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Diani, Andrea (2014) Experimental and numerical analysis of microstructured surfaces. [Tesi di dottorato]

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Abstract (inglese)

Heat dissipation is one of the most important issues for the reliability of electronics equipment. Up today, air represents the most safe, cheap, and common working fluid for electronics thermal management applications. Due to its poor heat transfer characteristics, air always flow through enhanced surfaces, such as plain and louvered fins, pin fins, offset strip fins and wire screens, in order to increase the heat transfer area and to create turbulence. Recently, metal foams have been proposed as promising enhanced surfaces to improve the overall heat transfer performance of the cooling system.
In several applications air might be not enough for high level of heat dissipation, thus two-phase systems can represent a viable solution. Boiling is the heat transfer mechanism with the highest heat transfer coefficients, thus it can be used to spread high heat fluxes to maintain the wall temperature at low values with compact heat sinks. Microstructured surfaces, such as metal foams and microfin tubes, can exploit positive benefits on the flow boiling mechanism, i.e. they can promote bubble nucleation, reduce onset of nucleate boiling, augment two-phase mixing, enhance critical heat flux. On the other hand, the environmental issues associated to the use of synthetic refrigerants call for a continuous improvement of the technical solutions. Recently, new low-GWP refrigerants, in particular R1234ze(E) and R1234yf, have been proposed as possible alternatives of the traditional R134a.
This PhD thesis explores the use of microstructured surfaces for thermal management applications. Metal foams, plain finned and pin finned surfaces are experimentally and numerically investigated during air forced convection. In addition, single- and two-phase flow (vaporization) of refrigerants through a copper foam and in a microfin tube is experimentally studied.
The first chapter is focused on the air forced convection through metal foams. Nine copper foams are experimentally tested, and the overall heat transfer coefficients and pressure drops are calculated from the experimental measurements. The effects of the geometrical parameters (foam core height, pore density, and porosity) on the thermal and hydraulic behaviour of such materials are discussed. The experimental data points, coupled with other measurements previously obtained on aluminum foams, have permitted the development of a new semi-empirical equation for the estimation of the foam finned surface efficiency and of the heat transfer coefficient.
The air forced convection through plain finned and pin fin surfaces is discussed in the second chapter. Numerical simulations are performed on different geometrical configurations of fin thickness, pitch, and height for the plain finned surfaces, and different configurations of pin diameter, longitudinal and transverse pin pitch, and pin height for the pin fin surfaces. The effects of the geometrical characteristics on the thermal and hydraulic behaviour are reported. From the numerical results, four correlations have been developed for the estimation of the Colburn j-factor and friction factor for plain finned and pin fin surfaces. In the end, an optimization of a plain finned surface is reported.
The third chapter proposes a numerical approach to study the air forced convection through metal foams. The real structure of four copper foams, whose experimental results are reported in the first chapter, is obtained by micro-computed tomography scanned images. Once reconstructed, the real foams are meshed and the air flow simulated with a commercial software. Numerical results of pressure drop and heat transfer coefficient are compared against the experimental values.
The design and development of a new experimental facility to study the phenomenon of the flow boiling inside microstructured surfaces is reported in the fourth chapter. The numerical design of the test section, which hosts a 200 mm long metal foam, is presented. Every component of the set up is discussed in details. The results of the calibration tests are reported.
The flow boiling of refrigerants inside a metal foam is shown in the fifth chapter. The tested copper foam is 200 mm long, 10 mm wide, and 5 mm high. Three different refrigerants are studied: R134a, R1234ze(E), and R1234yf. R1234ze(E) and R1234yf (GWP=6 and 4, respectively) are possible substitutes of R134a (GWP=1400). Tests are run at a saturation temperature of 30 °C, which can be considered suitable for the case of electronic cooling applications, at different working conditions, in order to highlight the effects of the vapour quality, mass velocity, and heat flux on the thermal and hydraulic performance.
Finally, the sixth chapter reports some results about the flow boiling of refrigerants inside a 3.4 ID microfin tube. Three different refrigerants are studied: R134a, R1234ze(E), and R1234yf. As for the case of flow boiling inside a metal foam, tests are run at a saturation temperature of 30 °C under different working conditions, i.e. different vapour quality, mass velocity, and heat flux. The experimental results of heat transfer coefficient, vapour quality at the onset of the dryout, and pressure drop are compared against values predicted by correlations from the open literature

Abstract (italiano)

Lo smaltimento di calore è uno degli aspetti più importanti per l’affidibilità di componenti elettronici. Ad oggi, l’aria è il più sicuro, economico e utilizzato fluido operativo in applicazioni di raffreddamento di componentistica elettronica. A causa delle sue scarse qualità di scambio termico, l’aria fluisce attraverso superficie estese, come alette piane, cilindriche e louvered, “offset strip fins” e “wire screens”, per aumentare la superficie di scambio termico e per creare turbolenza. Recentemente, le schiume metalliche sono state proposte come promettenti superfici estese per incrementare le prestazioni termiche del sistema di raffreddamento.
Tuttavia, l’aria potrebbe non essere sufficiente nel caso in cui i flussi termici da asportare siano particolarmente alti e pertanto i sistemi bifase possono essere una soluzione attuabile. La vaporizzazione è il meccanismo di scambio termico con i maggiori coefficienti di scambio termico, pertanto può essere usato per dissipare elevati flussi termico e mantenere la temperatura di parete del dissipatore entro limiti che siano compatibili con quelli delle apparecchiature elettroniche. Superfici microstrutturate, come schiume metalliche e tubi microalettati, possono avere benefici nella vaporizzazione, cioè possono incrementare i siti di nucleazione delle bolle, anticipare l’ebollizione nucleata, aumentare il miscelamento tra la fase liquida e vapore, aumentare il flusso termico critico. Importanti sono anche gli aspetti ambientali associati a refrigeranti sintetici, situazione che richiede un miglioramento delle soluzioni tecniche attualmente impiegate. Recentemente, nuovi refrigeranti a basso impatto ambientale, in particolare l’R1234ze(E) e l’R1234yf, sono stati proposti come alternative al tradizionale R134a.
Questa tesi di dottorato esplora l’uso di superfici microstrutturate in sistemi di raffreddamento. Sono state studiate sperimentalmente e numericamente schiume metalliche, alette piane e cilindriche durante la convezione forzata di aria. Inoltre,è stato sperimentalmente studiato il deflusso monofase e bifase (vaporizzazione) di refrigeranti in una schiuma metallica in rame e all’interno di un tubo microalettato.
Il primo capitolo si focalizza sulla convezione forzata di aria attraverso schiume metalliche. Nove schiume in rame sono sperimentalmente studiate e dalle misure sperimentali vengono calcolati i coefficienti globali di scambio termico e le perdite di carico. Vengono discussi gli effetti dei parametri geometrici (altezza della schiuma, densità di pori e porosità) sul comportamento termico e idraulico di tali materiali. I punti sperimentali raccolti, insieme ad altre misure sperimentali precedentemente ottenute su schiume in alluminio, hanno permesso lo sviluppo di una correlazione per la stima dell’efficienza e del coefficiente di scambio termico.
La convezione forzata di aria attraverso alette piane e cilindriche è discussa nel secondo capitolo. Sono state condotte simulazioni numeriche su differenti configurazioni geometriche di spessore, passo e altezze delle alette nel caso di alette piane, e di diametro, passo longitudinale e trasversale e altezza nel caso di alette cilindriche. Vengono riportati gli effetti delle caratteristiche geometriche sul comportamento termico e idraulico. Dai risultati numerici, sono state sviluppate quattro correlazioni per la stima del fattore j di Colburn e del fattore f di attrito per alette piane e cilindriche. Infine, è riportato un esempio di ottimizzazione di una superficie con alette piane.
Il terzo capitolo propone un approccio numerico alla modellizazione della convezione forzata di aria in schiume metalliche. La reale struttura di quattro schiume in rame, i cui risultati sperimentali sono riportati nel primo capitolo, è ottenuta mediante immagini ottenute con la tecnica della microtomografia. Il deflusso di aria è quindi simulato con un software commerciale. I risultati numerici sulle perdite di carico e sui coefficienti di scambio termico sono quindi confrontati con i risultati sperimentali.
Il dimensionamento e lo sviluppo di un nuovo impianto sperimentale per lo studio del fenomeno della vaporizzazione in superfici microstrutturate è riportato nel quarto capitolo. Viene presentato lo sviluppo mediante un codice numerico della sezione di prove, che alloggerà una schiuma metallica lunga 200 mm. Ogni componente dell’impianto è discusso in dettaglio. Infine vengono riportati i risultati della calibrazione dell’impianto.
I risultati relativi alla vaporizzazione di refrigeranti all’interno di una schiumametallica sono presentati nel quinto capitolo. La schiuma metallica in rame è lunga 200 mm, larga 10 mm e alta 5 mm. Tre diversi refrigeranti sono studiati: R134a, R1234ze(E), and R1234yf. L’R1234ze(E) e l’R1234yf (GWP=6 e 4, rispettivamente) sono possibili sostituti dell’R134a (GWP=1400). Le prove sperimentali sono state condotte ad una temperatura di saturazione di 30 °C, che è un valore idoneo al caso di raffreddamento di componenti elettronici, in diverse condizioni operative, al fine di evidenziare gli effetti del titolo di vapore, della portata specifica e del flusso termico sulle performance termiche ed idrauliche.
Nel sesto ed ultimo capitolo vengono riportati alcuni risultati sulla vaporizzazione di refrigeranti all’interno di tubo microalettato avente un diametro interno di 3.4 mm. Tre diversi refrigeranti sono studiati: R134a, R1234ze(E), and R1234yf. Come nel caso precedente, le prove sono state condotte ad una temperatura di saturazione di 30 °C in diverse condizioni operative, cioè a diverso titolo di vapore, portata specifica e flusso termico. I risultati sperimentali del coefficiente di scambio termico, del titolo di vapore all’inizio della crisi termica e delle perdite di carico sono confrontati con i valori stimati da alcune correlazioni empiriche proposte in letteratura

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Tipo di EPrint:Tesi di dottorato
Relatore:Rossetto, Luisa
Dottorato (corsi e scuole):Ciclo 26 > Scuole 26 > INGEGNERIA INDUSTRIALE > INGEGNERIA DELL' ENERGIA
Data di deposito della tesi:30 Gennaio 2014
Anno di Pubblicazione:30 Gennaio 2014
Parole chiave (italiano / inglese):convezione forzata, microtomografia, CFD, coefficienti di scambio termico, perdite di carico, crisi termica, vaporizzazione, schiume metalliche, tubo microalettato / forced convection, microtomography, CFD, heat transfer coefficient, pressure drop, dryout, flow boiling, metal foam, microfin tube
Settori scientifico-disciplinari MIUR:Area 09 - Ingegneria industriale e dell'informazione > ING-IND/10 Fisica tecnica industriale
Struttura di riferimento:Dipartimenti > Dipartimento di Ingegneria Industriale
Codice ID:6657
Depositato il:12 Nov 2014 12:16
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