Droplet Internal Circulation Research Articles

Numerical Modeling of In-Flight Molten Metal Droplet Dynamics and Particle Oxidation in Atmospheric Plasma Spraying

In Atmospheric Plasma Spraying (APS), it is important to understand the controlling factors dominating molten metal droplets’ reaction dynamics to control final coating compositions and properties. The present paper focuses on a novel Computational Fluid Dynamics (CFD) modeling of mass transfer mechanisms of an inflight molten metal droplet leading to droplet oxidation control during APS. The onset conditions of droplet internal circulation and its effect on the deoxidizer oxidation behavior during APS are theoretically examined. Moreover, rapidly supplying these elements from the inside to the droplet surface requires the rapid switching of the mass transfer mechanism to rapid convective flow due to a very limited in-flight time of less than 1ms. The temperature and velocity variations within the droplet would influence the thermo-physical properties of the droplet and the plasma gas and it controls the onset of the internal circulations within the droplets. A novel User Defined Function (UDF) is developed to capture the onset of Hill’s vortex formation and its effect on the rapid-rate convective mass transfer mechanism during APS. Further, a detailed analysis of particle size effect on in-flight in-situ deoxidation during APS of NiAlC droplets was performed. It is observed that the smaller size droplets will undergo more severe oxidation during a flight than larger size droplets due to more molecules of O2 available for reaction in the smaller droplet.

Experimental and numerical study of droplet formation with Marangoni instability

The Marangoni effect produces some complex phenomena during droplet formation and has an important influence on the mass transfer state. In this paper, the droplet formation process of toluene/acetone/water droplet extraction system accompanied by Marangoni instability was numerically simulated, and the actual mass transfer states of droplets were photographed using schlieren imaging technology. The results reveal that: suspended droplets may have mass transfer states such as diffusion layer, natural convection, jet and turbulence, and the Marangoni instability causes the droplets to jet solute intermittently. Compared to droplets with constant surface tension, the Marangoni effect increases the mass transfer rate during droplet formation, and the Marangoni stress generates Marangoni convection at the interface, whose mutual annexation with the internal circulation leads to droplet oscillations. The Marangoni effect also causes pulsations in droplet vorticity, increasing the solute concentration exacerbates the vortex disorder, while increasing the injection rate may suppress the pulsations, which are physically dependent on the relative advantage of droplet internal circulation over Marangoni convection.

Insights into the breaking and dynamic mixing of microemulsion (W/O) in the T-junction microchannel

In this paper we conduct a numerical simulation study of droplet generation and dynamic mixing in T-junction microchannels to investigate the effects of two-phase flow rate, viscosity of the dispersed phase and wall wettability on the mixing quality of droplets. It is found that both droplet size and contact length of droplets with the wall are key factors affecting mixing. The larger the droplet size, the worse the mixing effect, and the larger the contact length of droplets with the wall, the better the mixing effect. At the same time, the size of the droplet also constrains the contact length of droplets with the wall. The larger the size of the droplet, the greater the contact length of droplets with the wall, so that an equilibrium is reached, and when the size of the droplet is large enough, this equilibrium will be broken. As the viscosity of the dispersed phase increases, the internal circulation of the droplets becomes difficult and the droplets break up later, which reduces the mixing efficiency. As the contact angle increases, the inner wall of the channel becomes more hydrophobic and the viscous shear of the continuous phase dominates the mixing, changing the direction of the droplet's internal circulation. The precondition for the continuous phase viscous shear to dominate the mixing is that, there exists the velocity difference between the continuous phase fluid and the droplet, and there will be no good mixing effect otherwise. The droplets with good mixing quality should meet the following conditions: reduce the viscosity of the dispersed phase, prepare droplets with smaller size, make the droplets contact with the wall as much as possible, and let the viscous shear of the wall dominate the mixing.

Substance transfer behavior controlled by droplet internal circulation

Abstract Internal flow can be induced by the external flow around a droplet. In the present work, the three-dimensional flow pattern and dynamic characteristics of the internal vortex of adhered droplet were demonstrated firstly, and then the substance transfer behavior within the droplet dominated by the internal circulation was revealed by adopting two systems, namely fluorescence reaction and suspended particle capture, and combining with an interface reaction model developed. It was found that the internal circulation could not only transfer the reactants from the inside onto the surface of the droplet quickly, but also carry the particles captured on the droplet surface into the vortex core rapidly in a vortex manner, which could improve the reaction rate and control the capture process. This work uniquely demonstrates fundamental insights into the internal flow pattern of adhered droplet and promotes an understanding of the multiphase droplet-based mass transfer mechanism and particle capture process.

Multiple Bounces and Oscillatory Movement of a Microdroplet in Superhydrophobic Minichannels

A droplet jumping intensification device of droplets surrounded by gas has effectively integrated small droplets (d = 0.30–0.60 mm) generated by an electrohydrodynamics system (EHD) and superhydrophobic minichannel to enhance gas absorption or chemical reaction due to the multiple bounces of the droplets. Droplet internal circulation was formed after impact on the superhydrophobic surface in the process of droplet bounce back and forth, with deformation in the superhydrophobic minichannel, which was like a liquid slug moving in the meandering channels. It was shown that the bouncing frequency went up with the decreasing height of the minichannel, meaning that the bouncing movement became stronger in the confined space. The droplet trajectory could be better controlled by the height of the minichannel, droplet size, and impact Weber number, which was investigated experimentally and analytically.

THE GAS PRESSURE INFLUENCE ON COMBUSTION CHAMBER FLOW CHARACTERISTICS

The atomization process influences combustion. The Sauter Mean Diameter (SMD), the pressure, and the temperature of the combustor play an important role in the progress of the combustion. The objective of this study is to develop a mathematical model to evaluate the effect of the gas pressure on the combustion chamber flow characteristics. Based on this model a computer program has been designed taking into account the effects of droplet internal circulation, change in combustion chamber gas temperature, the non-unitary Lewis number, the changing gas film thickness surrounding the droplet, the slip, the variable thermo-physical properties. The effect of gas pressure on Liquid Reynolds number, gas Reynolds number, non-dimensional radii, Lewis number, droplet velocity and droplet surface temperature is established. Increasing the gas pressure increases the vapor mass fraction, which affects all other parameters (thermal conductivity, diffusivity, specific heat of liquid and gas, mass and heat transfer number) and leads to a decrease in combustion time. The droplet surface is found to reach its maximum temperature in a shorter time if the pressure is higher. By increasing the gas pressure the droplet-evaporating rate increases, which leads to a decrease in evaporating time. Consequently the combustion chamber volume is decreased. Iterations on the chamber dimensions are done until the program converges.

Interfacial Rheology Through Microfluidics

The bulk properties and structural characteristics of emulsions arise substantially from their interfacial rheology, which depends strongly on surfactant mass transfer and its coupling to flow. Typical methods used to measure such properties often employ simpler flows and larger drops than those encountered in typical processing applications. Mass transfer mechanisms are governed by droplet size; therefore experimentation at length scales typical of those encountered in applications is desired. Utilizing a microfluidic approach allows high-throughput experimentation at relevant length scales and with adjustable flow dynamics. Using a microfluidic device that facilitates the measurement of interfacial tension in two-phase droplet flows, particle tracers are also used to determine the droplet internal circulation velocity as a measure of interfacial mobility. Combining these measurements in a single device, the coupling between interfacial tension, interfacial retardation, and surfactant mass transfer is explored and mass transfer coefficients and interfacial mobility are measured for a two-phase system containing a diffusing surfactant. Such a device is also used to probe the deformability of elastic capsules and viscoelastic biological cells.

Modeling of particle formation by spray pyrolysis using droplet internal circulation

A mathematical model is developed to predict the morphology of powders prepared by spray pyrolysis process. For high velocity solution droplets, the contours of constant solute concentration and temperature coincide with the streamlines. Based on this fact, the liquid phase transport equations are simplified and solved, numerically. Variation of droplet surface temperature and solute mass fraction versus time are presented. In addition, the temperature and the solute mass fraction distributions within the droplet are provided. Using the solute mass fraction distribution, the model predicts whether the final particle is fully-filled or hollow with two cavities.

Modeling the gas absorption in a spray scrubber with dissolving reactive particles

The absorption of a gaseous species into a spherical slurry droplet with internal circulation that contains reactive, sparingly soluble microparticles is studied. The droplet recirculation is assumed to be similar to Hill's vortex flow, and the chemical reaction is assumed to be either instantaneous or of second order. Reactive gas species diffusion, chemical reaction and particle dissolution are numerically modeled using droplet recirculation streamlines as a coordinate. Parametric calculations are performed, and are utilized to assess the adequacy of the widely used film theory, and to examine the effect of particle size variation on the absorption rate. A transient model is also developed that utilizes the aforementioned quasi-steady droplet mass transfer and reaction models and calculates the time variation of the average solid particle size in the droplet. Parametric calculations are presented that confirm the importance of particle size and its variation as a result of dissolution. Transient parametric calculations show a declining total absorption rate with time as a result of the shrinkage of the average particle size. The results are everywhere sensitive to droplet internal circulation, however. Partial suppression of droplet internal circulation leads to significant reduction in absorption rate, even with a high concentration of particles. The calculation results thus demonstrate that several parameters should be considered for optimal design of a slurry spray scrubber.

Mechanism of gas absorption enhancement in a slurry droplet containing reactive, sparingly soluble microparticles

The absorption of a gaseous species by a slurry droplet containing reactive and sparingly soluble microparticles is numerically simulated. The problem studied is relevant to spray flue gas desulfurization systems and the objective of this study was to elucidate the effect of the reactive solid particles on the parameters that determine the mass transfer processes. Spherical droplets with internal circulation similar to Hill’s vortex flow were considered. Quasi-steady conservation equations representing the absorbed and dissolved reactant species and equations representing the dissolution of particles were numerically solved using the droplet internal circulation streamline as a coordinate. Second-order and instantaneous chemical reactions were both addressed. The results show that the reactant microparticles enhance the absorption rate by increasing the gradient of the absorbed species beneath the droplet surface. The relative effect of solid particles depends strongly on the droplet internal circulation and diminishes as stronger recirculation occurs.

Unsteady absorption of sulfur dioxide by an atmospheric water droplet with internal circulation

Unsteady absorption characteristics of sulfur dioxide by an atmospheric water droplet in motion are predicted numerically and analyzed theoretically to recognize the physical mass transport processes inside an aerosol droplet, which is frequently encountered in the atmosphere. Considering the absorption of sulfur dioxide by a droplet in cloud or fog with various velocities, three different Reynolds numbers, viz., Re g=0.643, 1.287, and 12.87 are studied and compared with each other. The results indicate that for the Reynolds number of 0.643, sulfur dioxide always penetrates toward the droplet centerline throughout the entire absorption period. This is due to the mass transfer dominated by diffusion along the radial direction. In contrast, when the Reynolds number is 12.87, the strength of the vortex motion inside the droplet is strong enough. It results in that, most of the time the concentration contours parallel the streamlines and the lowest SO 2 concentration is located at the vortex center. As a consequence, the diffusion distance is reduced by a factor of three and the absorption time for the droplet reaching the saturated state is shortened in a significant way. With regard to an intermediate Reynolds number such as 1.287, a two-stage mass transfer process can be clearly identified. In the first stage, it is dominated by one-dimensional diffusion, in which over 50% sulfur dioxide is absorbed before the saturated state is reached. In the second stage, the vortex motion mainly controls the mass transfer. However, the contour core is inconsistent with the vortex center. This is because the characteristic time of mass diffusion is in a comparable state with that of droplet internal circulation. The present study elucidates that the strength of a droplet's internal motion plays a vital role in determining SO 2 absorption process.

Thermocapillary flow effects on convective droplet evaporation

The thermocapillary convection effect on the dynamic behavior of a vaporizing-convecting hydrocarbon droplet is investigated numerically in a laminar, axisymmetric environment. The model predicts that surface-tension gradients along the gas-liquid interface strengthen droplet internal circulation and result in shorter evaporation lifetimes. The relative effect of thermocapillary stresses on overall droplet evaporation rates is nearly independent of Reynolds number. While Nusselt numbers on the droplet surface are little affected, a significant reduction of drag coefficient is observed when thermocapillary convection is taken into account. A substantial enhancement of the corresponding Sherwood numbers is also predicted. The results indicate that the Marangoni stresses cause temporal flow oscillations along the droplet interface at early stages of the droplet evaporation lifetime.

Effects of Marangoni Convection on Transient Droplet Evaporation

Abstract Results from a computational model of transient droplet vaporization are presented. In this model, axisymmetric flows around translating single component droplets are assumed. The governing equations are expressed in finite volume form and solved numerically for transient velocity, species and temperature profiles. Fully variable properties in the gas and liquid phases are allowed (except for liquid densities, which are held constant), and pressures of 1 and 10 atm are considered. A unique feature of the calculations is the inclusion of surface-tension gradients resulting from droplet surface temperature variations. Results show that surface-tension gradients significantly affect droplet internal temperature and velocity fields even when initial droplet Reynolds numbers, based on droplet diameters and free-stream conditions, are as large as 50. When surface-tension gradients are allowed at high initial Reynolds numbers (SO), droplet internal circulation rates are found to be initially increased f...

Thermal-Hydraulics of OC-OTEC Spout Flash Evaporators

A mechanistic model was developed for the thermal-hydraulic processes in the spout flash evaporator of an OC-OTEC plant. Nonequilibrium, two-fluid, conservation equations were solved for the two-phase flow in the spout, accounting for evaporation at the gas-liquid interface, and using a two-phase flow regime map consisting of bubbly, churn-turbulent and dispersed droplet flow patterns. Solution of the two-phase conservation equations provided the flow conditions at the spout exit, which were used in modeling the fluid mechanics and heat transfer in the evaporator, where the liquid was assumed to shatter into a spray with a log-normal size distribution. Droplet size distribution was approximated by using 30 discrete droplet size groups. Droplet momentum conservation equations were numerically solved to obtain the residence time of various droplet size groups in the evaporator. Evaporative cooling of droplets was modeled by solving the 1-D heat conduction equation in spheres, and accounting for droplet internal circulation by an empirical thermal diffusivity multiplier. The model was shown to favorably predict the available single-spout experimental data.

A model for the evaporation of a slowly moving droplet

The evaporation of a moving liquid drop has been investigated. The drop experiences an evaporation-induced radial velocity field while undergoing slow translation. In view of the large evaporation velocity, the flow field is not in the Stokes regime. Consequently, both the viscous and the inertial terms have been retained in the governing equations. While the flow and the transport processes in the gaseous phase and the droplet internal circulation are treated as quasisteady, the droplet heating is regarded as a transient process. The transport properties of the gaseous phase have been considered variable. The transport equations of the gaseous phase require analysis by a singular perturbation technique. The transient heating of the droplet interior is solved by a series truncation method. The solution of the total problem is obtained by coupling the results for the gaseous and liquid phases. The enhancement in the evaporation rate due to convective motion has been predicted. The friction, the pressure, and the evaporation drag coefficients have been individually predicted, and the total drag coefficient behavior has been discussed.

A detailed study of burning fuel droplets

Solutions are presented of steady flowfields around buring fuel droplets at constant Reynolds number with finite-rate global kinetics. A finite-difference solution to the full set of conservation equations in two space dimensions provides droplet vaporization rates, drag coefficients, Nusselt numbers and flame structures over a range of Reynolds and Damkohler numbers. The study has been restricted to gas-phase combustion processes, therefore droplet internal circulation has been ignored and the droplet is assumed to be at a constant temperature near the boiling point. The results are interpreted in terms of the characteristic time scales for convection, diffusion and chemical reactions, and clearly demonstrate the effects of these scales on the presence of an envelope or wake flame. Comparisons are made with experimental measurements in the literature of envelope and wake flames on porous spheres for typical hydrocarbon fuels. Good qualitative agreement is obtained between numerical and experimental results for the two distinct flame structures.