Les échangeurs de chaleur constitués de microcanaux parallèles sont considérés une bonne solution technologique pour dissiper de grands flux de chaleur dans les composants et les systèmes miniaturisés. D’une manière générale cette réduction de taille permet une diminution des coûts des matériaux et l'utilisation de plus faible quantité de fluides frigorigènes pour les systèmes de refroidissement. Cette étude est divisée en deux parties complémentaires A et B. Elles visent à étudier le comportement thermo-hydraulique dans les échangeurs de chaleur constitués de microcanaux pour une meilleure compréhension des transferts de chaleur et des écoulements diphasiques dans les évaporateurs miniatures. Dans la partie A, nous étudions l'ébullition convective du réfrigérant R134a dans un mini échangeur composé de neuf microcanaux parallèles de section transversale circulaire, placés horizontalement, avec un diamètre interne de 0,77 mm et longueur et 150 mm. Les résultats expérimentaux montrent que la configuration d'écoulement a une forte influence sur le coefficient de transfert de chaleur, et que différents mécanismes de transfert de chaleur ont lieu dans chacune de ces configurations d'écoulement. En revanche la perte de pression est une fonction directe de la vitesse massique, du titre de vapeur et de la pression du système. La partie B, porte sur l’analyse de la distribution de l'écoulement diphasique en l’absence de transferts de chaleur et de changement de phase liquide-vapeur. A partir de ces résultats nous montrons que les effets du titre de gaz sur la répartition du liquide change considérablement selon la position de l'ensemble (tube d'alimentation, distributeur-canaux). / Heat exchangers consisting of parallel micro-channels are considered a good technological solution in response to the increasing demand for compact systems, which require high heat flux dissipation, ensuring a decrease in the material costs and the use of a lower quantity of refrigerants. The aim of this study was to investigate the thermo-hydraulic behavior inside these components provided by microchannels. This study is divided into two experimental studies (A and B) which are complementary. In part A, the convective boiling of the refrigerant R134a is analyzed within nine parallel microchannels of circular cross section, positioned horizontally, with internal diameter and length of 0.77 mm and 150 mm, respectively. The experimental results show that the flow pattern has a strong influence on the heat transfer coefficient, and that different heat transfer mechanisms are associated with each of the flow patterns observed, whereas the frictional pressure drop is a direct function of the mass velocity, vapor quality and pressure of the system. In part B, the two-phase flow distribution, using as the working fluid a mixture of air and water, is analyzed inside a circular header coupled to nine branched parallel microchannels of circular cross-section with internal diameter and length of 0.8 mm and 150 mm, respectively. The results show that the effect of the gas quality on the liquid distribution changes considerably depending on the configuration (feeder tube-header-channels).

Passive two-phase cooling devices such as capillary pump loops, loop heat pipes, and vapor chambers can utilize capillary-fed boiling in the porous evaporator wick to achieve high heat flux dissipation, while maintaining low thermal resistances. These systems typically rely only on passive capillary pumping through the porous wick to transport fluid. This inevitably leads to limits on the maximum heat flux and power dissipation based on the maximum capillary pressure available. To overcome these capillary pumping limitations in these passive devices, a mechanical pump can be added to the system to create a pump-assisted capillary loop (PACL). The pump can actively transport the fluid to overcome the pressure drop in liquid lines, reserving all of the available capillary action to draw liquid from a compensation chamber into the porous evaporator at the location of the heat input.<br>Previous studies on pump-assisted capillary loops have used a porous pathway to draw liquid to the heated evaporator surface from a liquid supply in the compensation chamber. This pathway typically comprises porous posts distributed over the heated surface area to ensure uniform liquid feeding during boiling and to avoid dryout regions. This thesis presents an evaporator design for a pump-assisted capillary loop system featuring a non-porous manifold connection between the compensation chamber and the evaporator wick base where boiling occurs. By using this approach, microscale liquid-feeding features can be implemented without the manufacturing restrictions associated with the use of porous wick pathways (such as sintered powder copper particles).<br>An analytical model for two-phase pressure drop prediction in the base wick is developed and used to define the evaporator geometry and feeding structure dimensions. A parametric analysis of the evaporator geometry is performed with the target of achieving a maximum heat dissipation of 1 kW/cm2 without a capillary limit. A 24 x 24 microtube array configuration with an outside tube diameter of 0.25 mm was identified as a result of this analysis. This manifold delivers liquid the base wick manufactured from sintered copper particles with a mean particle diameter of 90 microns. <br>The resulting evaporator geometry was translated into a manufacturable copper manifold design. A modular test section design consisting of a cover for attachment of fittings, a support structure for holding the manifold, a sintered copper wick base, and a carrier plate was created and manufactured, to accommodate for future testing scheduled to be performed by an external industry partner. The resulting design provides a testing vehicle to investigate the effect of different tubing arrangements and dimensions, as well as multiple base wick configurations. This knowledge can be used to engineer future evaporator architectures for enhanced performance. The improved understanding providing on the effect of liquid feeding distribution into the base wick, the effects of boiling on the base wick pressure drop, and the manufacturing limitations can each improve the performance prediction of evaporators with top feeding. <br>

The search for Accident tolerant fuels (ATF) which is the first part of this thesis is currently one of the most actual topics in the field of nuclear fuels. These fuels must be first successfully tested in operational and also accident conditions for their possible inclusion in commercial use. Following part of the thesis specifically focuses on the boiling crisis in nuclear reactors which can damage the nuclear fuel cladding. Therefore, it is necessary to know the critical heat flux value and the departure from nuclear boiling ratio. Calculations which determine critical heal flux value are placed in the practical part of the thesis. Calculations are compared with the data obtained during experiments. The ALTHAMC12 and the other correlations which are based on the previous measurements are used for the computational analysis.

This master’s thesis is focused on the design of electric steam generator for superheated steam. Using heating cartridge is the easiest way how to make a saturated or superheated steam. This device is part of gasification reactor, which is used for experimental purposes. The thesis includes schematic involvement of technology. Technology contains evaporator and superheater. The evaporator is calculated as pool boiling. The superheater is design for steam flowing through heating cartridge bundle. At the end of the thesis are made projection drawings, which are based on the calculations.

This thesis deals with a design concept of the facility for the educational objectives of the boiling crisis. In the first part, the issue of boiling crisis is explained. There are also examples of some experimental facilities in the world. The second part includes design concept of a loop, which is accompanied by designs of individual devices, such as separator, condenser, exchanger, pump, water tank, electric heater. The thesis also states designs of throttle control, temperature control and flow control.

In einem Verbundprojekt im Rahmen des Programms „Energie 2020+“ gefördert durch das BMBF koordiniert durch das HZDR arbeiteten 4 Universitäten, 2 Forschungszentren und ANSYS zusammen. Der vorliegende Bericht beschreibt die Arbeiten des HZDR, die im Zeitraum September 2009 bis Januar 2013 durchgeführt wurden. Das Vorhaben war auf die Entwicklung und Validierung von CFD-Modellen von unterkühltem Sieden bis zu Filmsieden gerichtet. Im Bericht werden die entwickelten und verwendeten Modelle dargestellt. Anhand der Nachanalyse von Experimenten wird auf die vorgeschlagene Kalibrierung der Modelle eingegangen. Wichtig ist hierbei eine genauere Beschreibung der Zwischenphasengrenzfläche, die durch Kopplung des Wandsiedemodells mit einem Populationsmodell erreicht werden kann. Anhand der Analyse von Bündelexperimenten konnte gezeigt werden, dass die gemessenen querschnittsgemittelten Messwerte mit einem Satz im Rahmen der Modellunsicherheiten kalibrierter Modellparameter reproduziert werden kann. Für die Berechnung der Verteilungsmuster des Dampfgehaltes im Kanalquerschnitt muss die Modellierung der Turbulenz beachtet werden. Die experimentellen Arbeiten waren auf die Untersuchung eines Brennelementbündels gerichtet. An einer Versuchsanordnung zu einem Brennelementbündel werden die turbulente einphasige Geschwindigkeit (PIV), der mittlere Gasgehalt (Gamma-Densitometrie) sowie der zeitlich und räumlich aufgelöste Gasgehalt (Hochgeschwindigkeits-Röntgentomographie) gemessen. Letztere Methode wurde in Rossendorf entwickelt.

Boiling is used in a wide variety of industries, including electronics cooling, distillation, and power generation. Fundamental studies on the boiling process are needed for effective implementation. Key performance characteristics of boiling are the heat transfer coefficient, which determines the amount of heat flux that can be dissipated for a given superheat, and critical heat flux(CHF), the failure point that occurs when vapor blankets the surface. The wettability of a surface is one of the key parameters that affects boiling behavior. Wetting surfaces(e.g., hydrophilic surfaces), typically characterized by a static contact angle below 90°,have better critical heat flux due to effective rewetting, but compromised heat transfer coefficients due to increased waiting times between nucleation of each bubble. Meanwhile, nonwetting surfaces (e.g., hydrophobic surfaces), characterized by static contact angles greater than 90°, have better heat transfer coefficients due to improved nucleation characteristic, but reach critical heat flux early due to surface dry out. However, recent studies have shown that the static contact angle alone offers and incomplete, and sometimes inaccurate, description of this behavior, which is instead governed entirely by the dynamic wettability. Specifically, the receding contact angle impacts the size and contact area of bubbles forming on a surface during boiling, while the advancing contact angle determines how the bubble departs. With this more complete set of wettability descriptors, three characteristic wetting regimes define the boiling behavior: hygrophilic surfaces having advancing and receding contact angles both under 90°; hygrophobic surfaces having both these dynamic contact angles over 90°;and ambiphilic surfaces having a receding contact angle less than 90°, but an advancing contact angle greater than 90°.The goal of this thesis is to experimentally characterize and compare the behavior of boiling surfaces in each of these regimes, observe the contact line behavior, and explain the mechanisms for their differences in performance.

In this master thesis an input-model of a Nordic BWR power plant has been developed in APROS. The plant model contains key systems and major thermohydraulic components of the steam cycle, including I&amp;C systems (i.e. power, pressure, level and flow controls). The plant model is primarily designed for balance of plant studies at discrete power levels. The input-model of the power plant focuses especially on the steam cycle which is crucial for analysing water and steam behaviour and its influence on the reactor power. At the current stage, the model primarily handles steady-state conditions of full-power operation, which has been the design point. It has also been shown that reduced-power operation can be simulated with a reasonable trendline of pressure and temperature progression over facility components. / Inom ramen för examensarbete har en indatafil (modell) av en nordisk kokvattenreaktor, BWR, utvecklats i simuleringsverktyget APROS. Anläggningsmodellen är främst utformad för att simulera diskreta effektnivåer och innehåller viktiga system och termohydrauliska komponenter som ingår i ångcykeln, inklusive instrumenterings- och kontrollutrustning (dvs. effekt-, tryck-, nivå- och flödesreglering). Fokus har lagts särskilt på att få till en bra representation av ångcykeln, vilket är avgörande för analys av vatten- och ångbeteendet och dess påverkan på reaktoreffekten. Modellen kan främst användas för simulering av jämviktstillstånd vid full effektdrift och till en viss grad även reducerad effektdrift.

This project deals with two-phase CFD model validation, a subject which is currently under active research due to its complexity. The goal is to create a model that predicts data profiles to an acceptable degree for a wide array of flow conditions. The applications within the nuclear field would mainly be for safety analysis, e.g. to predict phenomena such as the critical heat flux. The underlying physics were investigated, and simulations were performed of two-phase flow of the coolant R12 using the program OpenFOAM in an attempt to match radial profiles of void fraction, interfacial area concentration, vapour velocity and sauter mean diameter for different flow conditions provided by the DEBORA experiments. The proper set of models was found via sensitivity testing: trying combinations of different models and model coefficients. The effect on the simulation result was investigated, with the models that improved the result kept while the rest were discarded. The main strategy was to find models that accurately predicted the sauter mean diameter, as initial sensitivity tests showed the result to be highly dependent on this parameter. The impact of initial conditions and mesh refinement was also investigated, and a temperature validation study was done. The process was aided by a number of Matlab programs written by the author, to calculate and verify inputs and to post-process the result. A model was found that simulated the subcooled nucleate boiling datasets to an acceptable degree. The model failed to accurately simulate saturated nucleate boiling. / Detta projekt behandlar tvåfas CFD-modellvalidering, ett ämne som fortfarande ärunder aktiv forskning på grund av sin komplexitet. Syftet är att utveckla en modellsom förutser dataprofiler till en acceptabel grad för en mängd olika flödesregimer.Inom kärnkraftsbranchen skulle detta primärt vara applicerbart inom reaktorsäkerhet,t.ex. för att förutse fenomen så som critical heat flux. Den underliggande fysikenundersöktes, och simuleringar genomfördes på tvåfas flöde av kylmedlet R12 med hjälpav programmet OpenFOAM i ett försök att matcha 14 dataprofiler för void fraction,interfacial area, vapour velocity och sauter mean diameter för olika flödesregimertillhandahålla av DEBORA-experimenten.Den rätta uppsättningen modeller fanns via känslighetsanalys, genom att testa olikakombinationer av modeller och modellkoefficienter. Deras effektersimuleringsresultatet undersöktes, och de modeller som förbättrade resultatet behölls,medans resten förkastades. Huvudstrategin var att hitta modeller som förutsåg sautermean diameter, bubblornas medeldiameter, väl, då preliminär känslighetsanalysvisade att resultaten var mycket känsliga på denna parameter. Inflytandet avinitialvillkor och mesh-förfining undersöktes, och en temperaturprofilsvalideringgenomfördes. Till hjälp i processern var ett antal Matlab-program skrivna avförfattaren för att beräkna och verifiera inmatning och behandla och visualiserautdatan. En modell hittades som simulerade underkyld kokning till en acceptabel grad.Modellen misslyckades med att träffsäkert simulera mättad kokning.

This thesis is divided into four distinct chapters all linked by the topic of spray cooling. Chapter one gives a detailed categorization of future and current spray cooling applications, and reviews the major advantages and disadvantages that spray cooling has over other high heat flux cooling techniques. Chapter two outlines the developmental goals of spray cooling, which are to increase the output of a current system and to enable new technologies to be technically feasible. Furthermore, this chapter outlines in detail the impact that land, air, sea, and space environments have on the cooling system and what technologies could be enabled in each environment with the aid of spray cooling. In particular, the heat exchanger, condenser and radiator are analyzed in their corresponding environments. Chapter three presents an experimental investigation of a fluid management system for a large area multiple nozzle spray cooler. A fluid management or suction system was used to control the liquid film layer thickness needed for effective heat transfer. An array of sixteen pressure atomized spray nozzles along with an imbedded fluid suction system was constructed. Two surfaces were spray tested one being a clear grooved Plexiglas plate used for visualization and the other being a bottom heated grooved 4.5 x 4.5 cm2 copper plate used to determine the heat flux. The suction system utilized an array of thin copper tubes to extract excess liquid from the cooled surface. Pure water was ejected from two spray nozzle configurations at flow rates of 0.7 L/min to 1 L/min per nozzle. It was found that the fluid management system provided fluid removal efficiencies of 98% with a 4-nozzle array, and 90% with the full 16-nozzle array for the downward spraying orientation. The corresponding heat fluxes for the 16 nozzle configuration were found with and without the aid of the fluid management system. It was found that the fluid management system increased heat fluxes on the average of 30 W/cm2 at similar values of superheat. Unfortunately, the effectiveness of this array at removing heat at full levels of suction is approximately 50% & 40% of a single nozzle at respective 10[degrees]C & 15[degrees]C values of superheat. The heat transfer data more closely resembled convective pooling boiling. Thus, it was concluded that the poor heat transfer was due to flooding occurring which made the heat transfer mechanism mainly forced convective boiling and not spray cooling. Finally, Chapter four gives a detailed guide for the design and construction of a high heat flux heater for experimental uses where accurate measurements of surface temperatures and heat fluxes are extremely important. The heater designs presented allow for different testing applications; however, an emphasis is placed on heaters designed for use with spray cooling.