Characteristics Of Droplet Evaporation Research Articles

Heat and Mass Transfer Processes and Evaporation of a Liquid Droplet on a Structured Surface

The characteristics of water droplet heating and evaporation on structured hydrophobic and hydrophilic surfaces in the range of static contact angles from 73° to 155° were studied experimentally using high-speed video recording. Two fundamentally different technologies for applying coatings on a metal surface were used in comparison with the results on a polished surface. Microscopic studies were conducted to identify the features of the formed coatings. The wetting properties were characterized by means of the static contact angle and the contact angle hysteresis: on polished surface No. 1 (contact angle—73°, hysteresis—11°), on structured surface No. 2 (contact angle—125°, hysteresis—9°), and on structured surface No 3 (contact angle—155°, hysteresis—7°). The experimental dependences of the droplet evaporation rate on the different surfaces under normal conditions (ambient air temperature—293 K, atmospheric pressure, humidity—35%) were obtained. The evaporation regimes of droplets on the surfaces under study were identified. Water droplets evaporated in the pinning mode on surfaces No. 1 and No. 2. When a water droplet evaporated on surface No 3, the droplet was in the constant contact angle regime for ≈90% of its lifetime. Based on the experimental data obtained, a two-dimensional model of conjugate heat and mass transfer was developed, which describes the heating and evaporation of a liquid droplet on structured hydrophobic and hydrophilic surfaces at a wide range of contact angles. Satisfactory agreement was obtained between the numerical simulation results and experimental data. Using the model, the fields of temperature, concentration and other key characteristics were established at different points in time. Recommendations for its application in the development of gas–vapor–droplet applications were formulated.

Numerical investigation on the coupling effect of sessile ethanol droplet evaporation and the induced thermal flow in its pure vapor environment at low pressure

In order to reveal the coupling characteristics of sessile ethanol droplet evaporation and induced thermal flow in its pure vapor system at low pressure, a series of three-dimensional numerical simulations were carried out on the ethanol droplet evaporation on a copper substrate. The droplet radius is 2.5 mm. The droplet height ranges from 2.2 to 0.4 mm. To investigate the effect of vapor pressure, two pressure ratios (η) of 0.9 and 0.6 are chosen. The results show that, when η = 0.9, regardless of the droplet height, the surface temperature distribution in most areas is always uniform. However, when η = 0.6, several surface thermal patterns appear. Because of the evaporation, the substrate temperature distribution always has the lowest value near the contact line. The thermal pattern evolution will affect the evaporation rate and the substrate temperature distribution dynamically. When η = 0.6, the evaporation rate shows a non-monotonic variation trend with the decrease of the height. Although the droplet size is larger than the capillary length, buoyancy has little effect on the thermal pattern evolution and evaporation rate.

Experimental study on the evaporating progress of hexane lens on immiscible liquid：Spreading and receding

The change of evaporation liquid on another immiscible liquid has important guiding significance for many applications. In this experiment, the geometric temperature distribution and evaporation rate of n-hexane droplets were observed and recorded by changing the temperature of deionized water. The results show that with the increase of temperature of deionized water-based solution, the maximum diameter of n-hexane droplet spreading after titration increases gradually, while the minimum diameter of n-hexane droplet disappearing decreases gradually. Meanwhile, the evaporation rate of n-hexane droplet is constant during the whole evaporation process. It should also be mentioned that if the base solution is changed from deionized water to a certain concentration of salt solution, the maximum diameter of n-hexane droplet spreading will be reduced, and the evaporation intensity will be relatively reduced. These experimental results will give us a better understanding of the mechanism and characteristics of droplet evaporation.

Effects of Flow Compressibility on Two-Phase Mixing in Supersonic Droplet-Laden Flows

This research addresses a numerical analysis on the effects of flow compressibility on the characteristics of droplet dispersion, evaporation, and mixing of fuel and air according to the simulation of the spatially developing supersonic shear flows laden with evaporating n-decane droplets. A sixth-order hybrid WENO numerical scheme is employed for capturing the unsteady wave structures. The influence of inflow convective Mach number ( M c ), representing the high-speed flow compressibility, on the two-phase mixing is analyzed, in which M c is specified from 0.4 to 1.0. It is found that the shearing vortex is compressed spatially as M c increases, associated with the alternate distributions of compression and expansion regimes in the flow field. The flow compressibility changes not only the vortex structures but also the aerothermal parameters of the shear flows, and further influences the dispersion and evaporation of droplets. The two-phase mixing efficiency is observed to decrease as M c increases.

Direct numerical simulations of sessile droplet evaporation on a heated horizontal surface surrounded by moist air

In this paper, the newly developed multi-component multi-phase (MCMP) lattice Boltzmann method (LBM) is extended for direct simulation on effects of ambient humidity of air and wall temperatures on sessile droplet evaporation. The effect of repulsive strength between gas component and moisture component is taken into consideration by proposing an equivalent gas density based on equilibrium of total pressure between bulk liquid phase and moist air. Relative humidity (RH) is defined under different repulsive strengths, which can be adjusted according to moist air with different humidities. Based on this extended MCMP LBM, characteristics of sessile droplet evaporation into unsaturated humid air on a heated surface near Leidenfrost point under normal gravity conditions are investigated. Effects of surface wettability on interfacial temperature and evaporation flux distribution at liquid-vapor interface are analyzed. The simulated total evaporation time versus ambient RH is found following an exponential curve which is in agreement with an existing analytical model, suggesting the feasibility and reliability of simulating humidity effects based on this novel MCMP LBM. Transitional stage of sessile droplet evaporation on a heated surface near Leidenfrost point is also studied. When the droplet is detached from the wall above the Leidenfrost temperature, no buoyancy flow is observed inside the hovering droplet, and the maximum heat flux on the surface occurs where the vapor layer adjacent to the heating surface has a minimum thickness. It is found that the variation of total evaporation time versus surface temperature near the Leidenfrost point is a U-shape curve, which is in qualitative agreement with existing experimental data.

On the lifetime of evaporating confined sessile droplets

Confinement effects may trigger certain non-trivial characteristics of droplet evaporation, which may incur far-reaching implications in practical scenarios. In effect, the vapor mediated interactions in a confined droplet may alter its intrinsic evaporation characteristics altogether, primarily as a consequence of significant increase in droplet lifetime due to enriched accumulation of the vapor field in the vicinity. Here, we derive a universal scaling law for the evaporation lifetime of a geometrically confined sessile droplet, appropriate to a wide gamut of droplet-substrate combinations. With asymptotic convergence to the classical picture in case of an unconfined droplet, the validity of our scaling law for droplet lifetime is comprehensively tested and benchmarked against a wide range of experimental data sets. These findings may turn out to be of profound importance in several emerging applications including micro-scale combustion and droplet based microfluidic technology.

Determination of parameters of heat and mass transfer in evaporating drops

To compute parameters of heat and mass transfer in evaporating drops the emission-diffusion model is proposed. It is based on a dynamic balance between the vapor flow, obtained in the framework of the kinetic approach of Hertz-Knudsen, and diffusive flow of Maxwell. This approach allows taking into account characteristics of small droplet evaporation, where heat and mass transfer processes are not described by the regular diffusion model. The results of computations of the evaporation of free water droplets of different sizes according to regular diffusion and emission-diffusion models are compared. The effect of free convection flow and radiative heat transfer on droplets evaporation is determined. The suggested model is validated by comparison with experimental data obtained at the evaporation of water droplets suspended on polypropylene monofilaments. The form of droplet during evaporation is detected by microphotographing. The temperature of its surface is measured using infrared thermography. The numerical results and the experimental data show good agreement in time of evaporation and temperature of evaporating drops.

The features of heterogeneous water droplet evaporation in high-temperature combustion products of typical flammable liquids

This paper presents the experimental results on heating and evaporation features of heterogeneous (with opaque solid particles ? the size of 0.05-0.5 mm, relative mass concentration 0-1%) water droplets (the initial size ? radius 1-3 mm) during their motion through high-temperature (500-1800 K) gases. A significant increase in the integral characteristics of evaporation by introducing opaque inclusions into droplets was observed. The influence of energy accumulation on the conditions of droplet evaporation at the internal solid/liquid interfaces was established. For proportioned inclusions, the conditions of intensive vaporization (leading to the explosive disintegration of droplets) at internal inclusion/liquid interfaces was set. To summarize research results, experiments were conducted with the combustion products of kerosene, gasoline, industrial alcohol, acetone, and oil. The particles of graphite, carbon, and aluminum as solid inclusions were used. The investigation compared integral characteristics of heterogeneous droplet evaporation under the conditions of non-stationary (gas temperature varied from 1800 K to 500 K over the length of channel) and nearly stationary (gas temperature was maintained at about 1100 K) heating.

Integral characteristics of water droplet evaporation in high-temperature combustion products of typical flammable liquids using SP and IPI methods

This paper examines the evaporation features of water droplets moving through high-temperature combustion products of typical flammable liquids (on an example of gasoline, kerosene, ethanol, and acetone). Using Shadow Photography and Interferometric Particle Imaging methods contributed to perform the given experimental research. Such approach enabled to establish the integral characteristics of droplet motion (sizes and velocities) and evaporation process (evaporation rate). The initial droplet sizes (radii) and the temperatures of combustion products varied in a wide range: 0.05–4 mm and 400–1900 K, respectively. The choice of these ranges allowed us to summarize the research results, which can be applied in a group of promising high-temperature (usually more than 1000 K) gas-vapor-droplet technologies (thermal or flame methods of liquid waste treatment; heat carriers based on flue gases, steam, and water droplets; polydisperse firefighting). Conditions of almost the complete and insignificant (with minimum change of liquid mass) droplet evaporation were determined. In addition, the integral characteristics of water droplet evaporation under non-stationary heat transfer (at a temperature of combustion products from 1900 K to 400 K) were compared with the same characteristics under nearly stationary heat transfer (at a temperature of 1100 ± 30 K). During short-time heating (less than 0.5 s), values of these parameters correlated well (deviations did not exceed 8–12%).

The Characteristics of Water Droplet Evaporation in the Flames of Various Flammable Liquids

This paper investigates the integral characteristics of the evaporation of water droplets (the initial diameter 4–6 mm) during their motion in flames obtained through the combustion of typical flammable liquids for various purposes, such as kerosene and denatured alcohol. We performed the experiments using high-speed video cameras and panoramic cross-correlation techniques PTV and SP. The difference between evaporation rates under appropriate conditions has been defined. The influence of a droplet initial size on evaporation rate has been demonstrated.

Experimental Investigation of Droplet Evaporation of Suspension Based on Water with Subsurface Admixtures in a Flame Area

The influence of typical mixtures (clay, sand, soil, silt) on integral characteristics of droplet evaporation of suspension based on water in high-temperature (about 1100 K) gases was investigated experimentally by the optical methods and high-speed (to 10 5 frames per second) video recording system. The significant intensification of heat up and evaporation of suspension droplets was defined when varying the relative mass fraction of admixtures within the range γ=0−0.1 for a sand, γ=0−0.01 for clay, ground, and silt.

Investigation of Regularities of Heat and Mass Transfer and Phase Transitions during Water Droplets Motion through High-Temperature Gases

The macroscopic regularities of heat and mass transfer and phase transitions during water droplets motion through high-temperature (more than 1000 K) gases have been investigated numerically and experimentally. Water droplet evaporation rates have been established. Gas and water vapors concentrations and also temperature values of gas-vapor mixture in small neighborhood and water droplet trace have been singled out. Possible mechanisms of droplet coagulation in high-temperature gas area have been determined. Experiments have been carried out with the optical methods of two-phase gas-vapor-droplet mixtures diagnostics (“Particle Image Velocimetry” and “Interferometric Particle Imaging”) usage to assess the adequateness of developed heat and mass transfer models and the results of numerical investigations. The good agreement of numerical and experimental investigation results due to integral characteristics of water droplet evaporation has been received.

Numerical simulation of dynamics of droplet impact on heated flat solid surface

The process of the impinging of a droplet upon a heated flat solid surface was computationally studied to identify the key process occurring in a liquid droplet. A computational fluid dynamics model, based on the volume of fluid (VOF) approach, was used to simulate the impact of droplet dynamics on the heated flat solid surface. Surface tension and wall adhesion phenomena were taken into account in this model. The model was proved effective by comparing the simulation results with the experimental data in the literature. It could be concluded that the VOF-based model was able to simulate the dynamics of the impinging of a droplet upon a heated solid surface. The key characteristics of droplet evaporation and the interaction between the droplet and the solid surface could be captured. Copyright , Oxford University Press.

Water droplet evaporation on Cu-based hydrophobic surfaces with nano- and micro-structures

The characteristics of water droplet evaporation on three different hydrophobic surfaces, PCu (Plain Copper, θ=115°), MSCu (Micro-Structured Copper, θ=126°) and NSCuO (Nano-Structured Copper Oxide, θ=159°) with coating of the same SAM (Self-Assembled Monolayer) material, were experimentally investigated. For industrial heat transfer applications, copper material was used as the substrate, and the simple and cost-effective fabrication technique to prepare the superhydrophobic surface, NSCuO, was introduced. Based on the observations, the behavior of droplet evaporation was divided into three stages: Stage I (constant contact area stage), Stage II (constant contact angle stage) and Stage III (mixed stage). When studying the PCu surface, the Stages I, II, and III were observed, consistent with previous reports. For the MSCu surface, Stages I and III appeared without Stage II, and the pinning period of contact line was the longest among the test samples due to the formation of Wenzel state droplet. In the case of the superhydrophobic NSCuO surface, only Stage III occurred, and the contact line moved freely during the entire evaporation time because of the formation of Cassie state droplet. The total evaporation time of the NSCuO was the longest out of all the samples tested. At the last stage of evaporation, the edge of the droplet shrank at a much faster rate in all surfaces. On the other hand, the shrinking velocity of the droplet height drastically increased only on the NSCuO, which was considered as the unique behavior of superhydrophobic surface. In this experiment, it was found that the surface structure determines the motion of the contact line on the surface, which, in turn, strongly influences the characteristics of the droplet evaporation.