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Analisi sperimentale e modellazione numerica della condensazione a gocce
No full textLa condensazione è un processo di cambiamento di fase che si trova spesso in natura ed è anche sfruttato in molteplici applicazioni ingegneristiche come centrali termiche, raffinazione chimica, desalinizzazione dell'acqua di mare, refrigerazione e raffreddamento dell'elettronica. È noto che promuovere la condensazione goccia a goccia (DWC) invece della tradizionale condensazione a film (FWC) è una soluzione passiva per migliorare il trasferimento di calore bifase. La condensazione goccia a goccia sui metalli si ottiene solitamente mediante l'applicazione di rivestimenti idrofobici. È noto il vantaggio dato dall'utilizzo di superfici ingegnerizzate durante la condensazione di vapore puro, mentre non è ancora stato sufficientemente studiato. Sebbene lo studio teorico del DWC sia iniziato nella prima metà del secolo scorso, non esiste ancora un modello in grado di descrivere con precisione tutti i fenomeni coinvolti durante il DWC. Questo divario è dovuto alla difficoltà di descrivere i molteplici processi simultanei che coesistono su diverse scale temporali (da 1 μs a 1 s) e spaziali (dai nanometri ai millimetri).
Questo lavoro è diviso in due parti principali. Il primo è incentrato su misure sperimentali di scambio termico durante DWC con vapore puro e in presenza di aria umida. La seconda parte è incentrata sulla modellazione numerica dei processi coinvolti durante il DWC. Superfici di diversa bagnabilità sono state testate durante la condensazione sia di vapore puro che di aria umida, valutando gli effetti della bagnabilità superficiale sullo scambio termico bifase in diverse condizioni sperimentali. Inoltre, la distribuzione delle goccioline e lo scambio termico sono stati studiati attraverso simulazioni numeriche. L'attività di ricerca è stata svolta principalmente presso il Laboratorio Scambi Termici Bifase (Università di Padova, Dipartimento di Ingegneria Industriale) con la collaborazione del gruppo di ricerca NANOENG - Ingegneria dei nanomateriali per lo sviluppo e la caratterizzazione dei substrati. L'ultima parte dell'attività di ricerca è stata svolta presso il laboratorio LAPLACE dell'Università Paul Sabatier di Tolosa (FR) indagando il numero e la distribuzione dei siti di nucleazione durante la condensazione del vapore puro mediante tecniche di misura interferometriche.
Questa tesi è suddivisa in sei capitoli. Il primo capitolo è dedicato ad una breve rassegna della letteratura sulla bagnabilità superficiale e la condensazione goccia a goccia. Il secondo capitolo riassume i modelli più importanti che mirano a descrivere la distribuzione delle goccioline e il trasferimento di calore durante il DWC sia in presenza di aria umida che di vapore saturo. Il terzo capitolo presenta le misure del trasferimento di calore e della popolazione di goccioline ottenute durante la condensazione del vapore acqueo puro. È stato anche studiato l'effetto della velocità del vapore sul fenomeno DWC. Il quarto capitolo è incentrato sullo studio sperimentale della condensazione in presenza di aria umida concentrandosi sullo studio dello scambio termico durante DWC in presenza di NCG. Oltre alle misure di scambio termico, sono stati acquisiti video del processo di condensazione con il rilevamento di goccioline fino a pochi micron per ottenere informazioni sulla popolazione di goccioline e sulla densità dei siti di nucleazione. Nel quinto capitolo viene presentato un nuovo modello sviluppato in MATLAB® ibrido e codice C, basato sul calcolo parallelo e che può essere utilizzato per simulare il processo di condensazione delle goccioline. Nell'ultimo capitolo viene proposta una metodologia per lo studio della densità dei siti di nucleazione durante la condensazione di goccioline di vapore puro su una superficie rivestita di alluminio. La determinazione dei siti di nucleazione viene eseguita con tecniche interferometriche.Condensation is a phase change process that is often found in nature and is also exploited in multiple engineering applications such as thermal power plants, chemical refining, seawater desalination, refrigeration, and electronics cooling. It is known that promoting dropwise condensation (DWC) instead of traditional filmwise condensation (FWC) is a passive solution to improve two-phase heat transfer. Dropwise condensation on metals is usually achieved by applying hydrophobic coatings. The advantage given by the use of engineered surfaces during the condensation of pure vapor is known, while the possible advantage provided by the promotion of DWC in the presence of non-condensable gases (as in the case of humid air) has not yet been sufficiently studied. Although the theoretical study of DWC began in the first half of the last century, there is still no model capable of accurately describing all the phenomena involved during DWC. This gap is due to the difficulty of describing the multiple simultaneous processes that coexist on different temporal (from 1 μs to 1 s) and spatial (from nanometers to millimeters) scales.
This work is divided into two main parts. The first is centered on experimental measurements of heat transfer during DWC with pure steam and in the presence of humid air. The second part is focused on the numerical modeling of the processes involved during the DWC. Surfaces of different wettability were tested during the condensation of both pure steam and humid air, evaluating the effects of surface wettability on the two-phase heat exchange under different experimental conditions. Furthermore, the distribution of the droplets and the heat exchange were investigated through numerical simulations. The research activity was mainly carried out at the Two-Phase Thermal Exchange Laboratory (University of Padua, Department of Industrial Engineering) with the collaboration of the research group NANOENG - Engineering of nanomaterials for the development and characterization of substrates. The last part of the research activity was carried out in the LAPLACE laboratory of the Paul Sabatier University of Toulouse (FR) by investigating the number and distribution of nucleation sites during the condensation of pure steam using interferometric measurement techniques.
This thesis is divided into six chapters. The first chapter is dedicated to a brief review of the literature on surface wettability and dropwise condensation. The second chapter summarizes the most important models that aim to describe the droplet distribution and heat transfer during DWC both in the presence of humid air and saturated steam. The third chapter presents the measurements of heat transfer and droplet population obtained during the condensation of pure water vapor. The effect of steam velocity on the DWC phenomenon was also investigated. The fourth chapter is focused on the experimental study of condensation in the presence of humid air focusing on the study of heat exchange during DWC in the presence of NCGs. In addition to the heat transfer measurements, videos of the condensation process with the detection of droplets down to a few microns were acquired in order to obtain information on the droplet population and the density of the nucleation sites. In the fifth chapter, a new model developed in hybrid MATLAB® and C code is presented, based on parallel computation and which can be used to simulate the droplet condensation process. In the last chapter, a methodology is proposed for the study of the density of the nucleation sites during the droplet condensation of pure vapor on a coated aluminum surface. The determination of the nucleation sites is performed with interferometric techniques
Intelligenza artificiale e imposizione fiscale
No full textChe il mondo economico-industriale sia in perenne “werden”, ossia in continuo cambiamento e addivenire, costituisce un dato di fatto: dalla prima rivoluzione industriale consistita nella meccanizzazione che sfruttava la forza dell’acqua e del vapore, dalla seconda rivoluzione industriale che ha visto lo sviluppo della produzione di massa all’impiego delle catene di montaggio, da un sempre maggior impiego dei computer alla relativa automazione dei processi produttivi, oggi siamo di fronte ad una nuova rivoluzione industriale, quella dell’Industria 4.0
Modeling of growth and dynamics of droplets during dropwise condensation of steam
Full text linkComputational modeling is essential for understanding dropwise condensation (DWC) mechanisms, droplet lifecycle,
and predicting heat transfer. However, the multiscale nature of DWC increases the computational cost,
thus making the study of the droplet distribution more difficult. Population-based models available in the
literature rely on empirical or statistical methods for determining the drop-size distribution. Differently, in the
present study, a new individual-based model developed in hybrid MATLAB® and C codes and based on parallel
computing is developed to simulate the whole dropwise condensation process, addressing the growth of each
droplet, without making any assumption on the droplet population and considering a number of drops never
reached in previous similar studies. The proposed model’s computational efficiency is significantly improved
when considering more than 1 million drops in the computational domain. To optimize the calculation time, the
effects of time step, computational domain size, and simulation duration on the overall heat flux and drop-size
distribution are discussed. The numerical results are compared against predictions from population-based models
available in the literature. The proposed model is also used to study the droplet population and the instantaneous
heat flux during DWC at different positions along a vertical condensing surface (upper, middle and lower areas).
As a final step, a preliminary comparison is carried out between the present model and experimental data acquired
during dropwise condensation on a nearly hydrophobic vertical surface. Considering a nucleation size
density of 5 × 10^12 m-2 (11 × 10^6 drops in the computational domain), the simulation is able to predict the
experimental heat flux and the large drop-size distribution
VAPOR VELOCITY AND DROPLET DYNAMICS DURING DROPWISE CONDENSATION OF STEAM FLOWING OVER HYDROPHILIC SURFACES
No full textDropwise condensation (DWC) is a complex phase-change process that involves the nucleation, growth and removal of randomly distributed drops on the condensing surface. It is widely established that the promotion of dropwise condensation can significantly improve the heat transfer coefficient (HTC) as compared to filmwise condensation (FWC). The interaction between the condensing fluid and the surface (wettability) defines the condensation mode. Low wettability coatings with small contact angle hysteresis and low thermal resistance are a possible solution to obtain high heat transfer coefficients during DWC on metals. In energy applications, the condensing steam usually has a non-negligible velocity, but experimental data taken with flowing vapor are rare in the literature. Therefore, the investigation of DWC in presence of steam velocity would help to understand the physical mechanisms governing the phenomenon and to develop comprehensive dropwise condensation models.
In the present study, hydrophilic (advancing contact angle θa < 90°) sol-gel coated aluminum samples with reduced contact angle hysteresis (Δθ < 30°) were tested during DWC of pure steam. The engineered surfaces were characterized by dynamic contact angles and film thickness measurements. The experimental apparatus used for DWC investigation is a thermosyphon loop operating in steady-state conditions. The test rig is equipped with an optical system for the study of the droplet population and droplet dynamics. Heat transfer measurements and droplet population analyses were carried out at constant saturation temperature (~ 107.5 °C) while varying the heat flux in the range 290-1020 kW m-2 and increasing the inlet vapor velocity from 3 m s-1 to 13.5 m s-1. As a further step, the collected experimental data were compared against DWC models accounting for the effect of vapor velocity
Investigation of surface inclination effect during dropwise condensation of flowing saturated steam
Full text linkWhen a pure vapor condenses over a surface, it can form a continuous liquid film or a multitude of discrete droplets, thus realizing the so-called dropwise condensation (DWC). In the literature, most of the experimental data refer to DWC on vertical condensing surfaces with quiescent vapor. However, in many applications, the condensing vapor usually has a non-zero flow velocity with a consequent effect on the sliding motion of droplets. Moreover, the drag force due to vapor velocity may be the only mechanism for liquid removal on a horizontal surface or in space applications. A systematic investigation of the effects of vapor drag and surface inclination on the heat transfer and droplet population during DWC is needed and is addressed in the present paper.
Here, DWC of flowing steam is experimentally studied on sol-gel silica-based coated aluminium substrates at three different inclinations: vertical, inclined at 45°, and horizontal. Heat transfer coefficient (HTC) and droplet population measurements are performed in a wide range of heat flux (260–610 kW m−2) and average vapor velocity (3.3–13.8 m s−1). When decreasing the tilt angle, from vertical to horizontal, due to the lower contribution of the gravity force, the average droplet size increases, and a strong HTC reduction is observed above all at low vapor velocities. Because of the vapor drag force, the HTC increases with steam velocity and, at the highest mass velocity, the HTC is independent from the surface inclination. A model for the droplet departing radius in the presence of vapor velocity, initially proposed by the present authors for the sole case of vertical surfaces, is here modified to account also for the effect of surface inclination and then assessed against the present experimental data. Hence, we propose to predict the heat flux during DWC by coupling the new equation for the departing radius with the available models of heat transfer through a single droplet and drop-size distribution. The developed calculation method is found to provide satisfactory predictions of the HTC for the whole range of vapor velocity, heat flux and surface inclination
Simultaneous measurement of heat flux and droplet population during dropwise condensation from humid air flowing on a vertical surface
No full textA new experimental apparatus for the investigation of dropwise condensation from humid air flowing over a vertical aluminum surface at controlled velocity is presented. Differently from other works on this subject, here we measure simultaneously, during condensation of flowing moist air, the total heat flux (by a heat flux sensor), the latent heat flux (by weighing the mass of condensate), the small and the large droplet population. Experimental tests were performed on two aluminum specimens that display different wettability: a mirror-polished surface (56° advancing contact angle, 46° contact angle hysteresis) considered as a baseline, and a coated surface functionalized by using a sol–gel coating (87° advancing contact angle, 15° contact angle hysteresis). The effects of the wall subcooling, moisture content, relative humidity and air velocity on the heat and mass transfer during dropwise condensation (DWC) are investigated. Enhanced condensation is observed on the coated surface but, with the increase of relative humidity and wall subcooling, the advantage of using the coated surface diminishes. Time-lapse videos of the condensation process, featuring droplets detection down to three microns, are used to investigate droplet population (both large and small droplets) and nucleation sites density. Usually, DWC models assume tentative values of the nucleation site density and tentative trends of the droplet population. In this paper we measure and discuss such parameters. The determination of the nucleation site density is crucial because it affects the droplet interactions and the drop size distribution, determining the overall heat transfer.
The measurement of the nucleation site density is currently rare in the literature, especially during condensation of flowing humid air. Here the nucleation site density is determined with a double approach and it is found to vary between 3.3 × 108 sites/m2 and 6.1 × 108 sites/m2 in the investigated range of experimental conditions. The experimental droplet population is also compared against the predicted one, finding some disagreement which should be properly addressed for the development of improved DWC models
Heat transfer and droplet population during dropwise condensation on durable coatings
No full textLong-term sustainability of dropwise condensation (DWC) on treated surfaces is a key point for the exploitation of this heat transfer mechanism in industrial applications. A viable solution to achieve DWC, consisting of hybrid organic-inorganic sol-gel silica coatings containing hydrophobic moieties (methyl or phenyl group) is here presented. Different sol-gel coatings for DWC promotion were tested during condensation of steam in saturated conditions exhibiting heat transfer coefficient (HTC) around 100 kW m−2 K−1 in the heat flux range 100–500 kW m−2. Endurance tests have been performed at 400 kW m−2; an optimized sol-gel coating deposited on an aluminum substrate is shown to sustain DWC for more than 100 h without transition to filmwise condensation (FWC), which is an excellent result among those achieved on aluminum substrates. A comparison between the different coatings is done by surface characterization (contact angles measurements and Scanning Electron Microscopy) performed before and after condensation tests. Video analyses are carried out looking at droplet departing radius, droplets population and surface time renewal using a home-made software to detect the dimensions of the droplets. The present data are used to assess the expression proposed by Le Fevre and Rose (1966) for the droplet population, the equation by Kim and Kim (2011) for the departing radius and the model proposed by Chavan et al. (2016) for the heat transfer coefficient
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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