluidic devices. The present study analyses the effect of laser as an external optical source in modulating the evaporation characteristics of pendent nanofluid microdroplets (which are free from substrate effects) so as to capture the physics behind the interfacial mass transport. The study analyses the effect of the power of laser, nature and concentration of the particle on evaporation rate of the complex fluids. Evidence of internal circulation was observed with Particle Image Velocimetry (PIV) technique in colloidal systems which together with the volumetric heat generation due to laser irradiation can be attributed to be the cause behind the enhanced evaporation rate. The nanoparticles are observed to efficiently convert the energy of radiant photons to heat while water is observed to show poor evaporative performance under laser irradiation. Theoretical analysis of the evaporation rate with the classical diffusion driven evaporation model is found to fall short in predicting the evaporation rate in colloidal systems, even in the absence of laser irradiation. Thermal Marangoni and Rayleigh numbers are calculated from the theoretical examination and are found not potent enough to induce the circulation in such systems which could improve evaporation. Hence the observed weak internal circulation can be attributed to the solutal Marangoni arising out of the local concentration gradients of nanoparticles in the droplet and the enhanced Thermophoretic drift and Brownian dynamics of the nanoparticles. Also the enhancement in the diffusion coefficient and its strong dependence with the particle concentration is also contributing to the enhancement in enhanced evaporation rate in nanocolloidal systems. The augmentation in the evaporative process under laser radiation is found to be governed majorly by the optical heat generation with weak solutal Marangoni and a scaling model is able to accurately predict the experimental observations. The present findings could have implications in modulation of thermofluidic and species transport phenomena in microscale devices.

Breakup of heterogeneous water drop immersed in high-temperature air

The enhancement of the evaporation rate of a heterogeneous water drop is experimentally investigated. A water drop encapsulating a solid graphite inclusion is instantaneously placed in the very hot air. Under action of the high ambient temperature and due to different thermal properties between water and solid inclusion, the initial drop explodes and creates numerous very small droplets. The vaporization rate of the water drop is strongly increased leading to a short time life of the drop. High speed video recording is used to detail the explosive breakup mechanism of a drop immersed in hot air when its temperature varies in the range between 1073K and 1373K. The target of the study is to determine the possible conditions for the increase of the water/air surface of the drop after breakup, i.e., the ratio of the final to initial surface, Sout/Sin, and to verify that this enlargement is responsible of the evaporation rate enhancement. For this purpose, we investigate drops of different initial volumes (5–15µl) with graphite solid inclusions of different shapes and volumes: 2×2×1mm, 2×2×2mm, or 2×2×3mm. The water surface ratio Sout/Sin is given as function of Vw/Vinc, which is the ratio of initial volume of water drop Vw to the volume of solid graphite inclusion Vinc. The study has shown that the maximum value of the ratio Sout/Sin≈15 can be reached when Vw∼Vinc. When, the ratio of Sout/Sin varies in the range 1 to 15. The increase of the evaporating surface is of the highest interest to improve the heat transport and to be used and developed in heat technologies.

Experimental investigation and modeling of the evaporation rate of carbon nanotube blended emulsified Neem biodiesel

ABSTRACTNeem biodiesel is an abundant fuel in India. However, the higher viscosity and lower calorific value result in a lower evaporation rate, hence limiting its use in combustion applications. The evaporation rate of fuel plays a very important role in combustion reactions. An improvement in evaporation rate can be achieved using nanoparticles in Neem biodiesel. The present investigation was carried out to determine the evaporation characteristics of carbon nanotube suspended Neem biodiesel with different dosing ratios of nanoparticles with and without water. An improvement in evaporation rate with addition of carbon nanotubes was seen in Neem biodiesel. An improvement in calorific value and cetane number was obtained. It can be concluded that an increase in nanoparticle concentration results in an enhancement in evaporation rate. A regression model was developed and found to be statistically significant.

Hydrophobically modified nanoparticle suspensions to enhance water evaporation rate

The evaporation rates of water can be enhanced by adding the hydrophobically modified nanoparticles as a suspension. The magnitudes of enhancement are related to the diameter and mass concentration of nanoparticles. In particular, a 15% enhancement was achieved after adding the modified Al2O3 nanoparticle with a diameter of 13 nm and mass percentage of 0.02%. A theoretical model was established in order to estimate the evaporation rates of hydrophobic particle-based nanofluids. The obtained results indicate that the enhanced evaporation rates are attributed to the elevated saturated vapor pressures of the nanofluids. These results may have important applications for energy-efficient enhancement of water evaporation rates.

Coffee-Ring-Free Quantum Dot Thin Film Using Inkjet Printing from a Mixed-Solvent System on Modified ZnO Transport Layer for Light-Emitting Devices.

Inkjet printing has been considered an available way to achieve large size full-color RGB quantum dots LED display, and the key point is to obtain printed film with uniform and flat surface profile. In this work, mixed solvent of 20 vol % 1,2-dichlorobenzene (oDCB) with cyclohexylbenzene (CHB) was used to dissolve green quantum dots (QDs) with CdSe@ZnS/ZnS core/shell structure. Then, by inkjet printing, a flat dotlike QDs film without the coffee ring was successfully obtained on polyetherimide (PEI)-modified ZnO layer, and the printed dots array exhibited great stability and repeatability. Here, adding oDCB into CHB solutions was used to reduce surface tension, and employing ZnO nanoparticle layer with PEI-modified was used to increase the surface free energy. As a result, a small contact angle is formed, which leads to the enhancement of evaporation rate, and then the coffee ring effect was suppressed. The printed dots with flat surface profile were eventually realized. Moreover, inverted green QD-LEDs with PEI-modified ZnO film as electron transport layer (ETL) and printed green QDs film as emission layer were successfully fabricated. The QD-LEDs exhibited the maximum luminance of 12 000 cd/m2 and the peak current efficiency of 4.5 cd/A at luminance of 1500 cd/m2.

Evaporation Flux Distribution of Drops on a Hydrophilic or Hydrophobic Flat Surface by Molecular Simulations.

The evaporation flux distribution of sessile drops is investigated by molecular dynamic simulations. Three evaporating modes are classified, including the diffusion dominant mode, the substrate heating mode, and the environment heating mode. Both hydrophilic and hydrophobic drop-substrate interactions are considered. To count the evaporation flux distribution, which is position dependent, we proposed an azimuthal-angle-based division method under the assumption of spherical crown shape of drops. The modeling results show that the edge evaporation, i.e., near the contact line, is enhanced for hydrophilic drops in all the three modes. The surface diffusion of liquid molecular absorbed on solid substrate for hydrophilic cases plays an important role as well as the space diffusion on the enhanced evaporation rate at the edge. For hydrophobic drops, the edge evaporation flux is higher for the substrate heating mode, but lower than elsewhere of the drop for the diffusion dominant mode; however, a nearly uniform distribution is found for the environment heating mode. The evidence shows that the temperature distribution inside drops plays a key role in the position-dependent evaporation flux.

Floatable, Self-Cleaning, and Carbon-Black-Based Superhydrophobic Gauze for the Solar Evaporation Enhancement at the Air-Water Interface.

Efficient solar evaporation plays an indispensable role in nature as well as the industry process. However, the traditional evaporation process depends on the total temperature increase of bulk water. Recently, localized heating at the air-water interface has been demonstrated as a potential strategy for the improvement of solar evaporation. Here, we show that the carbon-black-based superhydrophobic gauze was able to float on the surface of water and selectively heat the surface water under irradiation, resulting in an enhanced evaporation rate. The fabrication process of the superhydrophobic black gauze was low-cost, scalable, and easy-to-prepare. Control experiments were conducted under different light intensities, and the results proved that the floating black gauze achieved an evaporation rate 2-3 times higher than that of the traditional process. A higher temperature of the surface water was observed in the floating gauze group, revealing a main reason for the evaporation enhancement. Furthermore, the self-cleaning ability of the superhydrophobic black gauze enabled a convenient recycling and reusing process toward practical application. The present material may open a new avenue for application of the superhydrophobic substrate and meet extensive requirements in the fields related to solar evaporation.

Stable Isotope Record of Human and Sheep Enamel Carbonate from the Ancient Middle Euphrates Valley (Syria)

AbstractStable isotope analyses of human tooth enamel have allowed us to reconstruct the isotope composition of dietary carbon, changes in the oxygen isotope composition of drinking water and the possible migration of humans in ancient Terqa and Tell Masaikh (SE Syria). δ18Ocarbonate values of human tooth enamel from the interval comprising the Neo‐Assyrian to the modern Islamic periods (from 900 BC to AD 1949) generally mirror the isotope composition of Euphrates water, which is believed to have been a major drinking water source. Lower δ18Ocarbonate values of human Bronze Age apatite are linked to a different hydrologic system that was present in the Middle Euphrates valley at that time (2650–1700 BC). Higher δ18Ocarbonate values of some individuals in the Neo‐Assyrian (900–700 BC) and Islamic periods (AD 600–1200) may indicate human migration from the interior of the Near East. Low δ13Ccarbonate values (−11.3 to −12.4‰) of human tooth enamel from the interval comprising the Early Bronze to the Islamic periods (from 2650 BC to AD 1200) indicate C3 plants as a predominant source of dietary carbon. Changes in human dietary customs in SE Syria (with inferred usage of C4 plants) occurred in the modern Islamic period only (AD 1850–1949).Oxygen and carbon isotope data of sheep enamel show the usage of water bodies characterised by an enhanced evaporation rate during the Neo‐Assyrian time (900–700 BC) and grazing sheep herds on drier areas during the Islamic and the modern Islamic periods (after AD 600). Copyright © 2015 John Wiley & Sons, Ltd.

Shallow Cumulus Mixing and Subcloud-Layer Responses to Variations in Aerosol Loading

Abstract The sensitivity of lightly precipitating trade wind shallow cumulus to both aerosol concentration and domain size is investigated using large-eddy simulations (LESs). The mean states of liquid water potential temperature, total water, and velocity field exhibit negligible change between all LES runs, offering the perfect opportunity to investigate microphysical–dynamical interactions solely due to variations in aerosol concentration and not changes in meteorology. As aerosol concentration increases, two cloud population responses are found: 1) cloud and cloud-core widths decrease while their strength increases and 2) cloud and core numbers increase. The narrowing of the polluted clouds is caused by enhanced evaporation rates surrounding the cloud cores, which in turn shrinks the diameter of the cumulus toroidal circulation. The more narrow toroidal circulation in polluted clouds has a faster rise rate and imparts weaker dynamical entrainment on the cloud cores, resulting in stronger clouds as aerosol concentration increases. The reduction in cloud number for more pristine conditions occurs from greater cold pool coverage, reducing the likelihood of subcloud-layer thermals reaching their lifting condensation level. The increase in cloud number as aerosol concentration increases is compensated by narrower and stronger clouds, resulting in a cumulus-core mass flux that appears to be unaffected by aerosol concentration variability. For the weakly precipitating case studied here, the trends in the response of the cumulus clouds to aerosol concentration are found to be insensitive to domain size.

A parametric study on phase change heat transfer due to Taylor-Bubble coalescence in a square minichannel

In this paper, a numerical investigation of the phase change characteristics of Taylor-Bubbles (T-B) during flow boiling of FC-72 in a square minichannel is carried out. Multiple Taylor-Bubbles starting from their nucleation, growth and coalescence along with the associated heat transfer mechanisms have been modeled. The temporal variation of bubble coalescence pattern is found to exhibit a good agreement with the in-house experimental measurements conducted in microgravity environment. A detailed parametric study is conducted to understand the effects of Reynolds number (Re), wall superheat (ΔTw), bubble nucleation radii, and the surface tension expressed in terms of Capillary number (Ca) on the T-B nucleation and coalescence characteristics. The parametric study reveals that the nucleating bubbles tend to grow and coalesce faster at Re=500 compared to Re=50 due to higher temperature gradients leading to enhanced evaporation rates. The phenomenon of bubble ‘roll-off’ is observed when the wall and liquid are both superheated to 2K due to absence of heat transfer between the top wall of the channel and the T-B. Also it is observed that the bubble coalescence time is reduced nearly by a factor of two for the coalescence of unequal bubble sizes. At higher values of Ca, both coalescence and break-up of T-B occur in succession while at lower values no coalescence is observed. The heat flux contours in the vicinity of the T-B contact line region predicted by the numerical model is found to exhibit a good qualitative agreement with the experimental measurement. It is inferred that of the parameters studied, Re and ΔTw are the two most significant factors that influence wall heat transfer during T-B coalescence.

INSTABILITY OF EVAPORATION FRONTS IN THE INTERSTELLAR MEDIUM

The neutral component of the interstellar medium is segregated into the cold neutral medium (CNM) and warm neutral medium (WNM) as a result of thermal instability. It was found that a plane-parallel CNM-WNM evaporation interface, across which the CNM undergoes thermal expansion, is linearly unstable to corrugational disturbances, in complete analogy with the Darrieus-Landau instability (DLI) of terrestrial flames. We perform a full linear stability analysis as well as nonlinear hydrodynamic simulations of the DLI of such evaporation fronts in the presence of thermal conduction. We find that the DLI is suppressed at short length scales by conduction. The length and time scales of the fastest growing mode are inversely proportional to the evaporation flow speed of the CNM and its square, respectively. In the nonlinear stage, the DLI saturates to a steady state where the front deforms to a finger-like shape protruding toward the WNM, without generating turbulence. The evaporation rate at nonlinear saturation is larger than the initial plane-parallel value by a factor of 2.4 when the equilibrium thermal pressure is 1800 k_B cm^-3 K. The degrees of front deformation and evaporation-rate enhancement at nonlinear saturation are determined primarily by the density ratio between the CNM and WNM. We demonstrate that the Field length in the thermally unstable medium should be resolved by at least four grid points to obtain reliable numerical outcomes involving thermal instability.

Wetting and evaporation of a sessile drop under an external electrical field: A review

The enhancement of heat and mass transfer using a static electric field is an interesting process for industrial applications, due to its low energy consumption and potentially high level of evaporation rate enhancement. However, to date, this phenomenon is still not understood in the context of the evaporation of sessile drops. We synthesise the current research concerning the effect of an electric field on sessile drops with a focus on the change of contact angle and shape and the influence of the evaporation rate. We also discuss the future prospects of this research field.

Integrated production of fresh water, sea salt and magnesium from sea water

Abstract Seawater desalination is becoming an important source of fresh water in several countries all around the world. One of the main drawbacks of desalination processes, however, is related to the disposal of large quantities of concentrated brine, which is an always-present by-product of the process. An integrated production of fresh water and salts may be achieved using the discharge brine from a desalination plant as a feed for conventional salt ponds, with the advantages of using brine more concentrated than sea water and, in the case of thermal desalination plants, warmer than sea water. By doing so, the process is faster as a consequence of the enhancement of evaporation rate on the surface of ponds. The above concept has been proposed already several years ago, but only rare examples exist of real applications. A pilot test has been performed in the last 4 years in Trapani (Italy), where a 36,000-m3/d multiple effects desalination with thermal vapour compression plant is operating very close to a traditional salt pond normally fed with sea water. Furthermore, the use of fractionated crystallisation process, typically adopted in conventional salt ponds, allows for the easy separation of salts like calcium carbonates and sulphates, sodium chloride and a final saturated brine which is extremely rich in magnesium as a sole bivalent cation. Thus, the possibility of a further exploitation of such saturated brine has been experimentally analysed by laboratory tests in order to produce high-purity magnesium to be commercialised in the pharmaceutical, food and metal industries. Results have shown a very promising enhancement of the salt pond production capacity, keeping at the same time the very high quality standards required for the production of food-grade salt from sea water. On the other side, laboratory experiments indicated the actual possibility of producing high-purity magnesium salts, thus encouraging towards further investigation for the development of a pilot process development and installation.

The effect of evaporation kinetics on nanoparticle structuring within contact line deposits of volatile drops

This paper deals with the evaporation behaviour of nano-suspensions. An investigation of the relationship between reduced environmental pressure and the ring-stain formation mechanism is presented. Monodisperse, nanosuspension sessile drops formed a variety of patterns, at the macro-scale, consisting of sets of rings, concentric rings and irregularly distributed stains. “Stick-slip” motion of the triple line (TL), constantly pinned TL and very rapid evolution of the evaporation were the reasons for each of the observed patterns respectively. At the particle level (nano-scale), the promotion of close-packed, hexagonal particle structures was observed as a result of the enhanced evaporation rate and hence increased particle velocity. Beyond an optimal/critical pressure, a combination of further enhanced particle velocity with very limited space (for particles to find their most thermodynamically favourable positions) led to the formation of a disordered region at the exterior region and, as expected, to more defects. Lastly, inside the rings, particles formed random groups of particle aggregates, resembling branches, following TL motion during the “slip” phase. On the other hand, particle structures resembling ripples formed on top of a particle monolayer with increasing surface coverage as pressure was reduced. This was a direct result of fluid flow being too weak to reach the TL but strong enough to carry particles over smaller distances during the last stages of the evaporation process.

Ordered Assembly of Sorted Single-Walled Carbon Nanotubes by Drying an Aqueous Droplet on a Meshed Substrate

We report on the ordered assembly of sorted single-walled carbon nanotubes (SWCNTs) (95% semiconducting tubes) by drying aqueous droplets on meshed grids with an enhanced evaporation rate. Besides the commonly observed "coffee ring" deposit of aligned SWCNTs on the droplet contact line after water evaporation, tubes self-organized in the central area of a grid and covered up to -1/3 of the whole surface area of a grid. In addition, parallel-aligned and straightened SWCNTs were seen to span across cracks in the SWCNT-surfactant film. The evaporation-driven assembly of sorted SWCNTs has the potential to produce ordered SWCNT structures that are attractive for the fabrication of electronic devices comprising mostly semiconducting or metallic tubes.

Investigation of EHD-Enhanced Water Evaporation and a Novel Empirical Model

This paper investigates the relationship between the enhanced evaporation rate by electric field, applied voltage (U) and electrode gap (d) in electrohydrodynamics-enhanced water evaporation. The electric fields produced by a single point-to-plane electrode configuration are used. In our experiments, the enhanced evaporation rate by electric field has shown increasing linearly with the U/d ratio. As regard the evaporation rate distribution on the plane electrode, the maximum evaporation rate is observed at the center of the plane (i.e., directly under the point electrode), and significantly decreased while the radial distance (i.e., radial distance from the point electrode centerline) increases. The evaporation rate is inversely proportional to the radial distance. A novel empirical model based on the dependence relation between the evaporation rate, U/d ratio and radial distance was developed. The model and the experimental results agree well.

Assessing pedogenic calcite stable-isotope values: Can positive linear covariant trends be used to quantify palaeo-evaporation rates?

Palaeoclimate models suggest enhanced evaporation rates in subtropical regions (15–30° latitude) during greenhouse-world conditions, however, there are no empirical data to support these estimates. Laboratory evaporation experiments have shown that calcites precipitated from variably saturated solutions yield a positive linear covariant trend (PLCT) in δ 18O calcite versus δ 13C calcite values. The goal of the present controlled laboratory experiments is to develop a method to quantify regional palaeo-evaporation rates from palaeosol calcrete PLCTs. Samples of powdered CaCO 3 were dissolved in de-ionized water in pressure sealed containers with a range of elevated atmospheric pCO 2 concentrations for 24 h. The solution was then allowed to evaporate completely from an open container within an incubator with the time of calcite crystallization noted, and aliquots removed for analysis every 24 h. The precipitated calcite produced an array of δ 18O calcite versus δ 13C calcite values that fall upon a PLCT projected from a theoretical meteoric calcite line (MCL). Water analyses yielded δ 18O water enrichments from an initial value of − 4.75‰ VSMOW ranging up to between + 10.0‰ and + 14.8‰. The experimental results show that solutions formed under higher pCO 2 conditions precipitated calcite very early on during evaporation, and thus have δ 18O calcite and δ 13C calcite values that are slightly enriched relative to the MCL. The solutions that formed under low pCO 2 conditions precipitated calcite after much of the fluid had evaporated, and thus yield more enriched δ 18O calcite and δ 13C calcite values. Repeat trials with varying temperature and relative humidity show that the PLCT is steeper under both higher temperature and low relative humidity. The wide range of pCO 2, temperature and relative humidity conditions used simulate meteoric phreatic and meteoric vadose conditions that may affect a calcrete horizon over time during numerous dissolution/precipitation reactions. The results of these experiments show that a dominant factor in the precipitation of vadose calcite is the saturation state of the fluid with respect to CaCO 3, while the primary factors affecting the magnitude and steepness of the PLCT are vadose zone temperature, relative humidity, the starting δ 18O water value and saturation state of the fluid with respect to CaCO 3. Since the pCO 2 of the rooting zone is cross-controlled by the local soil and surface ecology, the magnitude of the PLCT enrichment may not be a direct proxy for palaeo-evaporation rates.

Anomalous evaporation behavior of ZnO powder milled mechanically under high-energy conditions

ZnO powder showed anomalous evaporation behavior after its mechanical milling treatment under high-energy conditions. The amount of generated vapor is about 10 times higher in the first 15 min of annealing at 1300 °C than that of unmilled ZnO powders. The strong ball impacts are responsible for the greatly enhanced evaporation ability. Low-energy ball milling involving shearing actions and rare weak impacts leads only to a small evaporation rate enhancement. The possible explanation of the high evaporation rate of the heavily milled material is the existence of large fraction of weakly bonded atoms in grain boundaries, surface defects and strained areas.

Effect of polymer addition on the contact line crystallisation of paracetamol

In an evaporating aqueous paracetamol solution, crystallisation of the metastable orthorhombic polymorph can occur at the solution–substrate contact line due to a favourable meniscus geometry and enhanced evaporation rates at the edges. Upon the addition of polymers or other common excipients to these systems, it was found that crystallisation of this form was suppressed. This was attributed to the polymer blocking the nucleation sites at the edge of the meniscus, altering the evaporation rate or disrupting the flow of solution to the edges. These results are in contrast to previous conclusions by other groups suggesting that the addition of certain polymers may aid in the selective crystallisation of the orthorhombic polymorph from solution via the formation of specific interactions.