Droplet Evaporation Time Research Articles

Droplet boiling on heated surfaces with various wettabilities

Single droplet boiling is a basic problem for spray cooling which is an effective technology for high heat flux applications. Visualizations were used to investigate the effect of surface wettability on single droplet boiling with measurements of the droplet evaporation times. Four boiling regimes were identified on the hydrophilic surface and the bare aluminum surface, including nucleate boiling, nucleate boiling with violent atomization, transition boiling and film boiling. The nucleate boiling with violent atomization regime was not seen on the hydrophobic surface and the superhydrophobic surface. The transition boiling regime was not seen on the superhydrophobic surface. The Leidenfrost temperatures were approximately 220 °C on the hydrophilic surface, 200 °C on the bare aluminum surface, 170 °C on the hydrophobic surface and 170 °C on the superhydrophobic surface. The effect of surface wettability on the Leidenfrost temperature was discussed using the potential barrier existing at the transition to film boiling. A droplet film boiling model was proposed to relate the droplet evaporation time, initial droplet size and surface superheat with a semi-empirical correlation developed for the droplet evaporation time. The correlation agrees well with the current measured droplet evaporation times for film boiling and with data in the literature.

Effects of gas absorption with chemical dissociation reaction on single slurry droplet drying

A theoretical model of the absorption of soluble gases during the first stage of slurry droplet drying, at or below room temperature and at atmospheric pressure, is suggested as a basis for the development of alternative drying methods. The suggested approach is based on the theory of slurry droplet evaporation, in conjunction with a model of chemical absorption by evaporating droplets. Slurry droplet evaporation is described by a system of transient conjugate nonlinear energy and mass conservation equations. Numerical calculations are performed for a slurry droplet evaporating in various gaseous mixtures, including a gaseous mixture containing soluble gases. It is shown that the effect of gas absorption is a significant enhancement in the rate of evaporation of a slurry droplet, with higher concentrations of soluble gas in the gas phase leading to a shorter first stage of drying. In particular, in a ternary gaseous mixture (N2/NH3/H2O) with an ammonia mass fraction of 0.5 and at 288 K, the evaporation time of a coal-water slurry droplet with an initial radius of 100 was about 20% shorter than that in a mixture free of soluble gases. The existence of a maximum in the temporal dependence of the interfacial temperature of the droplet during evaporation is also revealed. This maximum becomes more pronounced with increasing ambient concentration of the absorbate. Calculations show that relative humidity exerts a minor influence on the dynamics of gas absorption by liquid droplets. By contrast, relative humidity strongly affects the rate of evaporation of slurry droplets. The model for slurry droplet evaporation in the presence of soluble gas presented herein allows for calculation of the appropriate concentration of soluble gas in the gaseous phase to achieve an optimal regime for slurry droplet drying.

Development of the experimental stand for research heat removal with a high power density by a dispersed heat carrier flow

A schematic and measuring diagram of an experimental stand has been developed for research heat removal with a high power density by a dispersed heat carrier flow. Methods for measuring thermophysical and electrical parameters are determined. The design of the experimental chamber is developed. The scheme and design of the induction heating system and steam condenser have been developed. An experimental setup for research the evaporation time of a droplet has been created. An experimental study of the droplet evaporation time was carried out. The experimental data were processed and compared with the theoretical dependence.

Влияние смачиваемости подложки и влажности воздуха на самосборку наночастиц при испарении капель коллоидных растворов

This article discusses the influence of substrate wettability and air humidity on the process of nanoparticle patterns formation in evaporating microdroplets. The process of self-assembly of polystyrene and aluminum oxide particles on the glass covered with titanium, tungsten, carbon, and teflon was investigated.<br> The droplet evaporation time and the obtained packing of particles with increasing hydrophobicity of substrates were evaluated. The influence of air humidity on the evaporation dynamics and on the process of nanoparticles self-assembly is revealed.

INTERACTIONS BETWEEN ADJUVANTS AND THE FUNGICIDE AZOXYSTROBIN+BENZOVINDIFLUPYR IN HYDRAULIC SPRAYING

ABSTRACT Adjuvants are tools to maximize pesticide spray quality. This study aimed to evaluate the interaction effects between adjuvants and the fungicide azoxystrobin + benzovindiflupyr on the physicochemical characteristics, droplet evaporation time, and spray droplet size. The experiment was carried out using a 2×6 factorial scheme, in which the first factor was the absence or presence of the fungicide mixture (water or water + fungicide) and the second factor was the presence of different adjuvants (water and five adjuvants). The parameters evaluated were surface tension, pH, viscosity, electrical conductivity, droplet evaporation time, volume median diameter, percentage of droplets smaller than 100 μm, and relative amplitude of the droplet spectrum. The silicone and propionic acid + soy lecithin adjuvants reduced the pH of the spray solution, and the sodium lauryl ether sulfate, propionic acid + soy lecithin, and orange essential oil adjuvants reduced droplet evaporation when mixed with the fungicide. All tested adjuvants improved spray quality related to the characteristics of the droplet spectrum, reducing the percentage of droplets smaller than 100 μm, and allowing more uniform droplet size. The fungicide azoxystrobin + benzovindiflupyr affected droplet evaporation time.

3D Hotspots Platform for Plasmon Enhanced Raman and Second Harmonic Generation Spectroscopies and Quantitative Analysis

AbstractConstructing plasmonic hotspots in metal nanostructures is essential for plasmon‐enhanced spectroscopies (PESs), but their application in quantitative analysis is a long‐standing challenge. Herein, highly uniform, reproducible, stable, and sensitive 3D hotspots platform substrates are assembled on hydrophobic silicon wafers using a drop of solution containing silica coated surface plasmon resonance (SPR) active nanoparticles (NPs). Plasmon‐enhanced second harmonic generation (PESHG) and surface‐enhanced Raman scattering (SERS) experimental signals increase sharply with the increase of droplet evaporation time, which is attributed to the rapid increase of 3D hotspots generated by gradual decrease of distance between adjacent NPs. Compared with 3D bare metal NP substrate, 3D metal shell‐isolated nanoparticles (SHINs) substrate demonstrates much more stable PESHG and SERS signals attributing to the protecting SiO2 shells. Besides that, uniform distribution of 3D “hotspots” on the hydrophobic substrate can facilitate the quantitative analysis of an analyte using SERS. Experimental and theoretical results demonstrate that the 3D hotspots platform has the potential to be developed as a plasmon‐enhanced substrate for linear and nonlinear spectroscopies with practical sensing applications.

Thermal Decomposition of a Single AdBlue® Droplet Including Wall–Film Formation in Turbulent Cross-Flow in an SCR System

The selective catalytic reduction (SCR) methodology is notably recognized as the widely applied strategy for NOX control in exhaust after-treatment technologies. In real SCR systems, complex unsteady turbulent multi-phase flow phenomena including poly-dispersed AdBlue® spray evolve with a wide ranging relative velocity between the droplet phase and carrier gas phase. This results from an AdBlue® spray that is injected into a mixing pipe which is cross-flowing by a hot exhaust gas. To reduce the complexity while gaining early information on the injected droplet size and velocity needed for a minimum deposition and optimal conversion, a single droplet with a specified diameter is addressed to mimic a spray featuring the same Sauter Mean Diameter. For that purpose, effects of turbulent hot cross-flow on thermal decomposition processes of a single AdBlue® droplet are numerically investigated. Thereby, a single AdBlue® droplet is injected into a hot cross-flowing stream within a mixing pipe in which it may experience phase change processes including interaction with the pipe wall along with liquid wall–film and possible solid deposit formation. First of all, the prediction capability of the multi-component evaporation model and thermal decomposition is evaluated against the detailed simulation results for standing droplet case for which experimental data is not available. Next, exploiting Large Eddy Simulation features the effect of hot turbulent co- and cross-flowing streams on the dynamic droplet characteristics and on the droplet/wall interaction is analyzed for various droplet diameters and operating conditions. This impact is highlighted in terms of droplet evaporation time, decomposition efficiency, droplet trajectories and wall–film formation. It turns out that smaller AdBlue® droplet diameter, higher gas temperature and relative velocity lead to shorter droplet life time as the droplet evaporates faster. Under such conditions, possible droplet/wall interaction processes on the pipe wall or at the entrance front of the monolith may be avoided. Since the ammonia (NH3) gas generated by urea decomposition is intended to reduce NOX emission in the SCR system, it is apparent for the prediction of high NOX removal performance that UWS injector system which allows to realize such operating conditions is favorable to support high conversion efficiency of urea into NH3.

Simulation of desulphurization wastewater evaporation through flue gas

As an advanced treatment of desulphurization wastewater, spray evaporation technology has received significant attention and broad research interests recently. However, the results of previous studies were based on the simplification that droplets of desulphurization wastewater were regarded as droplets of pure water. To effectively reveal the evaporation mechanism of desulphurization wastewater, the evaporation model of droplets with different salt contents was used. The effects of parameters such as nozzle arrangement, droplet diameter, temperature of flue gas and salt content were obtained by numerical simulation. And the operation situations of evaporation equipment under different working conditions were analyzed. Numerical results show that the symmetrical arrangement of nozzles should be adopted since there is no scaling or corrosion in flue. The evaporation time of droplets could be shortened by adopting uniform diameter distribution. Droplets with small diameter under high flue gas temperature require less time and shorter distance for complete evaporation. The optimum diameter and temperature of flue gas are found to be 54 μm and 403 K, respectively. The increase of salt content of droplets could lead to the increase of mean distance and time which are needed for complete evaporation of droplets. And the droplets cannot completely evaporate under 75% THA (turbine heat acceptance) in which the evaporation equipment should not be turned on.

Leidenfrost point and droplet dynamics on heated micropillar array surface

We studied the Leidenfrost Point (LFP) of a falling liquid droplet on various surfaces that were covered by micropillars. We used a Micro Electric Mechanical System to fabricate a bare smooth surface, and surfaces that had eight kinds of micropillar array. The pillar density was controlled by manipulating the distance between the micropillars. First, evaporation time of droplets of deionized water was measured under overheated condition (100–350 °C) to evaluate the LFP of the test samples. To analyze the LFP trend, the droplet dynamics were recorded at high speed, and their spreading diameter on impact, and the time that they took to rebound from the surface were measured. We then modelled the interaction between droplet and test samples and discussed the physical mechanism of the LFP change. We detected an optimal micropillar density at which the LFP was highest in this study. This study provides insights into the droplet impact and rebounding process on an overheated textured surface, and the resulting heat transfer.

Numerical investigation of motion of sulfuric and hydrochloric droplets with phase change in free-falling process

Hydrochloric droplets and sulfuric droplets are two common contaminants in indoor environments. The different mechanisms of phase change may cause different transport behaviours in them. This paper presents fundamental research conducted to numerically investigate the free fall of single hydrochloric droplets and sulfuric droplets. The influence of the initial diameter and relative humidity (RH) are considered. The diameter change and falling distance of these two types of droplets were compared and discussed. The results show that the phase change and motion of the two types of droplets are quite different. It was possible for the hydrochloric droplets to evaporate completely, while it was not for the sulfuric droplets. The sulfuric droplets have a minimum size and show a linear tendency of increase in size with the increase in initial diameter at a certain RH. The evaporation time of a hydrochloric droplet exhibits an exponential relationship with the initial diameter at a certain RH. A two-stage motion behaviour is observed on comparing the falling distances between the two types of droplets under identical initial conditions. The falling distances of the hydrochloric droplets at different initial sizes and RH were determined. Furthermore, it was found that the falling of single hydrochloric and sulfuric droplets could be severely mispredicted if the solution effect and/or HCl volatilisation is ignored. The results of this study could contribute to the accurate prediction and evaluation of the damage inflicted by hydrochloric and sulfuric droplets on human health, equipment, and building structures.

Heat transport for evaporating droplets on superhydrophilic, thin, nanoporous layers

Previous studies of droplet spreading on superhydrophilic nanostructured surfaces have demonstrated that enhanced wetting and wickability of such surfaces can produce more rapid and extensive spreading of liquid droplets. There is also ample evidence from prior studies that these types of superhydrophilic nanostructured surfaces can exhibit enhanced vaporization heat transfer performance in for droplet evaporation and pool boiling processes. This study specifically explored the mechanisms of droplet vaporization on superhydrophilic nanoporous thin layers, at a fundamental level, for conditions in which liquid viscous forces and pore capillarity forces dominate, and liquid inertia forces are low. The investigation, summarized here, experimentally explored the droplet evaporation heat transfer performance on superhydrophilic, nanoporous, thin layers on a heated metal substrate. A thermal growth process was used to fabricate a layer of ZnO nanopillars a few microns thick on a copper substrate. The resulting nanoporous layer exhibited ultra-low contact angles and high wickability. The experiments indicate that, on these surfaces, a deposited droplet undergoes an initial rapid spreading phase followed by a slower vaporization phase, which shrinks the droplet until evaporation is complete. Although droplet evaporation without bubble nucleation is the primary focus of this work, the experiments also examined the added effects of bubble nucleation on droplet evaporation at high surface superheats. Based on experimental observations and data, a model of heat transport during the evaporation process was developed. The model predictions are shown to agree well with droplet evaporation time measurements for droplets of different initial sizes and a range of surface temperatures. The experiments and model indicate that the surfaces tested enhance droplet evaporation heat transfer performance by as much as a factor of three compared to an ordinary copper surface at the same conditions. The model is also used to explore the potential for more extreme augmentation of droplet evaporation heat transfer by further enhancement of surface wetting and wickability. The results of this study provide a clearer picture of the interconnection of droplet spreading mechanisms and evaporation heat transfer for these circumstances.

Evaluation of Atomization Characteristics of Second Generation Biodiesel Using Air Blast Atomizer

Biodiesel is one of the well-known renewable fuels that can be produced from organic oils and animal fats. Biodiesel fuel that meets ASTM D6751 fuel standards can replace diesel for reciprocating engine. On the other hand, biodiesel can also be considered for gas turbine application in power generation. Nevertheless, inferior properties of biodiesel such as high viscosity, density and surface tension results in inferior atomization and high emission which consequently hinders the fuel for gas turbine utilisation and generate higher emission pollutants. Therefore, this work focused on the evaluation of atomization characteristics of second generation biodiesel which is produced using microwave assisted post treatment scheme. The atomisation characteristics of second generation biodiesel was evaluated using air blast atomiser in terms of spray angle and spray length. Subsequently, numerical evaluation was performed to evaluate sauter mean diameter and droplet evaporation time of second generation biodiesel. The results show, atomization characteristics of second generation biodiesel has improved in terms of spray angle and spray length, sauter mean diameter and shorter evaporation time compared to biodiesel which is commonly referred to as first generation biodiesel and fossil diesel.

Numerical investigation on evaporation characteristics of kerosene droplets with dilute concentrations of aluminum nanoparticles at elevated temperatures

We have numerically investigated the problem of kerosene droplets evaporation with dilute concentrations of aluminum nanoparticles at elevated temperatures. The aim of the research was to develop a mathematical model describing the evaporation process and obtain numerical simulation data which is in good agreement with an experimental data. The dependences of kerosene droplet evaporation time on its initial diameter and ambient temperature are presented in this paper.

Near-azeotropic volatility behavior of hydrous and anhydrous ethanol gasoline mixtures and impact on droplet evaporation dynamics

After fermentation, the concentration of bioethanol is only 8–12 wt%. To produce anhydrous ethanol fuel, a significant amount of energy is required for separation and dehydration. Once the azeotrope composition is reached, distillation can no longer be exploited for purification and more expensive methods must be used. Replacing anhydrous ethanol fuel with hydrous ethanol (at the azeotrope composition) can result in significant energy and cost savings during production. The goal of this study was to characterize the volatility behavior and the droplet evaporation dynamics of hydrous and anhydrous ethanol gasoline blends. Three hydrous ethanol-gasoline blends (10, 15, and 30 vol%) in which the hydrous ethanol was composed of the azeotropic proportions of ethanol and water, and three anhydrous ethanol gasoline blends (10, 15, and 30 vol%) were prepared and analyzed with the advanced distillation curve method. Distillation curves were obtained for all test fuels and distillate samples were taken during the distillation process. A droplet evaporation model validated with the distillation data was exploited to understand how the non-ideal volatility behavior of these blends, the high heat of vaporization of water, and altered fluid properties can affect the transient droplet evaporation phenomena and thus the fuel's potential to effectively mix with air in direct injection internal combustion engines. Minor differences in the distillation curves and vapor-liquid equilibrium between the hydrous and anhydrous fuels were measured. Droplet modeling results showed that the higher heat of vaporization and viscosity of water relative to ethanol can lead to significant differences in the net droplet evaporation time between the two types of blends, especially at the higher blending ratios evaluated. These results suggest that the presence of water in ethanol-gasoline blends may extend droplet lifetimes and increase the susceptibility of the fuel to form particulate matter emissions. This is the first study to use distillation methods to gain a better understanding of evaporation behavior and the role of water's non-linear vapor-liquid equilibrium on droplet evaporation dynamics.

Evaporation characteristics of fuel sessile droplets with nanoparticles

ABSTRACTThe nano-fuel are prepared by dispersing CNT, CeO2, and Co3O4 nanoparticles in n-tetradecane (C14). This experimental study investigates the effect of nanoparticles mass fraction as well as the variation of nanoparticles sizes on the viscosity and evaporation characteristics. It is revealed that the viscosity increases with the increase in mass fraction and the decrease in particle size. In addition, the presence of nanoparticles leads to a reduction of the evaporation rate during constant contact line phase. However, the droplet evaporation rate is augmented remarkably in constant contact angle and hybrid phase. As a result, the whole evaporation time of nano-fuel droplets is shorter than base fuel.

Evaporation and wettability of fungicide spray, with or without adjuvant, on leaves of vegetables

ABSTRACT On tomato, cucumber and bell pepper cultivation, commonly large quantities of plant-protection products are applied, to control pests and diseases, as a way to guarantee better productivity and final product quality. The knowledge of spreading and evaporation time of spray droplets is fundamental to understand the interaction between fungicides and target surface for proper distribution of this fungicide. This study was installed to determine the wetting area and droplet evaporation of sprays containing the fungicide Cabrio Top, with or without adjuvant Nimbus®, deposited on leaves and artificial surfaces (glass slide). A system was used which analyzes images composed of a droplet generator, a stereoscope camera for capturing images and a climatic chamber for controlling temperature and relative air humidity. Droplets of 600 µm in diameter containing spray solution were deposited on leaf surfaces and on glass slide and sequential images were used to quantify the wetting area and the evaporation time. The spray solution and the target surface are determinant for wetness and droplet evaporation after deposition. Evaporation time and surface tension were inversely proportional to the wetting areas. Addition of adjuvant Nimbus® (0.5%, v/v) reduced the surface tension and provided an increase in the wetting surface area, except on tomato leaves which had shown low wetting capacity in both fungicide solutions applied.

Effect of surfactants and leaf surface morphology on the evaporation time and coverage area of ZnIDHA droplets

The aim of the study was to determine the effects of the addition of surfactants to ZnIDHA fertilizer on the area of leaf coverage and water evaporation time taking into account the morphology of leaf blades. The effect of three surfactants (polyglucoside, alcohol EO (ethylene oxide) and trisiloxane) on the size of the leaf coverage area and evaporation time was compared. Adaxial leaf surfaces of maize (Zea mays L. `Wilga`), wheat (Triticum aestivum `Adana-99`) and rapeseed (Brassica napus L. `F1`) were analyzed by scanning electron microscopy and laser scanning confocal microscopy. The contact angle for aqueous ZnIDHA solutions with and without the addition of surfactants was determined. Measurements of droplet evaporation time and area of leaf coverage were conducted at the relative humidity of 60±1%. The adaxial surfaces of wheat and rapeseed leaves used in the study can be classified as highly non-wettable, whereas maize leaves are no-wettable. Out of the surfactants under study, the largest leaf coverage area was observed when adding the trisiloxane surfactant. Regardless of the surfactant used, the leaf coverage area decreased and the evaporation time increased for fertilizer droplets applied to the maize, rapeseed and wheat leaves. This study demonstrated that the addition of surfactants to the fertilizer solution increases the wettability of the adaxial leaf surface and induces a strong capillary effect which determines the direction of spread of fertilizer droplets. In addition to the amount of trichomes and waxes, also the capillary effect has a significant impact on water evaporation time and the area of leaf coverage by fertilizer droplets.

Sessile drop evaporation under an electric field

In this study, for the first time, the natural (diffusion-limited) evaporation of a sessile drop under an electric field was experimentally examined. A sessile drop natural evaporation is affected by the geometry of the drop, e.g. baseline, contact angle, and surface area, which all can be changed in the presence of an electric field. As such, first, the effect of electric field on sessile water droplet geometry was studied, together with how it differs for surfaces with various contact angle and contact angle hysteresis (for both hydrophilic and hydrophobic surfaces). Then, the dynamics (evaporation time and mode) of natural evaporation of sessile droplet under an electric field was studied by measuring the evaporation rate, surface area, contact angle, and baseline of the droplet. It is found that compare to when there is no electric field, the evaporation time of a sessile droplet increases when there is an electric field. This was statistically shown to be significant for drops placed on surfaces with contact angle hysteresis lower than 32°. For droplets that evaporate in constant baseline mode initially and then by evaporation in constant contact angle mode, the presence of the electric field resulted in a decrease in the duration of constant baseline mode when the surface had a low contact angle hysteresis.

Interaction of droplet dispersion and evaporation in a polydispersed spray

The interaction between droplet dispersion and evaporation in an acetone spray evaporating under ambient conditions is experimentally studied with an aim to understand the physics behind the spatial correlation between the local vapour mass fraction and droplets. The influence of gas-phase turbulence and droplet–gas slip velocity of such correlations is examined, while the focus is on the consequence of droplet clustering on collective evaporation of droplet clouds. Simultaneous and planar measurements of droplet size, velocity and number density, and vapour mass fraction around the droplets, were obtained by combining the interferometric laser imaging for droplet sizing and planar laser induced fluorescence techniques (Sahu et al., Exp. Fluids, vol. 55, 1673, 2014b, pp. 1–21). Comparison with droplet measurements in a non-evaporating water spray under the same flow conditions showed that droplet evaporation leads to higher fluctuations of droplet number density and velocity relative to the respective mean values. While the mean droplet–gas slip velocity was found to be negligibly small, the vaporization Damköhler number ($Da_{v}$) was approximately ‘one’, which means the droplet evaporation time and the characteristic time scale of large eddies are of the same order. Thus, the influence of the convective effect on droplet evaporation is not expected to be significant in comparison to the instantaneous fluctuations of slip velocity, which refers to the direct effect of turbulence. An overall linearly increasing trend was observed in the scatter plot of the instantaneous values of droplet number density ($N$) and vapour mass fraction ($Y_{F}$). Accordingly, the correlation coefficient of fluctuations of vapour mass fraction and droplet number density ($R_{n\ast y}$) was relatively high (${\approx}0.5$) implying moderately high correlation. However, considerable spread of the $N$ versus $Y_{F}$ scatter plot along both coordinates demonstrated the influence on droplet evaporation due to turbulent droplet dispersion, which leads to droplet clustering. The presence of droplet clustering was confirmed by the measurement of spatial correlation coefficient of the fluctuations of droplet number density for different size classes ($R_{n\ast n}$) and the radial distribution function (RDF) of the droplets. Also, the tendency of the droplets to form clusters was higher for the acetone spray than the water spray, indicating that droplet evaporation promoted droplet grouping in the spray. The instantaneous group evaporation number ($G$) was evaluated from the measured length scale of droplet clusters (by the RDF) and the average droplet size and spacing in instantaneous clusters. The mean value of $G$ suggests an internal group evaporation mode of the droplet clouds near the spray centre, while single droplet evaporation prevails near the spray boundary. However, the large fluctuations in the magnitude of instantaneous values of $G$ at all measurement locations implied temporal variations in the mode of droplet cloud evaporation.

Water droplet impacting on overheated random Si nanowires

Micro and nanostructured surfaces can significantly improve the heat transfer capability and shift the Leidenfrost temperature of impacting droplets. In this study, the effect of overheated random Si Nanowires on impacting water droplets is presented. The cooling performance was studied in term of droplet evaporation time and surface temperature while the boiling dynamics was observed through high-speed visualization. The Si Nanowires can increase almost twice the Leidenfrost temperature compare to the similar case of plain Si surface. At the same time, the heat transfer is enhanced by widening the transition boiling region about 150 K. In this regime, the film liquid lift-off behavior is observed in the range of the We numbers studied.