In a worldwide pursuit for more Accident Tolerant nuclear Fuel (ATF), the quest to obtain and certify alternative nuclear fuel cladding tubes for light-water nuclear power reactors is still a key challenge. One of the facets in this program to develop more ATF is the heat transfer evaluation between the various proposed clad tubes manufactured from suitable replacement materials and the current problematic zirconium-alloy based clad tubes used in nuclear power reactors. For the heat transfer analysis, the accurate measurement of the temperature on the heat transfer surface of heated tubes to be tested was one of the important objectives for the effective analysis of the heat transfer characteristics to the water coolant. After extensive investigations, a suitable technique was developed and validated against recognised forced-convection heat transfer correlations. The results showed that this technique was well suited for external forced convection heat transfer studies from heated surfaces exposed to forced convection water coolants. / Dissertation (MSc)--University of Pretoria, 2019. / Mechanical and Aeronautical Engineering / MSc (Applied Science - Mechanics) / Unrestricted

Nucleating boiling is a repeating cycle of bubble initiation, growth and departure at many nucleation sites at the heated wall. Thereby, the bubble growth process significantly affects the dynamics of bubble departure. Experiments were performed to study the influence of heater surface characteristics, such as wettability and roughness, on single bubble growth and departure dynamics for natural circulation and upward flow boiling conditions. Self-assembled monolayer (SAM) coating, wet-etching and femtosecond pulsed laser treatment were used to alter the surface wettability and produce nano- and microstructures on stainless steel surfaces with a roughness in the range of micrometers. These surface preparation techniques allowed to separately quantify the effect of surface wettability and roughness on the bubble dynamics. The surface wettability and roughness are represented by the liquid contact angle hysteresis (θhys) and root mean square roughness of the surface (Sq). Boiling experiments were conducted at atmospheric pressure with degassed deionized water at low-subcooling. Stainless steel heater surfaces were vertically oriented during natural circulation boiling. In the experiments, bubbles were generated from an artificial nucleation cavity on the treated stainless steel heater surfaces. High-resolution optical shadowgraphy has been used to record the bubble generation, departure, sliding, detachment and inception of the next bubble. Higher bulk liquid velocity yielded smaller bubble departure diameters and slower bubble growth rates for all heater surface types. The effect of surface wettability on single bubble dynamics was studied for smooth surfaces with different liquid contact angle hysteresis. Low wetting surfaces yielded a greater bubble growth rate and departure diameter. The bubble growth rate and departure diameter were found maximum for an intermediate surface roughness Sq between 0.108 and 0.218 m. The corresponding roughness height is referred to as the ‘optimal roughness height’ in this work. Surface roughness was found very influential to the bubble growth and departure, which can be explained by considering its interaction with the microlayer underneath a bubble. The role of the heater surface parameters for the bubble growth was qualitatively assessed by evaluating the microlayer thickness constant C2. Hence, an improved bubble growth model was derived in this work. The bubble growth model was formulated on the basis of the evaporation of the microlayer beneath a bubble with the dryout area, inertia and heat diffusion controlled bubble growth and condensation at the bubble cap. The model can also predict the superheated liquid layer around a bubble which helps to determine the portion of a bubble that is in contact with the subcooled liquid. As bubble growth Abstract is highly dependent on the effective interactions of heater surface roughness and microlayer, a term Ceff was introduced in the bubble growth model. The effective microlayer thickness constant Ceff incorporates the impact of heater surface characteristics on the bubble growth process until the departure of a bubble. The bubble growth model was utilized in the analysis of high-resolution experimental data of steam bubble growth and the values of Ceff were calculated for different heater surface characteristics. The value of Ceff was found to decrease with the increase of bubble growth rate. A simplified model for the bubble departure criterion was derived from the expressions of forces which act on a nucleating bubble throughout its growth cycle. It was found that 90% of the departing bubbles satisfy the bubble departure criterion model with ±25% deviation. The knowledge gained from this work shall be particularly useful to improve nucleate boiling models for numerical simulations. The findings are also useful for designing heater surfaces in the future.:Abstract v Kurzfassung vii Acknowledgements xiii Abbreviations and Symbols xv Chapter 1: Introduction and Motivation 1 1.1 General overview 1 1.2 Theoretical background 3 1.3 Objectives and outline of the thesis 7 Chapter 2: Fundamentals of Bubble Dynamics in Nucleate Boiling 9 2.1 Bubble growth in nucleate boiling 9 2.2 Bubble growth models 12 2.3 The physical process of bubble departure 16 2.4 Experimental investigations of bubble dynamics 20 2.4.1 Effects of heater surface characteristics 21 2.4.2 Effects of bulk liquid velocity 24 2.5 Chapter conclusion 26 Chapter 3: Heater Surface Preparation and Characterization 27 3.1 Surface properties 27 3.2 Surface preparation 29 3.2.1 Self-assembled monolayer coating 30 3.2.2 High-power pulsed laser irradiation 31 3.2.3 Wet-etching 32 3.3 Surface cleaning 32 3.4 Surface characterization 32 3.4.1 Wettability measurement 32 3.4.2 Roughness measurement 33 3.4.3 Analysis of surface characteristics 34 3.4.4 Uncertainty of surface parameters 38 3.5 Artificial cavity preparation 38 Chapter 4: Experimental Setup and Procedure 41 4.1 Natural circulation boiling (NCB 41 4.1.1 Experimental procedure and measurement techniques 41 4.1.2 Uncertainty analysis 44 4.2 Upward flow boiling (UFB) 45 4.2.1 Experimental procedure and measurement techniques 45 4.2.2 Uncertainty analysis 48 4.3 Image processing 50 Chapter 5: Experimental Results 53 5.1 Introduction to the analysis of the bubble dynamics 53 5.1.1 The bubble life cycle 53 5.1.2 Calculation of the bubble equivalent diameter 55 5.1.3 Bubble dynamics with the increase of heat flux 57 5.1.4 Qualitative assessment of the bubble dynamics for different parameters 60 5.2 Bubble dynamics 61 5.2.1 Effect of heater surface wettability 61 5.2.2 Effect of heater surface roughness 65 5.2.3 Effect of bulk liquid velocity 70 5.3 Bubble departure 76 5.3.1 Effect of heater surface wettablity 76 5.3.2 Effect of heater surface roughness 76 5.3.3 Effect of bulk liquid velocity 78 5.4 Chapter conclusion 79 Chapter 6: Analysis and Model Development 81 6.1 Numerical evaluation of the role of heater surface characteristics 81 6.1.1 Derivation of an improved bubble growth model 86 6.1.2 Calculation of Ceff 82 6.2 Effect of liquid velocity on the bubble growth 93 6.3 Improved modeling of bubble departure 95 6.3.1 Analysis of important parameters 95 6.3.2 Formulation of a bubble departure criterion 100 6.4 Chapter conclusion 102 Chapter 7: Summary and Outlook 105 Bibliography 109 List of Figures 121 List of Tables 127 Appendix: Surface Parameters and Profile 129 / Der Blasenabriss von einer Keimstellenkavität ist ein komplexer Ablösemechanismus und spielt eine wichtige Rolle beim Wärmetransport. Zur Beschreibung der Blasendynamik sind Kenntnisse über den Blasenwachstumsprozess sowie die Vorhersage eines Kriteriums für die Blasenablösung erforderlich. In den existierenden Blasenwachstums- und Blasenablösungsmodellen wird die Oberflächencharakteristik des Heizers bisher nicht berücksichtigt. Im Rahmen dieser Promotion wurden Experimente durchgeführt, um den Einfluss der Heizeroberfläche und der Hauptströmungsgeschwindigkeit auf diese Parameter für eine vertikale Heizfläche zu untersuchen. Hierbei wurden das Naturkonvektionssieden und das aufwärtsgerichtete Strömungssieden betrachtet. Die Experimente wurden mit vollentsalztem Wasser bei einer Unterkühlung zwischen 1,68 und 4,00 K bei Atmosphärendruck und einem aus Edelstahl gefertigten Heizer durchgeführt, dessen Oberfläche anhand der Parameter Oberflächenrauigkeit und Benetzbarkeit charakterisiert ist. Unterschiedliche Oberflächenbearbeitungstechniken, wie Beschichtung durch Self-Assembled Monolayer (SAM), Nass-Ätzen und Hochleistungspuls-Laserbestrahlung wurden genutzt, um die Oberflächenbenetzung und –rauigkeit zu modifizieren. Der Unterschied zwischen dem gemessenen Fortschritts- (θadv) und Rückzugskontaktwinkel (θrec) der Flüssigkeit wird als Flüssigkeitskontaktwinkelhysterese (θhys) bezeichnet und beschreibt die Oberflächenbenetzbarkeit. Die Oberflächenrauigkeit wurde durch ein Konfokal-Mikroskop bestimmt und durch das gemittelte Quadrat der Rauigkeit (Sq) und den Maximalwert der Rauigkeit (St) definiert. Insgesamt wurden 18 unterschiedliche Heizoberflächen mit einer Größe von 130 x 20 mm² untersucht. Davon kamen jeweils die Hälfte für das Naturkonvektionssieden bzw. aufwärtsgerichtetes Strömungssieden zur Anwendung. Der Einfluss der Oberflächenbenetzbarkeit auf die Blasendynamik wurde für polierte Oberflächen (Sq  0,01 μm) analysiert. Die Wirkung der Oberflächenrauigkeit auf die Blasendynamik wurde für konstante Flüssigkeitskontaktwinkelhysteresen von 40,05°±1,5° und 59,97°±1,5° für Naturzirkulation und Strömungssieden untersucht. Eine künstliche zylindrische Kavität mit einer Fläche von 1963,5 m² und einer Tiefe von 50 m wurde mittels Mikrolaser in die Heizoberflächen eingebracht, um die Blasen in einer spezifischen Position zu erzeugen. Während des Naturkonvektionssiedens betrug die Wärmestromdichte 19,22 bis 30,29 kW/m². Bei den Experimenten mit aufwärtsgerichtetem Strömungssieden wurde die Hauptströmungsgeschwindigkeit im Bereich von 0,052 bis 0,183 m/s variiert und eine Appendix: Surface Parameters and Profile Wärmestromdichte zwischen 39,41 und 45,47 kW/m² aufgeprägt. Daraus resultierten insgesamt 87 Experimentalserien. Um den Blasenlebenszyklus zu erfassen, wurde hochauflösende Bildgebungstechnik verwendet. Mit der Bildverarbeitungssoftware ImageJ wurden die erfassten Videos weiterverarbeitet. Die Temperatur der Hauptströmung wurde mit Typ-K Thermoelementen gemessen. Die zeit- und ortsgemittelten Heizerwandtemperaturen wurden für die Naturzirkulation durch Infrarotthermografie und für das aufwärtsgerichtete Strömungssieden durch Typ-K Thermoelemente erfasst. Die mittlere Flüssigkeitsgeschwindigkeit wurde bei der Naturzirkulation mittels Particle Image Velocimetry (PIV) und beim Strömungssieden mittels Coriolis-Durchflusszähler bestimmt. Eine hochauflösende optische Schattenbildtechnik diente zur Aufzeichnung der Hauptphasen des Blasenlebenszyklus: Blasenerzeugung, Blasenwachstum, Blasenablösung, Blasengleiten und Blasenabriss. In dieser Arbeit wurden die der Blasenablösung vorrausgehenden Phasen untersucht. Blasenhöhe, Blasenbreite, Blasenbasisdurchmesser und Schwerpunkt der Blase wurden mit Hilfe der Bildverarbeitung ermittelt. Der blasenäquivalente Durchmesser wurde mittels des geometrischen Mittelwertes, der Blasenbreite und der Blasenhöhe berechnet. Basierend auf den Messdaten können folgende Erkenntnisse für das Blasenwachstum und den Blasenablösemechanismus postuliert werden: (i) Eine höhere Wärmeströmedichte führen zu größen Blasen und kürzeren Wachstumsperioden. Der Einfluss der Oberflächenbenetzbarkeit und der Oberflächenrauigkeit auf die Blasendynamik zeigt ähnliche Tendenzen für Naturkonvektion und aufwärtsgerichtetes Strömungssieden. (ii) Eine höhere Flüssigkeitskontaktwinkelhysterese führt zu einer schnelleren Expansion der Blasenbasis und zu einem schnellern Blasenwachstum. Für gut benetzbare Oberflächen bewegt sich der Blasenschwerpunkt schneller entlang der Strömungsrichtung. Für Oberflächen mit geringer Benetzbarkeit ist die Blasengröße vor der Blasenablösung größer und die Ablöseperiode länger. Der mittlere Blasenablösedurchmesser für unterschiedliche Hauptströmungsgeschwindigkeiten der Flüssigkeit erhöht sich von 0,75 auf 1,75 mm bei zunehmender Flüssigkeitskontaktwinkelhysterese von 42,32° auf 62,30°. (iii) Eine, bezogen auf die Mikrogrenzschichtdicke, optimale Oberflächenrauigkeit erhöht die Blasenwachstumsrate und die Blasengröße. Dieses Ergebnis ist bisher einzigartig bei der Untersuchung der Einzelblasendynamik beim Blasensieden. Die Expansion der Blasenbasis und der Blasenwachstumsrate erreicht ein Maximum für das gemittelte Quadrat der Rauigkeit (Sq) im Bereich zwischen 0,156 und 0,202 m für Naturzirkulation. Für aufwärtsgerichtetes Strömungssieden war die Expansion der Blasenbasis und die Blasenwachstumsrate für Sq-Werte zwischen 0,108 und 0,218 m maximal. Der Blasenablösedurchmesser wurde für einen großen Bereich der Hauptströmungsgeschwindigkeiten und Wärmestromedichte gemittelt. Das Maximum des mittleren Ablösedurchmessers wurde für die Oberfläche mit einem Wert von Sq = 0,218 m erreicht. Die Oberflächenrauigkeit erweitert die Wärmeübertragungsoberfläche neben der Blasenbasis. Der Einfluss der Oberflächenrauigkeitshöhe auf die Blasen hängt von der Mikrogrenzschichtdicke sowie vom Blasenbasisradius ab. Das Modell der Mikrogrenzschichtdicke von Cooper und Lloyd [1] und die konzeptionelle Idee zur Störung der Mikrogrenzschicht durch die Rautiefe von Sriraman [2] wurden analysiert. Es wurde nachgewiesen, dass die Oberflächenrauigkeit die effektive Mikrogrenzschichtdicke und die dazugehörige Wärmeübertragung beeinflusst. (iv) Es wurden geringere Blasenwachstumsraten für höhere Hauptströmungs-geschwindigkeiten gemessen. Weiterhin reduzieren sich der Blasenablösedurchmesser sowie Ablöseperioden mit zunehmender Hauptströmungsgeschwindigkeit bei unterschiedlichen Wärmeoberflächencharakteristiken. Bei niedrigen Hauptströmungs-geschwindigkeiten im Bereich zwischen ungefähr 0,052 und 0,16 m/s reduziert sich der durchschnittliche Blasenablösedurchmesser deutlich. Die experimentellen Ergebnisse zeigen einen wesentlichen Einfluss der Oberflächenbeschaffenheit auf das Blasenwachstum und den Ablöseprozess beim Blasensieden. Um diesen Einfluss numerisch zu charakterisieren, wurde ein neues Blasenwachstumsmodel entwickelt. Existierende Blasenwachstumsmodelle berücksichtigen den umfangreichen Einfluss der Oberfläche des Heizers bisher nicht. Das vorgeschlagene Model bezieht die plausibelsten Mechanismen des Blasensiedens mit ein. Dazu zählen: Mikrogrenzschichtverdampfung im Bereich der Austrocknung, trägheits- und wärmediffusionskontrolliertes Blasenwachstum und Kondensation an der Blasenoberseite. Das Modell berücksichtigt, dass die überhitzte Flüssigkeitsschicht an der Heizerwand durch die wachsende Blase nach außen verdrängt wird und die so gestreckte Flüssigkeitsschicht einen Teil der Blase einhüllt. Kondensation erfolgt an der Blasengrenze, die in Kontakt mit der unterkühlten Flüssigkeit steht, und demzufolge mit der überhitzen Flüssigkeitsschicht nicht in Kontakt kommt. Das vorgeschlagene Blasenwachstumsmodel arbeitet mit drei Konstanten für die beschriebenen Wärmeübertragungsmechanismen beim Blasenwachstum. Dabei handelt es sich um eine Konstante für die effektive Mikrogrenzschichtdicke (Ceff ), eine weitere Konstante 𝑏 ́ für die Wärmediffusion hin zur Blase und der Trägheit sowie letztendlich einer Konstante S zur Abbildung des Kondensationswärmeübergangs, anhand der Beschreibung des Anteils der Blase, welcher in Kontakt mit der unterkühlten Flüssigkeit steht. Die effektive Mikrogrenzschichtdickenkonstante (Ceff) definiert den Einfluss der Heizoberflächencharakteristik auf die Verdampfung der Mikrogrenzschicht und somit die Blasenwachstumsrate beim Blasensieden. Die numerisch berechnete und experimentell gemessene Blasengröße wurde verglichen, um die Mikrogrenzschichtdickenkonstante Ceff zu definieren. Der Einfluss der Kondensation auf Ceff wurde geprüft.:Abstract v Kurzfassung vii Acknowledgements xiii Abbreviations and Symbols xv Chapter 1: Introduction and Motivation 1 1.1 General overview 1 1.2 Theoretical background 3 1.3 Objectives and outline of the thesis 7 Chapter 2: Fundamentals of Bubble Dynamics in Nucleate Boiling 9 2.1 Bubble growth in nucleate boiling 9 2.2 Bubble growth models 12 2.3 The physical process of bubble departure 16 2.4 Experimental investigations of bubble dynamics 20 2.4.1 Effects of heater surface characteristics 21 2.4.2 Effects of bulk liquid velocity 24 2.5 Chapter conclusion 26 Chapter 3: Heater Surface Preparation and Characterization 27 3.1 Surface properties 27 3.2 Surface preparation 29 3.2.1 Self-assembled monolayer coating 30 3.2.2 High-power pulsed laser irradiation 31 3.2.3 Wet-etching 32 3.3 Surface cleaning 32 3.4 Surface characterization 32 3.4.1 Wettability measurement 32 3.4.2 Roughness measurement 33 3.4.3 Analysis of surface characteristics 34 3.4.4 Uncertainty of surface parameters 38 3.5 Artificial cavity preparation 38 Chapter 4: Experimental Setup and Procedure 41 4.1 Natural circulation boiling (NCB 41 4.1.1 Experimental procedure and measurement techniques 41 4.1.2 Uncertainty analysis 44 4.2 Upward flow boiling (UFB) 45 4.2.1 Experimental procedure and measurement techniques 45 4.2.2 Uncertainty analysis 48 4.3 Image processing 50 Chapter 5: Experimental Results 53 5.1 Introduction to the analysis of the bubble dynamics 53 5.1.1 The bubble life cycle 53 5.1.2 Calculation of the bubble equivalent diameter 55 5.1.3 Bubble dynamics with the increase of heat flux 57 5.1.4 Qualitative assessment of the bubble dynamics for different parameters 60 5.2 Bubble dynamics 61 5.2.1 Effect of heater surface wettability 61 5.2.2 Effect of heater surface roughness 65 5.2.3 Effect of bulk liquid velocity 70 5.3 Bubble departure 76 5.3.1 Effect of heater surface wettablity 76 5.3.2 Effect of heater surface roughness 76 5.3.3 Effect of bulk liquid velocity 78 5.4 Chapter conclusion 79 Chapter 6: Analysis and Model Development 81 6.1 Numerical evaluation of the role of heater surface characteristics 81 6.1.1 Derivation of an improved bubble growth model 86 6.1.2 Calculation of Ceff 82 6.2 Effect of liquid velocity on the bubble growth 93 6.3 Improved modeling of bubble departure 95 6.3.1 Analysis of important parameters 95 6.3.2 Formulation of a bubble departure criterion 100 6.4 Chapter conclusion 102 Chapter 7: Summary and Outlook 105 Bibliography 109 List of Figures 121 List of Tables 127 Appendix: Surface Parameters and Profile 129

<p>Heat dissipation requirements of next-generation power electronics in electric vehicles, high-performance computing, and radar systems will far exceed the capabilities of conventional heat sink technologies such as single-phase liquid cold plates and air-cooled heat sinks. The leading candidate technology that promises to meet these needs is microchannel flow boiling. Compared to conventional heat sink technologies, flow boiling provides some of the highest heat transfer coefficients available and can dissipate heat at a lower pumping power and with more uniform surface temperatures. However, there are unique challenges associated with flow boiling that currently prevent practical implementation of the technology, including limited modeling capabilities, inherent critical heat flux (CHF) limitations, and the presence of two-phase flow instabilities. This thesis is targeted primarily at addressing the impact of dynamic two-phase flow instabilities on heat transfer and CHF in microchannel heat sinks, in contrast with earlier literature that has focused on prediction and characterization of the flow dynamics.</p> <p><br></p> <p>Two dynamic instabilities of importance in microchannel heat sinks are pressure drop oscillations (PDO) and parallel channel instabilities, both resulting from an interaction between the inertia of a two-phase mixture within a heated channel and a source of compressibility outside of the channel. However, the individual impact of these instabilities on heat transfer performance has not been quantified. In this thesis, an experimental facility is developed to isolate the individual and combined impact of PDOs and parallel channel instabilities on surface temperature and CHF in single- and parallel-microchannels. This is achieved by introducing a measurable compressible volume directly upstream of the test section and isolating the test section from any unwanted compressibility within components throughout the rest of the system. Experiments are first performed targeting the investigation of PDOs in single channels and then targeting PDOs and parallel channel instabilities in multi-channel heat sinks. In the case of parallel channels, inlet restrictors are introduced to suppress channel-to-channel interactions and provide a baseline case of stable boiling. Throughout these experiments, only moderate increases in time-average surface temperature are observed (6 °C) and reduction of CHF is negligible, despite drastically different flow pattern observations when instabilities are present. These observations are in stark contrast with other cases in the literature, for which significant deterioration of surface temperatures and CHF have been attributed to the presence of PDOs. For example, significant temperature oscillations have been observed in the literature studying silicon-etched microchannel heat sinks experiencing PDOs. A predictive model is clearly required to understand and detect the conditions when dynamic instabilities should be considered in heat sink design.</p> <p><br></p> <p>To better understand the conditions when PDOs might have significant impact on heat transfer performance, an investigation of thermal capacitance is performed using a dynamic two-phase model and a targeted experimental approach in heat sinks having different thermal masses. The model reveals that, if thermal capacitance is low, PDOs become more severe, and the amplitude of temperature oscillations increase. These predictions are confirmed by experimental observations, and, in addition, premature CHF is observed in the heat sink with lower thermal mass. With sufficient thermal capacitance, the system recovers before triggering CHF, preventing deterioration of performance due to PDOs. Among the wide range of flow conditions considered in this thesis, the reduction of thermal mass resulted in the greatest impact on transient response of a heat sink during flow boiling instabilities. This reveals thermal capacitance as a critical parameter when determining if dynamic instabilities will deteriorate performance in a microchannel heat sink application. This allows engineers to make an informed judgement on whether adding features to suppress instabilities, at the cost of increased pumping power, is warranted. In order for the practical implementation of two-phase heat sinks to be realized, further development of dynamic modeling capabilities is required, and these models should be backed by systematic experimental investigations into conditions where instabilities should be considered.</p>

No description available.

The study of the instabilities in boiling water reactors is of significant importance to the safety withwhich they can be operated, as they can cause damage to the reactor posing risks to both equipmentand personnel. The instabilities that concern this paper are progressive growths in the oscillatingpower of boiling-water reactors. As thermal power is oscillatory is important to be able to identifywhether or not the power amplitude is stable. The main focus of this paper has been the development of a neural network estimator of these insta-bilities, fitting a non-linear model function to data by estimating it’s parameters. In doing this, theambition was to optimize the networks to the point that it can deliver near ”best-guess” estimationsof the parameters which define these instabilities, evaluating the usefulness of these networks whenapplied to problems like this. The goal was to design both MLP(Multi-Layer Perceptron) and SVR/KRR(Support Vector Regres-sion/Kernel Rigde Regression) networks and improve them to the point that they provide reliableand useful information about the waves in question. This goal was accomplished only in part asthe SVR/KRR networks proved to have some difficulty in ascertaining the phase shift of the waves.Overall, however, these networks prove very useful in this kind of task, succeeding with a reasonabledegree of confidence to calculating the different parameters of the waves studied.

<p>An experimental investigation has been carried out in order to investigate the effect of surface initial conditions, concentration, nanoparticles size and deposition pattern on pool boiling and jet impingement boiling of nanofluids. A flat copper surface with initial conditions of Ra = 420 nm, Ra = 80 nm and Ra = 20 nm has been used as the boiling surface. Al₂O₃ and CuO nanoparticles have been used with de-ionized water to prepare the nanofluids. At 0.01 vol. % concentration of Al₂O_3, the rate of heat transfer enhanced by 41% and 34% for the Ra = 80 nm and Ra = 20 nm, respectively. While, in the case of Ra = 420 nm, the rate of heat transfer deteriorated by 49%. At 0.005 vol. % concentration the rate of heat transfer deteriorated for all three surfaces. It is believed that the deterioration was due to the uniformity of the deposition. Using 0.01 vol. % concentration of CuO nanofluids resulted in the same trend, however, the rate of heat transfer is less compared to using Al₂O₃nanofluids. For example, in the case of Ra = 80 nm, the rate of heat transfer was reduced by 14%.</p> <p>The effect of nanoparticles size has been investigated by changing the nanoparticles size from 50 nm to 10 nm. The change in nanoparticles size resulted in a significant deterioration in the rate of heat transfer for all three surfaces. It is believed that the deterioration was due to the deposition uniformity. As the deposition uniformity has been found to be a major factor that affects the rate of heat transfer, new approach was introduced to quantify the effect of the rate of deposition on the pool boiling of nanofluids.</p> <p>An experimental investigation has been carried out in order to investigate using submerged impingement jet on the rate of heat transfer using nanofluids. At of 0.005 vol. % concentration of Al₂O₃, surface with Ra = 80 nm, jet to surface vertical distance of 3 mm and Reynolds number of 101311, the rate of heat transfer deteriorated by 19%.</p> <p>Comparing the pool boiling and jet impingement boiling of nanofluids showed that, in the case of jet impingement boiling, the rate of heat transfer was enhanced compared to the case of pool boiling and the deposition was less. However, jet impingement boiling experiments showed deterioration in the rate of heat transfer by 19% compared with pure water.</p> / Master of Applied Science (MASc)

[ES] La inyección de combustible es, entre los temas de investigación de motores, una de las piezas críticas para obtener un motor eficiente. El papel es aún más significativo cuando se persigue una estrategia de inyección directa. La geometría interna y el movimiento de la aguja determinan el comportamiento del flujo del inyector, que se sabe que afecta enormemente al desarrollo externo del spray y, en última instancia, al rendimiento de la combustión dentro de la cámara. La conciencia sobre el cambio climático y los contaminantes ha ido creciendo, impulsando el esfuerzo en motores más limpios. En este sentido, los motores de gasolina tienen un margen más amplio para mejo- rar que los motores diesel. La evolución de los antiguos PFI a las modernas estrategias de inyección directa, que se utilizan en los motores de nueva generación, demuestra esta tendencia. Los sistemas GDI tienen el potencial de cumplir con las estrictas emisiones y aumentar el ahorro de combustible, sin embargo, todavía se enfrenta a muchos desafíos. Este trabajo implica el uso de dos inyectores, uno es una moderna tobera de GDI de investigación designada por el Engine Combustion Network (ECN), y el otro es una unidad de inyección de producción (PIU) con la misma tecnología y una geometría ligeramente diferente. Ambos equipos se someten a una completa caracterización (flujo interno y externo) que abarca las técnicas más avanzadas en diversas instalaciones experimentales. Además, se diseña y construye una nueva instalación para realizar experimentos en condiciones de evaporación instantánea (cuando la presión de vapor del combustible inyectado es superior a la presión del volumen de descarga). La instalación construida está diseñada para simular un ambiente de descarga en ciertas condiciones del motor en las que podrían producirse fenómenos de flash boiling. Así, debido a las propiedades típicas del combustible de gasolina, era un requisito operar con presiones de cámara de 0,2 a 15 bares. Además, la temperatura ambiente se controlaba mediante la implementación de una resistencia que puede calentar el gas ambiente. La instalación funciona en un bucle abierto, pudiendo renovar el volumen de gas entre las inyecciones. Por último, se construyeron tres amplios accesos ópticos para acomodar muchas técnicas de diagnóstico óptico como DBI, MIE, shadowgraphy o PDA, entre otros. Para la evaluación del flujo interno se determinó la geometría de las toberas y la orientación de los agujeros, el movimiento de la aguja y, por último, la caracterización del ratio de inyección (ROM) y el momento de inyección (ROI) de ambas toberas. La geometría de las toberas y la elevación de la aguja se midieron mediante técnicas avanzadas de rayos X en el Laboratorio Nacional de Argonne (ANL). Las mediciones de ROI y ROM se realizaron utilizando las instalaciones de CMT-Motores Térmicos siguiendo los conocimientos técnicos aplicados en los inyectores de gasóleo y adaptándolos a las toberas de GDI. El ROI nos permitió comparar las boquillas, cuyo número de orificios y geometría eran diferentes, aunque entregan aproximadamente la misma cantidad de combustible. Se ensayó la respuesta a condiciones típicas de motor como variaciones en la presión del rail, la presión de descarga, la temperatura del combustible, etc. Para el inyector de investigación "Spray G", se desarrolló un modelo 0-D de la velocidad de inyección que permite obtener la señal para diferentes condiciones y duración de la inyección, lo cual es útil para la calibración del motor y la validación del CFD. Además, para la caracterización de la ROM, se desarrolló la metodología de la técnica de deformación plástica para obtener la orientación del cono del spray y orientar adecuadamente los chorros de combustible para la medición de ROM. En el análisis hidráulico se combinaron los datos para estudiar los bajos valores del coeficiente de descarga y / [CA] La injecció de combustible és, entre els temes d'investigació de motors, una de les peces crítiques per a obtindre un motor eficient. El paper és encara més significatiu quan es persegueix una estratègia d'injecció directa. La geometria interna i el moviment de l'agulla determinen el comportament del flux de l'injector, que se sap que afecta enormement el desenvolupament extern de l'esprai i, en última instància, al rendiment de la combustió dins de la cambra. La consciència sobre el canvi climàtic i els contaminants ha anat creixent, impulsant l'esforç en motors més nets. En aquest sentit, els motors de gasolina tenen un marge més ampli per a millorar que els motors dièsel. L'evolució dels antics PFI a les modernes estratègies d'injecció directa, que s'utilitzen en els motors de nova generació, demostra aquesta tendència. Els sistemes GDI tenen el potencial de complir amb les estrictes emissions i aug- mentar l'estalvi de combustible, no obstant això, encara s'enfronta a molts desafiaments. Aquest treball implica l'ús de dos injectors, un és una moderna tovera de GDI d'investigació designada pel Engine Combustion Network (ECN), i l'altre és una unitat d'injecció de producció (PIU) amb la mateixa tecnologia i una geometria lleugerament diferent. Tots dos equips se sotmeten a una completa caracterització (flux intern i extern) que abasta les tècniques més avançades en diverses instal·lacions experimentals. A més, es dissenya i construeix una nova instal·lació per a realitzar experiments en condicions d'evaporació instantània (quan la pressió de vapor del combustible injectat és superior a la pressió del volum de descàrrega). La instal·lació construïda està dissenyada per a simular un ambient de descàrrega en certes condicions del motor en les quals podrien produir-se fenòmens de flash boiling. Així, a causa de les propietats típiques del combustible de gasolina, era un requisit operar amb pressions de cambra de 0,2 a 15 bars. A més, la temperatura ambient es controlava mitjançant la implementació d'una resistència que pot calfar el gas ambiente. La instal·lació funciona en un bucle obert, podent renovar el volum de gas entre les injeccions. Finalment, es van construir tres amplis accessos òptics per a acomodar moltes tècniques de diagnòstic òptic com DBI, MIE, shadowgraphy o PDA, entre altres. Per a l'avaluació del flux intern es va determinar la geometria de les toveres i l'orientació dels forats, el moviment de l'agulla i, finalment, la caracterització del ràtio d'injecció (ROM) i el moment d'injecció (ROI) de totes dues toveres. La geometria de les toveres i l'elevació de l'agulla es van mesurar mitjançant tècniques avançades de raigs X en el Laboratori Nacional de Argonne (ANL). Els mesuraments de ROI i ROM es van realitzar utilitzant les instal·lacions de CMT-Motores Térmicos seguint els coneixements tècnics aplicats en els injectors de gasoil i adaptant-los a les toveres de GDI. El ROI ens va permetre comparar els filtres, el nombre d'orificis dels quals i geometria eren diferents, encara que entreguen aproximadament la mateixa quantitat de combustible. Es va assajar la resposta a condicions típiques de motor com a variacions en la pressió del rail, la pressió de descàrrega, la temperatura del combustible, etc. Per a l'injector d'investigació "Esprai G", es va desenvolupar un model 0-D de la velocitat d'injecció que permet obtindre el senyal per a diferents condicions i duració de la injecció, la qual cosa és útil per al calibratge del motor i la validació del CFD. A més, per a la caracterització de la ROM, es va desenvolupar la metodologia de la tècnica de deformació plàstica per a obtindre l'orientació del con de l'esprai i orientar adequadament els dolls de combustible per al mesurament de ROM. En l'anàlisi hidràulica es van combinar les dades per a estudiar els baixos valors del coeficient de descàrrega i del coeficient d'àr / [EN] Fuel injection is among the engine research topics one of the critical pieces to obtain an efficient engine. The role is even more significant when a direct injection strategy is pursued. The internal geometry and pintle movement determine the injector flow behavior, which is known to hugely affect the external spray development and, ultimately, the combustion performance inside the chamber. Climate change and pollutants awareness has been growing, pushing forward the effort on cleaner engines. In this regard, gasoline en- gines have a wider margin to improve than diesel engines. The evolution from old Port Fuel Injectors to modern direct injection strategies, which are used in new generation engines, demonstrates this trend. GDI systems have the potential to comply with stringent emissions and increase fuel economy, however, it still faces many challenges. This work involves the use of two injectors, one is a modern research GDI nozzle appointed by the Engine Combustion Network (ECN), and the other is a production injector unit (PIU) with the same technology and slightly different geometry. Both hardware's undergo a complete characterization (internal and external flow) covering the state- of-the-art techniques in various experimental facilities. Furthermore, a new facility is designed and built to perform experiments under flash boiling conditions (when the fuel injected's vapor pressure is higher than the pressure in the discharge volume). The developed facility is designed to simulate a discharge ambient at certain engine conditions in which flash boiling phenomena could occur. Thus, due to typical gasoline fuel properties, it was a requirement to operate from chamber pressures from 0.2 bar to 15 bar. Also, the ambient temperature was controlled by implementing a resistor that can heat the ambient gas. The facility operates in an open loop, being able to renovate the gas volume between injections. Finally, three wide optical accesses were built to accommodate many optical diagnostic techniques such as DBI, MIE, shadowgraphy, or PDA, among others. For the internal flow description, it was determined the nozzles geometry and holes orientation, the pintle movement, and finally, the characterization of the rate of momentum (ROM) and rate of injection (ROI) of both nozzles. The nozzles geometry and needle lift were measured using advanced optical x-ray techniques at Argonne National Laboratory (ANL). The ROI and ROM measurements were performed using CMT-Motores Térmicos facilities follow- ing the know-how applied in diesel injectors and adapting it to GDI nozzles. The ROI allowed us to compare the nozzles, whose orifices number and geometry were different, although they deliver approximately the same amount of fuel. It was tested their response to typical boundary conditions such as rail pressure, discharge pressure, fuel temperature, etc. For the research nozzle "Spray G", it was developed a 0-D model of the rate of injection allowing to obtain the signal for different injection duration and conditions, which is useful in engine calibration and CFD validation. Furthermore, for the ROM characterization, the plastic deformation technique methodology was developed to obtain spray cone orientation and adequately guide the fuel jets for measuring ROM. The hydraulic analysis combined the data to study the low discharge coefficient and area coefficient values, which could result from low needle lift combined with novel hole designs in both nozzles that promote cavitation and air interaction from inside the orifice. In the external flow characterization, it was used the new developed vessel to study the external spray covering flash boiling conditions. It was employed four surrogate fuels to simulate different volatility properties of gasoline com- pounds and ultimately reproduce more extreme flashing conditions. It was used lateral visualization using DBI and Schlieren in addition to frontal MIE visualization. Some of t / Bautista Rodríguez, A. (2021). Study of the Gasoline Direct Injection Process under Novel Operating Conditions [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/167809

<p>This study comprises experimental investigations and theoretical/empirical analyses of three forced-convective (pumped) boiling schemes: (i) confined round single jet and jet array impingement boiling, and flow boiling through conventional-sized (ii) concentric circular annuli and (iii) rectangular channels. These schemes could be utilized in the thermal management of various applications including high-heat-flux electronic devices, power devices, electric vehicle charging cables, avionics, future space vehicles, etc.</p>

Direct injection spark ignition (DISI) engines are the next generation of gasoline fuelled engines. Their greater fuel economy and reduced CO2 emissions compared with port fuel injection (PFI) engines has led to their popularity. However, DISI engines produce a greater number of particulate matter (PM) emissions than PFI engines. Concern over the health effects of PM emissions, and forthcoming European legislation to regulate them from gasoline powered vehicles has led to an increased interest in the study of PM formation, measurement, and characterisation. A model was developed by Aikawa et al, the PM index, correlating PM emissions with fuel composition. PM emissions are thought to be linked both to the vapour pressure (VP) and the double bond equivalent (DBE) of the components of the fuel. However, there was no independent control of these parameters and the study was undertaken on a PFI engine. In this thesis, experiments have been conducted to validate this model and extend it, as the PN index, to DISI engines. Fuels have been designed using Raoult’s law and UNIFAC (with careful consideration of octane number) such that the DBE and VP of the fuel mix could be varied independently. The design of the fuels was such that the component parts would co-evaporate upon injection into the cylinder, ensuring a homogeneous mixture of the components at the point of ignition. The PN index has been tested on a single cylinder engine, at a matrix of test points, using these model fuels, and their PM emissions have been analysed using a Cambustion DMS500. The results show that the PN index is followed closely using model fuels, provided that these model fuels contain a ‘light-end’ (in this case 5 % v/v n-pentane). Imaging of in-cylinder evaporation and in-cylinder measurement of hydrocarbons shows how the composition of model fuels affects their PM emissions. The PN index has also been tested using commercial fuels on a single cylinder engine and a Jaguar V8 engine; the results again show that the PN index is also an excellent predictor of PN emissions for market fuels from both of these engines. PN emissions have been evaluated from two fuels representing the EU5 reference fuel specification, developed using the PN index to give a difference in PM emissions. Testing these fuels on both a single cylinder engine and a Jaguar V8 engine has shown up to a factor of three variation in observed PN emissions. This has important implications for forthcoming European emissions legislation. The results of these tests were fed into the recommendations for the EU6 reference fuel specification. The PN index has also been investigated in a Jaguar V6 engine with five different fuels with a spread of calculated PN indices over a simulated NEDC. Here the PN emissions have been measured using two PN, and one PM instrument and the results compared. The results show that the trends of the PN index are followed, but not as closely as predicted. Detailed analysis shows that this discrepancy is due to other effects, for example cold start, dominating the PN emissions in certain phases. PN emissions have been measured from a highly boosted engine at a variety of operating points using 14 different fuels. It has been shown that for a large variety of engine operating parameters PN emissions from highly boosted engines behave as expected. When changing the fuels, the results show that a variation of over three orders of magnitude can be observed. The predictions of the PN index are inconclusive however, with further work suggested to fully evaluate the PN index on highly boosted engines.

The increasing demand for the domestic refrigeration system urges the development of greener form of refrigeration. The eighty-year-old single pressure absorption refrigeration system invented by Albert Einstein and Leo Szilard is attractive as it has no mechanical moving parts and can be driven by heat alone. However, the literature on either the refrigeration system or its components is scarce. The bubble pump is the crucial component of the refrigeration system, but it is poorly understood as its mass flow rate cannot be readily predicted. Two new time correlations in the mass flow rate prediction are presented to increase the accuracy when heat losses occur in the bubble pump. These time correlations are verified with the experimental results. When either the heat input or submergence ratio increases, the accuracy of the prediction increases. The percentage of error for the high heat input or submergence ratio is within ±10%. Working conditions and system dimension have a direct influence to the bubble pump performance. For instance, the bubble pump experimental results show that the mass flow rate of the bubble pump increases when either the submergence ratio or the concentration of ammonia increases. However, the performance of the bubble pump drops when the tube diameter or the system pressure increases. The Einstein refrigeration system has only been rebuilt once since it invention. In order to redesign and rebuilt a practical Einstein refrigeration system, some challenges are revealed. With the combination of the good features of the designs of Einstein and Delano, a new prototype has been rebuilt and tested. The practical results obtained from the five experimental setups are the first set of experimental result that has ever been presented. The highest cooling capacity and coefficient of performance (COP) obtained are 5 W and 0.04 respectively.