1,721,251 research outputs found
"Scambio termico e fluidodinamica durante i deflussi bifase e monofase su superfici estese e in microgeometrie" "Two-phase and single-phase heat transfer and fluid flow through enhanced surfaces and in microgeometries"
No full textThe heat transfer between two fluids or between a fluid and a surface, during both single-phase and two-phase flow, is intimately linked to the geometrical characteristic of the heat transfer surface involved. Generally speaking, if the heat transfer area increases, the global heat transfer also increases, but if the surface is developed to improve heat transfer, the overall performance can be higher.
Compact and efficient heat exchangers and heat sinks are more and more demanded for electronics cooling applications, refrigeration and air conditioning systems. For this reason, the development of new enhanced surfaces is a critical issue of thermal research.
In this Ph. D. thesis, two arguments connected with enhanced heat transfer surfaces have been experimentally and analytically studied. In particular, the condensation of R410A inside a microfin tube and air forced convection through different enhanced surfaces have been analysed.
With respect to two-phase flow, the heat transfer coefficients and the pressure drops during the condensation of R410A at 40 °C of saturation temperature inside a microfin tube have been measured. The experimental analysis have been carried out at the Dipartimento di Fisica Tecnica of the University of Padova.
Using a database of 4000 experimental data points, a new correlation for the computation of the heat transfer coefficient during the condensation of several natural and synthetic refrigerants inside different enhanced microfinned tubes has been developed.
With respect to single-phase flow, a new experimental test rig has been developed, designed and built at the laboratory of the Dipartimento di Fisica Tecnica of the University of Padova. This test rig allows to carry out accurate heat transfer and pressure drop measurements during air forced convection through different enhanced surfaces, such as traditional finned surfaces, metal foams and microgeometries, etc.
The apparatus has been calibrated by testing a traditional finned surface for electronics cooling applications. Then, the heat transfer coefficients and the pressure drops during air forced convection through five different aluminum foams have been measured. All tested samples have been heated by imposing different heat fluxes, which have been varied between 150 W and 400 W. The experimental results have been compared with those obtained for the traditional finned surface.Lo scambio termico tra due fluidi o tra un fluido e una superficie, sia esso monofase o bifase, è profondamente legato alle caratteristiche geometriche delle superfici di scambio con le quali i fluidi stessi sono in contatto. Aumentare la superficie di scambio termico significa, in generale, incrementare lo scambio termico globale. Se poi questa superficie viene sviluppata per migliorare l’efficienza dello scambio termico, le prestazioni possono crescere ulteriormente. Il raffreddamento di componenti elettronici, la refrigerazione e il condizionamento dell’aria richiedono scambiatori di calore sempre più efficienti e compatti, pertanto lo sviluppo di nuove superfici di scambio termico risulta essere un obiettivo fondamentale della ricerca.
In questo lavoro di tesi sono stati affrontati, sia sperimentalmente che analiticamente, due argomenti connessi allo scambio termico su superfici estese. In particolare, si sono studiati la condensazione all’interno di tubi micro-alettati e la convezione forzata di aria su schiume metalliche e su superfici alettate tradizionali. Per quanto riguarda lo scambio termico bifase, sono stati misurati i coefficienti di scambio termico e le perdite di carico durante la condensazione di R410A all’interno di un tubo micro-alettato, alla temperatura di saturazione di 40 °C. Le prove sono state realizzate utilizzando l’impianto presente presso il Dipartimento di Fisica Tecnica dell’Università di Padova.
Successivamente, utilizzando un database contenente più di 4000 dati sperimentali, è stato sviluppato un nuovo modello per la stima del coefficiente di scambio termico durante la condensazione di refrigeranti naturali e sintetici all’interno di tubi micro-alettati.
Con riferimento allo scambio termico monofase, un nuovo impianto sperimentale è stato sviluppato, progettato e costruito presso il Dipartimento di Fisica Tecnica dell’Università di Padova. Questo apparato permette di eseguire accurate misure di scambio termico e perdite di carico durante la convezione forzata di aria attraverso superfici estese, quali tradizionali superfici alettate, schiume metalliche, microgeometrie, ecc. L’impianto sperimentale è stato calibrato testando una superficie alettata tradizionale per il raffreddamento di componenti elettronici. Successivamente, sono stati misurati i coefficienti di scambio e le perdite di carico di cinque schiume di alluminio durante il deflusso di aria. Tutti i provini sono stati riscaldati elettricamente imponendo differenti flussi temici tra 150 W e 400 W. I risultati sperimentali sono stati confrontati con quelli ottenuti per la superficie alettata tradizionale
An assesment on forced convection in metal foams
Full text linkMetal foams are a class of cellular structured materials with open cells randomly
oriented and mostly homogeneous in size and shape. In the last decade, several authors have
discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for
air conditioning, refrigeration, and electronic cooling applications. This paper reports an
assessment on the forced convection through metal foams presenting experimental and
analytical results carried out during air heat transfer through twelve aluminum foam samples
and nine copper foam samples. The metal foam samples present different numbers of pores per
linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896-0.956;
samples of different heights have been studied. From the experimental measurements two
correlations for the heat transfer coefficient and pressure drop calculations have been
developed. These models can be successfully used to optimize different foam heat exchangers
for any given application
Shell and tube carbon dioxide gas coolers - Experimental results and modelling
No full textThis paper experimentally compares the heat transfer performance of four different shell and tube gas coolers implemented in a 5 kW, R744 water/water heat pump controlled by a back pressure valve as expansion device. The tubes bundle consists of 10 tubes in a 30° arrangement for all the gas coolers with different tube geometries: smooth, corrugated, internally grooved, and corrugated and internally grooved, respectively. The results were carried out at fixed gas cooler inlet water temperature of around 25 °C and by imposing two inlet gas cooling pressures: 8 MPa and 10 MPa, and by varying the water flow rate from 340 to 786 l h-1. A step-by-step procedure for the simulation of the heat transfer cooling process of the carbon dioxide in shell and tubes gas coolers is proposed and validated, allowing for a direct comparison of the heat transfer performance of the shell and tube gas coolers
Effects of lubricant on carbon dioxide heat transfer in transcritical refrigerating cycles
No full textSimultaneous Optical and Electrical Impedance-Based Monitoring Of the Liquid Fraction during Solidification inside a Vertical Enclosure
No full textLatent thermal energy storage (LTES) with phase change materials (PCMs) plays a critical role in the decarbonization of the cooling and heating sectors. The real-time monitoring of the liquid fraction during phase transitions—melting and solidification—offers essential insights into optimizing LTES design and facilitating the implementation of smart control strategies that maximize the utilization of renewable energy sources [1]. However, monitoring the liquid fraction during solid-liquid phase change poses a trade-off between accuracy and ease of implementation. Conventional methods for sensing thermal and flow variables, such as temperature, pressure, and flow rate, often exhibit limited precision in estimating the liquid fraction [2]. More advanced techniques, such as optical, X-ray, and ultrasonic imaging, can provide detailed information regarding the phase distribution but can be difficult to implement in real-world LTES [3], [4], [5]. Within this framework, electrical impedance-based sensing appears to be an accurate and easy-to-implement approach for liquid fraction measurement [6], [7], [8], [9]. The current study evaluates the feasibility of an electrical impedance-based sensor for liquid fraction measurement. The experimental study analyses the solidification of demineralized water inside vertical rectangular enclosure. A novel and specifically designed test section has been developed, incorporating Peltier cooling modules to regulate the temperature of the enclosure's side walls. Demineralized water is contained in a vertical enclosure with a width of 10 mm, a height of 100 mm and a depth of 100 mm. The complex impedance between two 90 mm x 90 mm square electrodes located on the side walls of the enclosure is measured with an LCR meter. Furthermore, the front and back walls of the test section are transparent, facilitating direct visualization of the phase distribution with a USB optical camera. For the solidification experiments, the water is allowed to reach thermal equilibrium at 10 °C. Then, the external temperature of the side walls suddenly changed to -12 °C. Optical images and electrical impedance measurements are continuously acquired until the solidification process is complete. During solidification, a layer of ice starts forming on each of the side walls. The ice layers grow until they merge in the centre of the enclosure. The development of ice layers on the electrode surface results in a continuous increase in electrical impedance between the electrodes, attributable to the electrically insulating properties of ice as opposed to those of liquid water. An equivalent electrical circuit model is employed to estimate the temporal evolution of the liquid fraction based on electrical impedance measurements. Concurrently, image processing techniques are used to estimate the liquid fraction from the optical images, in which the solid-liquid interface is clearly distinguishable. Both measurement methodologies demonstrate a nearly linear decrease in liquid fraction over time, with the liquid fraction estimates from each technique showing excellent concordance, exhibiting a maximum deviation of approximately 0.08. Hence, electrical impedance-based sensing is found to be an accurate and nonintrusive approach to determine the liquid. The relative simplicity of impedancebased sensing as compared with direct optical visualization favours its implementation in real-world LTES
Characterization of electrical properties of phase change materials for liquid fraction sensing via electrical tomography
No full textElectrical impedance tomography could be an accurate and non-intrusive alternative to estimate liquid fraction in phase change materials (PCMs). However, the electrical properties of most PCMs have not yet been characterized. In the present study, Rubitherm RT54HC and Sodium Acetate Trihydrate (SAT) are characterized via electrical impedance spectroscopy. The measurements are performed in liquid and solid state in a temperature range close to the melting point and a frequency range between 4 Hz to 500 MHz. Rubitherm RT54HC is found to be a dielectric with low electrical conductivity and low electrical permittivity in both liquid and solid phases. Hence, material development strategies are required to make electrical tomography sensing feasible. On the other hand, SAT is electrically conductive due to ion transport and presents an innate contrast in electrical conductivity between the liquid and solid phases. Consequently, SAT seems to be better suited for electrical tomography sensing
Grey-box modelling of melting inside a rectangular enclosure for predictive control strategies of latent thermal energy storages
No full textAn optimized operation of latent thermal energy storage requires the implementation of smart control strategies, which are based on simplified models that can predict the behavior of the system under diverse operating conditions. Numerical simulations are too computationally expensive to be used in real-time control. The present study proposes a grey-box modelling approach for melting inside a rectangular enclosure with an isothermal side wall. Parametric numerical simulations are performed that consider a range of wall temperature values to generate a reference dataset. The grey-box model uses interpolation on the reference dataset for the non-dimensional groups that describe the problem. The performance of the model is assessed for a case with a transient wall temperature profile and the results of the grey-box model closely match the simulation results. The proposed grey-box modelling approach captures the main dynamics of the solid-liquid phase change process including latent and sensible heat effects
Sensitivity of an electrical impedance-based sensor for the liquid fraction estimation during melting and solidification inside a vertical rectangular enclosure
No full textMonitoring the liquid fraction in latent thermal energy storages (LTESs) can enable the implementation of smart control strategies for improved performance and increased utilization of renewable energy sources. However, measuring the liquid fraction is challenging because solid-liquid phase change processes occur at nearly constant temperature. The present study explores an electrical impedance-based sensing technique that could become a non-intrusive and accurate alternative to determine the liquid fraction. The study examines the melting and solidification of a phase change material (PCM) inside a vertical rectangular enclosure with an isothermal wall. Two sets of electrodes are located on each of the side walls of the enclosure. Initially, numerical simulations of melting and solidification are performed to generate physically meaningful solid and liquid phase distributions. Then, these phase distributions are used as input for electrical simulations to estimate the response to changes in the liquid fraction of the electrical impedance between electrode pairs in in-line, front-facing and crosswise configurations. Also, the effect of the contrast ratios between the electrical conductivity of the solid and liquid phase on the sensitivity is assessed. The front-facing electrodes are found to have the best performance across a large range of conductivity ratios. The in-line electrodes perform the lowest because the sensitivity drastically decreases as a layer of the more conductive phase starts forming on the wall where the electrodes are located. An improvement in the performance of in-line electrodes is observed when the conductivity ratio is decreased
Electrical impedance solid fraction sensor for a panel filled with nanocomposite phase change material
No full textMonitoring the solid fraction in a latent thermal energy storage system can enable the implementation of smart control strategies for improved performance. However, measuring the solid fraction in a non-invasive, easy-to-implement and accurate manner is challenging. The present study analyzes a parallel plate impedance sensor to measure the solid fraction in a panel filled with phase change material (PCM). The use of nano-additives is proposed to increase the contrast in electrical properties between the solid and liquid phases, which increases the sensor sensitivity. An equivalent electrical circuit model is used to estimate the sensor impedance. The sensor sensitivity is assessed for a nanocomposite PCM (NPCM) of hexadecane with functionalized carbon nanotubes. The electrical properties of the solid and liquid NPCM are drastically different at low frequencies, but the impedance for the melted PCM can be unpractically large. As the frequency increases, the impedance and sensor sensitivity decrease simultaneously. Hence, proper sensor design requires a balance between sensor size and excitation frequency
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