Diversity of microbial communities at shallow-hydrothermal vents off Dominica, Lesser Antilles

Hydrothermal vents are often compared to desert oases, because of the presence of highly diverse and abundant biotic communities inhabiting these extreme environments. Nevertheless, the microbial communities associated with shallow-hydrothermal systems have been poorly studied. Hydrothermal activity at Dominica Island is quite well known under the geological and geochemical aspects, but no previous information existed about the microbial communities associated to this area. This thesis is therefore targeting the microbiology of hydrothermal sediments combining geochemical and molecular biological investigations, focusing on differences between hydrothermal vents and background (i.e. control) areas, and between hydrothermal sites. It was also intended to assess relationship between geochemical parameters and microbial diversity at the two hydrothermally impacted sites. Two shallow-sea hydrothermal vents located south-west off Dominica Island (Lesser Antilles) have been investigated in this study: Champagne Hot Springs and Soufriere Bay offshore vent. During this study, sediments for geochemical and molecular analyses were collected every 2 cm from the two impacted areas and from two control sites not associated with hydrothermal activity; in situ temperatures measurements were also taken every 5 cm deep in the sediment for all the sites. A geochemical characterization of the sediment porewater was performed through the analysis of several elements’ concentrations (i.e. H2S, Cl-, Br-, SO42-, Fe2+, Na+, K+, B+, Si+). Microbial communities at the different sites were studied by Automated Intergenic Spacer Analysis (ARISA). Inspection of the operational taxonomic units (OTUs) distribution was performed, as well as statistical analyses for communities’ structure and composition differences, and for changes of β-diversity along with sediment geochemistry. Data suggested that mixing between hydrothermal fluids and seawater results in distinct different environmental gradients and potential ecological niches between the two investigated hydrothermal vents, reflecting a difference in microbial community structures between them.

Dynamic development of hydrofractures

Natürliche hydraulische Bruchbildung ist in allen Bereichen der Erdkruste ein wichtiger und stark verbreiteter Prozess. Sie beeinflusst die effektive Permeabilität und Fluidtransport auf mehreren Größenordnungen, indem sie hydraulische Konnektivität bewirkt. Der Prozess der Bruchbildung ist sowohl sehr dynamisch als auch hoch komplex. Die Dynamik stammt von der starken Wechselwirkung tektonischer und hydraulischer Prozesse, während sich die Komplexität aus der potentiellen Abhängigkeit der poroelastischen Eigenschaften von Fluiddruck und Bruchbildung ergibt. Die Bildung hydraulischer Brüche besteht aus drei Phasen: 1) Nukleation, 2) zeitabhängiges quasi-statisches Wachstum so lange der Fluiddruck die Zugfestigkeit des Gesteins übersteigt, und 3) in heterogenen Gesteinen der Einfluss von Lagen unterschiedlicher mechanischer oder sedimentärer Eigenschaften auf die Bruchausbreitung. Auch die mechanische Heterogenität, die durch präexistierende Brüche und Gesteinsdeformation erzeugt wird, hat großen Einfluß auf den Wachstumsverlauf. Die Richtung der Bruchausbreitung wird entweder durch die Verbindung von Diskontinuitäten mit geringer Zugfestigkeit im Bereich vor der Bruchfront bestimmt, oder die Bruchausbreitung kann enden, wenn der Bruch auf Diskontinuitäten mit hoher Festigkeit trifft. Durch diese Wechselwirkungen entsteht ein Kluftnetzwerk mit komplexer Geometrie, das die lokale Deformationsgeschichte und die Dynamik der unterliegenden physikalischen Prozesse reflektiert. rnrnNatürliche hydraulische Bruchbildung hat wesentliche Implikationen für akademische und kommerzielle Fragestellungen in verschiedenen Feldern der Geowissenschaften. Seit den 50er Jahren wird hydraulisches Fracturing eingesetzt, um die Permeabilität von Gas und Öllagerstätten zu erhöhen. Geländebeobachtungen, Isotopenstudien, Laborexperimente und numerische Analysen bestätigen die entscheidende Rolle des Fluiddruckgefälles in Verbindung mit poroelastischen Effekten für den lokalen Spannungszustand und für die Bedingungen, unter denen sich hydraulische Brüche bilden und ausbreiten. Die meisten numerischen hydromechanischen Modelle nehmen für die Kopplung zwischen Fluid und propagierenden Brüchen vordefinierte Bruchgeometrien mit konstantem Fluiddruck an, um das Problem rechnerisch eingrenzen zu können. Da natürliche Gesteine kaum so einfach strukturiert sind, sind diese Modelle generell nicht sonderlich effektiv in der Analyse dieses komplexen Prozesses. Insbesondere unterschätzen sie die Rückkopplung von poroelastischen Effekten und gekoppelte Fluid-Festgestein Prozesse, d.h. die Entwicklung des Porendrucks in Abhängigkeit vom Gesteinsversagen und umgekehrt.rnrnIn dieser Arbeit wird ein zweidimensionales gekoppeltes poro-elasto-plastisches Computer-Model für die qualitative und zum Teil auch quantitativ Analyse der Rolle lokalisierter oder homogen verteilter Fluiddrücke auf die dynamische Ausbreitung von hydraulischen Brüchen und die zeitgleiche Evolution der effektiven Permeabilität entwickelt. Das Programm ist rechnerisch effizient, indem es die Fluiddynamik mittels einer Druckdiffusions-Gleichung nach Darcy ohne redundante Komponenten beschreibt. Es berücksichtigt auch die Biot-Kompressibilität poröser Gesteine, die implementiert wurde um die Kontrollparameter in der Mechanik hydraulischer Bruchbildung in verschiedenen geologischen Szenarien mit homogenen und heterogenen Sedimentären Abfolgen zu bestimmen. Als Resultat ergibt sich, dass der Fluiddruck-Gradient in geschlossenen Systemen lokal zu Störungen des homogenen Spannungsfeldes führen. Abhängig von den Randbedingungen können sich diese Störungen eine Neuausrichtung der Bruchausbreitung zur Folge haben kann. Durch den Effekt auf den lokalen Spannungszustand können hohe Druckgradienten auch schichtparallele Bruchbildung oder Schlupf in nicht-entwässerten heterogenen Medien erzeugen. Ein Beispiel von besonderer Bedeutung ist die Evolution von Akkretionskeilen, wo die große Dynamik der tektonischen Aktivität zusammen mit extremen Porendrücken lokal starke Störungen des Spannungsfeldes erzeugt, die eine hoch-komplexe strukturelle Entwicklung inklusive vertikaler und horizontaler hydraulischer Bruch-Netzwerke bewirkt. Die Transport-Eigenschaften der Gesteine werden stark durch die Dynamik in der Entwicklung lokaler Permeabilitäten durch Dehnungsbrüche und Störungen bestimmt. Möglicherweise besteht ein enger Zusammenhang zwischen der Bildung von Grabenstrukturen und großmaßstäblicher Fluid-Migration. rnrnDie Konsistenz zwischen den Resultaten der Simulationen und vorhergehender experimenteller Untersuchungen deutet darauf hin, dass das beschriebene numerische Verfahren zur qualitativen Analyse hydraulischer Brüche gut geeignet ist. Das Schema hat auch Nachteile wenn es um die quantitative Analyse des Fluidflusses durch induzierte Bruchflächen in deformierten Gesteinen geht. Es empfiehlt sich zudem, das vorgestellte numerische Schema um die Kopplung mit thermo-chemischen Prozessen zu erweitern, um dynamische Probleme im Zusammenhang mit dem Wachstum von Kluftfüllungen in hydraulischen Brüchen zu untersuchen.

Imaging and modeling of pore scale processes in porous media using X-ray computed tomography and lattice Boltzmann solver

In der Erdöl– und Gasindustrie sind bildgebende Verfahren und Simulationen auf der Porenskala im Begriff Routineanwendungen zu werden. Ihr weiteres Potential lässt sich im Umweltbereich anwenden, wie z.B. für den Transport und Verbleib von Schadstoffen im Untergrund, die Speicherung von Kohlendioxid und dem natürlichen Abbau von Schadstoffen in Böden. Mit der Röntgen-Computertomografie (XCT) steht ein zerstörungsfreies 3D bildgebendes Verfahren zur Verfügung, das auch häufig für die Untersuchung der internen Struktur geologischer Proben herangezogen wird. Das erste Ziel dieser Dissertation war die Implementierung einer Bildverarbeitungstechnik, die die Strahlenaufhärtung der Röntgen-Computertomografie beseitigt und den Segmentierungsprozess dessen Daten vereinfacht. Das zweite Ziel dieser Arbeit untersuchte die kombinierten Effekte von Porenraumcharakteristika, Porentortuosität, sowie die Strömungssimulation und Transportmodellierung in Porenräumen mit der Gitter-Boltzmann-Methode. In einer zylindrischen geologischen Probe war die Position jeder Phase auf Grundlage der Beobachtung durch das Vorhandensein der Strahlenaufhärtung in den rekonstruierten Bildern, das eine radiale Funktion vom Probenrand zum Zentrum darstellt, extrahierbar und die unterschiedlichen Phasen ließen sich automatisch segmentieren. Weiterhin wurden Strahlungsaufhärtungeffekte von beliebig geformten Objekten durch einen Oberflächenanpassungsalgorithmus korrigiert. Die Methode der „least square support vector machine” (LSSVM) ist durch einen modularen Aufbau charakterisiert und ist sehr gut für die Erkennung und Klassifizierung von Mustern geeignet. Aus diesem Grund wurde die Methode der LSSVM als pixelbasierte Klassifikationsmethode implementiert. Dieser Algorithmus ist in der Lage komplexe geologische Proben korrekt zu klassifizieren, benötigt für den Fall aber längere Rechenzeiten, so dass mehrdimensionale Trainingsdatensätze verwendet werden müssen. Die Dynamik von den unmischbaren Phasen Luft und Wasser wird durch eine Kombination von Porenmorphologie und Gitter Boltzmann Methode für Drainage und Imbibition Prozessen in 3D Datensätzen von Böden, die durch synchrotron-basierte XCT gewonnen wurden, untersucht. Obwohl die Porenmorphologie eine einfache Methode ist Kugeln in den verfügbaren Porenraum einzupassen, kann sie dennoch die komplexe kapillare Hysterese als eine Funktion der Wassersättigung erklären. Eine Hysterese ist für den Kapillardruck und die hydraulische Leitfähigkeit beobachtet worden, welche durch die hauptsächlich verbundenen Porennetzwerke und der verfügbaren Porenraumgrößenverteilung verursacht sind. Die hydraulische Konduktivität ist eine Funktion des Wassersättigungslevels und wird mit einer makroskopischen Berechnung empirischer Modelle verglichen. Die Daten stimmen vor allem für hohe Wassersättigungen gut überein. Um die Gegenwart von Krankheitserregern im Grundwasser und Abwässern vorhersagen zu können, wurde in einem Bodenaggregat der Einfluss von Korngröße, Porengeometrie und Fluidflussgeschwindigkeit z.B. mit dem Mikroorganismus Escherichia coli studiert. Die asymmetrischen und langschweifigen Durchbruchskurven, besonders bei höheren Wassersättigungen, wurden durch dispersiven Transport aufgrund des verbundenen Porennetzwerks und durch die Heterogenität des Strömungsfeldes verursacht. Es wurde beobachtet, dass die biokolloidale Verweilzeit eine Funktion des Druckgradienten als auch der Kolloidgröße ist. Unsere Modellierungsergebnisse stimmen sehr gut mit den bereits veröffentlichten Daten überein.

Hydrodynamics and solid dosage form disintegration/dissolution : immediate release tablets and novel in situ polyelectrolyte gastroretentive drug delivery systems

Solid oral dosage form disintegration in the human stomach is a highly complex process dependent on physicochemical properties of the stomach contents as well as on physical variables such as hydrodynamics and mechanical stress. Understanding the role of hydrodynamics and forces in disintegration of oral solid dosage forms can help to improve in vitro disintegration testing and the predictive power of the in vitro test. The aim of this work was to obtain a deep understanding of the influence of changing hydrodynamic conditions on solid oral dosage form performance. Therefore, the hydrodynamic conditions and forces present in the compendial PhEur/USP disintegration test device were characterized using a computational fluid dynamics (CFD) approach. Furthermore, a modified device was developed and the hydrodynamic conditions present were simulated using CFD. This modified device was applied in two case studies comprising immediate release (IR) tablets and gastroretentive drug delivery systems (GRDDS). Due to the description of movement provided in the PhEur, the movement velocity of the basket-rack assembly follows a sinusoidal profile. Therefore, hydrodynamic conditions are changing continually throughout the movement cycle. CFD simulations revealed that the dosage form is exposed to a wide range of fluid velocities and shear forces during the test. The hydrodynamic conditions in the compendial device are highly variable and cannot be controlled. A new, modified disintegration test device based on computerized numerical control (CNC) technique was developed. The modified device can be moved in all three dimensions and radial movement is also possible. Simple and complex moving profiles can be developed and the influence of the hydrodynamic conditions on oral solid dosage form performance can be evaluated. Furthermore, a modified basket was designed that allows two-sided fluid flow. CFD simulations of the hydrodynamics and forces in the modified device revealed significant differences in the fluid flow field and forces when compared to the compendial device. Due to the CNC technique moving velocity and direction are arbitrary and hydrodynamics become controllable. The modified disintegration test device was utilized to examine the influence of moving velocity on disintegration times of IR tablets. Insights into the influence of moving speed, medium viscosity and basket design on disintegration times were obtained. An exponential relationship between moving velocity of the modified basket and disintegration times was established in simulated gastric fluid. The same relationship was found between the disintegration times and the CFD predicted average shear stress on the tablet surface. Furthermore, a GRDDS was developed based on the approach of an in situ polyelectrolyte complex (PEC). Different complexes composed of different grades of chitosan and carrageenan and different ratios of those were investigated for their swelling behavior, mechanical stability, and in vitro drug release. With an optimized formulation the influence of changing hydrodynamic conditions on the swelling behavior and the drug release profile was demonstrated using the modified disintegration test device. Both, swelling behavior and drug release, were largely dependent on the hydrodynamic conditions. Concluding, it has been shown within this thesis that the application of the modified disintegration test device allows for detailed insights into the influence of hydrodynamic conditions on solid oral dosage form disintegration and dissolution. By the application of appropriate test conditions, the predictive power of in vitro disintegration testing can be improved using the modified disintegration test device. Furthermore, CFD has proven a powerful tool to examine the hydrodynamics and forces in the compendial as well as in the modified disintegration test device. rn

The NutriChip project--translating technology into nutritional knowledge

Advances in food transformation have dramatically increased the diversity of products on the market and, consequently, exposed consumers to a complex spectrum of bioactive nutrients whose potential risks and benefits have mostly not been confidently demonstrated. Therefore, tools are needed to efficiently screen products for selected physiological properties before they enter the market. NutriChip is an interdisciplinary modular project funded by the Swiss programme Nano-Tera, which groups scientists from several areas of research with the aim of developing analytical strategies that will enable functional screening of foods. The project focuses on postprandial inflammatory stress, which potentially contributes to the development of chronic inflammatory diseases. The first module of the NutriChip project is composed of three in vitro biochemical steps that mimic the digestion process, intestinal absorption, and subsequent modulation of immune cells by the bioavailable nutrients. The second module is a miniaturised form of the first module (gut-on-a-chip) that integrates a microfluidic-based cell co-culture system and super-resolution imaging technologies to provide a physiologically relevant fluid flow environment and allows sensitive real-time analysis of the products screened in vitro. The third module aims at validating the in vitro screening model by assessing the nutritional properties of selected food products in humans. Because of the immunomodulatory properties of milk as well as its amenability to technological transformation, dairy products have been selected as model foods. The NutriChip project reflects the opening of food and nutrition sciences to state-of-the-art technologies, a key step in the translation of transdisciplinary knowledge into nutritional advice.

Assessing the Validity of Brain Glucose Concentration Measurement Using Microdialysis

Brain functions, such as learning, orchestrating locomotion, memory recall, and processing information, all require glucose as a source of energy. During these functions, the glucose concentration decreases as the glucose is being consumed by brain cells. By measuring this drop in concentration, it is possible to determine which parts of the brain are used during specific functions and consequently, how much energy the brain requires to complete the function. One way to measure in vivo brain glucose levels is with a microdialysis probe. The drawback of this analytical procedure, as with many steadystate fluid flow systems, is that the probe fluid will not reach equilibrium with the brain fluid. Therefore, brain concentration is inferred by taking samples at multiple inlet glucose concentrations and finding a point of convergence. The goal of this thesis is to create a three-dimensional, time-dependent, finite element representation of the brainprobe system in COMSOL 4.2 that describes the diffusion and convection of glucose. Once validated with experimental results, this model can then be used to test parameters that experiments cannot access. When simulations were run using published values for physical constants (i.e. diffusivities, density and viscosity), the resulting glucose model concentrations were within the error of the experimental data. This verifies that the model is an accurate representation of the physical system. In addition to accurately describing the experimental brain-probe system, the model I created is able to show the validity of zero-net-flux for a given experiment. A useful discovery is that the slope of the zero-net-flux line is dependent on perfusate flow rate and diffusion coefficients, but it is independent of brain glucose concentrations. The model was simplified with the realization that the perfusate is at thermal equilibrium with the brain throughout the active region of the probe. This allowed for the assumption that all model parameters are temperature independent. The time to steady-state for the probe is approximately one minute. However, the signal degrades in the exit tubing due to Taylor dispersion, on the order of two minutes for two meters of tubing. Given an analytical instrument requiring a five μL aliquot, the smallest brain process measurable for this system is 13 minutes.

Use of a 3D perfusion bioreactor with osteoblasts and osteoblast/endothelial cell co-cultures to improve tissue-engineered bone

The delivery of oxygen, nutrients, and the removal of waste are essential for cellular survival. Culture systems for 3D bone tissue engineering have addressed this issue by utilizing perfusion flow bioreactors that stimulate osteogenic activity through the delivery of oxygen and nutrients by low-shear fluid flow. It is also well established that bone responds to mechanical stimulation, but may desensitize under continuous loading. While perfusion flow and mechanical stimulation are used to increase cellular survival in vitro, 3D tissue-engineered constructs face additional limitations upon in vivo implantation. As it requires significant amounts of time for vascular infiltration by the host, implants are subject to an increased risk of necrosis. One solution is to introduce tissue-engineered bone that has been pre-vascularized through the co-culture of osteoblasts and endothelial cells on 3D constructs. It is unclear from previous studies: 1) how 3D bone tissue constructs will respond to partitioned mechanical stimulation, 2) how gene expression compares in 2D and in 3D, 3) how co-cultures will affect osteoblast activity, and 4) how perfusion flow will affect co-cultures of osteoblasts and endothelial cells. We have used an integrated approach to address these questions by utilizing mechanical stimulation, perfusion flow, and a co-culture technique to increase the success of 3D bone tissue engineering. We measured gene expression of several osteogenic and angiogenic genes in both 2D and 3D (static culture and mechanical stimulation), as well as in 3D cultures subjected to perfusion flow, mechanical stimulation and partitioned mechanical stimulation. Finally, we co-cultured osteoblasts and endothelial cells on 3D scaffolds and subjected them to long-term incubation in either static culture or under perfusion flow to determine changes in gene expression as well as histological measures of osteogenic and angiogenic activity. We discovered that 2D and 3D osteoblast cultures react differently to shear stress, and that partitioning mechanical stimulation does not affect gene expression in our model. Furthermore, our results suggest that perfusion flow may rescue 3D tissue-engineered constructs from hypoxic-like conditions by reducing hypoxia-specific gene expression and increasing histological indices of both osteogenic and angiogenic activity. Future research to elucidate the mechanisms behind these results may contribute to a more mature bone-like structure that integrates more quickly into host tissue, increasing the potential of bone tissue engineering.

Reynolds-stress turbulence model in the KIVA code for engine simulation

A Reynolds-Stress Turbulence Model has been incorporated with success into the KIVA code, a computational fluid dynamics hydrocode for three-dimensional simulation of fluid flow in engines. The newly implemented Reynolds-stress turbulence model greatly improves the robustness of KIVA, which in its original version has only eddy-viscosity turbulence models. Validation of the Reynolds-stress turbulence model is accomplished by conducting pipe-flow and channel-flow simulations, and comparing the computed results with experimental and direct numerical simulation data. Flows in engines of various geometry and operating conditions are calculated using the model, to study the complex flow fields as well as confirm the model’s validity. Results show that the Reynolds-stress turbulence model is able to resolve flow details such as swirl and recirculation bubbles. The model is proven to be an appropriate choice for engine simulations, with consistency and robustness, while requiring relatively low computational effort.

Simulation of four stroke engine cycle for a 4 - valve pentroof engine in KIVA 3VR2

The main purpose of this project is to understand the process of engine simulation using the open source CFD code called KIVA. This report mainly discusses the simulation of the 4-valve Pentroof engine through KIVA 3VR2. KIVA is an open source FORTRAN code which is used to solve the fluid flow field in the engines with the transient 2D and 3D chemically reactive flow with spray. It also focuses on the complete procedure to simulate an engine cycle starting from pre- processing until the final results. This report will serve a handbook for the using the KIVA code.

Non-intrusive pressure measurement in microchannels

A non-intrusive interferometric measurement technique has been successfully developed to measure fluid compressibility in both gas and liquid phases via refractive index (RI) changes. The technique, consisting of an unfocused laser beam impinging a glass channel, can be used to separate and quantify cell deflection, fluid flow rates, and pressure variations in microchannels. Currently in fields such as microfluidics, pressure and flow rate measurement devices are orders of magnitude larger than the channel cross-sections making direct pressure and fluid flow rate measurements impossible. Due to the non-intrusive nature of this technique, such measurements are now possible, opening the door for a myriad of new scientific research and experimentation. This technique, adapted from the concept of Micro Interferometric Backscatter Detection (MIBD), boasts the ability to provide comparable sensitivities in a variety of channel types and provides quantification capability not previously demonstrated in backscatter detection techniques. Measurement sensitivity depends heavily on experimental parameters such as beam impingement angle, fluid volume, photodetector sensitivity, and a channel’s dimensional tolerances. The current apparatus readily quantifies fluid RI changes of 10-5 refractive index units (RIU) corresponding to pressures of approximately 14 psi and 1 psi in water and air, respectively. MIBD reports detection capability as low as 10-9 RIU and the newly adapted technique has the potential to meet and exceed this limit providing quantification in the place of detection. Specific device sensitivities are discussed and suggestions are provided on how the technique may be refined to provide optimal quantification capabilities based on experimental conditions.

Implementation of monotonic higher order upwind scheme in KIVA 4

KIVA is a FORTRAN code developed by Los Alamos national lab to simulate complete engine cycle. KIVA is a flow solver code which is used to perform calculation of properties in a fluid flow field. It involves using various numerical schemes and methods to solve the Navier-Stokes equation. This project involves improving the accuracy of one such scheme by upgrading it to a higher order scheme. The numerical scheme to be modified is used in the critical final stage calculation called as rezoning phase. The primitive objective of this project is to implement a higher order numerical scheme, to validate and verify that the new scheme is better than the existing scheme. The latest version of the KIVA family (KIVA 4) is used for implementing the higher order scheme to support handling the unstructured mesh. The code is validated using the traditional shock tube problem and the results are verified to be more accurate than the existing schemes in reference with the analytical result. The convection test is performed to compare the computational accuracy on convective transfer; it is found that the new scheme has less numerical diffusion compared to the existing schemes. A four valve pentroof engine, an example case of KIVA package is used as application to ensure the stability of the scheme in practical application. The results are compared for the temperature profile. In spite of all the positive results, the numerical scheme implemented has a downside of consuming more CPU time for the computational analysis. The detailed comparison is provided. However, in an overview, the implementation of the higher order scheme in the latest code KIVA 4 is verified to be successful and it gives better results than the existing scheme which satisfies the objective of this project.

LATTICE BOLTZMANN METHOD AND CELLULAR AUTOMATA SIMULATION OF PARTICLE MOTION AND DEPOSITION IN 2-D CASE

This technical report discusses the application of the Lattice Boltzmann Method (LBM) and Cellular Automata (CA) simulation in fluid flow and particle deposition. The current work focuses on incompressible flow simulation passing cylinders, in which we incorporate the LBM D2Q9 and CA techniques to simulate the fluid flow and particle loading respectively. For the LBM part, the theories of boundary conditions are studied and verified using the Poiseuille flow test. For the CA part, several models regarding simulation of particles are explained. And a new Digital Differential Analyzer (DDA) algorithm is introduced to simulate particle motion in the Boolean model. The numerical results are compared with a previous probability velocity model by Masselot [Masselot 2000], which shows a satisfactory result.

EXPERIMENTAL INVESTIGATION OF CERTAIN INTERNAL CONDENSING AND BOILING FLOWS: THEIR SENSITIVITY TO PRESSURE FLUCTUATIONS AND HEAT TRANSFER ENHANCEMENTS

Space-based (satellite, scientific probe, space station, etc.) and millimeter – to – microscale (such as are used in high power electronics cooling, weapons cooling in aircraft, etc.) condensers and boilers are shear/pressure driven. They are of increasing interest to system engineers for thermal management because flow boilers and flow condensers offer both high fluid flow-rate-specific heat transfer capacity and very low thermal resistance between the fluid and the heat exchange surface, so large amounts of heat may be removed using reasonably-sized devices without the need for excessive temperature differences. However, flow stability issues and degradation of performance of shear/pressure driven condensers and boilers due to non-desirable flow morphology over large portions of their lengths have mostly prevented their use in these applications. This research is part of an ongoing investigation seeking to close the gap between science and engineering by analyzing two key innovations which could help address these problems. First, it is recommended that the condenser and boiler be operated in an innovative flow configuration which provides a non-participating core vapor stream to stabilize the annular flow regime throughout the device length, accomplished in an energy-efficient manner by means of ducted vapor re-circulation. This is demonstrated experimentally. Second, suitable pulsations applied to the vapor entering the condenser or boiler (from the re-circulating vapor stream) greatly reduce the thermal resistance of the already effective annular flow regime. For experiments reported here, application of pulsations increased time-averaged heat-flux up to 900 % at a location within the flow condenser and up to 200 % at a location within the flow boiler, measured at the heat-exchange surface. Traditional fully condensing flows, reported here for comparison purposes, show similar heat-flux enhancements due to imposed pulsations over a range of frequencies. Shear/pressure driven condensing and boiling flow experiments are carried out in horizontal mm-scale channels with heat exchange through the bottom surface. The sides and top of the flow channel are insulated. The fluid is FC-72 from 3M Corporation.

Mixing-cell boundary conditions and apparent mass balance errors for advective-dispersive solute transport

In many field or laboratory situations, well-mixed reservoirs like, for instance, injection or detection wells and gas distribution or sampling chambers define boundaries of transport domains. Exchange of solutes or gases across such boundaries can occur through advective or diffusive processes. First we analyzed situations, where the inlet region consists of a well-mixed reservoir, in a systematic way by interpreting them in terms of injection type. Second, we discussed the mass balance errors that seem to appear in case of resident injections. Mixing cells (MC) can be coupled mathematically in different ways to a domain where advective-dispersive transport occurs: by assuming a continuous solute flux at the interface (flux injection, MC-FI), or by assuming a continuous resident concentration (resident injection). In the latter case, the flux leaving the mixing cell can be defined in two ways: either as the value when the interface is approached from the mixing-cell side (MC-RT -), or as the value when it is approached from the column side (MC-RT +). Solutions of these injection types with constant or-in one case-distance-dependent transport parameters were compared to each other as well as to a solution of a two-layer system, where the first layer was characterized by a large dispersion coefficient. These solutions differ mainly at small Peclet numbers. For most real situations, the model for resident injection MC-RI + is considered to be relevant. This type of injection was modeled with a constant or with an exponentially varying dispersion coefficient within the porous medium. A constant dispersion coefficient will be appropriate for gases because of the Eulerian nature of the usually dominating gaseous diffusion coefficient, whereas the asymptotically growing dispersion coefficient will be more appropriate for solutes due to the Lagrangian nature of mechanical dispersion, which evolves only with the fluid flow. Assuming a continuous resident concentration at the interface between a mixing cell and a column, as in case of the MC-RI + model, entails a flux discontinuity. This flux discontinuity arises inherently from the definition of a mixing cell: the mixing process is included in the balance equation, but does not appear in the description of the flux through the mixing cell. There, only convection appears because of the homogeneous concentration within the mixing cell. Thus, the solute flux through a mixing cell in close contact with a transport domain is generally underestimated. This leads to (apparent) mass balance errors, which are often reported for similar situations and erroneously used to judge the validity of such models. Finally, the mixing cell model MC-RI + defines a universal basis regarding the type of solute injection at a boundary. Depending on the mixing cell parameters, it represents, in its limits, flux as well as resident injections. (C) 1998 Elsevier Science B.V. All rights reserved.

Intracranial meningiomas associated with cervical syringohydromyelia in a cat

Eine dreizehnjährige, weibliche, nicht kastrierte Hauskatze wurde zur chirurgischen Therapie eines multiplen Meningeoms in der Kleintierklinik des Tierspital Bern vorgestellt. Eine leichtgradige generalisierte Ataxie wurde beobachtet, die Propriozeption war generalisiert herabgesetzt und der Drohreflex war beidseitig reduziert. Es wurden eine prä- und eine postoperative Magnetrezonanztomographie durchgeführt. Drei supratentoriale extra-axiale Raumforderungen wurden diagnostiziert. Eine vierte Masse wurde infratentorial extra-axial über der linken zerebellären Hemisphere lokalisiert. Eine Herniation des kaudoventralen Kleinhirn (Vermis) von ungefähr einem Zentimeter Länge durch das Foramen magnum wurde beobachtet. Eine zervikale Syringohydromyelie wurde als Zufallsbefund diagnostiziert. Die Meningeome wurden durch 3 Kraniotomiestellen entfernt. Zwei Jahre nach der Operation ist die Katze normal. Anhand der vorhandenen Literatur wird die tumor-assoziierte Syringohydromyelie besprochen. Die Therapie der Syringohydromyelie sollte gegen den kausalen pathologischen Prozess (z.B. intrakranieller Tumor) für die Liquorzirkulationsstörung gerichtet sein.