Surface Of Liquid Film Research Articles

Numerical study on characteristics of single droplet impacting on wetted surface

In this paper, the mathematical and physical models of the single droplet impacting on liquid film are established. The axisymmetric numerical simulation of the single droplet impacting on liquid film is carried out by using the Couple Level-set and VOF (Volume of Fluid) numerical simulation method, which is verified by experiment, studying the influence of the liquid film thickness, surface tension, viscosity, and density of liquid on the liquid crown, and analyzing the thickness effect on the neck ejecta sheet. Through the analysis of pressure and velocity field, the mechanism of crown formation is studied. The results show that: (1) experiment and simulation are in good agreement, so CLSVOF method (the Couple Level-set and VOF) is suitable for research of droplet impacting on liquid film; (2) with the increase of liquid film thickness, the expansion radius and height of crown decrease, and the crater depth increases; (3) the surface tension, density, and viscosity of the fluid have influence on the crown; (4) ejecta sheet velocity decreases with the increase of the liquid film thickness but when H* > 1.0 (ratio of film thickness to droplet diameter), the ejecta sheet velocity does not change; (5) neck ejecta sheet is caused by neck pressure difference.

Equilibrium, kinetic and thermodynamic studies on the adsorption of atrazine in soils of the water fluctuation zone in the Three-Gorges Reservoir

BackgroundEnvironmental behavior of pesticide in soils is a key current research focus. Studying the adsorption characteristics of pesticides in soils as a parameter for evaluating the risk of groundwater pollution by pesticides is commonly applied in agriculture.ResultsTo provide a theoretical basis for environment risk assessment and pollution remediation, the thermodynamics and kinetics of the equilibrium of atrazine adsorption in the Three-Gorges Reservoir area were assessed and analyzed via batch experiments. Results showed that the sorption of atrazine was an exothermic and spontaneous process at temperatures of 298–318 K. Atrazine was more easily adsorbed by soils at concentrations of 0–30 mg L−1, with low-temperature adsorption being more effective than high-temperature adsorption. The adsorption of atrazine to the two assessed soils was well fitted by the Freundlich and Langmuir models. The adsorption kinetics of atrazine in soils were consistent with the quasi-second-order kinetic model and intraparticle diffusion was found not to be the only control step. The monolayer adsorption occurred with non-uniform energy distribution on the soil surface, indicating that the adsorption of atrazine by the two kinds of soil was controlled by internal diffusion surface adsorption and liquid film diffusion, leading to the complexity of its adsorption kinetics. The values of standard free energy Delta rG_{m}^{0} < 0, indicated that the adsorption of atrazine in soils was spontaneous and dominated by physical adsorption. Changes in standard enthalpy (Delta {text{r}}H_{m}^{0}) indicated that the adsorption was exothermic.ConclusionsAtrazine exhibited a weak adsorption capacity in both soils, indicating it is highly mobile in the soil–water environment and can easily cause groundwater pollution. Therefore, much attention should be paid to the environmental behavior of pesticide soil moisture fluctuations, potentially leading to the broad transfer and spread of pollution.

Flow Accelerated Corrosion of Carbon Steel with Droplet Impingement Using a Modified Rotating Cylinder Electrode Experiment

In power plant cooling systems, water droplets and condensate films form due to heat transfer through cooling tube walls. Condensate films are known to cause flow accelerated corrosion on carbon steels used in air-cooled condensers. Corrosion is further accelerated by droplets suspended in the accelerating steam that impinge on walls, T-joints, or valves, further damaging protective oxide layers on pipe walls. Droplet impingement and flow accelerated corrosion were studied using a modified rotating cylinder electrode system coupled with electrochemical impedance spectroscopy. Surface liquid films caused by droplet impingement were found to correlate directly with flow accelerated corrosion caused by condensate films. In the absence of a stable liquid film, droplet impingement increased corrosion rates and resulted in pit formation. Select corrosion inhibitors were found to be ineffective under flow accelerated corrosion or droplet impingement.

Preparation of activated carbon from Dipterocarpus alatus fruit and its application for methylene blue adsorption.

Activated carbons were prepared from three parts of Dipterocarpus alatus fruit (wing, endocarp and pericarp), an abundant and renewable waste in Southeast Asia, by chemical activation using ZnCl2, FeCl3, H3PO4 and KOH and physical activation using CO2 and steam. This study indicated that activated carbon prepared from Dipterocarpus alatus fruit could be employed as a promising adsorbent for the removal of methylene blue from aqueous solution. ZnCl2 activation led to an activated carbon with a surface area of 843 m2 g−1 and was able to remove methylene blue from aqueous solution. Adsorption studies were performed and analysed using Langmuir and Freundlich isotherm equations. Adsorption data demonstrated an excellent fit with the Langmuir isotherm model, with the maximum adsorption capacity of 269.3 mg g−1 at equilibrium. Pseudo-first order and pseudo-second order kinetic models were used in this study to describe the adsorption mechanism. The results show that methylene blue adsorption is pseudo-second order, indicating that liquid film diffusion, intra-particle diffusion and surface adsorption coexisted during methylene blue adsorption on the activated carbon. The activated carbon prepared from Dipterocarpus alatus fruit is a low cost and effective adsorbent with a fast rate for the removal of methylene blue from aqueous solutions when compared with a number of activated carbons studied in the literature.

Central uprising sheet in simultaneous and near-simultaneous impact of two high kinetic energy droplets onto dry surface and thin liquid film

Droplet impact on both dry and wet surfaces is present in several applications, and often multiple droplets, instead of one single droplet, are involved. This paper focuses on the problem of two-droplet impingement on dry and wet surfaces with two Weber numbers (We) of 115 and 230, corresponding to two Reynolds numbers (Re) of 6100 and 8620, respectively. We study impact dynamics phenomena, compare simultaneous and time-delayed impact dynamics of two droplets, and investigate the time evolution of a central uprising sheet formed between the two droplets impinged on dry or wet surfaces, a problem that has been barely studied. A central uprising sheet forms between two impinging droplets at sufficiently high Re and We and short droplet to droplet spacing (high kinetic energy at the point of spread contact). Three different shapes for the central uprising sheet are observed for two droplet impact on a dry surface with various time delays: ordered two-dimensional (2D) semilunar shape (vertical and inclined), curved or C-shaped three-dimensional (3D) shape, and irregular splash. Our experiments show that the central uprising sheet undergoes splashing under conditions not predicted by existing correlations; also, during the early formation of the central uprising sheet, the effect of gravity force on the sheet evolution is negligible. Mixing and surface waves are also studied, taking advantage of liquids with three different colors.

Wave characteristics of the falling liquid film in the development region at high Reynolds numbers

The wave characteristics of the turbulent falling liquid film in the development region along the vertical Perspex plate were studied by using an Ultrasonic Doppler Velocimetry (a non-intrusive technique). The instantaneous thickness of the falling film at the center of the plate with axial distance ranges from 50 mm to 1300 mm was measured and the Rel (the liquid film Reynolds number) ranges from 2.28 × 103 to 1.43 × 104. The results show that the mean thickness, wave frequency and amplitude increase while the growth rate decreases with the increase of Rel. The Probability Density Function of the liquid film instantaneous thickness tends to an approximately normal distribution with increasing Rel. Furthermore, the Plateau-Rayleigh instability can cause the breakup of the liquid film surface and resist the increase of the liquid film thickness, which results in the fluctuation range of the liquid film approximate to a fixed value when the Rel > 1.20 × 104.

A numerical analysis of air entrapment during droplet impact on an immiscible liquid film

The air entrapment during droplet impingement is responsible for spontaneous droplet bouncing on an arbitrary solid surface at low Weber numbers. However, for the impact on liquid film surfaces, the outcome would significantly change, making it more favorable for the fabrication of non-wetting lubricant impregnated surfaces (LIS/SLIPS). In this paper, we describe a problem associated with the impact on a liquid surface using a three-phase flow model that captures the details of the gas layer thickness and dynamics of fluid motions. The numerical model was based on the finite volume solution coupled with the volume of fluid method to track the phases. The model was validated with the analytical solution. Consequently, the numerical tool was utilized to investigate the thickness of the entrapped air during the impact process while the behavior of droplet and the immiscible liquid film was quantitatively measured. The morphology of the interfacial gas layer was analyzed for key parameters including impact velocity and film thickness. It was observed that the presence of liquid film can reduce the probability of rupturing the gas layer. The results for the profile of liquid film during droplet impact illustrated that the effect of film thickness can considerably influence the bouncing behavior.

Surface orientation effects on heat transfer performance of spray cooling

In this study, the heat transfer performances of spray cooling for upward, vertical, and downward surfaces under different nozzle-to-surface distances, inlet pressures, nozzle types, and surface areas were investigated experimentally. Almost no difference in heat transfer performance can be found among the different orientations in single-phase and two-phase regimes. A considerable orientation dependency of the critical heat flux (CHF) can be observed in medium-spray volumetric fluxes: the CHF of the vertical surface is higher than that of the upward and downward surfaces. The orientation dependency of the CHF reduces as the inlet pressure increases, implying that an increase in droplet diameter or decrease in droplet velocity would enhance this dependency. The surface orientation has no significant effect on surface liquid film flow and temperature distribution. In this paper, interaction between the vapour and spray (or surface wall) is proposed and verified to be the main mechanism that affects orientation dependency by intermittent spray cooling. A macrolayer model for the CHF is semi-empirically introduced to account for the orientation effects and provide a quantitative evaluation of these effects. Finally, based upon this model, a general correlation for the CHF is proposed.

Effect of thermocapillary instability on liquid film breakdown

The film of water flowing down along a vertical surface with a heater was studied experimentally at Re = 10–50. The initial temperature of the water film varied from 15 to 70 °C and heat fluxes on the heater varied from 0 to 6.5 W/cm2. Simultaneous measurements of the film thickness and surface temperature carried out. The effect of development of thermocapillary instability type A on wave amplitudes, deformation of the liquid film surface, and formation of the first stable dry spot on the heater was investigated. It is shown that when the longitudinal temperature gradients reach values larger then 7–10 K/mm formation of thermocapillary structures begins. At the leading edge of the heater, X/mm < 10–15, the thermocapillary structures in the form of a series of rivulets with a thin film between them are formed on the surface of residual liquid film after wave front propagation. The distance between the rivulets is λ/mm = 10. It is shown that the formation of the first stainable dry spots occurs in areas where deformation liquid film reaches its maximum value, and the value of the wave’s amplitudes decreases. The interaction of waves with thermocapillary structure type A leads to an increase in the critical heat flux corresponding to the liquid film breakdown on 75% in comparison with the data known in literature. A new mechanism of action on the film flow was first identified and studied in detail.

Experimental Investigations of Spray Characteristics of a Pressure-Swirl Atomizer

Spray characteristics of a pressure-swirl atomizer are investigated using high-speed shadowgraph technique under different pressure drops (Δ P) and fuel temperatures ( T). An image processing method is developed using MATLAB. The results illustrate that the mass flow rate climbs with the increase of Δ P, while the discharge coefficient ( Cd) decreases firstly and then climbs with the increase of Δ P. Δ P has larger effect on the cone angle relative to fuel temperature. With the increase of Δ P, the shape of liquid film changes from ‘onion’ to ‘tulip’ and finally be fully developed spray cone. Meanwhile, the surface of liquid film becomes smoother with the increase of Δ P. The average breakup length climbs, then decreases to nearly a constant value with the increase of Δ P, which is induced by the “Impact wave,” surface wave, and turbulent energy. There are little differences on the shape of the liquid film under different temperatures, and temperature has different influence on breakup length under different Δ P. Both the fuel temperature and Δ P have significant impact on the surface wavelength ( λ) and velocities ( U, V) of surface wave. The width of fuel stream becomes larger with the increase of Δ P and fuel temperature. The results can further deepen the understanding of spray characteristics of pressure-swirl atomizer.

An Adaptable Device for Scalable Electrospinning of Low- and High-Viscosity Solutions

This paper summarizes the design and construction of an adaptable electrospinner capable of spinning fluids over a large range of viscosities. The design accommodates needless electrospinning technologies and enables researchers to explore a large range of testing parameters. Modular parts can be exchanged for alternative versions that adapt to the research question at hand. A rotating drum electrode immersed halfway into a solution bath provides the liquid film surface from which electrospinning occurs. We tested and assessed several electrode designs and their electrospinning performance at higher (&lt;500 poise) viscosities. Relative humidity was found to affect the onset of electrospinning of highly viscous solutions. We demonstrate robust device performance at applied voltage up to 90 kV between the electrospinning electrode and the collector. Design and fabrication aspects are discussed in practical terms, with the intent of making this device reproducible in an academic student machine shop.

Limitations of Matching Condensing Film Profile on a Micro Fin with the Groove: Critical Effect of Disjoining Pressure

ABSTRACTCondensation on a fin top terminating with a groove involves several simultaneous phenomena including vapor–liquid boundaries whose shapes are unknown a priori, fluid flow due to capillary and disjoining pressure gradients, and condensation over thin films. This problem occurs in grooved heat pipes, where the condensation is predominantly present on fin tops due to the thinner liquid film – having a lower thermal resistance compared to inside the groove where the fluid is substantially thicker. Majority of the studies in the literature assume an approximate profile for the liquid film surface and apply an integral balance for conservation laws, accounting for the effect of the capillary pressure only. In addition, this approximate profile is matched with the liquid profile inside the groove, which serves as a boundary condition. Although intuitive, validity of the matching is not straightforward, and its limitations have never been discussed in the literature, despite the presence of experimental findings to the contrary. In the current study, the effect of disjoining pressure and matching conditions with the groove is investigated using a comprehensive model. The results suggest that for small temperature differences and small slopes, the effect of disjoining pressure is non-negligible, and beyond limiting values of edge angles, the effect of disjoining pressure precludes solutions where the fin top film matches the groove in a smooth transition.

A Mechanistic Model of Liquid Film Movements in Pipe Elbows for Annular Flow

A mechanistic model of film movements is developed based on the treatments on the annular flow field. The initial conditions at the inlet are determined by adopting a validated film thickness correlation of fully developed upward annular flow in vertical pipes. The overall pressure gradient is assumed to be uniform all along the axial distance within the elbow and the static pressure is also uniform on every cross section. The axial velocities of the liquid film and the core region are both uniform on the cross-sectional plane. The droplets are assumed to travel in straight lines normal to the inlet plane until colliding on and absorbed by the liquid film surface. The liquid film motion is divided into the axial and radial directions. Energy conservation law and Newton's second law are, respectively, used in the two directions. The film motion calculation is executed by using a discrete method with an explicit solution. The average film thickness and the circumferential thickness distribution on an arbitrary cross section can be obtained for the given flow conditions. The mechanistic model is verified by comparing the predicted circumferential distribution of film thickness with three series of experimental data from the literature. Parametric studies are also conducted to investigate the parameter effects and the range of application. The present work proves that the variation and distribution of film thickness within the elbows can be efficiently described by the mechanistic model.

Effects of microstructure formation on the stability of vapor-deposited glasses

Glasses formed by physical vapor deposition (PVD) are an interesting new class of materials, exhibiting properties thought to be equivalent to those of glasses aged for thousands of years. Exerting control over the structure and properties of PVD glasses formed with different types of glass-forming molecules is now an emerging challenge. In this work, we study coarse-grained models of organic glass formers containing fluorocarbon tails of increasing length, corresponding to an increased tendency to form microstructures. We use simulated PVD to examine how the presence of the microphase-separated domains in the supercooled liquid influences the ability to form stable glasses. This model suggests that increasing molecule tail length results in decreased thermodynamic stability of the molecules in PVD films. The reduced stability is further linked to the reduced ability of these molecules to equilibrate at the free surface during PVD. We find that, as the tail length is increased, the relaxation times near the surface of the supercooled equilibrium liquid films of these molecules are slowed and become essentially bulk-like, due to the segregation of the fluorocarbon tails to the free surface. Surface diffusion is also markedly reduced due to clustering of the molecules at the surface. Based on these results, we propose a trapping mechanism where tails are unable to move between local phase-separated domains on the relevant deposition time scales.

Inwardly Rotating Spirals in a Nonoscillatory Medium.

We report the spontaneous formation of spiral patterns observed at a downward-facing free surface of a horizontal liquid film. The surface is unstable to the Rayleigh-Taylor instability and the resulting liquid discharge from the film can occur in the form of propagating liquid curtains. They are born at the film circular periphery and exhibit patterns of inwardly rotating spiral arms. With the help of a phenomenologically constructed cellular automaton, we show that the patterns arise from the phase locking leading to periodic liquid discharge at constant flow rate over the whole film surface.

Contact angle and heat transfer characteristics of a gravity-driven film flow of a particulate liquid metal on smooth and rough surfaces

In the present research, an experimental investigation was conducted to identify the behaviour of gallium as a promising heat transfer fluid for cooling a high heat flux surface. Bismuth nanoparticles were dispersed within the gallium to compensate its poor thermal conductivity and to enhance its wettability. A high-fidelity experimental setup was fabricated to provide conditions for measuring the contact angle between the gallium and the surface and also to measure the heat transfer coefficient of a free surface liquid film flowing on the heating surface. Influence of different operating parameters including tilt angle of the surface, heat flux of the surface, surface roughness and time on the contact angle and heat transfer coefficient of the liquid film was experimentally investigated. Results showed that with an increase in the tilt angle of the surface, higher heat transfer coefficient can be achieved, which was attributed to the enhancement of the terminal velocity of the liquid metal. Also, an increase in the roughness of the surface intensified the contact angle of the liquid metal and caused a decrease in the wettability. It was also found that the contact angle of gallium was a function of time such that it decreased with time spanning and reached a constant value.

Heat transfer characteristics of obliquely dispensed evaporating falling films on an elliptic tube

This study examines the evaporation heat transfer characteristics of obliquely dispensed falling films over a horizontal elliptic tube. The theoretical model applied the volume-of-fluid (VOF) method to simulate the spreading development of falling liquid films for resolving the distributions of velocity, temperature, volume fraction in conjunction with the convective heat transfer coefficient. Specifically, a user defined function (UDF) was formulated to treat the evaporative effect at the liquid film surface to the environment. The predicted heat transfer coefficients and film thickness distributions around the circular and elliptic tube surfaces were validated by the experimental data from the available literatures. Based on the test results via comparing different flow models, the SST k-ω model provides the best capabilities in terms of the better prediction accuracy and less calculation time. CFD simulations were then extended using the validated computational model to explore the effects of inlet liquid mass flow rate and obliquely dispensed angle on the liquid film distribution and heat transfer characteristics over the horizontal elliptic tube. At a low liquid flow rate of 0.093 kg/m-s, the liquid films failed to fully enfold the tube surface with the appearance of dry areas, causing the substantial decline of heat transfer effectiveness. The condition at a high liquid flow rate of 0.186 kg/m-s achieved the average heat transfer coefficient up to 4.18 kW/m2 K, indicating good thermal performance similar to literature reports. Reasonable average heat transfer coefficients can be attained for the variation of dispensed angle within 30°.

Removal of ciprofloxacin from aqueous solution by rabbit manure biochar

ABSTRACT Biochar was prepared from rabbit faeces at 400°C, 500°C, 600°C, and 700°C, respectively (labelled RFB400, RFB500, RFB600, and RFB700, respectively), and was characterized by elemental analysis, BET, SEM and FTIR. The adsorption factors, kinetics, isothermal adsorption and thermodynamics of the adsorption properties were investigated in batch experiments. The results showed that RFB possessed a large specific surface area and was rich in pore structure, and the aromaticity and stability increased with the pyrolysis temperature of the biochar. When the solution pH was 11, adsorption achieved equilibrium at approximately 180 min. The kinetic data were well-represented by the pseudo-second-order model, indicating that the adsorption rate was jointly controlled by liquid film diffusion, surface adsorption and intra-particle diffusion. The results of isothermal adsorption and thermodynamics showed that the adsorption behaviour of CIP (ciprofloxacin) onto RFB was better fitted with the Langmuir model, and the adsorption process was spontaneous and endothermic. FTIR studies showed that RFB was rich in oxygen-containing functional groups and that hydrogen bonds and π-π bonds were closely related to the adsorption process. This work showed that the rabbit faeces-derived biochar has promise as an effective adsorbent to remove ciprofloxacin from wastewater.

Multi-phase smoothed particle hydrodynamics modeling of supercooled large droplet dynamics for in-flight icing conditions

Understanding the dynamics of a single large water droplet is needed for accurate simulations of the in-flight icing phenomenon. Obtaining information on the ratio of ejected to deposited water and the post-impact droplet distribution should improve the numerical modeling of the bulk of impinging droplets. In this study, a weakly compressible multi-phase Smoothed Particle Hydrodynamics (SPH) method with shifting algorithm and surface tension model is presented to simulate the single droplet dynamics. The validity of the approach has been proved by modeling the classical problems of Rayleigh–Taylor instability, dam break, and droplet formation by comparing against other numerical and experimental data in the literature. Finally, droplet impingement on a liquid film and dry solid surface has been simulated and compared against the experimental data. The effect of impact angle and film thickness on the crown formation is studied to demonstrate the importance of modeling SLD impingement for in-flight icing conditions.

Adsorption characteristics of methylene blue by biochar prepared using sheep, rabbit and pig manure.

Sheep manure biochar (SMB500), rabbit faeces biochar (RFB500) and pig manure biochar (PMB500) prepared by controlled thermal decomposition at 500°C were used to adsorb methylene blue (MB) in water. Elemental analysis, BET and SEM results showed that the specific surface area, total pore volume and average pore diameter of SMB500 were 7.59, 4.20 and 1.16 times greater than those of RFB500, which were also 12.02, 6.88 and 1.37 times greater than those of PMB500, respectively. SMB500 had stronger stability and aromaticity. When the initial concentration of MB was 50mgL-1 and pH was 11, adsorption achieved equilibrium at approximately 210min. The adsorption followed pseudo-second-order kinetics (R2 > 0.96), indicating that liquid film diffusion, surface adsorption and intraparticle diffusion all contributed to the adsorption rate. The results of isothermal adsorption showed that the adsorption performance of SMB500 was more consistent with a Freundlich model, whereas the performance of RFB500 and PMB500 was more consistent with a Langmuir model with a maximum adsorption capacity of 53.68 to 238.31mgg-1. Thermodynamic and FTIR studies showed that the adsorption of MB on SMB500, RFB500 and PMB500 was spontaneous and endothermic, and hydrogen bonds and π-π bonds were closely related to the adsorption process. The results of regeneration show that the optimal number of cycles for SMB500, RFB500 and PMB500 are 8, 5 and 3, respectively.