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INDAGINE SULLA TRASMISSIONE DI CALORE E SULLA POPOLAZIONE DI GOCCE DURANTE LA CONDENSAZIONE A GOCCE DI VAPORE PURO E ARIA UMIDA
No full textLa condensazione di vapor d’acqua è un processo di cambiamento di fase che trova riscontro in molte applicazioni industriali. È ben noto che promuovere la condensazione a gocce (DWC) al posto di quella a film (FWC) è un’ottima strategia per migliorare il trasferimento di calore durante la condensazione di vapore puro. Anche se il vantaggio nell'utilizzare superfici idrofobiche durante la condensazione di vapore puro è stato accertato nel secolo scorso, il possibile beneficio fornito dalla promozione della DWC in presenza di gas non condensabili necessita di ulteriori investigazioni. Infatti, la presenza dell’aria complica lo studio del fenomeno. La presente tesi è focalizzata sulle misure di scambio termico e sulla modellistica durante la condensazione a gocce. L’obiettivo è quello di migliorare le prestazioni dei condensatori attraverso il controllo della bagnabilità delle superfici attive nello scambio termico, con un occhio di riguardo ai sistemi per il raffreddamento e la deumidificazione dell'aria. Nello specifico, sono state testate superfici con diversa bagnabilità durante la condensazione di vapore puro e in presenza di aria umida, andando a valutare gli effetti della bagnabilità superficiale sullo scambio termico bifase al variare delle condizioni sperimentali.
Questa tesi è suddivisa in cinque Capitoli. Il primo capitolo è dedicato ad una breve rassegna della letteratura sulla bagnabilità superficiale e sulla condensazione a gocce. Nel secondo capitolo sono riassunte le misure di scambio termico e della popolazione di gocce ottenute dallo studio della condensazione a gocce di vapore puro. Diversi substrati funzionalizzati con rivestimenti a bassa bagnabilità hanno permesso di ottenere coefficienti di scambio termico (HTC) di circa 80-160 kW m-2 K-1 nell'intervallo di flussi termici 100-600 kW m-2. I test per valutare la resistenza dei rivestimenti, che sono eseguiti a flusso termico costante, hanno dimostrato che l'ottimizzazione chimica del rivestimento può migliorare drasticamente la durata del trattamento, allungandone la vita ben oltre le 100 ore senza passaggio alla condensazione a film. Inoltre, partendo da una carenza della letteratura, è stato valutato l'effetto della velocità del vapore sul fenomeno della condensazione a gocce. Il terzo capitolo è incentrato nello studio sperimentale della condensazione da aria umida, tenendo conto quindi della presenza di gas incondensabili nello studio del fenomeno. Viene presentata una nuova tecnica sperimentale per misurare simultaneamente il flusso termico totale (mediante un termoflussimetro) e il flusso termico latente (pesando la massa di condensato) durante la condensazione dell’umidità presente in un flusso di aria umida. Oltre alle misure di scambio termico, sono stati acquisiti dei video time-lapse del processo per fornire misure della popolazione di gocce. Nel quarto capitolo vengono presentati i modelli più importanti per descrivere la popolazione di gocce e lo scambio termico durante la DWC. Inoltre, viene presentato un nuovo metodo per valutare l'aumento del coefficiente di scambio termico dovuto alla velocità del vapore durante la DWC e lo si valida con i dati sperimentali. Nell'ultimo Capitolo, si presenta l'attività sperimentale svolta presso il laboratorio di Eurapo S.r.l. sugli scambiatori di calore a batteria alettata per il raffreddamento e la deumidificazione dell'aria. I dati sperimentali raccolti sono stati utilizzati per validare il modello numerico di scambiatore di calore utilizzato dall'azienda quando il fluido che scorre dentro ai tubi è una miscela di acqua e glicole. All’interno del capitolo, è stata studiata anche la possibilità di aumentare l'efficienza dei ventilconvettori con l'applicazione di rivestimenti superficiali. Questo lavoro è stato finanziato da Eurapo S.r.l.Condensation of the water vapor is a phase change process encountered in many industrial applications. It is well accepted that promoting dropwise condensation (DWC) in place of the traditional filmwise condensation (FWC) has been identified as a method to enhance the heat transfer during condensation of pure steam. Even if the benefit of using hydrophobic surfaces during condensation of pure steam has been assessed over the past century, the possible advantage provided by promoting DWC in the presence of non-condensable gases deserves further investigation.
The present thesis focuses on heat transfer measurements and modeling during dropwise condensation to improve the performance of condensers by controlling and modifying the wettability of the heat transfer surfaces, with particular attention to the units for cooling and dehumidification of the air. Surfaces of different wettability were tested during condensation of both pure steam and humidity, evaluating the effects of surface wettability on the two-phase heat transfer under varying experimental conditions.
The thesis is divided into five Chapters. The first Chapter is dedicated to a brief review of the literature on surface wettability and dropwise condensation. The second chapter summarizes the heat transfer and droplet population measurements obtained investigating dropwise condensation of pure vapor. Several low-wettability coated substrates exhibited heat transfer coefficients (HTCs) of approximately 80-160 kW m-2 K-1 in the heat flux range 100-600 kW m-2. Endurance tests performed at constant heat flux showed that the optimization of the coating’s chemistry can drastically improve the coating lifetime well beyond 100 hours with no transition to FWC. Furthermore, starting from a lack in the literature, the effect of steam velocity on the DWC phenomenon was assessed. The third Chapter is focused on the experimental study of condensation from humid air, thus taking into account the presence of non-condensable gases in the study of the phenomenon. A novel experimental technique for the simultaneous measure of total heat flux (by a heat flux sensor) and latent heat flux (by weighing the mass of condensate) during condensation of flowing moist air is presented. In addition to the heat transfer measurements, time-lapse videos of the condensation process were acquired to provide droplet population and nucleation sites density measurements. In the fourth Chapter, the most important models that aim to describe the droplet population and the heat transfer during DWC are discussed. A new approach developed by the present authors to predict the HTC increase due to the vapor velocity is here presented and assessed against experimental data. In the last Chapter, the application of condensation in the field of air cooling and dehumidification is experimentally studied. Tests during condensation from humid air were carried out in the laboratory of Eurapo S.r.l. testing finned-coil heat exchangers. The experimental data acquired with water-ethylene mixtures have been used to assess the numerical model of heat exchanger used by the company when the fluid flowing in the tubes is a water-glycol mixture. Furthermore, the possibility of increasing the efficiency of fan coil units by applying surface coatings has been investigated.
This work is financially supported by Eurapo S.r.l
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
Comparative analysis of CO2 and propane heat pumps for water heating: seasonal performance of air and hybrid solar-air systems
No full textThis study numerically compares the seasonal heating performance (SCOP) of three 15 kW heat pumps using natural refrigerants: two with carbon dioxide and one with propane. The propane system is an air-source heat pump (R290-AHP). The carbon dioxide systems include an air-source heat pump (R744-AHP) and a dual-source solar-air heat pump (R744-SAHP) equipped with finned-coil and photovoltaic-thermal evaporators that can work simultaneously. In the case of a transcritical carbon dioxide cycle, the use of several low-temperature sources is a promising solution to improve the performance of the system by enhancing the exploitation of renewable energy sources. Since the efficiency of air and solar-based systems is related to weather conditions and location, there is the need for accurate models to evaluate the SCOP. In this work, a numerical model has been developed to design the three 15 kW heat pumps and assess the SCOPs under variable space heating and domestic hot water demand profiles, using climatic data for Rome and Strasbourg. The results indicate that the R290-AHP consistently achieves the highest SCOP, while the R744-AHP performs the lowest. The R744-SAHP overperforms the R744-AHP by approximately 4 % regardless of heating demand characteristics. In particular, the results show that the performance of the three heat pumps is significantly influenced by the distribution of the thermal load throughout the day. Specifically, when the thermal load is concentrated during daylight hours, the heat pumps can operate at a higher SCOP, especially for the R744-SAHP, and also increase, up to 44 %, the self-consumed photovoltaic energy produced
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
Understanding the Role of Superhydrophobicity on Heat Transfer Enhancement During Dropwise Condensation in Humid Air Flow
No full textSuperhydrophobic surfaces have been extensively studied to enhance heat transfer during moisture condensation. However, the existing literature presents conflicting results, with some studies reporting enhanced performance while others observe a decline compared to hydrophilic surfaces. Furthermore, the effect of air velocity has been marginally addressed. In this work, superhydrophobic aluminum surfaces (advancing contact angle of 160°, contact angle hysteresis <1°) are fabricated by chemical etching followed by fluorosilane coating. Condensation tests are performed at constant air temperature (28 °C), while varying relative humidity (70%, 90%), dew-to-wall temperature difference (7–13 K) and air velocity (0.4–6 m s−1). It is found that, compared to the hydrophilic untreated surface, superhydrophobic surfaces do not offer any advantage at low air velocities (0.4 and 1 m s−1), while a condensation heat transfer coefficient increased by 30%–40% is achieved at high air velocities (4 and 6 m s−1). The performance is attributed to a more efficient droplet removal mechanism and enhanced vapor mass transfer through the non-condensable gas layer, which is also associated with droplet-induced vorticity, as confirmed by video analysis. The results clarify the operating conditions under which superhydrophobic surfaces are advantageous for applications of condensation from humid air, including dehumidification
Investigation of dropwise condensation of water at atmospheric and sub-atmospheric pressure through an individual-based model
No full textSteam condensation at sub-atmospheric pressure is a crucial process in a broad range of industrial applications and energy systems, such as in Rankine cycles and sorption desalination plants. Increasing the effectiveness of heat exchangers is fundamental to improve the efficiency of the whole system and a passive solution is offered by promoting dropwise condensation (DWC) instead of filmwise condensation. The few studies on DWC at sub-atmospheric pressures indicate a reduction of the condensation heat transfer coefficient (HTC) when lowering the saturation pressure. However, the extent of this reduction remains unclear, and the effect of critical parameters, such as surface wettability, coating thermal resistance, and nucleation sites density requires further clarification. This lack of knowledge is here addressed by employing an individual-based model (IBM) that exploits parallel computing to investigate the effect of sub-atmospheric saturation pressure on steam DWC. Firstly, the model is validated against measurements from the literature referring to sub-atmospheric conditions. Then, simulations are performed on a 3 × 3 mm2 computational domain, varying saturation pressures (in the range 4.2–143.3 kPa) and nucleation sites densities (Ns) from 109 m−2 to 1011 m−2. A decrease in HTC of 35 % is found when decreasing saturation temperature from 110 °C to 30 °C, regardless of Ns. Such penalization is associated with an increase of the droplet liquid–vapor interfacial (+ 1400 %) and conduction (+ 60 %) thermal resistances. Finally, a parametric study is conducted to unveil the influence of advancing contact angle (from 50° to 130°), contact angle hysteresis (from 5° to 25°), and coating thermal resistance (in the range 0.5–8 K m2 MW−1) on the DWC heat transfer at sub-atmospheric pressure
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
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