Author: Omar Jameel Abdulrazzaq Al-RawiPublisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
Dynamics of Sessile Droplet Evaporation in Laminar Convective Environments
Author: Omar Jameel Abdulrazzaq Al-RawiPublisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
Behavior of Droplets Evaporating in High Temperature Convective Environments
Author: Joannes Baptista WijffelsPublisher:
ISBN:
Category :
Languages : en
Pages : 182
Book Description
Liquid-Phase Circulation and Mixing in Multicomponent Droplets Vaporizing in a Laminar Convective Environment
Author: Publisher:
ISBN:
Category :
Languages : en
Pages : 92
Book Description
A combined theoretical/experimental study of internal circulation induced by surface rotation of mm-sized pendant droplets was completed. The study consistently revealed predictable helical three-dimensional circulation patterns that bear little resemblance to the two-dimensional toroidal internal flows established within droplets under axisymmetric convective conditions. The observed complex flow patterns suggested substantially enhanced liquid mixing rates, thus indicating that droplet spinning may be very important in practical situations involving droplet convective transport. In a parallel study, charged droplets up to 100 micrometers in diameter were stably levitated and exposed to uniform air streams with Reynolds numbers up to 3. The evaporation rate of levitated dodecanol droplets was measured under quiescent conditions and followed the familiar d to the power of 2 -law. No detailed information could be obtained on internal circulation of levitated droplets impregnated with scattering or fluorescent agents.
Evaporation, Precipitation Dynamics and Instability of Acoustically Levitated Functional Droplets
Author: Abhishek SahaPublisher:
ISBN:
Category :
Languages : en
Pages : 250
Book Description
Evaporation of pure and binary liquid droplets is of interest in thermal sprays and spray drying of food, ceramics and pharmaceutical products. Understanding the rate of heat and mass transfer in any drying process is important not only to enhance evaporation rate or vapor-gas mixing, but also to predict and control the final morphology and microstructure of the precipitates. Acoustic levitation is an alternative method to study micron-sized droplets without wall effects, which eliminates chemical and thermal contamination with surfaces. This work uses an ultrasonic levitation technique to investigate the vaporization dynamics under radiative heating, with focus on evaporation characteristics, precipitation kinetics, particle agglomeration, structure formation and droplet stability. Timescale and temperature scales are developed to compare convective heating in actual sprays and radiative heating in the current experiments. These relationships show that simple experiments can be conducted in a levitator to extrapolate information in realistic convective environments in spray drying. The effect of acoustic streaming, droplet size and liquid properties on internal flow is important to understand as the heat and mass transfer and particle motion within the droplet is significantly controlled by internal motion. Therefore, the droplet internal flow is characterized by Particle Image Velocimetry for different dropsize and viscosity. Nanosuspension droplets suspended under levitation show preferential accumulation and agglomeration kinetics. Under certain conditions, they form bowl shaped structures upon complete evaporation. At higher concentrations, this initial bowl shaped structure morphs into a ring structure. Nanoparticle migration due to internal recirculation forms a density stratification, the location of which depends on initial particle concentration. The time scale of density stratification is similar to that of perikinetic-driven agglomeration of particle flocculation. The density stratification ultimately leads to force imbalance leading to a unique bowl-shaped structure. Chemically active precursor droplet under acoustic levitation shows events such as vaporization, precipitation and chemical reaction leading to nanoceria formation with a porous morphology. The cerium nitrate droplet undergoes phase and shape changes throughout the vaporization process followed by formation of precipitate. Ex-situ analyses using TEM and SEM reveal highly porous morphology with trapped gas pockets and nanoceria crystalline structures at 70°C. Inhomogeneity in acoustic pressure around the heated droplet can induce thermal instability. Short wavelength (Kelvin-Helmholtz) instability for diesel and bio-diesel droplets triggers this secondary atomization, which occurs due to relative velocity between liquid and gas phase at the droplet equator. On the other hand, liquids such as Kerosene and FC43 show uncontrollable stretching followed by a catastrophic break-up due to reduction in surface tension and viscosity coupled with inhomogeneity of pressure around the droplet. Finally, a scaling analysis has been established between vaporizing droplets in a convective and radiative environment. The transient temperature normalized by the respective scales exhibits a unified profile for both modes of heating. The analysis allows for the prediction of required laser flux in the levitator experiments to show its equivalence in a corresponding heated gas stream. The theoretical equivalence shows good agreement with experiments for a range of droplet sizes.
Scientific and Technical Aerospace Reports
Author: Droplet Wetting and Evaporation
Author: David BrutinPublisher: Academic Press
ISBN: 0128008083
Category : Science
Languages : en
Pages : 464
Book Description
Droplet Wetting and Evaporation provides engineers, students, and researchers with the first comprehensive guide to the theory and applications of droplet wetting and evaporation. Beginning with a relevant theoretical background, the book moves on to consider specific aspects, including heat transfer, flow instabilities, and the drying of complex fluid droplets. Each chapter covers the principles of the subject, addressing corresponding practical issues and problems. The text is ideal for a broad range of domains, from aerospace and materials, to biomedical applications, comprehensively relaying the challenges and approaches from the different communities leading the way in droplet research and development. Provides a broad, cross-subject coverage of theory and application that is ideal for engineers, students and researchers who need to follow all major developments in this interdisciplinary field Includes comprehensive discussions of heat transfer, flow instabilities, and the drying of complex fluid droplets Begins with an accessible summary of fundamental theory before moving on to specific areas such as heat transfer, flow instabilities, and the drying of complex fluid droplets
Influence of Forced Convection on Sessile Droplet Evaporation
Author: Henry FungPublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Evaporation is a phase change process with potential for achieving large heat transfer rates under high air temperatures due to the latent heat of vaporization. Bioin-spired artificial perspiration systems can leverage the evaporation of sessile droplets to implement this effect for different cooling applications. In the case of human perspiration, droplet evaporation typically occurs under exposure to moving air, or forced convection. However, current approaches to understanding droplet evaporation primarily use a vapour-diffusion limited model. Experiments using an open-loop wind tunnel and computer-vision based control system were conducted to measure evaporation rates of continuously-fed sessile droplets under forced convection. Results demonstrated increases to the evaporation rate with the inclusion of forced convection and removal of the vapour-diffusion limit, but also shows evidence for a limit based on thermal behavior. Additional experiments also demonstrate boundary layer effects caused by adjacent droplets suppresses increases to the evaporation rates from forced convection.
Experimental Investigation and Data-Driven Analysis of Binary and Multi-Component Droplet Evaporation
Author: Sahar AndalibPublisher:
ISBN:
Category :
Languages : en
Pages : 146
Book Description
Sessile droplet evaporation is an omnipresent phenomenon both in nature and technologies such as biodiagnostics, microfabrication, inkjet printing, spray cooling, and agriculture irrigation. Evolution of single component sessile droplets has been extensively studied under various parameters. However, in real applications, sessile droplets usually consist of two or more components. It has been shown that the environmental conditions such as humidity and temperature substantially change the behavior of sessile droplets. The strong tendency of organic fluids to absorb water is an important factor in evaporation of these fluids in humid environment. The water vapor present in the surrounding adsorbs/absorbs and possibly condenses into the droplet transforming the droplet into a binary system. While the humidity of surrounding is typically an imposed condition resulting in unwanted effects for many industries, we have proposed that these undesired effects can be controlled and eliminated by tuning the temperature of the substrate. We have studied the combined effect of relative humidity of surrounding and substrate temperature on evaporation of methanol droplets. Our results demonstrated that the diffusion of water into the droplet can be limited by changing temperature of the substrate by both shortening the lifetime of droplet or increasing the temperature of the liquid-gas interface above the due point. Additionally, we have developed machine learning, classification and regression, models to analyze the behavior of droplet under different conditions. We have shown that the regime of droplet evaporation can be accurately classified by analyzing the profile of the evolution of droplet macroscopic parameters. We have also demonstrated that the humidity of surrounding can be accurately estimated by analysis of droplet profile. Furthermore, the time evolution of diameter and contact angle are estimated by the regression model. As the number of components in the droplet increases, the underlying mechanisms become more complex. The proposed approach to analyze the dynamics of sessile droplet evaporation through data-driven techniques opens up ways to better understand the complicated physics behind multi-component droplet evaporation and in general intricate interfacial fluid mechanics problems. The combined effect of surrounding humidity and substrate temperature has been experimentally studied on the behavior of ternary droplet consisting of methanol, anise oil, and water. The simultaneous optical microscopy and infrared thermography revealed different mechanisms in the droplet such as hydrothermal waves, oil microdroplet nucleation, etc. Our results showed three stages in the evolution of hydrothermal waves during droplet lifetime. The experimental procedures and results in this work introduce easy and inexpensive method to control sessile droplet behavior which are crucial for the final product resolution in numerous applications.
Flow Motion in Sessile Droplets
Author: Florian CarlePublisher:
ISBN:
Category :
Languages : en
Pages : 0
Book Description
Sessile droplets are widely found in day to day life: it might be a coffee spilt, rain onto a waterproof raincoat or again, water falling onto a cooking plate. However, despite the vast number of studies devoted to droplets for almost half a century, the fundamental phenomenon of the evaporation of sessile droplets is still a field that attracts a high level of interest due to its wide applicability and the development of new visualisation techniques or new types of complex fluids. This experimental study is focused two distinct aspects:- The evaporation of pure fluids has allow to study hydrothermal waves that appear in the droplets of volatile fluids during phase change. The influence of the type of fluid --different alcohols and alkanes-- and the gravity levels --Terrestrial, Lunar and Martian-- is investigated to have a better understanding of the flow motion inside droplet. Moreover, the use of different gravity levels allows to experimentally evidence the contribution of the atmospheric convective transport to sessile droplet evaporation. This investigation has allowed to develop an empirical model to take account of natural convection which greatly increases the evaporation rate in the quasi-steady diffusion-controlled evaporation model.- If complex fluids exhibit an evaporation dynamic similar to pure fluid, other mechanisms come into play, such as gelation, particles organisation and cracks formation. Wetting and different functional groups on the particles graphs will be studied in relation to the final pattern of cracks.
Author: Khosrovan N. Mallah