A thermal hydraulic analysis has been conducted to investigate the conditions leading to the channel flow reversal and the subsequent effects that may have on the Primary Heat Transfer System(PHTS) thermohydraulic parameters during the natural circulation under the specific accident scenarios for a generic CANDU 900 MW plant model similar to Darlington NGS. The assumed initiating events are the combination of a small Loss Of Coolant Accident (LOCA) with a loss of Class IV power, as well as the unavailability of Emergency Coolant Injection (ECI) system. No makeup inventory is taken into account in this study, and there is no fuel sheath temperature excursion or fuel centerline melting, i.e., the integrity of fuel is always maintained. A one-dimensional quasi-steady state Homogeneous Equilibrium Model(HEM) has been constructed for the study. A specific node-link structure is adopted to represent the primary heat transfer loop: The whole loop and different components in HTS are represented by a series of nodes that have quasi-static thermal hydraulic characteristics such as pressure and enthalpy, etc. Dynamic characteristics are delivered by the links between nodes, e.g., flow rate and pressure drop. The channel powers (decay heat), the secondary side pressure and the pressure at Reactor Inlet Header(RIH) are chosen as boundary conditions to describe the assumed initiating incidents for the iii model. With ongoing loss of inventory and system depressurization, vapor lock occurs in Steam Generator(SG), and it increases the pressure drop from Reactor Outlet Header(ROH) to RIH across SG and forms an increasingly negative RIH-to-ROH pressure differential. Flow reversal occurs in the channel due to the counter force balance between negative RIH-to-ROH pressure difference and the driving force derived from the density difference between the hot and cold legs. It is found that channels in row A have the highest reversal preference, then followed by the channels in row B, and in that order subsequently. Row A reverses when inventory decreases to about 79.5% of initial value, with following boundary conditions: decay heat is 1.5% of Full Power(FP) and secondary side pressure is 5.070 MPa. In addition, it is found that the decrease in channel power accelerates the depressurization process and brings forward the occurrence of flow reversal in fuel channels. / Thesis / Master of Applied Science (MASc)

This research is aimed to study the system performance for a large solar hot water system constructed by connecting a series of small domestic natural circulation systems. There are few studies on this type of large solar hot water system available. The major concern is that when circulation pump is on, there forms a short flow between inlet and outlet of each storage tank of natural circulation solar hot water unit. Therefore, water does not have chance to flow though the collector by thermosyphon and system performance can be lowered down drastically. This thesis presents the numerical simulation study for the control and system operating parameters effects on the system performance to provide important information both for users and system designers.

Experimental thermal-hydraulic study of a rectangular supercritical CO2 natural-circulation loop with a horizontal heated channel was conducted at different steady-state conditions. These included different system pressures and three different inlet temperatures, with different inlet and outlet valve openings. Approximately, 450 experimental steady-state data-points were collected. The data include measurements of pressure-drop along the heated channel, pressure-drop across inlet and outlet valves, applied heat on the heated channel, pressure, temperature and flow-rate. Steady-state curves of mass flow-rate versus power, outlet temperature versus power, and detailed information of frictional pressure drop and local head loss coefficients were produced. Comparison showed that for the available experimental set-up, computed frictional pressure-drops fell within 1-1.20 of the Blasius formula prediction. Moreover, flow oscillations were observed in several cases when outlet temperature of CO2 was higher than the pseudo-critical temperature on the negative slope part of the mass flow-rate versus power curve.

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The water utilized by passive air-conditioning systems in buildings is typically required at higher elevations. The thermally driven natural circulation water pump (TDNCWP) is a passively driven pumping system for delivering water from ground level against gravity to a higher elevation. It consists of a humid air closed duct loop to which a temperature difference is applied, resulting in a density gradient driven flow. A hot water evaporation tray inside the loop at ground level introduces water vapour to the loop air flow, and a cold condensation plate inside the loop at the elevated level removes this water vapour for passive airconditioning usage. In this thesis, a one-dimensional theoretical and numerical simulation model is developed. Experiments were conducted on two experimental TDNCWP set-ups of different cross sectional areas to evaluate the pump design and the theoretical model. It is shown in this thesis that the TDNCWP can provide water at varied elevations using non-mechanical, passive means. A temperature difference of 9 to 12.5 °C induced an average velocity of 0.4 to 0.6 m/s for a duct cross section of 100 mm2. For a larger cross section of 400 mm2, a temperature difference of 2 to 5 °C induced an average velocity of 0.25 to 0.3 m/s. An asymmetrical velocity profile was observed which varied at different points in the loop. A water delivery rate of 1.2 to 7.5 L/day was experimentally determined which compares well to the passive air-conditioning water requirements of a small building. The theoretical model over-predicted the delivery rate at increased duct cross sectional areas but fared well when compared to the smaller experimental model results. Further refinement of the numerical model and the TDNCWP design is required, and recommendations were made regarding this. It is clear however that the TDNCWP provides an alternative to a conventional water pump for low-volume water pumping requirements. / AFRIKAANSE OPSOMMING: Die water wat gebruik word deur passiewe lugversorgingstelsels in geboue word tipies benodig op hoër vlakte. Die termies gedrewe natuurlike sirkulasie waterpomp (TDNCWP) is ʼn passiewe gedrewe pomp stelsel vir die lewering van water vanaf die grondvlak teen swaartekrag na ʼn hoër vlak. Dit bestaan uit 'n vogtige geslote lug geut siklus waarop ʼn temperatuur verskil toegepas word, dit lei tot vloei gedrewe deur ʼn digtheids gradiënt. ʼn Warm water verdampings-pan binne die geut op grondvlak stel waterdamp aan die geut lugvloei toe, en ʼn koue kondensasie plaat binne die geut op die verhoogde vlak verwyder hierdie waterdamp vir passiewe lugversorgings gebruik. In hierdie tesis word ʼn eendimensionele teoretiese en numeriese simulasie model ontwikkel. Eksperimente is uitgevoer op twee eksperimentele TDNCWP stelsels van verskillende deursnee grootes om die pomp ontwerp en die teoretiese model te evalueer. Die tesis dui aan dat die TDNCWP water kan voorsien teen verskillende hoogtes op ʼn nie-meganiese, passiewe wyse. ʼn Temperatuur verskil van 9 tot 12.5 °C veroorsaak ʼn gemiddelde snelheid van 0.4 tot 0.6 m/s vir ʼn geut deursnit van 100 mm2.Vir ʼn groter deursnit van 400 mm2, het ʼn temperatuur verskil van 2 tot 5 °C ʼn gemiddelde snelheid van 0.25 tot 0.3 m/s veroorsaak. ʼn Asimmetriese snelheidsprofiel was waargeneem wat gewissel het op verskillende punte in die siklus. ʼn Water voorsienings tempo van 1.2 tot 7.5 L / dag was eksperimenteel waargeneem wat goed vergelyk met die passiewe water lugversorging vereistes van 'n klein gebou. Die teoretiese model het ʼn groter voorsienings tempo voorspel vir die groot deursneë, maar het goed gevaar in vergelyking met die kleiner eksperimentele model. Verdere verfyning van die numeriese model en die TDNCWP ontwerp word vereis, en aanbevelings is gemaak ten opsigte van hiervan. Dit is egter duidelik dat die TDNCWP ʼn alternatief is vir konvensionele lae-volume water pomp applikasies. / National Research Foundation (NRF)

Neste trabalho são apresentados resultados experimentais, de um circuito operando em circulação natural, que permitem analisar o comportamento de um sistema de resfriamento de emergência quando é aberta uma linha de desvio entre a fonte quente e a fonte fria. O trabalho tem ainda a importância de documentar os testes de caracterização hidráulica do circuito experimental, fornecendo inclusive os fatores de perda de pressão específicos para o circuito. Observou-se que, para uma mesma potência, quando é aberta a linha de desvio, a temperatura na saída da fonte quente aumenta substancialmente. Esse aumento ocorre porque a vazão através do aquecedor diminui. A vazão através do trocador de calor (fonte fria) aumenta ligeiramente, sendo sempre a soma das vazões na linha de desvio e no aquecedor. O trabalho mostra ainda que a posição de conexão da linha de desvio com a perna quente determina o sentido de escoamento, podendo ocorrer a inversão a partir de uma determinada cota. Para comprovar a possibilidade de simulação precisa dos experimentos foi ainda desenvolvido um modelo numérico das equações de conservação, utilizando o programa “Engineering Equation Solver" (EES). Esse modelo foi utilizado para reproduzir os experimentos de circulação natural pelo circuito externo. / This work presents experimental results of a circuit when operating in natural circulation. These results allow to analyze the behavior of an emergency core cooling system when a bypass line that connects the hot source with the cold source is opened. This work also reports the hydraulic characterization of the experimental loop, given geometric and hydraulic data including experimental friction factors specific to this circuit. It was observed that, to a fixed thermal power, when the bypass line is opened, the heater outlet temperature increases. This temperature increase is due to the decrease in the flow rate through the heater. The heat exchanger's flow rate is subjected to a small increase. This flow rate is the sum of the bypass line and heater mass flow rates. This work also shows that the vertical position of the connection of the bypass line in the hot-leg determines the flow direction in the bypass line. If the bypass line connection is in the lowest position, the flow is from the cold to the hot-leg. If the bypass connection is in the highest position, the flow is from the hot to the cold-leg. A numerical model used to evaluate friction factors and heat transfer coefficients influence was developed. It was used to confirm the possibility of precise experiments simulation. The conservation equations are solved using “Engineering Equation Solver" (EES), a thermal hydraulics analysis tool. The model was adjusted with natural circulation experimental data and was tested with results of natural circulation without bypass lines.

Neste trabalho são apresentados resultados experimentais, de um circuito operando em circulação natural, que permitem analisar o comportamento de um sistema de resfriamento de emergência quando é aberta uma linha de desvio entre a fonte quente e a fonte fria. O trabalho tem ainda a importância de documentar os testes de caracterização hidráulica do circuito experimental, fornecendo inclusive os fatores de perda de pressão específicos para o circuito. Observou-se que, para uma mesma potência, quando é aberta a linha de desvio, a temperatura na saída da fonte quente aumenta substancialmente. Esse aumento ocorre porque a vazão através do aquecedor diminui. A vazão através do trocador de calor (fonte fria) aumenta ligeiramente, sendo sempre a soma das vazões na linha de desvio e no aquecedor. O trabalho mostra ainda que a posição de conexão da linha de desvio com a perna quente determina o sentido de escoamento, podendo ocorrer a inversão a partir de uma determinada cota. Para comprovar a possibilidade de simulação precisa dos experimentos foi ainda desenvolvido um modelo numérico das equações de conservação, utilizando o programa “Engineering Equation Solver” (EES). Esse modelo foi utilizado para reproduzir os experimentos de circulação natural pelo circuito externo. / This work presents experimental results of a circuit when operating in natural circulation. These results allow to analyze the behavior of an emergency core cooling system when a bypass line that connects the hot source with the cold source is opened. This work also reports the hydraulic characterization of the experimental loop, given geometric and hydraulic data including experimental friction factors specific to this circuit. It was observed that, to a fixed thermal power, when the bypass line is opened, the heater outlet temperature increases. This temperature increase is due to the decrease in the flow rate through the heater. The heat exchanger's flow rate is subjected to a small increase. This flow rate is the sum of the bypass line and heater mass flow rates. This work also shows that the vertical position of the connection of the bypass line in the hot-leg determines the flow direction in the bypass line. If the bypass line connection is in the lowest position, the flow is from the cold to the hot-leg. If the bypass connection is in the highest position, the flow is from the hot to the cold-leg. A numerical model used to evaluate friction factors and heat transfer coefficients influence was developed. It was used to confirm the possibility of precise experiments simulation. The conservation equations are solved using “Engineering Equation Solver” (EES), a thermal hydraulics analysis tool. The model was adjusted with natural circulation experimental data and was tested with results of natural circulation without bypass lines.

The removal of heat from the containment building is an important consideration in the design of a nuclear power plant. In this investigation a simple rectangular natural circulation loop was simulated to determine whether it could possibly be used to remove usable quantities of heat from a containment building. The loop had a vertical pipe on the inside and outside of the containment building. These pipes acted as heat exchangers. Single phase and two phase cases were simulated by imposing a temperature on the respective vertical leg pipe walls and determining the heat absorption from the containment building. The heat was conveyed from the inside of the building to the outside via the natural circulation phenomenon. A literature study was done to cover topics relevant to this investigation. A theoretical model using conservation equations and control volumes was derived. This model was based largely on knowledge gleaned from the literature study. The theoretical model was a simple homogenous model, which was sufficiently detailed for a conceptual investigation. The theoretical model was then manipulated into a form suitable for use in a computer simulation program. Simplifications were made to the simulation model and underlying theory due to the nature of the investigation. The simulation model was validated against published experimental results. During the simulation phase a number of cases were investigated. These cases were divided into base cases and parametric studies. During the base case simulations the change of key fluid variables along the loop was examined. During the parametric studies the hot and cold leg inside wall temperatures, loop geometry and pipe diameter were varied. The effect of these parameters on the heat absorption from the containment was determined. The simulations showed that with the current assumptions about 75 to 120 of the natural circulation loops are needed depending on their geometry and containment conditions. The heat removal rates that were calculated varied from 50 kW to 600 kW for a single loop. As explained in the final chapter, there are many factors that influence the results obtained. The natural circulation concept was deemed to be able to remove usable quantities of heat from the containment building. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.

The removal of heat from the containment building is an important consideration in the design of a nuclear power plant. In this investigation a simple rectangular natural circulation loop was simulated to determine whether it could possibly be used to remove usable quantities of heat from a containment building. The loop had a vertical pipe on the inside and outside of the containment building. These pipes acted as heat exchangers. Single phase and two phase cases were simulated by imposing a temperature on the respective vertical leg pipe walls and determining the heat absorption from the containment building. The heat was conveyed from the inside of the building to the outside via the natural circulation phenomenon. A literature study was done to cover topics relevant to this investigation. A theoretical model using conservation equations and control volumes was derived. This model was based largely on knowledge gleaned from the literature study. The theoretical model was a simple homogenous model, which was sufficiently detailed for a conceptual investigation. The theoretical model was then manipulated into a form suitable for use in a computer simulation program. Simplifications were made to the simulation model and underlying theory due to the nature of the investigation. The simulation model was validated against published experimental results. During the simulation phase a number of cases were investigated. These cases were divided into base cases and parametric studies. During the base case simulations the change of key fluid variables along the loop was examined. During the parametric studies the hot and cold leg inside wall temperatures, loop geometry and pipe diameter were varied. The effect of these parameters on the heat absorption from the containment was determined. The simulations showed that with the current assumptions about 75 to 120 of the natural circulation loops are needed depending on their geometry and containment conditions. The heat removal rates that were calculated varied from 50 kW to 600 kW for a single loop. As explained in the final chapter, there are many factors that influence the results obtained. The natural circulation concept was deemed to be able to remove usable quantities of heat from the containment building. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.

Les boucles de circulation naturelle d'hélium diphasiques sont utilisées comme systèmes de refroidissement d'aimants supraconducteurs de grande envergure, vus leurs avantages inhérents de sûreté et d'entretien. Des exemples sont le détecteur CMS au CERN (déjà en opération) ou les aimants du spectromètre R3B-GLAD au GSI (en installation). Une des préoccupations majeures lors du refroidissement par ébullition est la crise d'ébullition : la dégradation soudaine du transfert de chaleur pariétal au-delà d'une certaine valeur de flux de chaleur, dénommée critique. L'augmentation de température de paroi qui en résulte peut entraîner la perte de l'état supraconducteur de l'aimant.Les boucles de circulation naturelle à l'hélium ont déjà été étudiées expérimentalement et numériquement en régime permanent, spécialement en régimes pré-critiques (ébullition nucléée). Les travaux sur les transferts de masse et de chaleur en hélium en ébullition en régime transitoire présents dans la littérature ciblent principalement des systèmes de petites dimensions, des canaux très étroits ou trop courts, ou l'ébullition en bain. Bien que des comportements qualitativement similaires sont attendus, l'extrapolation de ces résultats à une boucle de circulation naturelle n'est pas évident, si possible. C'est pourquoi une étude particulière du comportement thermohydraulique transitoire de boucles d'hélium en circulation naturelle, lors d'une augmentation soudaine de la charge thermique, est nécessaire. Une partie de cette étude consiste en des expériences sur une boucle d'hélium diphasique en circulation naturelle de 2 m de haut, à 4,2 K. Deux sections chauffées verticales de diamètre différent (10 et 6 mm) et d'environ 1 m de longueur ont été testées. Les transitoires sont induits par une marche soudaine de puissance. Deux types de condition initiale ont été considérés : statique (sans puissance initiale), et en équilibre dynamique (puissance initiale non-nulle). L'évolution de la température de paroi le long de la section, le débit massique et la perte de charge a été mesurée. Parmi d'autres phénomènes, un fort intérêt a été porté au début de la crise d'ébullition. Les valeurs limites de flux de chaleur final auxquelles la crise arrive ont été déterminées. D'un côté, on a observé que la crise peut avoir lieu de façon temporaire ou permanente à une puissance appréciablement plus faible qu'en régime permanente. De l'autre côté, l'augmentation de la circulation initiale, à travers le flux de chaleur initial, peut inhiber partiellement ou totalement cette crise d'ébullition prématurée. On a déterminé que cette dégradation du transfert de chaleur est l'issu de deux phénomènes en compétition, véritablement inhérents à la circulation naturelle : une étape initiale d'accumulation uniforme de vapeur, avec inversion ou diminution de la vitesse d'entrée, et l'établissement ultérieur de la circulation, avec le transit d'un front froid depuis l'entrée. Une analyse semi-empirique nous a permis de déterminer un critère, basé sur l'évolution dynamique du profile spatial du titre massique, pour prédire le déclenchement de la crise. Néanmoins, il est nécessaire de connaître à priori l'évolution du débit massique pour pouvoir appliquer ce critère. La dernière partie de ce travail est dévouée à la production et validation de modèles et outils de calcul pour la simulation du comportement thermohydraulique d'un tel système. Deux options de modélisation sont présentées. L'une est une simplification des équations du modèle homogène 1D des écoulements diphasiques (mise en place en COMSOL) ; l'autre reprend le modèle homogène tel quel (programmé en C). Les simulations d'évolution du débit massique sont en assez bon accord avec les mesures, à l'exception d'un léger déphasage temporel. Ceci pourrait être dû à la combinaison d'un retard de l'instrumentation pour la mesure du débit et de l'inexactitude des hypothèses de base du modèle homogène lors de transitoires très violents. / Boiling helium natural circulation loops are used as the cooling system of large superconducting magnets because of their inherent safety and maintenance advantages. Examples are the cooling systems of the CMS detector solenoid magnet at CERN (already in operation) or the R3B-GLAD spectrometer magnet at GSI (in installation phase). A major concern in boiling cooling systems is that of boiling crisis: a sudden deterioration of the wall heat transfer takes place when the surface heat flux exceeds a certain value, called the critical heat flux (CHF). The resulting high temperatures on the wall could ultimately entail the loss of superconducting state of the magnet.Helium natural circulation loops have already been studied experimentally and numerically in steady state, especially in the pre-critical heat and mass transfer regimes (nucleate boiling). Works on transient boiling heat and mass transfer in helium present in the literature are mostly focused on small systems, very narrow channels, too short pipes or pool boiling. Although it is expected to find qualitative similarities with already observed behavior, the extrapolation to a natural circulation loop is not easy, if even possible. Hence the need for a particular study on the transient thermohydraulic behavior of helium natural circulation loops, after sudden increases in the heat load of the circuit.A part of this study consists of experiments conducted in a 2-meter high two-phase helium natural circulation loop at 4.2 K temperature. Two vertical heated sections with different diameters (10 and 6 mm) and around 1 m length were tested. Heat load transients were driven by a step-pulsed heat load. Transients with two types of initial conditions have been studied: static loop (no initial power applied) and in-dynamic-equilibrium loop (non-zero initial power applied). The evolutions of wall temperature along the heated section, total mass flow rate and pressure drop were measured. Among other phenomena, the nature of the onset of boiling crisis has received a special attention. The values of final heat flux limits for its occurrence have been determined. On the one hand, we observed that boiling crisis can take place in temporary or stable fashion at power significantly lower than in steady state. On the other hand, the increase of initial circulation, by raising initial heat flux, can inhibit partially or completely this power-premature boiling crisis. We could determine that this heat transfer deterioration is the result of two competing phenomena, veritably inherent to the natural circulation feature of the system: an initial stage of uniform vapor accumulation with inlet back-flow or velocity reduction, and the ulterior onset of circulation with the transit of a cold front from the entrance. A semi-empirical analysis of data allowed determining a criterion, based on the dynamic evolution of the quality profile in the section, to predict the incipience of boiling crisis. It became evident that it is necessary to know how the mass flow rate of the system is going to evolve, in order to apply the mentioned criterion.Hence, the other part of this work is aimed to the production and validation of models and calculation tools in order to simulate the thermohydraulic behavior of a two-phase helium natural circulation loop. Two modeling options are proposed. One of them consists of a simplification of the 1D two-phase homogeneous model equations (implemented in COMSOL) and the other of their full version (coded in C language). The simulated mass flow rate represents reasonably well the measured evolution except for a relatively small time phase-shift. This could be due to a combination of the delay of flow-metering instrumentation with the inaccuracy of the basic homogeneous model assumptions during violent transients.

This work studies the performance of heat pipe solar collector for water heating. Experimental results are validated using numerical modeling. Homemade heat pipes with distilled water as a working fluid were used for experimental tests. Both natural and forced convective heat pipe condensing mechanisms are studied and their results are compared with conventional natural circulation solar water heating system. Cross flow and parallel flow heat exchanger were tested in forced type heat pipe condensing mechanism. Experimental and numerical results showed good agreement. Heat pipe solar collectors outperformed conventional solar collector because of their efficient heat transport method. Forced convective heat exchanger was found to give higher efficiency compared to natural convective heat pipe condensing system. However, natural convective heat pipe condensing is free from parasitic power and low system weight. It also showed appreciable system efficiency and can be further developed to be used in rural areas where grid electricity is scarce. Cross flow and parallel flow heat exchanger have been tested for forced convective heat pipe condensing mechanism and no appreciable difference was found due to higher fluid velocity in heat exchangers.