Droplet Flow Research Articles

Optimal placement of droplets for UV nanoimprint lithography

Optimizing the locations and sizes of droplets are key to reduce defects and increase throughput of ultraviolet nanoimprint lithography (UVNIL). Previously, simulation models describing the flow and coalescence of multiple fluid drops were developed and used to predict the fluid flow during spreading and feature filling. However, the effect of small channels on the resulting capillary pressure is neglected even though the flow along small channels or grooves is encountered in UVNIL. In this study, an accurate method to estimate capillary pressure boundary conditions at the fluid-air interface is developed along with fluid simulation. A method to estimate and compute the anisotropic capillary pressure for templates with line and space patterns is presented. It is found that the droplets flow faster in the direction of the pattern not only because of anisotropic permeability but also the nanostructure of the template, which further propels anisotropic spreading of the droplets by exerting larger capillary pressure. Using the new capillary pressure boundary condition, the simulation of multidrop spreading in UVNIL is performed to study the effects of anisotropic capillary pressures caused by the template structure. A simulation approach to determine optimal droplet placement was developed. From the simulation, a general rule of thumb is developed: delaying droplet merging as much as possible and reducing the number of merging events.

Droplet factories: Synthesis and assembly of metal nanoparticles on magnetic supports

The interface between two immiscible liquids represent an ideal substrate for the assembly of nanomaterials. The defect free surface provides a reproducible support for creating densely packed ordered materials. Here a droplet flow reactor is presented for the synthesis and/or assembly of nanomaterials at the interface of the emulsion. Each droplet acts as a microreactor for a reaction between decamethylferrocene (DmFc) within the hexane and metal salts (Ag+/Pd2+) in the aqueous phase. The hypothesis was that a spontaneous, interfacial reaction would lead to the assembly of nanomaterials creating a Pickering emulsion. The subsequent removal of the solvents showed how the Ag nanoparticles remain trapped at the interface and retain the shape of the droplet, however the Pd nanoparticles were dispersed with no tertiary structure. To further exploit this, a one-step process where the particles are synthesised and then assembled into core-shell materials was proposed. The same reactions were performed in the presence of oleic acid stabilised iron oxide nanoparticles dispersed within the hexane. It was shown that by changing the reaction rate and ratio between metal and iron oxide a continuous coating of metal nanoparticles can be formed on top of an iron oxide microsphere, or form a uniform composite. These insights offer a new method and chemistry within flow reactors for the creation of palladium and silver nanoparticles. We use the technique to create metal coated iron oxide nanomaterials but the methodology could be easily transferred to the assembly of other materials.

Methodology of Measuring Spraying the Droplet Flow of Polymers from Nozzle

With a purpose for textile material coating applications, a process of spraying the polymer flow is considered. The equipment for determining the polymer drop size is designed. The formulas for estimating the size of polymer droplets are validated. For controlling and measuring the polymer drop size in real time, the multi-frequency phase method is suggested to be used. Classification is needed to find a category, within which the measurement will be done. Photos of various particles are obtained while experiments of spraying polymers are conducted. The sizes of those particles are determined. It is ascertained with the laser measurement and the corresponding software that when polymer drops get onto a textile material, they cool rapidly. The quality of the material is maintained. For properly spraying the polymer, the pressure of 6 MPa and the nozzle diameter of 2 mm are sufficient. The structure of the textile material along with the coating is maintained best with these parameters.

Flow and heat transfer characteristics of droplet obliquely impact on a stationary liquid film

The flow and heat transfer characteristics of a droplet obliquely impact on a stationary liquid film is numerically studied by using a coupled level set and volume of fluid method (CLSVOF). The effects of impact angle, Weber number, and film thickness are analyzed. As compared with the normal impact, the flow features and surface heat flux distribution for oblique impact are asymmetric and where the secondary droplets only can be observed on one side. Corresponding mechanisms have also been interpreted. The maximum value of the local surface heat flux decreases and moves to the right with the increase in impact angle, while the minimum local surface heat flux on both sides increases as the impact angle increases. With a greater Weber number or a thinner film thickness, the maximum local surface heat flux is higher, while the minimum local surface heat flux on both sides appears to be an opposite trend. In addition, the impact angle has little effect on the average surface heat flux.

Experimental examination of utilizing novel radially grooved surfaces in the evaporator of a thermosyphon heat pipe

The application of heat pipes with flat evaporators in cooling electronic devices has attracted a lot of attention in recent years. Increasing the rate of heat transfer in their evaporator by utilizing structured surfaces is considered as a prominent method for reducing the thermal resistance of the heat pipes. In this study, the performance of a thermosyphon heat pipe with novel radially rectangular-grooved and radially inclined triangular-grooved evaporator surfaces was evaluated experimentally. It is hypothesized that the radial grooves may enhance the performance by inducing rotational motions and increasing the heat transfer coefficients. Based on the results, the optimum filling ratio of the heat pipe was 20% for all of the considered surfaces. The average thermal resistance of radially rectangular-grooved and radially inclined triangular-grooved surfaces with the same heat transfer area improved by 22.9% and 36.13%, respectively, compared to the smooth surface. Further, experimental heat transfer coefficients on the proposed radially inclined triangular grooves can be predicted by theoretical correlations with an average deviation of less than 4%. Visualization of the boiling process of droplets on different surfaces indicated the existence of the rotational flow of droplets on the radially inclined triangular-grooved surface.

Numerical analysis of the mean structure of gaseous detonation with dilute water spray

Two-dimensional (2-D) numerical simulations based on the Eulerian–Lagrangian method that take droplet break-up into account are conducted to clarify the mean structure of gaseous detonation laden with a dilute water spray. The premixed mixture is a slightly diluted stoichiometric hydrogen–oxygen mixture at low pressure. The simulated results are analysed via 2-D flow fields and statistical Favre spatiotemporal averaging techniques. Gaseous detonation with water droplets (WD) propagates stably with a velocity decrease compared with the dry Chapman–Jouguet speed. The mean structure of gaseous detonation with dilute water spray shares a similar structure as the one without water spray. However, the hydrodynamic thickness is changed due to the interaction with water spray. Overall interphase exchanges (mass, momentum and energy) that take place within the hydrodynamic thickness induce a decrease of the detonation velocity and lower the level of fluctuations downstream of the mean leading shock wave. Droplet break-up occurs downstream of the induction zone and in our case, the water vapour from the evaporation of water spray does not affect the reactivity of gaseous detonation. The laminar master equation for gaseous detonation laden with inert WD shows that the hydrodynamic thickness should rely on the gaseous sound speed, and works well as the working mixture is weakly unstable and its cellular structure is regular. The droplet flow regimes and break-up modes have also been determined. The characteristic lengths of detonation and interphase exchanges have been ordered under the present simulation conditions and have been shown to be intimately intertwined.

Experimental Investigation of Droplet Entrainment in the Feed Section of Distillation Columns

AbstractDroplet entrainment was investigated in the feed section of distillation columns for multiple feed inlet devices under different flow conditions. Entrainments from two‐phase flow regimes were measured in the feed section of a column with an air and water system. A new feed inlet device named demister flash box was developed. Measurements with two‐phase flow regime in the feed pipe indicated that the demister flash box had the lowest entrainment. Experiments with a surfactant in water due to low surface tension demonstrated that the entrainment was higher compared to water. In addition, entrainment determinations with high droplet flow rate from a spray nozzle in feed were conducted. The results indicated that the V baffle had the lowest entrainment.

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.

Simulation and experimental research on droplet flow characteristics and deposition in airflow field

In order to study the motion law of droplet flow under the airflow action of long-range air-blast sprayers, a CFD-based 3D model was established for the air-blast sprayer duct and its external airflow field, and the discrete phase model was introduced to simulate the motion of droplet flow in the airflow. The simulation data of the droplet flow trajectory, droplet flow parameters and droplet deposition were obtained by establishing the monitoring sections and bilateral coupling calculation in the airflow field. Results showed that gravity had an obvious effect on droplets and large droplets settled faster. Some of the larger droplets were formed by polymerization in droplets motion. The smaller droplets were transported further along with the airflow, and the long-range sprayer has a significant effect on the directional transport of small droplets. Besides, the spraying swath in the direction perpendicular to the range enlarged gradually with the increase of the spraying range. At the end of the range, the diffuse and drifting of the droplets were dominant. Given that the outlet airflow velocity of the sprayer duct was 25.01 m/s and the spray pressure 1.8 MPa, the maximum motion distances of aerosol, mist, fine mist and coarse mist in the airflow field were 18.5 m, 19.5 m, 17.5 m and 10.5 m, respectively. Droplet size and number as well as number density and volume density of droplet flow on all monitoring sections showed a regression function with changes in the distance of the spraying range. The simulation results of the model adopted in this paper were verified by Chi-square test between the simulation results of the droplet deposition and the spray measurement results. Research results provide a new method for the study of orchard air-blast spraying technology and references for the optimization of spraying technology. Keywords: droplet flow, droplet density, air-blast sprayer, computational fluid dynamics, droplet deposition DOI: 10.25165/j.ijabe.20201306.5455 Citation: Xiahou B, Sun D Z, Song S R, Xue X Y, Dai Q F. Simulation and experimental research on droplet flow characteristics and deposition in airflow field. Int J Agric & Biol Eng, 2020; 13(6): 16–24.

On the Problem of Rejection of Low-Potential Heat from High-Power Space Systems

Consideration has been given to the problems of organization of the operating process in spacecraft heat-exchange devices for rejecting low-potential heat to outer space. It has been shown that as far as the heat-rejection efficiency and weight-size characteristics are concerned, droplet radiant coolers outperform all the existing designs of heat exchangers. The use of droplet radiant coolers at heat-rejection powers exceeding 100 kW, i.e., at megawatt powers, is particularly efficient. A description of individual elements has been given, and results of investigations of the processes of generation of flows of monodisperse droplets and their collection under ground conditions and under microgravity and high-vacuum conditions have been presented. The latter were investigated from the results obtained in experiments on spacecraft.

Experiments and analytical solutions of light driven flow in nanofluid droplets

Adding nanoparticles with high light response characteristics to a light-transmitting fluid medium can form a light-driven nanofluid and achieve efficient use of light energy. This paper conducts the experimental observation and theoretical analysis of the light driven nanofluid flow behavior, which is the theoretical basis for achieving the precise control of optical drive nanofluid. To realize the efficient conversion of light energy into kinetic energy, here, the motion of Fe<sub>3</sub>O<sub>4</sub> particles with a diameter of 300 nm in droplets induced by the Marangoni effect is studied under different light sources by using the particle image velocimetry (PIV). The experimental results show that when the number density of particles is higher than the critical value, the vertical vortices with symmetrical structure can be induced. At the bottom of the droplet, the particles move from the periphery to the center of droplet, and at the top of the droplet, the particles move from the center to the periphery of droplet. In addition, the frequency of light source and the number density of particles are the dominant factors in this process. Subsequently, for the light driven nanofluid experiment in this paper, the analytical solution of the flow field distribution is achieved by using the Stokes equation and the surface tension gradient boundary condition. The analytical solution of the flow field distribution obtained here is consistent with the experimental results, confirming the validity of the quantitative theory. Finally, the correlation between various driving modes, including surface tension at the top surface, surface pressure at the bottom surface or concentrated light radiation force in bulk phase, is discussed. This research provides theoretical support for the precise regulation of flow behavior and efficient conversion of light energy in the optical microfluidic system.

Experimental investigation of droplet flow in wire mesh demisters: comparing plastic and metallic types

Experimental investigation of droplet flow in wire mesh demisters: comparing plastic and metallic types

Transition of Marangoni convection instability patterns during evaporation of sessile droplet at constant contact line mode

Due to continuous variation of contact angle of a sessile droplet during evaporation at constant contact line mode, the behaviors of Marangoni convection inside the droplet would inevitably vary with evaporation. In order to understand these behaviors, the Marangoni convection instability patterns and their transition phenomena in a sessile droplet during evaporation are investigated through a series of experiments. For easily volatile droplet of methanol the hydrothermal waves occur firstly near the three-phase contact line and then they are transformed to Bénard-Marangoni cells when the contact angle is lower than a critical value. The hydrothermal waves finally disappear with only Bénard-Marangoni cells remaining in the droplet when the contact angle is below the second threshold. For highly volatile droplet of R-113 the stationary longitudinal rolls appear firstly and then they are replaced by the Bénard-Marangoni cells. The basic linear flow in R-113 droplet caused by fast evaporation is a crucial reason to form the stationary longitudinal rolls in it. The critical conditions for the transitions between these kinds of the Marangoni convection instability patterns are determined and the underlying mechanisms of these transition phenomena are analyzed carefully.

A microfluidic platform for screening and optimization of organic reactions in droplets

We report an automated microfluidic system for screening and optimization of chemical reactions performed inside microliter liquid droplets. The system offers precise control over generation, merging and flow of droplet “micro-reactors” and over reaction conditions, including the volumes of the reagents, temperature and time. The high level of control coupled with the ability to quickly screen multiple reaction conditions allow us to thoroughly monitor the impact of input parameters on the yield of the reaction. In addition, the reagent consumption is kept remarkably low. As an exemplary use of our system we demonstrate a comprehensive study of acid-catalyzed (para-toluenesulfonic acid, p-TsOH) model imine formation (condensation of ortho-nitrobenzaldehyde and phenylethylamine) in an organic solvent (ethanol). By use of novel screening methods described herein, we unfold that the acid-catalyzed model imine formation in the organic medium can be considered as an assembly of acid-mediated and non-catalyzed reactions, both of which accelerate with increasing temperature.

The experimental investigation of water spray disperse parameters behind the water injecting system at the aircraft engine inlet by applying PSV

To provide an aircraft engine certification tests for water ingestion, it is necessary to control accurately the parameters of the water drops generated by the water injecting device simulating rain at the engine inlet. The paper presents the results of autonomous tests of a single-channel centrifugal nozzle for the water injecting device. The tests purpose is to investigate the disperse parameters of the water spray for the typical operation modes of the nozzle as a part of the water injecting system. Tests were carried out at the experimental setup using the PSV (Particle Shadow Velocimetry) technique at three typical nozzle operation modes. As a result of the tests, the distribution graphs of the D10, D32 and Dv50 diameters, axial velocity (U) and mass flow (Mabs) of water droplets over the radius of the spray pattern have been obtained. To assess Dv50 mean median diameter of spray droplets the cumulative distribution functions of the droplets mass fraction depending on the size for each of the operation modes of the nozzle have been plotted. On the basis of these functions, the mean median diameters of water spray droplets have been calculated.

Steady flow of pressure-driven water-in-oil droplets in closed-open-closed microchannels

Open microfluidics is an emerging field of bio/medical applications that need direct energy/matter exchange between microfluids and environment. This paper presents the design, simulation, fabrication, and test of a microfluidic chip for a water-in-oil (WiO) two-phase flow in closed-open-closed microchannels. The chip, fabricated from PDMS using soft lithography, consists of a flow-focusing structure for WiO droplet generation and a long closed-open-closed channel for droplet flow. A negative pressure applied to the end of the channel is used as the driving force for WiO droplets to flow through the open channel. It is found that the negative pressure that is capable of driving a steady flow for a given flow rate, without overflow and air suction, falls into a pressure range instead of being an exact value. The mechanism for the pressure range is investigated theoretically and experimentally and is attributed to the surface tension. Yeast cells have been incorporated in the droplets, and the successful flow through the open channels verifies the function of the chips.

Splat Behaviour Under Substrate Temperature Of 673K Via Coupling Simulation

The process of collision flattening is difficult to evaluate via experiment because is normally under rapid conditioned (in μsec). Via simulation, under compressible flow condition we propose a moving particle semi implicit method (MPS) coupling with finite volume method (FVM) to examine the deformation, splitting and conjoining of liquid. The substrate temperature has been constant at 673K. The result shows that the phenomena of splat behavior have been successfully being observed. The validation of droplet flow field also being confirm visually. Besides, the substrate temperature also influences the shape of the splat.

Supersonic flow of two-phase gas- droplet flows in nozzles

For practical applications, the description of processes occurring during the flow of two-phase gas-liquid mixtures requires a simple physical and mathematical model that describes the behavior of a two-phase medium in the entire range of phase concentrations changes and in a wide range of pressure changes. Problems of this kind arise in various branches of industry and technology. In the space industry, one often has to deal with the movement of various gases in rocket nozzles, consider the combustion, condensation of various vapors on the nozzle walls and their further impact on the velocity sublayer at the nozzle wall. The large acoustic effect arising from the engines affects the gas-liquid mixture in the nozzles of rocket engines. In the metal industry, metal cooling occurs with the help of nozzles in which the emulsion mixture is supplied under high overpressure. But this is only a short list of applied issues in which one has to deal with a problem of this type. The paper presents the results and directions of study of the problems of two-phase dispersed gas-droplet flows in the nozzles. The main methods of investigation of two- phase heterogeneous flows are described. The main characteristics of heterogeneous two-phase flows in the nozzles, which were confirmed by experimental results, are presented. The calculation of the air-droplet flow in the Laval nozzle is given. The technique, which is based on integral energy equations for two-phase dispersed flows, is described. The main problems and questions concerning the further description and studying of two-component flows are stated.

Understanding and Interpreting Laser Diagnostics in Flames: A Review of Experimental Measurement Techniques

There is a wealth of existing experimental data of flames collected using laser diagnostics. The primary objective of this review is to provide context and guidance in interpreting these laser diagnostic data. This educational piece is intended to benefit those new to laser diagnostics or with specialisation in other facets of combustion science, such as computational modelling. This review focuses on laser-diagnostics in the context of the commonly used canonical jet-in-hot-coflow (JHC) burner, although the content is applicable to a wide variety of configurations including, but not restricted to, simple jet, bluff body, swirling and stratified flames. The JHC burner configuration has been used for fundamental studies of moderate or intense low oxygen dilution (MILD) combustion, autoignition and flame stabilisation in hot environments. These environments emulate sequential combustion or exhaust gas recirculation. The JHC configuration has been applied in several burners for parametric studies of MILD combustion, flame reaction zone structure, behaviour of fuels covering a significant range of chemical complexity, and the collection of data for numerical model validation. Studies of unconfined JHC burners using gaseous fuels have employed point-based Rayleigh-Raman or two-dimensional Rayleigh scattering measurements for the temperature field. While the former also provides simultaneous measurements of major species concentrations, the latter has often been used in conjunction with planar laser-induced fluorescence (PLIF) to simultaneously provide quantitative or qualitative measurements of radical and intermediary species. These established scattering-based thermography techniques are not, however, effective in droplet or particle laden flows, or in confined burners with significant background scattering. Techniques including coherent anti-Stokes Raman scattering (CARS) and non-linear excitation regime two-line atomic fluorescence (NTLAF) have, however, been successfully demonstrated in both sooting and spray flames. This review gives an overview of diagnostics techniques undertaken in canonical burners, with the intention of providing an introduction to laser-based measurements in combustion. The efficacy, applicability and accuracy of the experimental techniques are also discussed, with examples from studies of flames in JHC burners. Finally, current and future directions for studies of flames using the JHC configuration including spray flames and studies and elevated pressures are summarised.

Lattice Boltzmann Simulation on Droplet Flow through 3D Metal Foam

The hydrodynamics of droplets passing through metal foam is investigated using the lattice Boltzmann method (LBM). The accurate 3D porous structure for the simulation is generated by X-ray micro-computed tomography. The simulated results are in good agreement with the experimental ones using high-speed video. The simulated results show that for droplets passing metal foam, there is a critical capillary number, Cac (around 0.061), above which the droplet continues to deform until it breaks up. The simulated results show that the capillary number, droplet size, pores diameter, and thickness of metal foam have the significant effect of droplets deforming and breaking up when the droplets pass through the metal foam. To avoid the calescence of two droplets at the inlet zone of the metal foam, the distance between droplets should be larger than three times the diameter of the droplet.