Master thesis focused on the research of the effervescent atomizers. Effervescent atomizers belong to a group of two-phase atomizers, which are often used in combustion applications. Right there in combustion applications, the degree of the stability sprays has a significant impact on combustion efficiency and exhaust gas emissions. The main aim of this work was to asses the level of spray unsteadiness depending on the atomizer design and its operating mode. The effect of construction was studied on the diameter and length of mixing chamber, and then on the size, number and location of aeration holes. Seventeen specific variants of the atomizer were constructed by different combinations of these design parameters. Each of these variants was measured in three operating modes, which were represented by a liquid pressure at the inlet to the atomizer and gas-to-liquid mass flow ratio (GLR). To evaluate the level of spray unsteadiness was used a methodology, which compares the ideal element´s distribution into the interparticle time bin, defined for the ideal (stable) spray, with the experimentally observed distribution. The laser measurement system P/DPA (Phase Doppler Particle Analyzer) was used to determine the experimental interparticle distribution. The result of the comparison of the ideal and the experimental distribution was the parameter , which expresses the level of spray unsteadiness for a specific atomizer and operating mode. With that parameter it was possible to compare the individual atomizers and determinate to the benefit of various construction´s correction of the atomizer. The results showed the recommendation for the modifications of the atomizer, creating a spray with a minimum level of spray´s unsteadiness. For the surveyed atomizer and his individual costruction´s variations the drawing was made.

A great deal of work has been conducted on in-tube condensation in horizontal and vertical smooth tubes. The available literature points to mechanisms governing two-phase condensation heat transfer coefficients and pressure drops, which are directly linked to the local flow pattern for both horizontal and inclined configurations. However, the work has been limited to flow pattern observations, heat transfer, pressure drops and void fractions for both horizontal and inclined tubes at high mass fluxes. No work has been conducted on the analysis of the observed flow patterns and the effect of temperature difference between the average wall temperature and average saturation temperature for different inclination angles at mass fluxes of 100 kg/m2.s and below. The purpose of this study is to carry out a qualitative analysis of flow patterns, and show the effect of temperature difference on the heat transfer coefficient for inclination angles from +90° (upward flow) to -90° (downward flow) at mass fluxes below 100 kg/m2.s. An experimental set-up provided the measurements for the two-phase condensation of R-143a in a smooth tube with an inside diameter of 8.38 mm and a length of 1.5 m. The mass fluxes were 25 kg/m2.s to 100 kg/m2.s, the saturation temperature was 40 °C and the mean qualities were 0.1 to 0.9. A high-speed camera was used to visually analyse and determine the flow patterns for both the inlet and the outlet of the test section. Through the results, eight flow patterns were observed: stratified-wavy, stratified, wavy, wavy-churn, intermittent, churn, annular and wavy-annular. The maximum heat transfer was observed for downward flow between inclination angles of -15° and -30°. The Thome-Hajal flow pattern map correctly predicted horizontal flow patterns, but failed to predict most of the inclined flow patterns. Various flow pattern transitions were identified and proposed for all the investigated inclination angles in this study. Finally, the heat transfer coefficient was found to be dependent on quality, mass flux, temperature difference and inclination angle. / Dissertation (MSc)--University of Pretoria, 2016. / Mechanical and Aeronautical Engineering / MSc / Unrestricted

(néant) / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

A novel technique to study the two-phase flow field around an asymmetric diaphragm in a vertical pipe is presented, that enables producing data for CFD code validation in complex geometries. Main feature is a translocation of the diaphragm to scan the 3D void field with a stationary wire-mesh sensor. Besides the measurement of time-averaged void fraction fields, a novel data evaluation method was developed to extract estimated liquid velocity profiles from the wire-mesh sensor data. The flow around an obstacle of the chosen geometry has many topological similarities with complex flow situations in bends, T-junctions, valves, safety valves and other components of power plant equipment and flow phenomena like curved stream lines, which form significant angles with the gravity vector, flow separation at sharp edges and recirculation zones in their wake are present. In order to assess the quality of the CFD code and their underlying multiphase flow and turbulence models pre-test calculations by ANSYS CFX 10.0 were carried out. A comparison between the calculation results and the experimental data shows a good agreement in term of all significant qualitative details of the void fraction and liquid velocity distributions. Furthermore, the report contains a method to assess the lateral components of bubble velocities in the form of a basic theoretical description and visualisation examples. The plots show the deviation of the flow around the obstacle in term of vectors represented the average velocities of the instantaneous cross-sections of all bubbles in the time interval when they pass the measuring plane. A detailed uncertainty analyse of the velocity assessments concludes the presented report. It includes remarks about the comparison with a second method for calculating bubble velocity profiles - the cross-correlation. In addition, this chapter gives an overview about the influence of acceleration and deceleration effects on the velocity estimation.

The works aimed at the further development and validation of models for CFD codes. For this reason, the new thermal-hydraulic test facility TOPFLOW was erected and equipped with wire-mesh sensors with high spatial and time resolution. Vertical test sections with nominal diameters of DN50 and DN200 operating with air-water as well as steam-water two-phase flows provided results on the evaluation of flow patterns, on the be¬haviour of the interfacial area as well as on interfacial momentum and heat transfer. The validation of the CFD-code for complex geometries was carried out using 3D void fraction and velocity distributions obtained in an experiment with an asymmetric obstacle in the large DN200 test section. With respect to free surface flows, stratified co- and counter-current flows as well as slug flows were studied in two horizontal test channels made from acrylic glass. Post-test calculations of these experiments succeeded in predicting the slug formation process. Corresponding to the main goal of the project, the experimental data was used for the model development. For vertical flows, the emphasis was put on lateral bubble forces (e.g. lift force). Different constitutive laws were tested using a Multi Bubble Size Class Test Solver that has been developed for this purpose. Basing on the results a generalized inhomogeneous Multiple Size Group (MUSIG) Model has been proposed and implemented into the CFD code CFX (ANSYS). Validation calculations with the new code resulted in the conclusion that particularly the models for bubble coalescence and fragmentation need further optimisation. Studies of single effects, like the assessment of turbulent dissipation in a bubbly flow and the analysis of trajectories of single bubbles near the wall, supplied other important results of the project.

Two-phase flow experiments at vertical pipes are much suitable for studying the action of different constitutive relations characterizing the momentum exchange at the gas/liquid interface as well as the dynamic behaviour of the gas/liquid interface itself. The flow can be observed in its movement along the pipe and, in particular, within the shear field close to the pipe wall over a considerable vertical distance and, consequently, over a comparatively long time without the immediate separation of gas and liquid characteristic for horizontal flows. Wire-mesh sensors, which were the working horse in the described experiments, supplied sequences of instantaneous two-dimensional gas fraction distributions with a high-resolution in space and time. This allows to derive from the data not only void fraction and bubble velocity profiles, but also bubble size distributions, bubble-size resolved radial gas fraction profiles as well as the axial evolution of these distributions. An interfacial surface reconstruction algorithm was developed in order to extract the extension of interfacial area from the wire-mesh sensor data. The sensors were upgraded to withstand parameters that are close to nuclear reactor conditions. Most of the experiments were performed for both air/water flow at ambient pressure and steam/water flow of up to 6.5 MPa at identical combinations of the gas and liquid superficial velocities. This offers excellent conditions for studying the influence of the fluid properties.

Ziel der Arbeiten war die Weiterentwicklung und Validierung von Modellen in CFD-Codes. Hierzu wurde am FZD die thermohydraulische Versuchsanlage TOPFLOW aufgebaut und mit räumlich und zeitlich hochauflösenden Gittersensoren ausgestattet. Vertikale Teststrecken mit Nenndurchmessern von DN50 bzw. DN200 für Luft/Wasser- sowie Dampf/Wasser-Strömungen lieferten Ergebnisse zur Entwicklung von Strömungsformen, zum Verhalten der Zwischenphasengrenzfläche sowie zum Wärme- und Impulsaustausch zwischen den Phasen. Die Validierung des CFD-Codes in komplexen Geometrien erfolgte anhand von 3D Gasgehalts- und Geschwindigkeitsfeldern, die bei Umströmung eines asymmetrischen Hindernisses auftreten, das in der Teststrecke DN200 eingebaut war. Im Hinblick auf Strömungen mit freier Oberfläche untersuchte das FZD in zwei horizontalen Acrylglas-Kanälen geschichtete Zweiphasenströmungen im Gleich- bzw. Gegenstrom sowie Schwallströmungen. Bei den Nachrechnungen dieser Versuche gelang die Simulation der Schwallentstehung. Entsprechend des Projektziels wurden die experimentellen Ergebnisse zur Modellentwicklung genutzt. Bei vertikalen Strömungen stand die Wirkung der lateralen Blasenkräfte (z.B. Liftkraft) im Vordergrund. Zum Test unterschiedlicher Modellansätze wurde hierzu ein Mehrblasenklassen-Testsolver entwickelt und genutzt. Darauf aufbauend wurde ein neues Konzept für ein Mehrblasenklassenmodell, das Inhomogene MUSIG Modell erarbeitet und in den kommerziellen CFD Code CFX (ANSYS) implementiert. Bei Validierungsrechnungen zeigte sich, dass vor allem die Blasenkoaleszenz- und -zerfallsmodelle weiter optimiert werden müssen. Untersuchungen zu Einzeleffekten, wie z.B. die Abschätzung von Turbulenzkoeffizienten und die Analyse der Trajektoren von Einzelblasen in unmittelbarer Wandnähe, lieferten weitere wichtige Ergebnisse des Projekts.

Die in diesem Bericht vorgestellten Experimente betreffen die Kondensation von Dampf in horizontalen bzw. leicht geneigten Rohren bei hohen auftretenden Temperaturdifferenzen bis zu über 200 K. Weitere Besonderheiten sind die detaillierte Untersuchung des transienten Verhaltens eines nichtkondensierbaren Gases mit einer neuartigen Messtechnik sowie die Ermittlung des Einflusses des Gases auf den Kondensationsvorgang. Beim Experiment wurden schnelle Übergangsvorgänge ausgelöst, indem ein in einer Kühlwanne liegendes, leicht geneigtes Wärmeübertragerrohr plötzlich mit dem Dampfraum eines unter Druck stehenden Kessels verbunden wurde. Dabei wurden im Rohr unterschiedliche Anfangsbedingungen hinsichtlich der Vorlage von nichtkondensierbaren Gasen (in diesem Falle Luft) eingestellt. Es wurden Versuche mit Atmosphärendruck, mit erhöhtem Druck, aber auch mit vorheriger Evakuierung des Versuchsrohrs durchgeführt. Durch eine Instrumentierung mit neuartigen Nadelsonden, die eine Phasendetektion kombiniert mit einer schnellen lokalen Temperaturmessung ermöglichen, konnte die Umverteilung von Dampf, Kondensat und nichtkondensierbarem Gas als Funktion der Zeit beobachtet werden. Damit bieten die erhaltenen Daten die Möglichkeit, insbesondere die in den Thermohydraulikprogrammen vorhandenen Optionen zur Berechnung der Ausbreitung von nichtkondensierbaren Gasen unter transienten Bedingungen zu validieren.

For the qualification and validation of two-phase CFD-models for medium and large-scale industrial applications dedicated experiments providing data with high temporal and spatial resolution are required. Fluid dynamic parameter like gas volume fraction, bubble size distribution, velocity or turbulent kinetic energy should be measured locally. Considering the fact, that the used measurement techniques should not affect the flow characteristics, radiation based tomographic methods are the favourite candidate for such measurements. Here the recently developed ultrafast X-ray tomography, is applied to measure the local and temporal gas volume fraction distribution in a vertical pipe. To obtain the required frame rate a rotating X-ray source by a massless electron beam and a static detector ring are used. Experiments on a vertical pipe are well suited for development and validation of closure models for two-phase flows. While vertical pipe flows are axially symmetrically, the boundary conditions are well defined. The evolution of the flow along the pipe can be investigated as well. This report documents the experiments done for co-current upwards and downwards air-water and steam-water flows as well as for counter-current air-water flows. The details of the setup, measuring technique and data evaluation are given. The report also includes a discussion on selected results obtained and on uncertainties.

For ammonia-water absorption refrigeration technology it is suggested to use bubble type absorbers because the higher contact surface area provides a higher mass transfer rate. Furthermore, dispersion of bubbles in the bulk of liquid phase also exhibits better heat transfer characteristics that facilitate the recovery of dissipated heat of the exothermic absorption.In this context, plate heat exchangers are believed to be an option to be employed as absorber in some applications. Commercial plate heat exchangers have only one inlet and outlet for a working fluid and as a result, gas and liquid should be mixed before supplied to a gap between the two adjacent plates. The consequence is the high risk of bubble mergence to form a bigger bubble and to follow the shortest flow paths in vertical direction so that not all the heat transfer surface can be effectively used. Furthermore this feature makes plate heat exchangers sensitive to the angle of plate relative to the vertical which would be worst when it is laid to its side on a horizontal plane.Austrian Institute of Technology (AIT) develops an efficient Bubble Plate Absorber for applications in high-pressure absorption systems and this work tries to investigate design possibility of this Bubble Plate Absorber based on a plate heat exchanger equipped with microchannels between plates.Two sets of seven parallel microchannels same in shape and dimension were tested. The first set had a continuous wall which means fluids could flow independently along the microchannels; whereas, the other set was benefiting from some linkages between channels that fluids could cross from one microchannel to another one. Ammonia vapour was injected via one and two-holed distributors.It was found that microchannels with continuous wall deliver higher concentration and less unabsorbed bubbles at the microchannels outlet. In visual analysis by high-speed camera, changing the vapour distributors from single-hole to double-hole had no significant effect on the bubble distribution quality in lower flowrates; however, double-hole vapour distributor showed better performance in higher vapours flowrates.