Bubble columns and airlift reactors were modeled numerically to better understand the hydrodynamics and analyze the mixing characteristics for each configuration. An Eulerian-Eulerian approach was used to model air as the dispersed phase within a continuous phase of water using the commercial software FLUENT. The Schiller-Naumann drag model was employed along with virtual mass and the standard k-e turbulence model. The equations were discretized using the QUICK scheme and solved with the SIMPLE coupling algorithm. The flow regimes of a bubble column were investigated by varying the column diameter and the inlet gas velocity using two-dimensional simulations. The typical characteristics of a homogeneous, slug, and heterogeneous flow were shown by examining gas holdup. The flow field predicted using two-dimensional simulations of the airlift reactor showed a regular oscillation of the gas flow due to recirculation from the downcomer and connectors, whereas the bubble column oscillations were random and resulted in gas flow through the center of the column. The profiles of gas holdup, gas velocity, and liquid velocity showed that the airlift reactor flow was asymmetric and the bubble column flow was symmetric about the vertical axis of the column. The average gas holdup in a 10.2 cm diameter bubble column was calculated and the results for the two-dimensional simulation of varying inlet gas velocities were similar to published experimental results. The average gas holdup in the airlift reactor for the three-dimensional simulations compared well with the experiments, and the two-dimensional simulations underpredicted the average gas holdup. / Master of Science

Hydrothermal processes at oceanic spreading centers are largely influenced by changing magmatic heat input. I use the FISHES code to investigate the evolution of surface temperature and salinity as a function of time-varying heat flux at the base of a two-phase, vapor-brine hydrothermal system. I consider a two-dimensional rectangular box that is 1.5 km deep and 4 km long with homogeneous permeability. Impermeable, insulated conditions are imposed on the left and right hand boundaries. To simulate time-varying heat flux from a sub-axial magma chamber of 500 m long half-width, I consider a variety of basal boundary conditions: (1) a constant heat flux with an value of 130 W/m2; (2) a sinusoidal heat flux with a period of 6 years and an amplitude ranging between 100 and 50 W/m2; (3) step, random, and exponential heat fluxes ranging between 200 and 15 W/m2; and (4) an analytical function of temporally decaying heat flux resulting from a simulated cooling, crystallizing magmatic sill. As a result of the investigation I find: (1) changes in bottom temperature and salinity closely follow the temporal variations in magmatic heat inputs; (2) the surface temperature response is severely damped and high frequency variations in heat flow are not detected; (3) in regions where phase separation of vapor and brine occurs, surface salinity variations may be recorded in response to changing conditions at depth, but these are smaller in amplitude. / Master of Science

The critical heat flux (CHF) point for a reactor core system is one of the most important factors to discuss in regards to reactor safety. If this point is reached, standard coolant systems are not enough to handle the temperature increase in the cladding, and the likelihood of meltdown greatly increases. While the nucleate boiling and film boiling regimes have been well-investigated, the transition boiling regime between the point of departure from nucleate boiling (DNB) and the minimum film boiling temperature (T_min) remains difficult to study. This is due to both the complexity of the phenomena, as well as limitations in measurement, where experiments typically utilize thermocouples for temperature data acquisition. As a result of technological advancement in the field of fiber optics, it is possible to measure the quenching temperature to a much higher degree of precision. Optical fiber sensors are capable of taking many more measurements along a fuel simulator length than thermocouples, which are restricted to discrete points. In this way, optical fibers can act as an almost continuous sensor, calculating data at a resolution of less than one millimeter where a thermocouple would only be able to measure at one point. In this thesis, the results of a series of quenching experiments performed on stainless steel, Monel k500, and Inconel 600 rods at atmospheric pressure, with different subcooling levels and surface roughnesses, will be discussed. The rewetting temperature measurement is performed to compare results between thermocouples and optical fiber sensors in a 30 cm rod. These results are further discussed with regard to future application in two-phase flow experiments. / Master of Science / There are multiple types of boiling that can occur depending on the heat transfer capabilities of the system and the power applied to the coolant. The most common is nucleate boiling, where vapor produced at the surface forms bubbles and move away from the surface due to buoyancy. At a high enough power, the bubbles can coalesce into a film and lead to a point at which the liquid coolant can no longer contact the surface. Since vapor is not as effective at transferring heat from the surface, the temperature will increase drastically. In nuclear reactors, this situation (known as departure from nucleate boiling), can quickly lead to a meltdown of the fuel rods. Another important safety parameter in nuclear reactors is the minimum temperature at which this vapor film can be maintained, T_min. This parameter is a source of significant concern with regard to accident scenarios such as LOCA (loss of coolant accident), where reintroducing coolant to the rods efficiently is of top priority. While much research has been done on nucleate and film boiling, it has been difficult to study the transition period between the two regimes due to both its transient nature and the lack of continuous measurement capabilities. Typically, temperature is measured using thermocouples, which are point-source sensors that do not allow for high spatial resolution over a large area. This thesis deals with the utilization of optical fibers for temperature measurement, which are capable of calculating data at every millimeter, potentially a much more precise measurement system than with the thermocouples. The experiments performed in this paper are quenching experiments, where a rod embedded with thermocouples and an optical fiber is heated to well above T_min and quickly plunged into a volume of water, in order to view the transition from film to nucleate boiling.

Inlet headers in the primary heat transport system(PHTS) of CANDU type reactors, are used to collect the coolant coming from the steam generators and distribute them into the reactor core via several feeders. During a postulated loss of coolant accident (LOCA), depressurization and vapor supplement into the core may occur, which results a deterioration in the heat transfer from fuel to the coolant. When a depressurization occurs, &ldquo / Emergency Coolant Injection(ECI)&rdquo / system in the PHTS in CANDU reactors, is automatically become active and supply coolant is fed into the reactor core via the inlet header and feeders. . This study is focused on the experimental and computational investigation of the ECI effect during a LOCA in a CANDU inlet header. The experiments were carried out in METU Two-Phase Flow Test Facility which consists of a scaled CANDU inlet header having 5 connected feeders. The same tests were simulated with a one dimensional two-fluid computer code, CATHENA, developed by Atomic Energy of Canada Limited(AECL). The average void fraction and the two phase mass flowrate data measured in the experiments are compared with the results obtained from CATHENA simulation. Although a few mismatched points exist, the results coming from two different studies are mostly matching reasonably. Lack of three-dimensional modeling for headers in CATHENA and experimental errors are thought to be the reasons for these dismatches.

With continued development of the electronic industry, the demand for highly efficient heat removal solutions requires innovative cooling technologies. A computational fluid dynamic (CFD) study, including heat transfer, is performed for an axisymmetric, confined jet impingement experiencing boiling and coupled with vapor extraction. Boiling occurs at the target surface while extraction occurs at the wall confining the radial flow. The region between the target and confining wall is defined as a confined gap. Extraction is employed to enhance heat transfer and to minimize the potential negative influence of flow instabilities resulting from two-phase flow within a confined region. A three-dimensional sector of the confined jet is employed in the simulation. A single circular impinging jet with a constant jet diameter (4 mm) and variable gap height (0.5, 1.0 and 1.5 mm), also known as nozzle-to-target spacing, is considered. The effect of mass flux at the confined gap entrance is also investigated (200, 400 and 800 kg/m²-s) for a range of heat flux (5 to 50 W/cm²). Fluid flow and heat transfer are simulated using the Volume of Fluid (VOF) model and the wall-boiling sub-model within the Multiphase Segregated Flow (MSF) model. The boiling sub-model in the VOF model applies the Rohsenow boiling correlation, while in the MSF model, the Kurul-Podowski boiling sub-model is used. Also, vapor extraction is realized by different mechanisms for these two models. For the VOF model, a specific phase "wall porosity" can be assigned to a wall to make it porous. Over a range of pressure differentials across this porous wall such that the inertial transport influence is negligible, vapor transport should agree with Darcy's law. For the MSF model, a wall can be made permeability to one substance or phase while remaining impermeable to the other substance or phase. However, a portion of the substance or phase reaching the boundary allowed to pass through the surface must be specified. A pressure drop cannot be applied across the wall, thereby prohibiting Darcy flow modeling. The solutions of both models are at steady state. The boiling curves without vapor extraction from both models are provided and compared to experiments. Simulations matching experimental wall temperatures under-predict theoretical vapor generation and those matching vapor generation over-estimate wall superheat. For cases with no extraction, local temperature and velocity profiles from the VOF model are provided at several radial locations within the confined gap. Scalar temperature and pressure distributions and velocity vectors are presented to explain observations in profiles. / Graduation date: 2013

In order to investigate the two-phase flow behaviour in a complex reactor-typical geometry and to supply suitable data for CFD code validation, a model of the hot leg of a pressurised water reactor was built at FZD. The hot leg model is operated in the pressure chamber of the TOPFLOW test facility, which is used to perform high-pressure experiments under pressure equilibrium with the inside atmosphere of the chamber. This technique makes it possible to visualise the two-phase flow through large windows, also at reactor-typical pressure levels. In order to optimise the optical observation possibilities, the test section was designed with a rectangular cross-section. Experiments were performed with air and water at 1.5 and 3.0 bar at room temperature as well as with steam and water at 15, 30 and 50 bar and the corresponding saturation temperature (i.e. up to 264°C). The total of 194 runs are divided into 4 types of experiments covering stationary co-current flow, counter-current flow, flow without water circulation and transient counter-current flow limitation (CCFL) experiments. This report provides a detailed documentation of the experiments including information on the experimental setup, experimental procedure, test matrix and on the calibration of the measuring devices. The available data is described and data sheets were arranged for each experiment in order to give an overview of the most important parameters. For the cocurrent flow experiments, water level histograms were arranged and used to characterise the flow in the hot leg. In fact, the form of the probability distribution was found to be sensitive to the boundary conditions and, therefore, is useful for the CFD comparison. Furthermore, the flooding characteristics of the hot leg model plotted in terms of the classical Wallis parameter or Kutateladze number were found to fail to properly correlate the data of the air/water and steam/water series. Therefore, a modified Wallis parameter is proposed, which takes the effect of viscosity into account.

Das Ziel des Vorhabens bestand in der Ertüchtigung von CFD-Codes für Wasser-Dampf-Strömungen mit Phasenübergang. Während CFD-Verfahren für einphasige Strömungen bereits breite Anwendung in der Industrie finden, steht ein entsprechender Einsatz für Zweiphasenströmungen auf Grund der komplexen Phasengrenzfläche und den davon beeinflussten Wechselwirkungen erst am Anfang. Für die Weiterentwicklung und Validierung geeigneter Schließungsmodelle werden experimentelle Daten mit hoher Orts- und Zeitauflösung benötigt. Solche Daten wurden an der TOPFLOW-Versuchsanlage des HZDR durch Kombination von Experimenten bei praxisnahen Parametern für die Reaktorsicherheit (große Skalen, hohe Drücke und Temperaturen) und innovativer Messtechnik gewonnen. Die Gittersensortechnik, mit der detaillierte Informationen über die Phasengrenzfläche gewonnen werden können, wurde in adiabaten Wasser-Luft-Experimenten sowie Kondensations- und Druckentlastungsexperimenten in einem großen DN200-Rohr eingesetzt. Umfangreiche Datenbasen mit hoher Qualität stehen im Ergebnis des Vorhabens zur Verfügung. Die Technologie für die schnelle Röntgentomographie, die Messungen ohne Strömungsbeeinflussung ermöglicht, wurde weiter entwickelt und in einer ersten Messserie erfolgreich eingesetzt. Hochaufgelöste Daten wurden auch in Experimenten zu verschiedenen Strömungssituationen (z.B. Gegenstrombegrenzung) in einem Modell des heißen Strangs eines Druckwasserreaktors gewonnen. Für die Wasser-Dampf-Experimente bei Drücken von bis zu 5 MPa wurde dabei erstmals die neu entwickelte innovative Drucktanktechnologie eingesetzt. Zur Ertüchtigung von CFD-Codes für Zweiphasenströmungen wurde das Inhomogene MUSIG-Modell für Phasenübergänge in Kooperation mit ANSYS erweitert und anhand der o.g. TOPFLOW-Experimente validiert. Außerdem erfolgten Verbesserungen u.a. für die Turbulenzmodellierung in Blasenströmungen sowie Simulationen zur Validierung der Modelle für Blasenkräfte und Blasenkoaleszenz und -zerfall. Ein wesentlicher Fortschritt wurde bei der Modellierung freier Oberflächen durch die Verallgemeinerung des AIAD-Modells erreicht. Die am Heißstrangmodell ermittelten Flut¬kurven können unter Nutzung dieses Modells in guter Übereinstimmung berechnet werden.

A suspension flow model based on the "suspension balance" approach has been developed. This work modifies the model to allow the solution of suspension flows under general flow conditions. This requires the development of a frame-invariant constitutive model for the particle stress which can take into account the spatially-varying local kinematic conditions. The mass and momentum balances for the bulk suspension and particle phase are solved numerically using a finite volume method. The particle stress is based upon the computed rate of strain and the local kinematic conditions. A nonlocal stress contribution corrects the continuum approximation of the particle phase for finite particle size effects. Local kinematic conditions are accounted through the local ratio of rotation to extension in the flow field. The coordinates for the stress definition are the local principal axes of the rate of strain field. The developed model is applied to a range of problems. (i) Axially-developing conduit flows are computed using both the full two-dimensional solution and the more computationally efficient "marching" method. The model predictions are compared to experimental results for cross-stream particle concentration profiles and axial development lengths. (ii) Model predictions are compared to experiments for wide-gap circular Couette flow of a concentrated suspension in a shear-thinning liquid. With minor modification, the suspension flow model predicts the major trends and results observed in this flow. (iii) Comparisons are made to experiments for an axisymmetric contraction-expansion. Model predictions for a two-dimensional planar contraction flow test the influence of model formulation. The variation of the magnitude of an isotropic particle normal stress with local kinematic conditions and anisotropy in the in-plane normal stresses are both explored. The formulation of the particle phase stress is found to have significant effects on the solid fraction and velocity. (iv) Finally, for a rectangular piston-driven flow and an obstructed channel flow, a "computational suspension dynamics" study explores the effect of particle migration on the bulk flow field, system pressure drop and particle phase composition.

In CANDU reactors, under normal operating conditions, the inlet headers collect and distribute single-phase liquid flow (heavy water) to the fuel cooling channels via the feeders. However, under some postulated loss of coolant accidents, the inlet headers may receive two-phase fluid (steam/water) and the fluid forms a stratified region inside the header. To predict the thermalhydraulic behaviour of headers for the reactor safety analysis, the two-phase flow distribution within the headers and through the feeders must be modelled. In order to analyse the two-phase flow behaviour of a scaled CANDU inlet header / a transparent and instrumented version of a header with 5 feeders was previously built in the Mechanical Engineering Department of Middle East Technical University (METU-Two Phase Flow Test Facility / METU-TPFTF). The aim of this study is to investigate two-phase flow distribution through multiple outlets from such a horizontal drum both numerically and experimentally. For this purpose, three-dimensional incompressible finite difference equations in cylindrical coordinates were derived by using two-fluid model to simulate adiabatic two-phase flow (air/water) in the header numerically. The discretized equations were then programmed into a computer code which was developed specifically for modelling the header type geometry. A method based on the principles of Implicit Multi Field (IMF) technique has been utilised to solve those equations. The solution algorithm was tested by using some numerical benchmark problems. A number of experimental tests covering single and two-phase flow distribution through outlet pairs from the header were performed. Void fractions and flow rates obtained from these tests are in good agreement with the code results. The code also predicts the void fraction and pressure distribution in the header satisfactorily.

This thesis addresses the experimental investigation of the cavitation phenomenon and its main consequences on the normal operation of a safety relief valve (SRV). More particularly, limitation of the mass flux discharged and alteration of the hydraulic fluid forces behavior is of main interest for the proper design and sizing of such devices. In nuclear or thermal engineering systems, the use of SRVs is mandatory since it represents the ultimate protection device before an accident occurs, caused by a sudden pressurization of the system. A careful design and sizing of the SRV is therefore essential. The complete understanding of the physics taking place in the flow through the valve is required to guaranty and optimize the security of the protected process.<p><p>In order to investigate the above effects of cavitation in a SRV, two different orifice sized valves (API 2J3 type and a transparent model based on an API 1 1/2G3 type) are tested in two different experimental facilities expressly built for this purpose. Instead of using a spring, the design of both valves allows the adjustment of the disc at any desired lift. Hence the static behavior of the valves is investigated. Both facilities, operating at different magnitude scales, allow the study of single phase and cavitating flow conditions required to properly determine the most important hydraulic characteristics, and access on any potential scaling effect between both sized SRVs. Experimental techniques used for the determination of the hydraulic characteristics include temperature, flow rate, fluid forces and pressure measurements both upstream and downstream the test sections. <p><p>Results show a similar influence of cavitation on the flow characteristics of both valves, minimizing any potential scaling effect. The liquid pressure recovery factor FL, which is normally used to identify a choked flow condition in a control valve, is experimentally determined for the first time in a SRV. The existence of a local minimum located at small openings of the lift indicates a change on the flow characteristics of both valves, which is related to the location of the minimum cross section of the flow that does not remain constant for every lift position. An extended experimental campaign is performed to analyse the effect of the blowdown ring adjustment located around the nozzle of the API 2J3 valve. Results confirm that the position of the ring has an important contribution for the hydraulic forces acting on the valve disc. <p><p>In the second part of the research, precise optical diagnostic techniques are successfully applied in the transparent valve to locally characterize the flow topology in a SRV experiencing cavitation. These results are innovative and enrich the experimental database available in the literature for the characterization and understanding of the flow physics in such devices. In a first configuration, high speed visualization is applied to observe qualitatively the flow pattern and the inception of liquid vaporization. Particle tracking results suggest that vapor bubbles are formed in the core of vortices detached from the shear layers attached to the valve. These rotational structures promote lower pressure regions allowing the liquid to vaporize. In the second configuration, particle image velocimetry is applied to extract the velocity field in both single phase and cavitating flow conditions. Results of PIV confirm the existence of a submerged jet just downstream the minimum section. This jet is characterized by two non-symmetric shear layers at its sides. Under cavitation conditions, PIV results confirm that vapor bubbles are formed preferentially inside the jet shear layers. The phenomenon of mass flux limitation caused by cavitation is reproduced at small openings of the valve and interaction with the flow topology is highlighted. It is observed that limitation of the flow occurs when the vena contracta is shifted towards the minimum geometrical section of the flow. Finally, instabilities of the flow downstream the critical section are investigated in the frequency domain by means of time resolved data. Results suggest that vortex shedding mechanism is dominated by a constant Strouhal number which is slightly affected by the valve opening. <p><p>In the last part of the research, the methodology used in water is extended and applied to cryogenic liquids. Two different geometries are investigated experimentally and numerically using water and liquid nitrogen as working fluids. Results suggest that both the flow coefficient (determined at single flow conditions), and the liquid recovery factor (used to identify choked flows), are independent on the fluid properties and therefore, an hydraulic similarity relation can be proposed.<p><p>This research project was carried out at the von Karman Institute for Fluid Dynamics (VKI), in Belgium, in close collaboration and with the funding of Centre Technique des Industries Mécaniques (CETIM) in France. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished