Evaporation Behavior Of Droplets Research Articles

Analysis on Evaporation Characteristics of Palm Oil Biodiesel using a HSDT Method

The purpose of this study is to investigate the evaporation characteristics of different types of fuels by applying the hot surface deposition test (HSDT). Generally, HSDT method is a simplified method to simulate fuel deposition in diesel engines. In this study, diesel fuel (DF) and palm oil biodiesel with different blend ratios (B10-B50) were used to evaluate the fuel droplet evaporation behavior on a heated aluminum alloy plate surface. Single fuel droplet was impinged on the heated plate at various surface temperatures (Ts). Important data from the evaporation characteristics was the maximum evaporation point (MEP), which is the temperature point where a droplet evaporates in the shortest lifetime (tlife). MEP is referred to identify the surface temperature range in which fewer deposits will be produced by the test fuel. Furthermore, MEP is also used to determine the appropriate droplet interval to create wet and dry conditions in the fuel deposition test. The obtained MEP was 360°C (DF), 365°C (B10, B50), 375°C (B20, B40), and 385°C (B30).

Evaporation Dynamics of Macro- and Nanodroplets on Heated Hydrophilic Rough Substrates: The Effect of Roughness and Scale.

Droplet evaporation on rough substrates plays an essential role in cooling and micro/nanoparticle assembly. Currently, there are numerous macroscopic experiments and theoretical models to investigate the droplet evaporation behavior on rough substrates. However, due to the complexity of this phenomenon, understanding its mechanisms solely through macroscale studies is difficult. To this end, molecular dynamics simulations of the models with distinct roughness factors are performed, and the obtained results are compared with those of relevant experiments of droplet evaporation on three hydrophilic substrates with different roughness average of 0.1, 0.15, and 0.2 μm, respectively. In this way, we assess the evaporation on these rough systems and the effect of scale on macro- and nanodroplets, which allows us to explore deeper the mechanism of droplet evaporation on rough hydrophilic substrates. In particular, we find that in the case of macroscale droplets, the evaporation mode remains the same with increasing roughness, pointing to a combined mixed and constant-contact-radius (CCR) mode. In the case of nanoscale droplets, the evaporation model is the constant-contact-angle mode when the roughness factor r = 1, while the mixed and CCR modes are found for r = 1.5 and 2, respectively. The scale effect has significant influence on the evaporation pattern of droplets on rough hydrophilic substrates. Moreover, it is also found that increasing the roughness of substrates expands the substrate-droplet contact area on both the macro- and nanoscale, which in turn enhances the heat transfer from the substrate toward the droplet. We anticipate that this first systematic analysis of scale effects provides further insights into the evaporation dynamics of droplets on rough hydrophilic substrates and has significant implications for the advancement of nanotechnology.

Numerical Simulation of Droplet Dispersion and Passenger Infection Risk Assessment in a Subway Carriage

Droplet transmission is a critical pathway for the spread of respiratory infectious viruses. A thorough understanding of the mechanisms of droplet dispersion within subway carriages is crucial to curb the widespread transmission of the virus. This study utilizes computational fluid dynamics (CFD) to establish a full-scale numerical model of a subway carriage. The numerical model and droplet evaporation behavior are validated using experimental data and literature. The impact of primary parameters such as the initial droplet size, release velocity, release position, relative humidity, and passenger density on the droplet diffusion and probability of infection for passengers is investigated. The results indicate that large droplets (100 μm) are deposited on the carriage floor before complete evaporation, while tiny droplets (10 μm) evaporate rapidly, leading to a longer suspension time in the air within the carriage. The infected passenger’s position influences the ventilation system’s efficiency in removing the droplets; removal takes significantly longer when the infected passenger is closer to the carriage end. Additionally, a low relative humidity (35%) and high passenger density (4 p/m2) result in more droplets being trapped by passengers’ bodies. The infection probability for passengers depends on the initial size and quantity of droplets trapped by their bodies. Maintaining higher relative humidity levels and limiting the passenger numbers within the subway carriage can reduce the number of droplets captured by passengers’ bodies, thus helping to reduce the infection probability of fellow passengers.

Application prospect of reaction engineering approach in studying drying behavior of single droplet during spray drying of traditional Chinese medicine

Spray drying technology is one of the most commonly used unit operations in the production of traditional Chinese medicine(TCM) preparations, offering advantages such as short drying time and uniform product quality. However, due to the properties of TCM extracts, such as high viscosity, strong hygroscopicity, and poor flowability, there is limited scope to solve the problems of wall adhesion and clumping in spray drying from the macroscopic perspective of pharmaceutical production. Therefore, it has become a trend to study and optimize the spray drying process from the microscopic point of view by investigating single droplet evaporation behavior. Based on the reaction engineering approach(REA), the single droplet drying system, as a novel method for studying droplets, collects parameter data on individual TCM droplets during the drying process and uses the REA to process the data and establish predictive models. This approach is crucial for understanding the mechanism of TCM spray drying. This paper summarized and analyzed the cha-racteristics of various single droplet systems, the application of REA in single droplet drying systems, and its significance in optimizing the process, predicting drying states, and shortening the development cycle in the field of TCM spray drying, and looked ahead to the prospects of this method, including the introduction of new parameters and imaging techniques, aiming to provide a reference for further research in the field of TCM spray drying.

Lattice Boltzmann Study of Microdroplet Evaporation Process in Pixel Pits

Inkjet printing has the advantages of high material utilization, low cost, and large-area production and is a promising manufacturing technology for organic light-emitting diode (OLED) displays. However, the droplet evaporation in micron-size pixel pits is highly influenced by the pit wall. Such a process is extremely difficult to control, leading to the appearance of defects such as the coffee ring in the printing process of OLED displays. In this work, a multiphase thermal lattice Boltzmann (LB) model based on multiple distribution functions is established to study the evaporation process of micron-size droplets in pits. According to the characteristics of the largest number of the three-phase contact line (TCL) appearing in the evaporation process, the evaporation modes can be divided into three types, i.e., one, two, and three TCLs. In the 1-TCL mode, the droplet stays in constant contact radius (CCR) for the shortest time; in 2-TCL and 3-TCL modes, the liquid film fracture behavior of evaporating droplets in the pit is well captured. The effects of the pit height and the contact angle on the droplet evaporation mode are investigated in detail. The phase diagrams of evaporation modes with different parameters are also established. The revealed evaporation mechanism is supposed to be useful for regulating the droplet evaporation behavior and controlling the cured film shape in the OLED printing process.

Evaporation behavior of liquid microdroplets in atmospheric-pressure nonequilibrium plasma

In recent years, atmospheric-pressure nonequilibrium plasma processing using microdroplets has attracted significant attention. To improve the controllability of this process, an understanding of the evaporation behavior of droplets in plasma is highly desirable. In this study, we examine the evaporation behavior of well-controlled inkjet droplets in atmospheric-pressure nonequilibrium argon plasma through both experiments and modeling. A comparison of the droplet evaporation model based on energy balance considering gas temperature, electron and ion collisions, and recombination reactions with experimental evaporation behavior suggests that droplet evaporation is enhanced in high-density plasma environments with electron and ion densities exceeding 1019 m−3 when compared with that in non-ionized gaseous environments at a gas temperature below 1000 K.

Effects of nanoparticle concentration and peclet number on nanofluid droplet evaporation behavior

Effects of droplet parameters are reported using model simulations on the evaporation characteristics of droplets containing insoluble nanoparticles and related to the formation of shells of particles. The droplet parameters are initial droplet diameter (do), initial particle mass fraction (Yvo), and the Peclet number (Pe) based on the fluid evaporation rate constant (K) to particle diffusivity in the fluid (Dpl). Time-dependent phenomena of the instantaneous droplet size (d), d2 vs time (for example, following the d2-law or the deviation from it), instantaneous evaporation efficiency (i.e., effective area of evaporation due to particle inclusion on the droplet surface compared to a pure fluid droplet) are reported. Analytical expression of shell parameters at shell formation such as time to shell formation, shell size, and evaporation efficiency are derived using the initial droplet parameters and compared with the computational results. It is found that the evaporation behavior of droplets containing insoluble nanoparticles, including the deviation from d2-law, is independent of do but is strongly influenced by the product of Pe·Yvo.

Development of device to study evaporation behavior of micro-droplets at high temperatures.

Leidenfrost phenomenon is one of the unique behaviors of droplets. In Leidenfrost phenomenon, liquid closing to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. This non-contact state between the droplet and the hot surface leads to the decrease in heat transfer and the increase in evaporation time. This phenomenon occurs for the micro-order droplets. For example, it is influence the combustion efficiency of the engine in which fuel sprayed. For another example, the mist CVD, which is one of the functional thin film fabrication methods, also makes a good use of the Leidenfrost phenomenon to make high quality thin films. This has been indirectly proven by the previous experiments, but there was no direct observation, which needs to be verified. In recent years, the evaporation behavior of microdroplets is also involved in the removal of bacteria from the air due to the spread of infectious diseases. In this study, we developed an observation device for checking the evaporation behavior of micro droplets and a program for measuring the droplet diameter, analyzed the evaporation behavior of droplets by optical observation. The experimental results shown that the range of observation conditions was improved and the accuracy of droplet measurement was enhanced. In the future, we will continue to enhance the accuracy of machine and study the observation methods to further improve the measurement accuracy.

Manipulating Nano-suspension Droplet Evaporation by Particle Surface Modification.

The evaporation of a nano-suspension droplet on a substrate has gained extensive attention recently due to its potential application in the rising industry of functional coating. In this paper, we reported that the droplet evaporation behavior can be controlled by the nanoparticles' concentration and the functional group on the surface of nanoparticles. Experimental results indicated that the suspension of nanoparticles notably enhanced the evaporation rate of droplets and decreased the duration time of the continuous-contact-radius (CCR) stage. This effect was more obvious when the nanoparticles were modified by the perfluorodecyltrimethoxysilane (PDTS), which made the particles more hydrophobic. Besides, the modified nanoparticles can effectively inhibit the formation of coffee rings during evaporation. These results may have important applications for the energy-efficient enhancement of the water evaporation rate.

Evaporation of Sessile Droplets of Polyelectrolyte/Surfactant Mixtures on Silicon Wafers

The wetting and evaporation behavior of droplets of aqueous solutions of mixtures of poly(diallyldimethylammonium chloride) solution, PDADMAC, with two different anionic surfactants, sodium laureth sulfate, SLES, and sodium N-lauroyl N-methyl taurate, SLMT, were studied in terms of the changes of the contact angle θ and contact length L of sessile droplets of the mixtures on silicon wafers at a temperature of 25 °C and different relative humidities in the range of 30–90%. The advancing contact angle θa was found to depend on the surfactant concentration, independent of the relative humidity, with the mixtures containing SLES presenting improved wetting behaviors. Furthermore, a constant droplet contact angle was not observed during evaporation due to pinning of the droplet at the coffee-ring that was formed. The kinetics for the first evaporation stage of the mixture were independent of the relative humidity, with the evaporation behavior being well described in terms of the universal law for evaporation.

Development of a system for steady state observation of micro droplet evaporation behavior

Development of a system for steady state observation of micro droplet evaporation behavior

Investigation on the droplet evaporation process on local heated substrates with different wettability

AbstractMarangoni effect is one of the critical factors in the droplet evaporation process, which is caused by surface tension gradient in the droplet interface. In this study, local heating is adopted to provide a more complicated temperature distribution on the droplet surface, and a detailed numerical investigation is carried out to address the effect of Marangoni flow on the droplet evaporation behaviour. Results show that asymmetric heat source position could result in the droplet morphology being asymmetric, especially for droplets on super-hydrophilic surfaces. The evaporation rate could be affected both by the heat source position and the droplet contact angle. When placed on a smooth substrate, the droplet will slip horizontally as a result of the asymmetric heating condition. The slipping behaviour is affected by both the heat source position and the surface wettability.

Surface declination governed asymmetric sessile droplet evaporation

The article reports droplet evaporation kinetics on inclined substrates. Comprehensive experimental and theoretical analyses of the droplet evaporation behaviour for different substrate declination, wettability and temperatures have been presented. Sessile droplets with substrate declination exhibit distorted shape and evaporate at different rates compared to droplets on the same horizontal substrate and is characterized by more often changes in regimes of evaporation. The slip stick and jump stick modes are prominent during evaporation. For droplets on inclined substrates, the evaporative flux is also asymmetric and governed by the initial contact angle dissimilarity. Due to smaller contact angle at the rear contact line, it is the zone of a higher evaporative flux. Particle image velocimetry shows the increased internal circulation velocity within the inclined droplets. Asymmetry in the evaporative flux leads to higher temperature gradients, which ultimately enhances the thermal Marangoni circulation near the rear of the droplet where the evaporative flux is highest. A model is adopted to predict the thermal Marangoni advection velocity, and good match is obtained. The declination angle and imposed thermal conditions interplay and lead to morphed evaporation kinetics than droplets on horizontal heated surfaces. Even weak movements of the TL alter the evaporation dynamics significantly, by changing the shape of the droplet from ideally elliptical to almost spherical cap, which ultimately reduces the evaporative flux. The life time of the droplet is modelled by modifying available models for non-heated substrate, to account for the shape asymmetry. The present findings may find strong implications towards microscale thermo-hydrodynamics.

Sessile droplet evaporation behavior in simulated multi-droplets system

Sessile droplet evaporation behavior in simulated multi-droplets system

Nanopore-Enhanced Drop Evaporation: When Cooler or More Saline Water Droplets Evaporate Faster.

The evaporation of water droplets on surfaces is a ubiquitous phenomenon in nature and has critical importance in a broad range of technical applications. Here, we show a substantial enhancement of liquid evaporation rate when droplets are on nanoporous thin film surfaces. We also reveal how this nanopore-enhanced evaporation leads to counterintuitive phenomena: cooler or more saline water droplets evaporate faster. We find indeed that, contrary to typical evaporation behavior of sessile droplets on nonporous surfaces, the droplets placed on nanoporous thin films evaporate more rapidly when salt concentration increases or when the temperature decreases. This peculiar droplet evaporation behavior is related to the key role of the steady wetted annulus that is self-generated into the nanopore network in the drop periphery, which leads to an effectively enhanced evaporation area that controls the overall evaporation process. Our results provide the prospect of conceiving fresh scenarios in the evaporation of drops on surfaces in both relevant applications and fundamental insights.

Micro-patterning of titanium surface and its effect on droplet evaporation

Two types of the superhydrophobic/hydrophilic micro-patterned surfaces were prepared through a ultraviolet photocatalytic etching technology. One is hydrophilic dot patterned surface another is hydrophilic square patterned surface. The hydrophilic dot patterned surface is mainly composed of hydrophobic surfaces, while the hydrophilic sites are arrayed in the hydrophobic region. The hydrophilic square patterned surface is mainly composed of hydrophilic surface.The latticed hydrophobic region cut the hydrophilic area into several independent small blocks. The surfaces with the superhydrophobic/hydrophilic pattern has a good control of the distribution of the liquid, which can influence the evaporation behavior of droplets and the evaporation rate of the dispersion droplets on the surface in a nonstructural manner. The effects of the two patterned surfaces on the evaporation rate of droplets by carrying out natural evaporation experiments and forced convection experiments with 4 × 4 droplets of two patterned surfaces at normal temperature and pressure respectively and the effect of the two surfaces on the droplet evaporation rate was compared with that of the homogeneous hydrophilic surface. The gas-liquid contact area of the droplets on the hydrophilic dot patterned surface is 76.9% higher than that of the homogeneous hydrophilic surface and the gas-liquid contact area of the droplets on the hydrophilic square patterned surface is increased 61.23% relative to the homogeneous hydrophilic surface. The increased gas-liquid contact area promotes the evaporation of droplets in the early stage. The droplet evaporation rate decreased in the late stage. It is mainly due to the anisotropy of humidity. In the horizontal direction between the drops along the wall, the gradient of humidity is lower than in the vertical direction over the top point of the droplet. As a result, the vapor flow on the droplet surface is significantly heterogeneous for many droplets.

Evaporation of pure and blended droplets of diesel and alcohols (C2–C9) under diesel engine conditions

ABSTRACTAn improved multicomponent evaporation model was developed to study the evaporation characteristics of the pure diesel and alcohol (C2–C9) as well as their blended droplets. In this model, the Predictive Soave–Redlich–Kwong (PSRK) equation of state (EOS) was employed to evaluate the vapor–liquid equilibrium for pure and blended droplets. The results indicated that the model predictions agree very well with the experimental measurements. The evaporation characteristics of different pure alcohol droplets were analyzed, and the influence of the addition of alcohols in diesel on the evaporation behavior of droplets under various conditions was discussed.

Droplet evaporation behaviour of a liquid fuel from chicken litter

Experiments on the evaporation of single droplets of a refined fraction of a liquid fuel produced by fast pyrolysis of chicken litter are presented. A computational model for the evaporation of the fuel is devised using the techniques of continuous thermodynamics, with distribution functions selected to represent the five main chemical groups in the fuel (n-paraffins, 1-alkenes, n-alkylbenzenes, n-alkyl fatty acids and n-diols) as determined through Py-FIMS and other analyses. The distribution parameters are determined directly from these analyses. This model is found to give accurate predictions of droplet temperature and diameter histories during evaporation. At ambient temperatures of 500°C and above bubbling and disruption of the droplet surface was observed, with the bubbles exhibiting a thin skin like that of a balloon. This was attributed to polymerization of the surface layer, causing the underlying liquid to reach higher temperatures. Under most conditions a solid residue remained after evaporation, believed also to be the product of polymerization, and, at higher temperatures, of pyrolysis as well.

Evaporation of single droplets of ethanol–fuel oil mixtures

Measurements of droplet evaporation behaviour are reported for mixtures of pure and denatured ethanol with No. 2 fuel oil. A model for the evaporation of droplets of these mixtures is presented which uses continuous thermodynamics to describe the fuel oil fraction. Activity coefficients were used to describe non-ideal phase equilibrium behaviour. Distribution function parameters for the fuel oil were fitted from a continuous thermodynamics simulation of the ASTM distillation test; this simulation was also used to test the accuracy of the phase equilibrium model. The model is shown to give very good agreement with the measurements. Internal boiling of the liquid is seen to take place during evaporation of the alcohol, but is not sufficiently violent to eject mass from the droplet.

HEAT AND MASS TRANSFER MODELING OF SUBMICROMETER DROPLETS UNDER ATMOSPHERIC PRESSURE CONDITIONS

The mathematical modeling of heat and mass transfers of submicrometer droplets vaporizing in gaseous surroundings at atmospheric pressure is investigated, including the effect of equilibrium/nonequilibrium assumption at the liquid/gas interface and of surface curvature on the droplet evaporation behavior. The numerical limitations of some of the most frequently used models for academic and industrial research activities on sprays have been enlightened. Emphasis is given to the verification of the validity of the assumptions imposed when they are extrapolated outside the range of operating conditions within which they have been derived, for instance, when they are used to predict the heat and mass transport phenomena occurring in submicrometer aerosol particles.