Inertial dynamics of air bubbles crossing a horizontal fluid–fluid interface

The dynamics of isolated air bubbles crossing the horizontal interface separating two Newtonian immiscible liquids initially at rest are studied both experimentally and computationally. High-speed video imaging is used to obtain a detailed evolution of the various interfaces involved in the system. The size of the bubbles and the viscosity contrast between the two liquids are varied by more than one and four orders of magnitude,respectively, making it possible to obtain bubble shapes ranging from spherical to toroidal. A variety of flow regimes isobserved,including that of small bubbles remaining trapped at the fluid–fluid interface in a film-drainage configuration.In most cases, the bubble succeeds in crossing the interface without being stopped near its undisturbed position and, during a certain period of time, tows a significant column of lower fluid which sometimes exhibits a complex dynamics as it lengthens in the upper fluid. Direct numerical simulations of several selected experimental situations are performed with a code employing a volume of-fluid type formulation of the incompressible Navier–Stokes equations. Comparisons between experimental and numerical results confirm the reliability of the computational approach in most situations but also points out the need for improvements to capture some subtle but important physical processes, most notably those related to film drainage. Influence of the physical parameters highlighted by experiments and computations, especially that of the density and viscosity contrasts between the two fluids and of the various interfacial tensions, is discussed and analysed in the light of simple models and available theories

Measurement and Modelling of Soft Solid Layers in Cleaning Applications

Fouling and effective cleaning are important operating issues in the food and pharmaceutical industries, which need to be managed well to ensure process hygiene and productivity. In order to understand fouling and cleaning mechanisms, methods are required to quantify soft solid soil or deposit characteristics by measuring these in situ. This dissertation describes the development of millimanipulation and two new fluid dynamic gauging (FDG) devices, namely sideways FDG (SiDG) and integrated FDG (iFDG), and demonstrates their application for in situ measurement. Computational fluid dynamic (CFD) simulations of the flow patterns using the volume of fluid method were performed which enables previously inaccessible information to be extracted from the experimental data. The millimanipulation device reported by Magens et al., J Food Eng, 197 (2017) 48-59 measures the force experienced by a blade as it is pushed through a soil layer. The deformation of layers of viscoplastic petroleum jelly, soft white paraffin and toothpaste were studied and simulated using the regularized Bingham and the bi-viscosity models. The simulations gave good agreement with experimental results: combining visualisation and an interrupted testing mode allowed the material’s yield stress to be estimated. The SiDG device allows one to study the initial and long-term swelling of soft solid layers. The concept was implemented, commissioned and demonstrated by monitoring the swelling of gelatin, poly(vinyl acetate) (PVAc) and complex model soil layers at different pH and temperatures. All these materials underwent rapid initial hydration, followed by different longer term behaviour: gelatin and PVAc layers at pH < 11 exhibited Fickian diffusion control while at pH ≥ 11, PVAc exhibited relaxation control associated with hydrolysis. In the iFDG system the distances between the nozzle head and the soil layer, and the metallic substrate, are measured simultaneously by incorporating an inductive sensor in the gauging nozzle. The iFDG device was taken from concept to demonstration with a range of gauging liquids: water, UHT milk (opaque), a more viscous Newtonian fluid (washing-up liquid) and non-Newtonian aqueous solutions of 1 wt% and 3 wt% carboxymethyl cellulose (CMC). The CFD simulations gave good agreement with the experimental data. An ice growth experiment was performed using the iFDG device to demonstrate its application to monitoring growth of fouling layers. The CFD simulations were extended to consider the coupled flows which can arise in FDG measurements, where the stressed imposed by the gauging liquid flow can deform the soft solid soil layer. Two-fluid simulations were performed to estimate the change of topography of a petroleum jelly layer subject to FDG testing at different clearances. Acceptable agreement between the simulation results and measurements was obtained, and the observed differences were attributed to tubing-induced artefacts. A short feasibility study considered extending the approach to the three-fluid problem when a coherent liquid jet impinges horizontally on a horizontal plane coated with a soft solid soil layer. The results showed promising agreement with experiments on water jet cleaning of petroleum jelly layers.Taiwan Cambridge Scholarship (Cambridge Trust and Ministry of Education, Taiwan

An acoustic water tank disdrometer

Microwave engineers and geomorphologists require rainfall data with a much greater temporal resolution and a better representation of the numbers of large raindrops than is available from current commercial instruments. This thesis describes the development of an acoustic instrument that determines rain parameters from the sound of raindrops falling into a tank of water. It is known as the acoustic water tank disdrometer (AWTD).There is a direct relationship between the kinetic energy of a raindrop and the acoustic energy generated upon impact. Rain kinetic energy flux density (KE) is estimated from measurements of the sound field in the tank and these have been compared to measurements from a co-sited commercial disdrometer.Furthermore, using an array of hydrophones it is possible to determine the drop size and impact position of each raindrop falling into the tank. Accumulating the information from many impacts allows a drop size distribution (DSD) to be calculated.Eight months of data have been collected in the eastern UK. The two methods of parameter estimation are developed and analysed to show that the acoustic instrument can produce rain KE measurements with a one-second integration times and DSDs with accurate large drop-size tails

Mass Flux Distribution Measurements and Visualizations of a Fluid Sheet Generated by a Partially Immersed Dip-Lubricated Gear

A simplified model of gearbox lubrication flow is investigated. A metric gear with module size 4 mm, tooth number 38, tip radius 80.25 mm and width 40 mm, is partially immersed and rotated between 0–12 1/s frequencies in room temperature distilled water. Flash and laser sheet visualizations of the global flow field and the boundary layer are presented. Resolved mass flow distributions in the fluid sheet above the water surface are obtained with in-situ liquid trapping (bucketing). The relation between the circumferential velocity and integrated mass flux of the fluid sheet is found to be linear above a critical speed, for which an analytical formula is given. Using dimensional analysis, we show that the relation between the mass flux and gear speed signifies a Reynolds-number independent behavior of the boundary layer around the immersed part of the gear

LBE-cooled tube receiver performance - Design aspects and high-flux operation in a solar furnace

Diese Arbeit untersucht die Eignung von flüssigem Blei-Bismut (Pb-Bi) als Wärmetransport-Medium in zentralen Rohr-Receivern von konzentrierenden solarthermischen Kraftwerken. Dazu werden zwei Ansätze verfolgt: a) auf theoretischer Ebene werden anhand existierender Literatur die Materialeigenschaften von Pb-Bi und existierende Auslegungswerkzeuge für Flüssigmetall-Anwendungen betrachtet und b) anhand eines Praxisbeispiels wird die Auslegung, der Bau, die Inbetriebnahme sowie erste Betriebserfahrung eines Pb-Bi-gekühlten Modellkreislaufs in einem eigens errichteten Solar-Ofen im Technikumsmaßstab erprobt. Der Pb-Bi-gekühlte Solarreceiver dieses Kreislaufs besitzt eine thermische Leistung von 10 kW. Pb-Bi besitzt einige hervorragende und einige ungünstige Stoffeigenschaften für seine Anwendung in Solarreceivern im kommerziellen Groß-Maßstab: eine hohe thermische Leitfähigkeit und eine geringe Viskosität stehen einer durchschnittlichen Wärmekapazität und einer sehr hohen Dichte gegenüber. Diese führen für Pb-Bi im Vergleich mit Alternativ-Wärmetransportmedien Solarsalz und Natrium beim Einsatz in Receiver-Rohren zu den höchsten Entropie-Erzeugungsraten. Pb-Bi ist darüber hinaus teuer. Im Pb-Bi muss eine definierte Sauerstoffkonzentration eingestellt werden, um die Korrosion der Edelstahl-Rohrwerkstoffe zu begrenzen. Dadurch wird allerdings der Bereich der Einsatztemperaturen eingeschränkt: Infolge dessen kann entweder der Vorteil einer geringen Schmelztemperatur von Pb-Bi nicht ausgenutzt werden, oder aber eine Betriebstemperatur oberhalb der Temperaturgrenze von Solarsalz kann nicht erreicht werden. Nach dem aktuellen Stand der Technik dient Solarsalz als Wärmetransportmedium. Die Literaturrecherche dieser Arbeit gibt deutliche Hinweise darauf, dass Fluide mit niedrigen Prandtl-Zahlen und insbesondere Pb-Bi empfindlicher als "konventionelle" Medien auf Randbedingungen mit inhomogener Wärmestromdichteverteilung reagieren. Tatsächlich werden die Rohrströmungen in konventionellen Solarreceivern nur einseitig beheizt. Unter dieser Randbedingung führen klassische Nusselt-Korrelationen, die für homogen beheizte, voll-turbulente Rohrströmung entwickelt wurden, bei Medien geringer Prandtl-Zahl zu einer deutlichen Unterschätzung der Wandtemperatur und damit einhergehend zu einer Unterschätzung der thermischen Spannungen in der Rohrwand. Daher werden alternative Auslegungsmethoden präsentiert. Erste Messungen am Pb-Bi-Kreislauf zeigen tatsächlich äußere Wandtemperaturen der Receiver-Rohre, die höher sind als von klassischen Nusselt-Korrelationen vorhergesagt. Trotz dieses Befundes hat Pb-Bi für seinen Einsatz in einem Modell-System seine Berechtigung, da es im Vergleich zu bspw. Natrium inhärent deutlich sicherer ist und kann für Receiver-Betrieb und die Untersuchung von Phänomenen, die spezifisch für Flüssigmetalle sind, verwendet werden. Diese Arbeit beschreibt die Auslegung der wichtigsten Teile des Pb-Bi-Kreislaufs und die ersten experimentellen Ergebnisse, darunter auch solche bei einer Wärmestromdichte von etwa 4 MW/m$^2$. Zur Messung derart hoher Wärmestromdichten wurde im Rahmen der Arbeit ein passiv-gekühlter Wärmestrom-Scanner entwickelt, dessen Funktion auf einem einzigen Heat-Flux-Mocro-Sensor basiert. Dieser Sensor bewegt sich während des Scannens auf einer Spiral-Bahn durch die Messebene. Dieses Messgerät wurde vor Messbeginn im Solarofen des DLR in Köln für den Betrieb kalibriert

Open source vacuum oven design for low-temperature drying: Performance evaluation for recycled pet and biomass

Vacuum drying can dehydrate materials further than dry heat methods, while protecting sensitive materials from thermal degradation. Many industries have shifted to vacuum drying as cost-or time-saving measures. Small-scale vacuum drying, however, has been limited by the high costs of specialty scientific tools. To make vacuum drying more accessible, this study provides design and performance information for a small-scale open source vacuum oven, which can be fabricated from off-the-shelf and 3-D printed components. The oven is tested for drying speed and effectiveness on both waste plastic polyethylene terephthalate (PET) and a consortium of bacteria developed for bioprocessing of terephthalate wastes to assist in distributed recycling of PET for both additive manufacturing as well as potential food. Both materials can be damaged when exposed to high temperatures, making vacuum drying a desirable solution. The results showed that the open source vacuum oven was effective at drying both plastic and biomaterials, drying at a higher rate than a hot-air dryer for small samples or for low volumes of water. The system can be constructed for less than 20% of commercial vacuum dryer costs for several laboratory-scale applications, including dehydration of bio-organisms, drying plastic for distributed recycling and additive manufacturing, and chemical processing