Exhausting the thermal power from a fusion tokamak is a critical engineering challenge. The life of components designed for these conditions has a strong influence on the availability of the machine. For a fusion power plant this dependence becomes increasingly important, as it will influence the cost of electricity. The most extreme thermal loading for a fusion power plant will occur in the divertor region, where components will be expected to survive heat fluxes in excess of 10 MW/m² over a number of years. This research focussed on the development of a heat sink module for operation under such conditions, drawing on advanced cooling strategies from the aerospace industry. A reference concept was developed using conjugate Computational Fluid Dynamics. The results were experimentally validated by matching Reynolds numbers on a scaled model. Heat transfer data was captured using a transient thermochromic liquid crystal technique. The results showed excellent agreement with the corresponding numerical simulations. To facilitate comparison against other divertor heat sink proposals, a nondimensional figure of merit for cooling performance was developed. When plotted against a non-dimensional mass flow rate, the reference heat sink was shown to have superior cooling performance to all other divertor proposals to date. Results from Finite Element Analysis were used in conjunction with the ITER structural design criteria to life the heat sink. The sensitivity of life to both boundary conditions, and local geometric features, were explored. The reference design was shown to be capable of exceeding the life requirements for heat fluxes in excess of 15 MW/m². A number of heat sinks, based on the reference design, were fabricated. These underwent non-destructive testing, before experimentation in a high-heat flux facility developed by the author. The heat transfer performance of the tested modules was found to exceed that predicted by numerical modelling, which was concluded to be caused by the fabrication processes used.

Material erosion, melting, splashing, bubbling, and ejection during disruption events in future large tokamak reactors are of serious concern to component longevity. The majority of the heat flux during disruptions will be incident on the divertor, which will be made from tungsten in the future large tokamak ITER. Electrothermal plasma sources operating in the confined controlled arc discharge regime produce heat fluxes in the range expected for hard disruptions in future large tokamaks. The radiative heat flux produced inside of the capillary discharge channel is from the formed high density (10^23 - 10^27/m^3) plasma with heat fluxes of up to 125 GW/m^2 over a period of 100s of microseconds, making such sources excellent simulators for ablation studies of plasma-facing materials in tokamaks during hard disruptions. Experiments have been carried out with the PIPE device exposing tungsten to these high heat flux plasmas. SEM images have been taken of the tungsten surfaces, cross sections of tungsten surfaces, and ejected material. Melting and bubble/void formation has been observed on the tungsten surface. The tungsten surface shows evidence of melt-layer flow and the existence of voids and cracks in the exposed material. The ejected material does not show direct evidence of liquid material ejection which would lead to splashing. EDS analysis has been performed on the ejected material which demonstrates a lack of deposited solid tungsten particulates greater than micron size. / Master of Science

L'énergie solaire concentrée est une source d'énergie alternative pour la conversion thermochimique de biomasse en vecteurs énergétiques ou en matériaux à haute valeur ajoutée. La production d'un gaz de synthèse à partir de biomasse lignocellulosique en est un exemple, de même que la production de résidus carbonés à propriétés contrôlées. Ces travaux portent sur l'étude du comportement d'un échantillon de hêtre thermiquement épais sous de hautes densités de flux solaire (supérieures à 1000 kW/m²). Deux approches ont été développées en parallèles : une étude expérimentale et le développement d'un modèle numérique. Les expériences ont permis de mettre en lumière le comportement particulier du hêtre sous de hautes densités de flux solaire. En effet, un cratère de char, dont la forme correspond à celle de la distribution du flux incident, se forme dans l'échantillon. Cette étude a aussi montré que la teneur en eau initiale de la biomasse a un fort impact sur son comportement. Les échantillons secs peuvent atteindre un rendement de conversion énergétique de 90 %, capturant jusqu'à 72 % de l'énergie solaire incidente sous forme chimique. Quant aux échantillons humides, ils produisent nettement plus d'hydrogène, au prix d'un rendement de conversion énergétique aux alentours de 59 %. De plus, le craquage thermique et le reformage des goudrons produits par la pyrolyse sont rendus possibles par les températures atteintes (supérieures à 1200 °C) et la présence d'eau. Enfin, il a été montré que l'orientation des fibres du bois n'a qu'un impact mineur sur son comportement. En parallèle, une modélisation des transferts couplés chaleur matière et des réactions chimiques mis en jeu lors de la gazéification solaire d'un échantillon a été développée. La construction du modèle a mis en avant la nécessité de recourir à des stratégies innovantes pour prendre en compte la pénétration du rayonnement dans la matière ainsi que la déformation du milieu par la gazéification. Les prédictions du modèle montrent un bon accord avec les observations expérimentales. Elles ont ainsi permis de mieux comprendre les couplages mis en jeu lors de la dégradation de biomasse sous haute densité de flux solaire. De plus, des analyses de sensibilités ont révélé que les modèles de type Arrhenius ne permettent pas de décrire finement le comportement de l'eau à l'intérieur de l'échantillon et que le choix du modèle de pyrolyse était capital pour décrire correctement le comportement la biomasse sous haute densité de flux solaire. / Concentrated solar energy is as an alternative energy source to power the thermochemical conversion of biomass into energy or materials with high added value. Production of syngas from lignocellulosic biomass is an example, as well as the production of carbonaceous residues with controlled properties. This work focuses on the study of the behaviour of a thermally thick beech wood sample under high solar heat flux (higher than 1000 kW/m²). Two approaches have been undertaken at the same time: an experimental study and the development of a numerical model. Experiments have highlighted a specific behaviour of beech wood under high solar heat flux. Indeed, a char crater, symmetrical to the incident heat flux distribution, forms in the sample. This study has also shown that biomass initial moisture content has a strong impact on its behaviour. The dry sample can achieve an energetic conversion efficiency of 90 %, capturing up to 72 % of the incident solar power in chemical form. While, high initial moisture content samples produce more hydrogen, at the price of an energetic conversion efficiency around 59 %. Furthermore, tar thermal cracking and steam reforming are enabled by the temperatures reached (higher than 1200 °C) and the presence of water. Finally, wood fiber orientation has been shown to have only a minor impact on its behaviour. At the same time, a modelling of the coupled reactions, heat and mass transfers at stake during solar gasification was undertaken. The development of this model has highlighted the necessity to implement innovative strategies to take into account radiation penetration into the medium as well as its deformation by gasification. Numerical model predictions are in good agreement with experimental observations. Based on the model predicted behaviour, further understanding of biomass behaviour under high solar heat flux was derived. In addition, sensitivity analyses revealed that Arrhenius type models are not fitted for precise intra-particular water behaviour description and that the choice of the pyrolysis scheme is key to properly model biomass behaviour under high solar heat flux.

Many critical applications today, in electronics, optics and aerospace fields, among others, demand advanced thermal management solutions for the acquisition of high heat loads they generate in order to operate reliably and efficiently. Current competing technologies for this challenging task include several single and two phase cooling options. When these cooling schemes are compared based on the high heat flux removal (100-1000 W/cm2) and isothermal operation (within several oC across the cooled device) aspects, as well as system mass, volume and power consumption, spray cooling appears to be the best choice. The current study focused on high heat flux spray cooling with ammonia on enhanced surfaces. Compared to some other commonly used coolants, ammonia possesses important advantages such as low saturation temperature, and high heat absorbing capability. Moreover, enhanced surfaces offer potential to greatly improve heat transfer performance. The main objectives of the study were to investigate the effect of surface enhancement on spray cooling performance, and contribute to the current understanding of spray cooling heat transfer mechanisms. These objectives were pursued through a two stage experimental study. While the first stage investigated enhanced surfaces for the highest heat transfer coefficient at heat fluxes of up to 500 W/cm2, the second stage investigated the optimized enhanced surfaces for critical heat flux (CHF). Surface modification techniques were utilized to obtain micro scale indentations and protrusions, and macro (mm) scale pyramidal, triangular, rectangular, and square pin fins. A third group, multi-scale structured surfaces, combined macro and micro scale structures. Experimental results indicated that micro- and macrostructured surfaces can provide heat transfer coefficients of up to 534,000 and 426,000 W/m2oC at 500 W/cm2, respectively. Multi-scale structured surfaces offered even a better performance, with heat transfer coefficients of up to 772,000 W/m2oC at 500 W/cm2, corresponding to a 161% increase over the reference smooth surface. In CHF tests, the optimized multi-scale structured surface helped increase maximum heat flux limit by 18%, to 910 W/cm2 at nominal liquid flow rate. During the additional CHF testing at higher flow rates, most heaters experienced failures before reaching CHF at heat fluxes above 950 W/cm2. However, the effect of flow rate was still characterized, suggesting that enhanced surfaces can achieve CHF values of up to 1,100 W/cm2 with 67% spray cooling efficiency. The results also helped shed some light on the current understanding of the spray cooling heat transfer mechanisms. Data clearly proved that in addition to fairly well established mechanisms of forced convection in the single phase regime, and free surface evaporation and boiling through secondary nucleation in the two phase regime, enhanced surfaces can substantially improve boiling through surface nucleation, which can also be supported by the concept of three phase contact lines, the regions where solid, liquid and vapor phases meet. Furthermore, enhanced surfaces are capable of retaining more liquid compared to a smooth surface, and efficiently spread the liquid film via capillary force within the structures. This unique advantage delays the occurrence of dry patches at high heat fluxes, and leads to higher CHF.

With the demand of computing power from electronic chips on a constant rise, innovative methods are needed for effective and efficient thermal management. Forced convection cooling through an array of micro pin-fins acts not only as a heat sink, but also allows for the electrical interconnection between stacked layers of integrated circuits. This work performs a multi-objective optimization of three shapes of pin-fins to maximize the efficiency of this cooling system. An inverse design approach that allows for the design of cooling configurations without prior knowledge of thermal mapping was proposed and validated. The optimization study showed that pin-fin configurations are capable of containing heat flux levels of next generation electronic chips. It was also shown that even under these high heat fluxes the structural integrity is not compromised. The inverse approach showed that configurations exist that are capable of cooling heat fluxes beyond those of next generation chips. Thin film heat spreaders made of diamond and graphene nano-platelets were also investigated and showed that further reduction in maximum temperature, increase in temperature uniformity and reduction in thermal stresses are possible.

A novel cooling mechanism based on evaporation of thin liquid films is presented for thermal management of confined heat sources, such as microprocessor hotspots. The underlying idea involves utilization of thin nanoporous membranes for maintaining microscopically thin liquid films by capillary action, while providing a pathway for the vapor generated due to evaporation at the liquid-vapor interface. The vapor generated by evaporation is continuously removed by using a dry sweeping gas keeping the membrane outlet dry. This thesis presents a detailed theoretical, computational and experimental investigation of the heat and mass transfer mechanisms that result in dissipating heat. Performance analysis of this cooling mechanism demonstrates heat fluxes over 600W/cm2 for sufficiently thin membrane and film thicknesses (~1-5µm) and by using air jet impingement for advection of vapor from the membrane surface. Based on the results from this performance analysis, a monolithic micro-fluidic device is designed and fabricated incorporating micro and nanoscale features. This MEMS/NEMS device serves multiple functionalities of hotspot simulation, temperature sensing, and evaporative cooling. Subsequent experimental investigations using this microfluidic device demonstrate heat fluxes in excess of 600W/cm2 at 90 C using water as the evaporating coolant. In order to further enhance the device performance, a comprehensive theoretical and computational analysis of heat and mass transfer at micro and nanoscales is carried out. Since the coolant is confined using a nanoporous membrane, a detailed study of evaporation inside a nanoscale cylindrical pore is performed. The continuum analysis of water confined within a cylindrical nanopore determines the effect of electrostatic interaction and Van der Waals forces in addition to capillarity on the interfacial transport characteristics during evaporation. The detailed analysis demonstrates that the effective thermal resistance offered by the interface is negligible in comparison to the thermal resistance due to the thin film and vapor advection. In order to determine the factors limiting the performance of the MEMS device on a micro-scale, a device-level detailed computational analysis of heat and mass transfer is carried out, which is supported by experimental investigation. Identifying the contribution of various simultaneously occurring cooling mechanisms at different operating conditions, this analysis proposes utilization of hydrophilic membranes for maintaining very thin liquid films and further enhancement in vapor advection at the membrane outlet to achieve higher heat fluxes.

Divertors are in-vessel, plasma-facing, components in magnetic-confinement fusion reactors. Their main function is to remove the fusion reaction ash (α-particles), unburned fuel, and eroded particles from the reactor, which adversely affect the quality of the plasma. A significant fraction (~15 %) of the total fusion thermal power is removed by the divertor coolant and must, therefore, be recovered at elevated temperature in order to enhance the overall thermal efficiency. Helium is the leading coolant because of its high thermal conductivity, material compatibility, and suitability as a working fluid for power conversion systems using a closed high temperature Brayton cycle. Peak surface heat fluxes on the order of 10 MW/m^2 are anticipated with surface temperatures in the region of 1,200°C to 1,500°C. Recently, several helium-cooled divertor designs have been proposed, including a modular T-tube design and a modular finger configuration with jet impingement cooling from perforated end caps. Design calculations performed using the FLUENT® CFD software package have shown that these designs can accommodate a peak heat load of 10 MW/m^2. Extremely high heat transfer coefficients (~50,000 W/(m^2 K)) were predicted by these calculations. Since these values of heat transfer coefficient are considered to be outside of the experience base for gas-cooled systems, an experimental investigation has been undertaken to validate the results of the numerical simulations. Attention has been focused on the thermal performance of the T-tube and the finger divertor designs. Experimental and numerical investigations have been performed to support both divertor geometries. Excellent agreement has been obtained between the experimental data and model predictions, thereby confirming the predicted performance of the leading helium-cooled divertor designs for near- and long-term magnetic fusion reactor designs. The results of this investigation provide confidence in the ability of state-of-the-art CFD codes to model gas-cooled high heat flux plasma-facing components such as divertors.

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>

No description available.

<p>To improve the prediction of two-phase local structure and heat transfer in subcooled boiling flow, the wall nucleation phenomenon was studied to accurately model the wall source term in the interfacial area transport equation (IATE) for the use with the two-fluid model. The existing experimental datasets and modeling works of departure diameter, departure frequency and active nucleation site density were comprehensively reviewed. Since these parameters are coupled in the bubble ebullition cycles, simultaneous measurements of departure diameter, departure frequency and active nucleation site density were performed in a vertical annular test section. The ranges of the existing experimental database were extended to high pressure and high heat flux conditions. The stochastic characteristics of the departure diameter and departure frequency measured from a single nucleation site and over multiple nucleation sites were investigated. Significant variations between different nucleation sites were observed. A parametric study of departure diameter, departure frequency and nucleation site density were conducted at varying system pressure, heat flux, flow rate and subcooling conditions. The existing models of these parameters were evaluated with the experimental dataset of the existing and the present works. Significant discrepancies were observed between model predictions and experimental data, which indicates that the mechanism of nucleate boiling is not fully understood. The heat flux partitioning model was also evaluated. The results show that the heat flux at high pressure or low flow rate conditions was significantly underestimated. This may suggest that major heat transfer mechanisms are missing in the heat flux partitioning model.</p>