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Φαινόμενα μεταφοράς κατά την εξάτμιση σταγόνας πάνω σε υπόστρωμαΠέτση, Αναστασία 21 October 2011 (has links)
Στην παρούσα διατριβή μελετάται η εξέλιξη του φαινομένου της εξάτμισης σταγονιδίου που βρίσκεται πάνω σε στερεό επίπεδο υπόστρωμα, καθώς, και η διεργασία απόθεσης των αιωρούμενων σωματιδίων κατά την εξάτμιση υγρών σωμάτων με κυλινδρική γεωμετρία που αποτελούνται από κολλοειδή αιωρήματα. Παρουσιάζεται η μοντελοποίηση της διεργασίας εξάτμισης μικροσταγονιδίων πάνω σε στερεά υποστρώματα. Υπολογίζεται αναλυτικά το πεδίο ροής στο εσωτερικό διδιάστατων σταγονιδίων κατά την εξάτμισή τους από επίπεδα υποστρώματα για τις περιπτώσεις δυναμικής και έρπουσας ροής. Εξετάζεται η επίδραση του μηχανισμού που ελέγχει την εξάτμιση (αλλαγή φάσης, διάχυση ατμών), καθώς, και ο ρόλος των γραμμών επαφής (σταθερές γραμμές επαφής, σταθερή γωνία επαφής). Η διεργασία απόθεσης των αιωρούμενων σωματιδίων προσομοιώνεται με τη μέθοδο τροχιών σωματιδίων και τη μέθοδο συναγωγής-διάχυσης δικτύου Boltzmann. Τα σωματίδια κινούνται υπό την επίδραση του πεδίου ροής και της θερμικής κίνησής τους. Μελετάται η επίδραση του υποστρώματος στη διάχυση των σωματιδίων και η αλληλεπίδραση των σωματιδίων με την ελεύθερη επιφάνεια του σταγονιδίου. Παρουσιάζονται αποτελέσματα προσομοιώσεων για διάφορους αριθμούς Peclet, τόσο για υδρόφιλα όσο και για ισχυρά υδρόφοβα υποστρώματα και εξάγονται συμπεράσματα για τις συνθήκες δημιουργίας ομοιόμορφων αποθέσεων. / In this thesis the evaporation of droplets lying on substrates and the deposition process during the evaporation of colloidal liquid lines are investigated. The evaporation process has been mathematically modeled. The flow field inside an evaporating two dimensional microdroplet has been analytically calculated for the cases of potential and creeping flow. The effect of the evaporation controlling mechanism (phase change, vapor diffusion) and the behavior of the contact lines (pinned contact lines, depinned contact lines with constant contact angle) have been investigated. The deposition process of the suspended particles is simulated using the method of particle trajectories as well as the lattice Boltzmann convection-diffusion method. The particles move due to the flow field and their Brownian motion. The effect of solid substrate on the diffusivity, as well as, the particles interactions with the free surface of the droplet have been, also, investigated. Simulations results are presented for different Peclet numbers for hydrophilic and strongly hydrophobic substrates and useful conclusions have been arrived about the conditions that favor the formation of uniform deposits.
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Droplet Heat and Mass Exchange with the Ambient During Dropwise Condensation and FreezingJulian Castillo (9466352) 16 December 2020 (has links)
<div>
<p>The distribution of local
water vapor in the surrounding air has been shown to be the driving mechanism for
several phase change phenomena during dropwise condensation and condensation frosting. This thesis uses reduced-order modeling approaches,
which account for the effects of the vapor distribution to predict the droplet
growth dynamics during dropwise condensation in systems of many droplets. High-fidelity modeling techniques are used to
further probe and quantify the heat and mass transport mechanisms that govern
the local interactions between a freezing droplet and its surrounding ambient,
including neighboring droplets. The
relative significance of these transport mechanisms in the propagation of frost
are investigated. A reduced-order analytical method is
first developed to calculate the condensation rate of each individual droplet
within a group of droplets on a surface by resolving the vapor concentration
field in the surrounding air. A point sink
superposition method is used to account for the interaction between all droplets
without requiring solution of the diffusion equation for a full
three-dimensional domain. For a
simplified scenario containing two neighboring condensing droplets, the rates
of growth are studied as a function of the inter-droplet distance and the relative
droplet size. Interactions between the
pair of droplets are discussed in terms of changes in the vapor concentration
field in the air domain around the droplets.
For representative systems of condensing droplets on a surface, the total
condensation rates predicted by the reduced-order model match numerical
simulations to within 15%. The results
show that assuming droplets grow as an equivalent film or in a completely
isolated manner can severely overpredict
condensation rates.</p>
<p>The point superposition model is then used to predict the condensation
rates measured during condensation experiments.
The results indicate that it is critical to consider a large number of
interacting droplets to accurately predict the condensation behavior. Even though
the intensity of the interaction between droplets decreases
sharply with their separation distance, droplets located relatively far away from a given droplet must
be considered to accurately predict the condensation rate, due to the large
aggregate effect of all such far away droplets.
By considering an appropriate number of interacting droplets in a
system, the point sink superposition method is able to predict experimental
condensation rates to within 5%. The
model was also capable of predicting the time-varying condensation rates of
individual droplets tracked over time. These
results confirm that diffusion-based models that neglect the interactions of
droplets located far away, or approximate droplet growth as an equivalent film,
overpredict condensation rates.</p>
<p>In dropwise condensation from humid air, a full description
of the interactions between droplets can be determined by solving the vapor
concentration field while neglecting heat transfer across the droplets. In contrast, the latent heat released during
condensation freezing processes cause droplet-to-ambient as well as droplet-to-droplet
interactions via coupled heat and mas transfer processes that are not well
understood, and their relative significance has not been quantified. As a first step in understanding these
mechanisms, high-fidelity modeling of the solidification process, along with
high-resolution infrared (IR) thermography measurements of the surface of a
freezing droplet, are used to quantify the pathways for latent heat dissipation
to the ambient surroundings of a droplet.
The IR measurements are used to show that the crystallization dynamics
are related to the size of the droplet, as the freezing front moves slower in
larger droplets. Numerical simulations
of the solidification process are performed using the IR temperature data at
the contact line of the droplet as a boundary condition. These simulations, which have good agreement
with experimentally measured freezing times, reveal that the heat transferred
to the substrate through the base contact area of the droplet is best described
by a time-dependent temperature boundary condition, contrary to the constant
values of base temperature and rates of heat transfer assumed in previous numerical
simulations reported in the literature.
In further contrast to the highly simplified descriptions of the
interaction between a droplet and its surrounding used in previous models, the
model developed in the current work accounts for heat conduction, convection,
and evaporative cooling at the droplet-air interface. The simulation results indicate that only a
small fraction of heat is lost through the droplet-air interface via conduction
and evaporative cooling. The heat
transfer rate to the substrate of the droplet is shown to be at least one order
of magnitude greater than the heat transferred to the ambient air.</p>
<p>Subsequently, the droplet-to-droplet interactions via heat
and mass exchange between a freezing droplet and a neighboring droplet, for
which asymmetries are observed in the final shape of the frozen droplet, are
investigated. Side-view infrared (IR)
thermography measurements of the surface temperature for a pair of freezing
droplets, along with three-dimensional numerical simulations of the
solidification process, are used to quantify the intensity and nature of these
interactions. Two droplet-to-droplet
interaction mechanisms causing asymmetric freezing are identified: (1)
non-uniform evaporative cooling on the surface of the freezing droplet caused
by vapor starvation in the air between the droplets; and (2) a non-uniform
thermal resistance at the contact area of the freezing droplet caused by the
heat conduction within the neighboring droplet.
The combined experimental and numerical results show that the size of
the freezing droplet relative to its neighbor can significantly impact the
intensity of the interaction between the droplets and, therefore, the degree of
asymmetry. A small droplet freezing in
the presence of a large droplet, which blocks vapor from freely diffusing to
the surface of the small droplet, causes substantial asymmetry in the
solidification process. The droplet-to-droplet
interactions investigated in thesis provide insights into the role of heat
dissipation in the evaporation of neighboring droplets and ice bridging, and
open new avenues for extending this understanding to a system-level description
for the propagation of frost.</p>
</div>
<br>
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Evaporation and Buckling Dynamics of Sessile Droplets Resting on Hydrophobic SubstratesBansal, Lalit Kumar January 2018 (has links) (PDF)
Droplet evaporation is ubiquitous to multitude of applications such as microfluidics, surface patterning and ink-jet printing. In many of the process like food processing tiny concentrations of suspended particles may alter the behavior of an evaporating droplet remarkably, leading to partially viscous and partially elastic dynamical characteristics. This, in turn, may lead to some striking mechanical instabilities, such as buckling and rupture. In this thesis, we provide a comprehensive physical description of the vaporization, self-assembly, agglomeration and buckling kinetics of sessile nanofluid droplet pinned on a hydrophobic substrate in various configurations. We have deciphered five distinct regimes of droplet lifecycle. Regime I-III consists of evaporation induced preferential agglomeration that leads to the formation of unique dome shaped inhomogeneous shell with stratified varying density liquid core. Regime IV involves capillary pressure initiated shell buckling and stress induced shell rupture. Regime V marks rupture induced cavity inception and growth. We provide a regime map explaining the droplet morphology and buckling characteristics for droplets evaporating on various substrates. Specifically, we find that final droplet volume and radius of curvature at buckling onset are universal functions of particle concentration. Furthermore, flow characteristics inside the heated and unheated droplets are investigated and found to be driven by the buoyancy effects. Velocity magnitudes are observed to increase by an order at higher temperatures with self-similar flow profiles. With an increase in the surface temperature, droplets exhibit buckling from multiple sites over a larger sector in the top half of the droplet. In addition, irrespective of the initial nanoparticle concentration and substrate temperature, hydrophobicity and roughness, growth of daughter cavity (subsequent to buckling) inside the droplet is found to be controlled by the solvent evaporation rate from the droplet periphery. The results are of great significance to a plethora of applications like DNA deposition and nanofabrication.
In the next part of the thesis, we deploy the droplet in a rectangular channel. The rich physics governing the universality in the underlying dynamics remains grossly elusive. Here, we bring out hitherto unexplored universal features of the evaporation dynamics of a sessile droplet entrapped in a 3D confined fluidic environment. Increment in channel length delays the completion of the evaporation process and leads to unique spatio-temporal evaporation flux and internal flow. We show, through extensive set of experiments and theoretical formulations, that the evaporationtimescale for such a droplet can be represented by a unique function of the initial conditions. Moreover, using same theoretical considerations, we are able to trace and universally merge the volume evolution history of the droplets along with evaporation lifetimes, irrespective of the extent of confinement. These results are explained in the light of increase in vapor concentration inside the channel due to greater accumulation of water vapor on account of increased channel length. We have formulated a theoretical framework which introduces two key parameters namely an enhanced concentration of the vapor field in the vicinity of the confined droplet and a corresponding accumulation lengthscale over which the accumulated vapor relaxes to the ambient concentration.
Lastly, we report the effect of confinement on particle agglomeration and buckling dynamics. Compared to unconfined scenario, we report non-intuitive suppression of rupturing beyond a critical confinement. We attribute this to confinement-induced dramatic alteration in the evaporating flux, leading to distinctive spatio-temporal characteristics of the internal flow leading to preferential particle transport and subsequent morphological transitions. We present a regime map quantifying buckling & non-buckling pathways. These results may turn out to be of profound importance towards achieving desired morphological features of a colloidal droplet, by aptly tuning the confinement space, initial particle concentration, as well as the initial droplet volume. These findings may have implications in designing functionalized droplet evaporation devices for emerging engineering and biomedical applications.
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Study of Droplet Dynamics in Heated EnvironmentPathak, Binita January 2013 (has links) (PDF)
Droplets as precursor are extensively applied in diverse fields of science and engineering. Various contributions are provided previously towards analysis of single phase and multi-phase droplets of single and multiple components.
This thesis describes modelling of multi-phase (nano fluid) droplet vaporization. The
evaporation of liquid phase along with migration of dispersed particles in two-dimensional plane within droplet is detailed using the governing transport equations
along with the appropriate boundary and interface conditions.
The evaporation model is incorporated with aggregate kinetics to study agglomeration
among nano silica particles in base water. Agglomeration model based on population
balance approach is used to track down the aggregation kinetics of nano particles in
the droplet. With the simulated model it is able to predict different types of final
structure of the aggregates formed as observed in experimental results available in
literature. High spatial resolution in terms of agglomeration dynamics is achieved
using current model. Comparison based study of aggregation dynamics is done by
heating droplet in convective environment as well as with radiations and using
different combination of heating and physical parameters. The effect of internal flow
field is also analysed with comparative study using levitation and without levitation
individually. For levitation, droplet is stabilized in an acoustic standing wave.
It is also attempted to study the transformation of cerium nitrate to ceria in droplets when heated under different environmental conditions. Reaction kinetics based on modified rate equation is modelled along with vaporization in aqueous cerium nitrate droplet. The thermo physical changes within the droplet along with dissociation
reaction is analysed under different modes of heating. The chemical conversion of
cerium nitrate to ceria during the process is predicted using Kramers' reaction velocity
equation in a modified form. The model is able to explain the kinetics behind
formation of ceria within droplet at low temperatures. Transformation of chemical
species is observed to be influenced by temperature and configuration of the system.
Reaction based model along with CFD (computational fluid dynamics) simulation
within the droplet is able to determine the rate of chemical dissociation of species and
predict formation of ceria within the droplet. The prediction shows good agreement
with experimental data which are obtained from literature.
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