Rozprawy doktorskie na temat „Gas solid interaction”

Title from PDF of title page (University of Missouri--Columbia, viewed on Feb 24, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Dissertation advisor: Dr. Jerry L. Atwood. Vita. Includes bibliographical references.

Cette thèse est consacrée à l’étude expérimentale et numérique de l’interaction entre gaz et surface solide. L’écoulement de gaz raréfié à travers un microcanal rectangulaire dans des conditions isothermes et nonisothermes ont été évalués expérimentalement. Le coefficient d’accommodation de la composante tangentielle de la quantité de mouvement pour le matériau PEEK (Poly Ether Ether Ketone) associé à cinq gaz (hélium, néon, azote, argon, krypton) a été extrait des écoulements induits par gradient de pression et gradient de température. De plus, des écoulements unidimensionnels stables d’un gaz polyatomique en présence d’une surface adsorbante-désorbante maintenue à température constante et uniforme sont simulés en résolvant numériquement l’équation cinétique de Boltzmann par la méthode Direct Simulation Monte Carlo (DSMC). Il est considéré ici l’écoulement de gaz entre deux parois planes et infinies, où une seule paroi est capable d’adsorber et de désorber les molécules, tandis que l’autre est imperméable. Enfin,des recherches expérimentales et numériques ont été effectuées pour analyser la séparation des espèces BTEX (benzène, toluène, éthylbenzène et xylènes) à l’intérieur d’une colonne chromatographique. De l’étalonnage des constantes d’adsorption et de désorption, le temps de rétention de chaque espèce peut être prévu pour différentes conditions de fonctionnement en utilisant le code numérique développé
This thesis is devoted to the experimental and numerical study of the interaction between gas and solid surface. Rarefied gas flows through a rectangular microchannel under both isothermal and non-isothermal conditions were experimentally evaluated. The tangential momentum accommodation coefficient for PEEK (Poly Ether Ether Ketone) material associated to five gases (helium, neon, nitrogen, argon, krypton) was extracted from both pressure and temperature gradient driven flows. Additionally, steady one-dimensional flows of a polyatomic gas in the presence of an adsorbing-desorbing surface kept at constant and uniform temperature are simulated by solving numerically the Boltzmann kinetic equation by the Direct Simulation Monte Carlo (DSMC) method. It is considered the flow of gas between two planar and infinite surfaces,where only one surface is able to adsorb and desorb molecules, while the other one is impermeable. Finally, experimental and numerical investigation were performed to analyze the BTEX (benzene, toluene,ethylbenzene and xylenes) species separation inside a chromatographic column. From calibrating the constants of adsorption and desorption, the retention time of each species can be predicted for different operating conditions using the numerical code developed

Studying the interactions of solid particles and deformable gas
bubbles in viscous liquids is very important in many applications,
especially in mining and chemical industries. These interactions
involve liquid-solid-air multiphase flows and an
arbitrary-Lagrangian-Eulerican (ALE) approach is used for the direct
numerical simulations. In the system of rigid particles and
deformable gas bubbles suspended in viscous liquids, the
Navier-Stokes equations coupled with the equations of motion of the
particles and deformable bubbles are solved in a finite-element
framework. A moving, unstructured, triangular mesh tracks the
deformation of the bubble and free surface with adaptive refinement.
In this dissertation, we study four problems. In the first three
problems the flow is assumed to be axisymmetric and two dimensional
(2D) in the fourth problem.

Firstly, we study the interaction between a rising deformable bubble
and a solid wall in highly viscous liquids. The mechanism of the
bubble deformation as it interacts with the wall is described in
terms of two nondimensional groups, namely the Morton number (Mo)
and Bond number (Bo). The film drainage process is also
considered. It is found that three modes of bubble-rigid wall
interaction exist as Bo changes at a moderate Mo.
The first mode prevails at small Bo where the bubble deformation
is small. For this mode, the bubble is
hard to break up and will bounce back and eventually attach
to the rigid wall. In the second mode, the bubble may break up after
it collides with the rigid wall, which is determined by the film
drainage. In the third mode, which prevails at high Bo, the bubble
breaks up due to the bottom surface catches up the top surface
during the interaction.

Secondly, we simulate the interaction between a rigid particle and a
free surface. In order to isolate the effects of viscous drag and
particle inertia, the gravitational force is neglected and the
particle gains its impact velocity by an external accelerating
force. The process of a rigid particle impacting a free surface and
then rebounding is simulated. Simplified theoretical models are
provided to illustrate the relationship between the particle
velocity and the time variation of film thickness between the
particle and free surface. Two film thicknesses are defined. The
first is the thickness achieved when the particle reaches its
highest position. The second is the thickness when the particle
falls to its lowest position. The smaller of these two thicknesses
is termed the minimum film thickness and its variation with the
impact velocity has been determined. We find that the interactions
between the free surface and rigid particle can be divided into
three regimes according to the trend of the first film thickness.
The three regimes are viscous regime, inertial regime and jetting
regime. In viscous regime, the first film thickness decreases as the
impact velocity increases. Then it rises slightly in the inertial
regime because the effect of liquid inertia becomes larger as the
impact velocity increases. Finally, the film thickness decreases
again due to Plateau-Rayleigh instability in the jetting regime.
We also find that the minimum film thickness corresponds to an
impact velocity on the demarcation point between the viscous and
inertial regimes. This fact is caused by the balance of viscous
drag, surface deformation and liquid inertia.

Thirdly, we consider the interaction between a rigid particle and a
deformable bubble. Two typical cases are simulated: (1) Collision of
a rigid particle with a gas bubble in water in the absence of
gravity, and (2) Collision of a buoyancy-driven rising bubble with a
falling particle in highly viscous liquids. We also compare our
simulation results with available experimental data. Good agreement
is obtained for the force on the particle and the shape of the
bubble.

Finally, we investigated the collisions of groups of bubbles and
particles in two dimensions. A preliminary example of the oblique
collision between a single particle and a single bubble is conducted
by giving the particle a constant acceleration. Then, to investigate
the possibility of particles attaching to bubbles, the interactions
between a group of 22 particles and rising bubbles are studied. Due
to the fluid motion, the particles involved in central collisions
with bubbles have higher possibilities to attach to the bubble.
Ph. D.

Thesis (Ph. D.)--West Virginia University, 2009.
Title from document title page. Document formatted into pages; contains xii, 122 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 115-122).

Master of Science
Department of Mechanical and Nuclear Engineering
Steven J. Eckels
NanoActiveTM metal oxide particles have the ability to destructively adsorb organophosphorus compounds and chlorocarbons. These nanomaterials with unique surface morphologies are subjected to separate, low concentrations of gaseous ammonia in air. NanoActiveTM materials based on magnesium oxide have large specific surface areas and defective sites that enhance surface reactivity and consequently improved adsorptivity. In gas contaminant removal by adsorption, presence of vast specific surface area is essential for effective gas-solid interaction to take place. This is also the case in many industrial and chemical applications such as purification of gases, separation and recovery of gases, catalysis etc,. Typically carbonaceous compounds are utilized and engineered in toxic gas control systems. The purpose of this study was to compare NanoActiveTM materials with carbon based compounds in the effectivity of toxic gas adsorption at low concentrations. A test facility was designed to investigate the adsorption properties of novel materials such as adorption capacity and adsorption rate. Adsorption capacity along with adsorption kinetics is a function of properties of the adsorbent and the adsorbate as well as experimental conditions. Nanomaterials were placed on a silica matrix and tested with increasing flow rates. Electrochemical sensing devices were placed at inlet and outlet of the facility to monitor real time continuous concentration profiles. Breakthrough curves were obtained from the packed bed column experiments and saturation limits of adsorbents were measured. Adsorption rates were obtained from the breakthrough curves using modified Wheeler-Jonas equation. The NanoActiveTM materials adsorbed ammonia though to a lesser extent than the Norit® compounds. This study also included measurement of pressure drop in packed beds. This information is useful in estimating energy losses in packed bed reactors. Brauner Emmet Teller tests were carried out for the calculation of surface area, pore volume and pore size of materials. These calculations suggest surface area alone had no notable influence on adsorption capacity and adsorption rates. This lead to the conclusion that adsorption was insignificant cause of absence of functional groups with affinity towards ammonia. In brief, adsorption of ammonia is possible on NanoActiveTM materials. However functional groups such as oxy-flouro compounds should be doped with novel materials to enhance the surface interactions.

Gas-solid fluidized bed reactors are widely applied in chemical and energy industries, and their design and scale-up are virtually empirical, extremely expensive and time consuming. This scenario has motivated the development of alternative theoretical tools, and two-fluid modeling, where gas and particulate are both treated as interpenetrating continuum phases, has appeared as a most promising approach. Owing to the large domains to be resolved in real-scale fluidized bed reactors, only filtered modeling approaches are feasible, and closure models become necessary to recover sub-grid effects that are filtered by the very coarse numerical grids that are imposed owing to computational limitations. Those closure models, which in hydrodynamic formulations account for filtered interphase momentum exchanges and filtered and residual stresses in the phases, can be derived from results of highly resolved simulations (HRS) performed over small size domains under refined numerical grids. One widely practiced approach consists of applying two-fluid modeling under micro-scale defined closures, generally known as microscopic two-fluid modeling. This approach includes microscopic closures for solid phase stresses derived from the kinetic theory of granular flows (KTGF), which accounts for kinetic-collisional effects only, and is adequate to dilute flows. Otherwise, the conventional KTGF does not account for interparticle friction effects, and its application to dense flow conditions is quite questionable. In this work a literature available modified version of KTGF is applied which also accounts for interparticle friction, and highly resolved simulations are performed for dense flow conditions in order to evaluate the effects of friction over relevant filtered parameters (namely effective drag coefficient, filtered and residual stresses). Ranges of domain average solid volume fractions and gas Reynolds numbers are considered (macro-scale conditions) embracing dense gas-solid fluidized flows from suspensions up to pneumatic transport. The MFIX open source code is used in all the simulations, which are performed over 2D periodical domains for a unique monodisperse particulate. The HRS results (i.e. meso-scale flow fields) are filtered over regions compatible with grid sizes in large scale simulations, and the relevant filtered parameters of concern are derived and classified by ranges of other filtered parameters taken as independent variables (filtered solid volume fraction, filtered slip velocity, and filtered kinetic energy of solid velocity fluctuations, which are referred to as markers). Results show that the relevant filtered parameters of concern are well correlated to all of those filtered markers, and also to all of the imposed macro-scale conditions. Otherwise, interparticle friction showed no significant effects over any filtered parameter. It is recognized that this issue clearly requires further investigation notably regarding the suitability of the markers that were assumed for classifying the filtered results. The current work is intended as a contribution for future developments of more accurate closure models for large scale simulations of gas-solid fluidized flows.
Reatores de leito fluidizado de escoamento gás-sólido são largamente utilizados nas indústrias química e de energia, e o seu projeto e escalonamento são virtualmente empíricos, extremamente caros e demorados. Este cenário tem motivado o desenvolvimento de ferramentas teóricas alternativas, e a modelagem de dois fluidos, onde gás e particulado são ambos tratados com fases contínuas interpenetrantes, tem surgido como uma aproximação muito promissora. Devido aos grandes domínios a serem resolvidos em reatores de leito fluidizado de escala real, apenas aproximações de modelagem filtradas são viáveis, e modelos de fechamento tornam-se necessários para recuperar efeitos sub-malha que são filtrados pelas malhas numéricas grosseiras que são impostas devido as limitações computacionais. Estes modelos de fechamento, que em formulações hidrodinâmicas respondem principalmente por trocas de momentum filtradas entre fases e tensões filtradas e residuais nas fases, podem ser obtidos de resultados de simulações altamente resolvidas (SAR) realizadas em domínios de dimensões reduzidas sob malhas numéricas refinadas. Uma aproximação largamente praticada consiste na aplicação de modelagem de dois fluidos sob fechamentos definidos na micro-escala, genericamente conhecida como modelagem microscópica de dois fluidos. Esta aproximação inclui fechamentos microscópicos para tensões da fase sólida obtidos da teoria cinética dos escoamentos granulares (TCEG), que considera apenas efeitos cinéticos-colisionais, e é adequada para escoamentos diluídos. Por outro lado, a TCEG convencional não leva em conta efeitos de fricção interpartículas, e sua aplicação para condições densas de escoamento é bastante questionável. Neste trabalho aplica-se uma versão modificada da TCEG disponível na literatura que também leva em conta fricção interpartículas, e simulações altamente resolvidas são realizadas para condições de escoamentos densos visando avaliar os efeitos da fricção sobre os parâmetros filtrados relevantes (coeficiente de arrasto efetivo, tensões filtradas e residuais). Considera-se faixas de frações volumétricas de sólido e números de Reynolds do gás médios no domínio (condições de macro-escala) abrangendo escoamentos gás-sólido fluidizados densos desde suspensões até transporte pneumático. O código aberto MFIX é utilizado em todas as simulações, que foram executadas sobre domínios periódicos 2D para um único particulado monodisperso. Os resultados das SAR (i.e., campos de escoamento de meso-escala) foram filtrados sobre regiões compatíveis com tamanhos de malha praticados em simulações de grandes escalas, e os parâmetros filtrados relevantes de interesse são calculados e classificados por faixas de outros parâmetros filtrados tomados como variáveis independentes (fração volumétrica de sólido filtrada, velocidade de deslizamento filtrada, e energia cinética das flutuações de velocidade da fase sólida filtrada, que são referidos como marcadores). Os resultados mostram que os parâmetros filtrados relevantes de interesse são bem correlacionados com todos os marcadores, e também com todas as condições de macro-escala impostas. Por outro lado, a fricção interpartículas não mostrou efeitos significativos sobre qualquer parâmetro filtrado. Reconhece-se que este aspecto claramente requer investigações adicionais, notadamente com respeito à adequação dos marcadores que foram considerados para classificação dos resultados filtrados. O trabalho corrente é posto como uma contribuição para o desenvolvimento futuro de modelos de fechamento mais acurados para simulações de grandes escalas de escoamentos gás-sólido fluidizados.

This project was compiled in co-operation with the Royal Institute of Technology, Stockholm and Veolia Environmental Services (Australia) at the Woodlawn Bioreactor in NSW, Australia. Hydrogen sulphide is an unwanted component of landfill gas, raising occupational health and safety concerns, whilst leading to acid gas corrosion of power generation equipment and increased emissions of SOx, a primary constituent of acidification. Australian governmental requirements to place a periodic cover over the unused proportion of the tipping surface of landfills and bioreactors create an interesting opportunity for the removal of the hydrogen sulphide component of landfill gas. Using waste materials containing a high concentration of metals as waste cover can enhance the precipitation of sulphur in the form of metal sulphides. The reduction of sulphate via sulphate reducing bacteria is prevalent in sites that have a sizeable inflow of sulphate. The Woodlawn Bioreactor is located in an area where the influence of sulphate has a critical influence of bioreactor performance and production of hydrogen sulphide. Through a series of experimental bioreactors it was established that from the use of metalliferous periodic waste covers, the hydrogen sulphide component of landfill gas was maintained at an extremely low level when compared to the levels of hydrogen sulphide produced in waste under the influence of high sulphate loads with no waste cover.
www.ima.kth.se

The present research project is carried out at the von Karman Institute for Fluid Dynamics (Rhode-Saint-Genèse, Belgium) with the financial support of the European Space Agency.

The first stage of spacecrafts (e.g. Ariane 5, Vega, Shuttle) generally consists of large solid propellant rocket motors (SRM), which often consist of segmented structure and incorporate a submerged nozzle. During the combustion, the regression of the solid propellant surrounding the nozzle integration part leads to the formation of a cavity around the nozzle lip. The propellant combustion generates liquefied alumina droplets coming from chemical reaction of the aluminum composing the propellant grain. The alumina droplets being carried away by the hot burnt gases are flowing towards the nozzle. Meanwhile the droplets may interact with the internal flow. As a consequence, some of the droplets are entrapped in the cavity forming an alumina puddle (slag) instead of being exhausted through the throat. This slag reduces the performances.

The aim of the present study is to characterize the slag accumulation process in a simplified model of the MPS P230 motor using primarily optical experimental techniques. Therefore, a 2D-like cold-gas model is designed, which represents the main geometrical features of the real motor (presence of an inhibitor, nozzle and cavity) and allows to approximate non-dimensional parameters of the internal two-phase flow (e.g. Stokes number, volume fraction). The model is attached to a wind-tunnel that provides quasi-axial flow (air) injection. A water spray device in the stagnation chamber realizes the models of the alumina droplets, which are accumulating in the aft-end cavity of the motor.

To be able to carry out experimental investigation, at first the the VKI Level Detection and Recording(LeDaR) and Particle Image Velocimetry (PIV) measurement techniques had to be adapted to the two-phase flow condition of the facility.

A parametric liquid accumulation assessment is performed experimentally using the LeDaR technique to identify the influence of various parameters on the liquid deposition rate. The obstacle tip to nozzle tip distance (OT2NT) is identified to be the most relevant, which indicates how much a droplet passing just at the inhibitor tip should deviate transversally to leave through the nozzle and not to be entrapped in the cavity.

As LeDaR gives no indication of the driving mechanisms, the flow field is analysed experimentally, which is supported by numerical simulations to understand the main driving forces of the accumulation process. A single-phase PIV measurement campaign provides detailed information about the statistical and instantaneous flow structures. The flow quantities are successfully compared to an equivalent 3D unsteady LES numerical model.

Two-phase flow CFD simulations suggest the importance of the droplet diameter on the accumulation rate. This observation is confirmed by two-phase flow PIV experiments as well. Accordingly, the droplet entrapment process is described by two mechanisms. The smaller droplets (representing a short characteristic time) appear to follow closely the air-phase. Thus, they may mix with the air-phase of the recirculation region downstream the inhibitor and can be carried into the cavity. On the other hand, the large droplets (representing a long characteristic time) are not able to follow the air-phase motion. Consequently, a large mean velocity difference is found between the droplets and the air-phase using the two-phase flow measurement data. Therefore, due to the inertia of the large droplets, they may fall into the cavity in function of the OT2NT and their velocity vector at the level of the inhibitor tip.

Finally, a third mechanism, dripping is identified as a contributor to the accumulation process. In the current quasi axial 2D-like set-up large drops are dripping from the inhibitor. In this configuration they are the main source of the accumulation process. Therefore, additional numerical simulations are performed to estimate the importance of dripping in more realistic configurations. The preliminary results suggest that dripping is not the main mechanism in the real slag accumulation process. However, it may still lead to a considerable contribution to the final amount of slag.

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

Modelling gas-solid two-phase flows using a two-fluid approach has two main difficulties: formulating constitutive laws for the particulate stresses and modelling the gas turbulence modulation. Due to the complex nature of the gas-particle interactions, there is no universal model covering every flow regime. In this thesis, three flow regimes with distinctive characteristics are studied, i.e. the very dense regime where the solid volume fraction, v₂>5%, the dense flow regime where 5%≥1%, and the relatively dilute regime where 1%≥v₂>0.1%. In the very dense flow regime, where the interstitial gas is normally neglected, the gas flow is assumed laminar and causes a viscous energy dissipation in the particulate phase. Numerical results for granular materials flowing down an inclined chute show that the interstitial gas may have a considerable effect in these flows. In the dense regime, where the inter-particle collisions are very important, a fluctuational energy transfer rate between the two phases is postulated, similar to that in a dilute Stokes flow. Consequently, the numerical solutions relax the restriction of elastic inter-particle collisions and show good agreement with experimental measurements. In the above two regimes, the kinetic theory of dry granular flow is adopted for the particulate stresses because the inter-particle collisions dominate the flows. The interstitial gas influence on the constitutive flow behaviour of the particulate phase is considered in the relatively dilute flow regime also, and a k-equation with a prescribed turbulent length scale is first used to address the gas turbulence modulation. Numerical results show that the gas turbulence has a significant effect on the microscopic flow behaviour of the particulate phase. The k-equation of Crowe & Gillandt (1998) has the best performance in predicting the experimentally observed phenomena. Finally, the influence of the particles on the k-^Ε model coefficients are studied and the turbulent motion is considered to be restricted by the particles, thereby reducing the turbulent length scale directly. The simulation results indicate that these coefficients should be modified in order to incorporate the effect of particles.

Thesis (PhD)--Stellenbosch University, 2014.
ENGLISH ABSTRACT: A Computational Fluid Dynamics (CFD) model was developed for the prediction of flotation rate constants in a stirred flotation tank and validated against experimental data. The model incorporated local, time-varying values of the turbulent flow field into an existing kinetic flotation model based on the Generalised Sutherland Equation to predict the overall flotation rate constant. Simulations were performed for the flotation of various minerals at different operational conditions and the predictions were compared with experimental data. It was found that the CFD-based model yielded improvements in the prediction of flotation rate constant for a range of hydrophobicities, agitation speeds and gas flow rates compared with existing methodologies, which use volume-averaged empirical expressions for flow variables. Moreover, comparing to the available CFD alternatives for the flotation modelling this approach eliminates the need for solving an extra partial differential equation resulting in a more computationally economic model. The model was developed in three stages. In the first, a single-phase model was used to establish the requirements for successful modelling of the velocity components and turbulent properties of water inside flotation tanks. Also, a novel use of the Grid Convergence Index for this application was carried out, which allowed determination of the maximum achievable reduction in numerical uncertainties through systematic grid refinement and adaptation. All subsequent simulations were performed at the optimal discretization level determined in this manner. It was found that the Moving Reference Frames (MRF) method was adequate for representation of the impeller movement when the rotational zone was located close to the impeller, using a time step advance of between 10◦ and 15◦ of impeller rotation. Comparison of the different turbulence models for the single-phase modelling revealed that the standard k-e and Large Eddy Simulation turbulence models both performed equally well and that the computational requirement was lower for the standard k-e model, making it the method of choice. Validation of the methodology was done by comparison with experimental data for two different stirred tanks including an unbaffled mixer and a fully baffled standard Rushton turbine tank. The validation against experimental data showed that the model was capable of predicting the flow pattern, turbulent properties and the generation of trailing vortices. The second stage of modelling used an Eulerian-Eulerian formulation for gasliquid modelling of gas-sparged fully baffled vessels (2.25 l, 10 l and 50 l) using a Rushton turbine. It was determined that the minimum model uncertainty resulting from simulation of the sparger was achieved using a disk sparger with a diameter equal to 40% of the impeller diameter. The only significant interfacial force was found to be the drag force, and this was included in the multiphase methodology. A parametric study on the available formulations for the drag coefficient was performed which showed that the effect of turbulence on the air bubbles can accurately be represented using the proposed model of Lane (Lane, 2006). Validation of the methodology was conducted by comparison of the available experimental gas holdup measurements with the numerical predictions for three different scales of Rushton turbine tanks. The results verified that the application of the designed sparger in conjunction with Lane drag coefficient can yield accurate predictions of the gas-liquid flow inside the flotation tank with the error percentage less than 6%, 13%, and 23% for laboratory, pilot and industrial scale Rushton turbine tanks, respectively. The last stage of this study broadened the Eulerian-Eulerian framework to predict the flotation rate constant. The spatially and temporally varying flow variables were incorporated into an established fundamental flotation model due to Pyke (Pyke, 2004) based on the Generalized Sutherland equation for the flotation rate constant. The computation of the efficiency of the flotation sub-processes also incorporated the turbulent fluctuating flow characteristics. Values of the flotation rate constants were computed and volume-weight averaged for validation against available experimental data. The numerical predictions of the flotation rate constants for quartz particles for a range of particle diameters showed improvements in the predictions when compared with values determined from existing methodologies which use spatially uniform values for the important hydrodynamic variables as obtained from empirical correlations. Further validations of the developed CFD-kinetic model were carried out for the prediction of the flotation rate constants of quartz and galena floating under different hydrophobicities, agitation speeds and gas flow rates. The good agreement between the numerical predictions and experimental data (less than 12% error) confirmed that the new model can be used for the flotation modelling, design and optimization. Considering the limited number of CFD studies for flotation modelling, the main contribution of this work is that it provides a validated and optimised numerical methodology that predicts the flotation macro response (i.e., flotation rate constant) by integrating the significance of the hydrodynamic flow features into the flotation micro-processes. This approach also provides a more economical model when it is compared to the available CFD models for the flotation process. Such an approach opens the possibility of extracting maximum advantage from the computed parameters of the flow field in developing more effective flotation devices.
AFRIKAANSE OPSOMMING: 'n Wye verskeidenheid van industriële toepassings gebruik meganies geroerde tenks vir doeleindes soos die meng van verskillende vloeistowwe, verspreiding van 'n afsonderlike fase in 'n deurlopende vloeistoffase en die skeiding van verskillende komponente in ‘n tenk. Die hoofdoel van die tesis is om ‘n numeriese model te ontwikkel vir ʼn flotteringstenk. Die kompleksiteit van die vloei (drie-dimensioneel, veelvuldige fases en volledig turbulent) maak die voorspelling van die werksverrigting van die flottasieproses moeilik. Konvensioneel word empiriese korrelasies gebruik vir modellering, ontwerp en die optimalisering van die flotteringstenks. In die huidige studie word ‘Computational Fluid Dynamics’ (CFD) egter gebruik vir die modellerings doel, aangesien dit ‘n alternatief bied vir empiriese vergelykings deurdat dit volledig inligting verskaf aangaande die gedrag van vloei in die tenk. Die model is ontwikkel in drie agtereenvolgende stadiums. Dit begin met ‘n strategie vir enkelfase modellering in die tenk, vorder dan na ‘n gas-vloeistof CFD model en brei dan die tweede stap uit om ‘n CFD model te skep vir die skeidingsproses deur flottering. ‘n Enkelfase model, gebaseer op die kontinuïteits- en momentumvergelykings, dien as basis vir die flottasie model. Die ‘Multiple Reference Frames’ (MRF) metode word gebruik om die rotasie van die stuwer na te boots, terwyl die dimensies van die rotasie-sone gekies is om die gepaardgaande onsekerhede, insluitend die model- en numeriese foute veroorsaak deur die dimensies van die roterende sones, te verminder. Die turbulensie model studie het getoon dat die standaard k-e turbulensie model redelike akkuraatheid kon lewer in die numeriese voorspellings en die resultate verskil in gemiddeld net minder as 15% van die eksperimentele lesings, terwyl die rekenaartyd min genoeg was om die simulasies op 'n persoonlike rekenaar uit te voer. Verder het die ‘Grid Convergence Index’ (GCI) metode die inherente onsekerhede in die numeriese voorspellings gerapporteer en gewys dat die onderskatting van die turbulensie wat algemeen plaasvind reggestel kan word deur van ‘Large Eddie’ (LES) of ‘Direct Numerical Simulations’ (DNS) gebruik te maak. Die metode wat ontwikkel is, is op twee tipes geroerde tenks getoets, naamlik 'n onafgeskorte menger en 'n standaard Rushton turbine tenk. Die numeriese resultate is teen eksperimentele data gevalideer en het gewys dat die model in staat is om die vloeipatrone, turbulensie einskappe en die vorming van agterblywende vortekse te voorspel. Die CFD resultate het getoon dat die vloeipatroon twee simmetriese rotasies siklusse bo en onder die roterende sone vorm, terwyl die vlak van die ooreenkoms tussen die numeriese voorspellings van die turbulente eienskappe en die eksperimentele lesings met minder as 25% verskil. As die tweede stap van hierdie navorsing is 'n Eulerian-Eulerian struktuur ontwikkel vir die gas-vloeistof modellering binne 'n standaard Rushton turbine flotteringstenk. Soos vir die enkelfase modellering is die Reynolds spanningstensor opgelos deur die standaard k-e turbulensie model, terwyl die lugborrels ingevoer/versamel is in/van die tenk deurmiddel van bron/sink terme. Verskeie ‘sparger’ rangskikkings is in die tenk geïmplementeer om die onsekerheid in die model weens die metode van luginspuiting te verminder. Verder is verskillende korrelasies vir die sleursyfer vergelyk vir laminêre en turbulente vloei in die tenk. Daar is gevind dat die skyf ‘sparger’, met 'n deursnee gelykstaande aan 40% van die stuwer deursnee, in samewerking met die voorgestelde model van Lane (Lane, 2006) vir die bepaalde sleursyfer die naaste ooreenkoms met die eksperimentele metings lewer (met 'n gemiddelde verskil van minder as 25%). 'n Vergelykende studie is ook uitgevoer om die gevolge van die gas vloeitempo en roerspoed vir drie verskillende geroerde tenks met volumes van 2.5 l, 10 l en 50 l te ondersoek. Die resultate van hierdie afdeling bevestig dat die CFD metode in staat was om die gas-vloeistof vloei in die flotteringstenk korrek te voorspel. Die veelvuldigefase model wat ontwikkel is, is uitgebrei vir flottasie modellering. Dit behels die integrasie van die CFD resultate met die fundamentele flottasie model van Pyke (Pyke, 2004) vir die flotteringstempo konstant. Die CFD model is toegerus met Pyke se model deur aanvullende gebruiker gedefinieerde funksies. Die CFD-kinetiese model is geëvalueer vir die flottering van kwartsdeeltjies en die resultate het die geloofwaardigheid van die model bevestig, aangesien die gemiddelde verskil tussen die numeriese voorspellings vir die flotteringstempo konstante en die eksperimentele data minder as 5% was. Die resultate is ook vergelyk met die analitiese berekeninge van Newell en daar is bevind dat die model vergelykbare voorspellings van die flotteringtempo konstantes lewer, met die ‘root mean square deviations’ (RMSD) gelyk of minder as die RMSD waardes vir die analitiese berekeninge. Verdere ondersoeke van die CFD-kinetiese model bestaan uit 'n parametriese studie wat die gevolge van die roertempo, gas vloeitempo en die oppervlak hidrofobisiteit op die flottering van kwarts- en galenietdeeltjies bestudeer. Die aanvaarbare ooreenkoms tussen die numeriese voorspellings en eksperimentele data (oor die algemeen minder as 12% fout) bewys dat die nuwe model gebruik kan word vir flotterings modellering en optimalisering.

The goal of current solid oxide fuel cell (SOFC) research is to design electrode materials and other system components that permit the fuel cell to be operated in the 400-700ºC range. Cell performance in this lower temperature range is limited by the oxygen reduction process at the SOFC cathode and by multiple contamination processes. The work presented demonstrates that Raman spectroscopy, a form of vibrational spectroscopy, can provide structural and compositional information complementary to that from traditional characterization methods. Initial experiments into the oxygen reduction mechanism on SOFC cathodes were unable to detect surface oxygen species on selected perovksite-based SOFC cathode materials. However, the Raman signal from the cathode surface was able to be enhanced by depositing silver or gold nanoparticles on the cathode, creating the so-called surface-enhanced Raman scattering (SERS) effect. The Raman sample chamber was also used to study two possible electrode contamination processes. First, the deposition of carbon on nickel and copper anodes was observed when exposed to different hydrocarbon fuel gases. Second, the poisoning of an SOFC cathode by chromium-containing vapor (usually generated by stainless steel SOFC system components) was monitored. Overall, Raman spectroscopy was shown to be useful in many areas crucial to the development of practical, cost-effective SOFCs. The techniques developed here could also be applied to other high temperature electrochemical and catalytic systems.

Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008.
Committee Chair: Liu, Meilin; Committee Member: El-Sayed, Mostafa; Committee Member: Sanders, Thomas; Committee Member: Speyer, Robert; Committee Member: Wilson, Lane.

The band structure and transport properties of the technologically important materials GaAs and InP and (GaIn)(AsP)/InP were studied as a function of temperature, pressure and magnetic field. The electron effective mass, m*, was measured as a function of energy gap, E[o], using hydrostatic pressure as a dummy variable to change the band structure. In InP it was found that the three band k-p model was sufficient to explain the energy gap dependence of the effective mass while in GaAs, a small contribution from higher bands was necessary. In Ga[x]In[1-x]As[y]P[1-y] alloys, dm* /dEo was found to be about 50% larger than in compound semiconductors. The variation, dm*/dE[o], as a function of composition follows the variation of alloy disorder. Based on the trends in band gap dependence of the effective mass in various alloy systems and compound semiconductors, a tentative explanation was offered to account for the large dm*/dE[o] value in these alloys. Photoconductivity measurements were made as a function of pressure, to study the variation of the pressure coefficient of the direct band gap with composition. The results indicate that dEo/dP is also influenced by alloy disorder. Measurements of the hole mobility, mu, as a function of pressure show that, while in GaAs and InP dmu/dP is positive, in the alloy dmu/dP is negative. These results were analysed in terms of the transport parameters and their pressure dependence to evaluate the pressure coefficient of the heavy hole effective mass, dm[ch]/dP. It was found that, while in InP and the alloys dm[ch]/dP is positive, in GaAs dm[ch]/dP is negative. Measurements of the activation energy of the Mn acceptor level in (GaIn)(AsP) and its pressure coefficient show that the closer the level is to the valence band the more it is influenced by it, contrary to the arguments that the deep levels are independent of the host material. This is explained on the assumption that the Mn impurity wave functions are somewhat delocalised and are influenced by the valence band. In Mn+Ge co-doped alloys, it was found that when the Mn activation energy is large, even in heavily doped, closely compensated p-type materials the mobility is dominated by phonon scattering. This anamoly is attributed to the formation of Mn+Ge dipoles assosiated with the diffusion of neutral Mn. This hypothesis was supported by the measurements of the diffusion profiles of Mn into various substrates.

Les effets liés à la présence d'eau liquide sur la capacité d'adsorption de CO₂ par une silice mésoporeuse de type SBA-15 fonctionnalisée au moyen des amines suivantes: aminopropyltrimethoxysilane (APS) et N-(2-aminoéthyl) -3 - (aminopropyl) trimethoxysilane (AEAPS) ont été examinés pour évaluer le potentiel de ce mode de contact dans des laveurs gaz-liquide-solide. Les résultats ont été comparés à la capacité d'adsorption de CO₂ des amines greffées dans des conditions humides et sèches ainsi qu'à la capacité d'absorption de CO₂ dans les systèmes gaz-liquide avec des solutions aqueuses d'aminés ayant des structures semblables à celles des amines greffées. Dans ces conditions, une estimation de l'adsorption physique de CO₂ a été obtenue par l'étude de la SB A-15 non-modifiée. En outre, afin d'évaluer l'efficacité et la stabilité à long terme de l'association amine/SBA-15, les amines greffées ont été soumises à huit cycles successifs d'immersion dans les milieux aqueux d'une durée de 24 h chacune. Les échantillons récupérés ont été caractérisés au moyen de la diffraction aux rayons, des isothermes de sorption d'azote et d'analyse élémentaire CHN. Jusqu'à 40% de la quantité d'aminés greffées a subi une lixiviation durant les quelques premiers cycles de régénération; par la suite, la teneur en azote de l'AEAPS est demeurée relativement stable, contrairement à l'APS qui a connu une moindre stabilité. Fait intéressant, les structures des deux matériaux greffés, APS et AEAPS, sont demeurées intactes après plusieurs expositions à l'eau. L'efficacité de capture de CO₂ la plus élevée a été obtenue dans le cas des amines aqueuses (voie homogène). Cependant, la capture de CO₂ à l'aide d'aminés greffées dans le cas du système triphasique (gaz-liquide-solide) a donné lieu, pour des conditions opératoires comparables, à des valeurs intermédiaires entre les voies sèche et humide du mode de contact gaz-solide.

Orientador: Milton Mori
Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Quimica
Made available in DSpace on 2018-08-14T18:54:23Z (GMT). No. of bitstreams: 1 Bastos_JaciCarloSchrammCamara_D.pdf: 4353350 bytes, checksum: 0de3633c8edcc4ae2ec7fc57a3b590d0 (MD5) Previous issue date: 2009
Resumo: Apresenta-se nesta pesquisa uma análise experimental e numérica do comportamento da fase dispersa em um jato circular bifásico confinado e uma comparação com jato circular bifásico livre. Nas análises experimentais, uma câmara pentagonal em acrílico foi utilizada como sistema de confinamento para a obtenção de perfis axiais e radiais de velocidade média, flutuação de velocidade (RMS) e intensidade de turbulência. Estes dados foram analisados a fim de desenvolver uma análise completa da região desenvolvida do jato. Três diferentes jatos foram utilizados para a alimentação da fase gás no topo da câmara, mas apenas o jato central foi carregado com partículas entre 60 e 90µm de diâmetro. Os outros dois foram utilizados para proporcionar uma maior interação entre as fases no interior da câmara. A técnica óptica Phase Doppler Anemometry (PDA), foi empregada na medição da velocidade instantânea da fase sólida e do diâmetro das partículas nas diferentes posições axiais a partir do bico do jato. Nove casos de estudo distintos são investigados individualmente e, em seguida, comparados entre si. Estes casos fornecem informações importantes sobre o comportamento e o efeito do confinamento dos jatos sobre o transporte macrocóspico e turbulento das partículas entre o centro e as regiões de contorno do jato. As análises numéricas tratam da modelagem matemática tridimensional, turbulenta e transiente do escoamento no jato bifásico confinado. O modelo trata as fases gás e sólida a partir de uma abordagem Euleriana. O fechamento das equações de transporte foi realizado utilizando o modelo de turbulência de duas equações k-e para a fase gás e modelos de turbulência de zero-equação para a fase sólida, e ainda em alguns casos esta última apenas sofreu efeitos turbulentos advindos da fase contínua. A acurácia das previsões do modelo em um jato de partículas confinadas com as características médias no tempo, assim como os coeficientes da correlação de turbulência foram avaliados. Perfis radiais de velocidade média e fração volumétrica das partículas foram capturados em quarenta e dois níveis, subdivididos em nove casos e comparados aos dados experimentais adquiridos. O diâmetro médio das partículas utilizado nas simulações foi de 75µm e as velocidades iniciais utilizadas variam entre 3 e 11m/s no jato central. O modelo matemático previu um escoamento desenvolvido semelhante ao que foi encontrado experimentalmente.
Abstract: It is presented in this research an experimental and numerical analisys of the dispersed phase behavior in a circular confined two-phase jet and a comparison with circular free two-phase jet. In the experimental analysis, a pentagonal plexiglass chamber was used as confined system for the axial and radial profiles investigation of mean velocity, fluctuation velocity known as RMS velocity and turbulence intensity. These data were analyzed in order to develop a complete analysis in the developed region of the jet. Three different nozzles were used to feed the gas phase at the top of the chamber, but just the central nozzle was loaded with particles between 60 and 90µm of diameter. The other two were used to increase the interaction between the phases in the chamber. An optical technique known as Phase Doppler Anemometry was used to measure the instantaneous velocity of the solid phase and particle diameter in different axial positions of the jet nozzle. Nine different cases of study are investigated individually and then compared among each other. These cases provide important information about the jets behavior and the confinement effect on the macrocospic and turbulent transport of particles between the jet center and the jet contour regions. The numerical analysis deals with three-dimensional, turbulent and transient mathematical modeling of a confined two-phase jet flow. The model treats the gas and the solid phases from an Eulerian approach. The closure of the transport equations have been accomplished by using the k-e turbulence model for the gas phase and the zero-equation turbulence model for the solid phase, and in some cases the latter suffered turbulent effects occuring only from the continuos phase. The accuracy of the model predictions in a particle-laden confined jet with the characteristics as well as turbulence correlation coefficients have been evaluated. Radial mean velocity profiles for the solid phase were computed on forty two axial levels, subdivided in nine cases and compared to the obtained experimental data. The mean particle diameter used in the simulations was 75µm and the initial velocities used vary between 3 and 11m/s. The mathematical model predicted a flow development similar to that found experimentally.
Doutorado
Desenvolvimento de Processos Químicos
Mestre em Engenharia Química

Packed beds of gas-solid systems are extensively used as reactors, separators, dryers, filters, heat exchangers and combustors. The design of packed beds requires a detailed knowledge of local dynamics of flow, composition and temperature. Unfortunately, investigations for the development of 3D modelling codes by computational fluid dynamics are still not sufficiently mature compared with those relying on 2D modelling or simplified pseudo-homogenous models. This project investigates non-uniform catalyst deactivation in packed bed reactors of low aspect ratios under steady-state and dynamic operations. Low aspect ratio packed beds were selected as they are known to generate non-uniform distribution of local flow. Detailed knowledge of flow dynamics in terms of local structure of the packed bed, pressure drops, interstitial flow, heat and mass rate distributions was examined. The discrete element method was used to generate various packing configurations and the results of profiles of porosity were in a good agreement with the semi-analytical models, especially, in the vicinity of the wall. Similar oscillation trends with damping profiles towards the centre of the packed beds were observed. Flow heterogeneity was assessed by tests of mass transfer dispersion through a Lagrangian approach. Interactions of fluid flow, mass and heat transfers, and local deactivation of alumina catalyst Al2O3 of CO oxidation were investigated under design and operating conditions. An increase in the activation energy of deactivation promoted the deactivation by accelerating the reaction rate and releasing additional thermal energy, which in turn accelerated the deactivation. The 3D modelling allowed observation of local catalyst deactivation at packing pore level which is typically not accessible by the 2D modelling or pseudo-homogeneous models. In addition, the deactivation was quite asymmetrical along axial and radial directions, leading to uneven rates of thermal expansion and contraction and causing local deactivation associated with temperature runaways.

Les développements de deux spéctromètres de saturation à ultra-haute résolution dans le domaine infrarouge, l'un dans la région spectrale 9-12 mu m et l'autre accordable de 2,3 a 3,2 mu m sont exposés en detail. La stabilisation en fréquence d'un laser à CO(2) conventionnel ou guide d'ondes au niveau de la dizaine de hertz ainsi que l'obtention d'une stabilité meilleure que 1 khz pour le laser à centres colores soulignent le role cle de ces oscillateurs dans la chaine de raccordement de fréquence de l'horloge à cesium vers le domaine visible. L'étude des interactions hyperfines est presentée dans le cas des molécules diatomiques héteronucleaires (hf) et des toupies spheriques du groupe ponctuel O(h) (sf(6)). Grace a l'enregistrement de structures hyperfines tres bien resolues, de nombreux effets ont etes mis en evidence et interpretes à l'aide du formalisme tensoriel (dans le groupe (l)o(3) x O(h)) en particulier : l'interaction de spin-vibration (bande upsilon (3) de sf(6)), les mélanges d'états de types de symétrie differents (sf(6)), les corrections ro-vibrationnelles à l'interaction de spin-rotation (hf et sf(6)), les structures superhyperfines (sf(6))

La thèse vise au développement de microsystèmes capteurs de gaz sélectifs à l’ammoniac (NH3) et au sulfure d’hydrogène (H2S). Ces capteurs sont destinés au contrôle, à la gestion de la production et la sécurité dans les usines de méthanisation. Pour ce faire, deux types de capteurs chimiques utilisant des matériaux sensibles différents ont été investigués pour la détection des gaz. L’objectif premier est la mise en oeuvre des matériaux sensibles capables de réagir avec les molécules gazeuses et la caractérisation physico-chimique des matériaux. Ainsi, notre choix s’est focalisé sur un composite polymère à base polyaniline (PANI) de nanotubes de carbone multiparois et de deux polymères pour la mise en forme et la stabilisation (PANI-MWCNT/PS/PMMA) pour la détection de NH3. Quant à la détection de H2S, nous avons opté pour des nanoparticules d’oxyde métallique (V2O5). Ces matériaux ont été synthétisés, et exposés aux gaz en définissant un protocole de mesure pour évaluer leurs performances. L’évaluation des performances de détection de ces capteurs a montré qu’ils présentent une sensibilité et une stabilité de réponses même en milieu humide. De plus, chacun des matériaux a également montré une sélectivité partielle vis-à-vis de gaz cible et une certaine indépendance vis-à-vis de gaz interférents
The thesis aims to develop microsystems gas sensors for selective detection of ammonia (NH3) and hydrogen sulfide (H2S). These sensors are dedicated to the control and production as well as security in the management of anaerobic digestion plants dedicated to bio-energy production. To do this,two types of chemical sensors were used for gas sensing. The first objective is the development of sensitive materials able to react with gaseous molecules and the physico-chemical characterization of matérials. Thus our choice focused on a polyaniline-based polymer composite (PANI-MWCNT/PS/PMMA) for the detection of NH3 and metal oxide (V2O5) nanoparticles for the detection of H2S. These materials were synthesized and exposed to gases to evaluate their sensors performances. Such an evaluation has given a proof that these materials were the best choice to respond positively to the target gases since they have shown higher sensitivity and stable responses even in a humid environment. In addition, these materials also showed a partial selectivity towards target gas

Dans le contexte des écoulements micro- et nano-fluidiques, ce travail porte sur l'étude des interactions à l'interface entre des flux de gaz rares et des surfaces métalliques via une approche de modélisation multi-physique et multi-échelle. Elle se concentre tout particulièrement sur l'interaction entre l'argon et une surface d'or. Pour ce faire la modélisation a été effectuée en deux étapes, une première partie utilisant la mécanique quantique à l'échelle atomique et une deuxième partie de dynamique moléculaire à l'échelle nanométrique. La première partie est consacrée à l'obtention de potentiels d'interaction entre un atome d'argon et les atomes d'or de la surface par des méthodes de calculs théoriques basés sur la DFT comportant des effets à longues distances. Deux approches, donnant des résultats comparables, ont été utilisées : la première est liée à la description périodique de la surface d'or par un modèle basé sur la description des électrons par des ondes planes alors que la seconde permet de récupérer séparément les parties répulsives et attractives de l'interaction d'un atome d'argon avec un petit cluster d'or. Ces potentiels d'interactions ont été décomposés en potentiels de paires Ar-Au utilisables par des simulations de dynamique moléculaire. Ces simulations ont consisté en la projection d'atomes d'argon sur des surfaces d'or ‘parfaites' dites lisses ou des surfaces rugueuses plus représentatives de la technologie actuelle. L'analyse statistique des vitesses réfléchies permet de déterminer le coefficient d'accommodation tangentiel de l'argon sur des surfaces d'or. Ce coefficient est la traduction du phénomène de glissement qui peut ainsi être modélisé dans une description plus macroscopique de l'écoulement d'un gaz dans une micro-conduite. L'approche multi-physique utilisée dans ce travail a permis la détermination numérique de coefficients d'accommodations tangentiels très précis et comparables à l'expérience pour le couple argon-or, et doit pouvoir être appliquée à d'autres couples
In the context of micro- and nano-flows, this work concentrates on the study of interactions at the interface of noble gas and metal surfaces by a multi-physics and multiscale model. Particularly, the interaction of an argon atom with a gold surface is the focus of the study. The work has been made in two steps: the first one occurred at the atomic scale in which Quantum Mechanics is employed and the second one at the nanoscale with the use of Molecular Dynamics.The first part of the work was devoted to the determination of interaction potentials between an argon atom and gold atoms from the surface by DFT calculation methods comporting long range effects. Two approaches, leading similar results, have been used: the first one is linked to a periodic description of the gold surface where electrons are defined by plane waves, the second one gives independently repulsive and attractive parts of the interaction of an argon atom with a small gold cluster. Those interaction potentials are then decomposed in pair potentials suitable for Molecular Dynamics simulations. These last ones consisted in multiple times projecting argon atoms on smooth or rough gold surfaces (which are more representative of the roughness of actual technologies). The statistical analysis of the reflected velocities yielded the tangential momentum accommodation (TMAC) coefficient of argon on gold surfaces. This coefficient is the transcription of slip phenomena which occur at the interface, and it can then be used in nano-flow simulations. The multi-physics approach of the thesis gives accurate TMAC values which are comparable to experiments. The accounted method could then be applied to other noble gas metal surface couples

Lorsqu'un défaut est introduit dans un cristal phononique, des états apparaissent dans les bandes interdites et se localisent au niveau des défauts. Ils décroissent rapidement loin du défaut. Par conséquent, il est possible de localiser et de guider la propagation des ondes en concevant des défauts dans un cristal phononique parfait. Le guide d’onde à résonateurs couplés, fondé sur le couplage d'une séquence de cavités, présente simultanément un fort confinement des ondes et une faible vitesse de groupe ; il peut être utilisé pour concevoir des circuits plutôt arbitraires. En outre, la propagation des ondes élastiques dans une matrice solide peut être contrôlée en remplissant des cavités d'un fluide, sur la base des systèmes couplés fluides-solides. Ils ont des applications essentielles pour la réduction des vibrations et l’isolation acoustique. Dans cette thèse, les ondes acoustiques et élastiques se propageant dans les guides d’ondes à résonateurs couplés périodiques et apériodiques sont étudiées. L’interaction fluide-solide dans les cristaux phononiques fluide / solide est étudiée. Les travaux sont menés en combinant simulation numérique, analyse par modèles théoriques et investigation expérimentale
When a defect is introduced into a phononic crystal, states localized at the defect appear in the band gaps. They decay rapidly far away from the defect. Therefore, it is possible to localize and guide wave propagation by designing defects in the perfect phononic crystal. Coupled-resonator waveguides based on the coupling effect between a sequence of defect cavities have simultaneously strong wave confinement and low group velocity, and can be used to design rather arbitrary circuits. Furthermore, the propagation of elastic waves in a solid matrix can be controlled through changing fluid fillings based on fluid-solid interaction. Thus, they have essential applications in vibration reduction and noise isolation. In this thesis, the acoustic and elastic waves propagating in both periodic and aperiodic coupled-resonator waveguides are investigated. The fluid-solid interaction in fluid/solid phononic crystals is studied. The work is conducted by combining numerical simulations, theoretical model analysis and experimental investigations

La rationalisation des processus chimiques élémentaires aux surfacesest d'intérêt primordial pour de nombreux phénomènes naturels ou d'intérêttechnologique. D'un point de vue fondamental, la façon dont l'énergie, concomitanteà toute réaction chimique, est distribuée parmi les degrés de liberté des moléculesformées et/ou transférée à la surface est loin d'être systématisée. Dans ce travail,des simulations, reposant sur la méthode des trajectoires quasi-classiques (QCT),sont réalisées pour examiner cette problématique lors de recombinaisons demolécules d'hydrogène (H2) et d'azote (N2) résultant de l'abstraction d'atomesadsorbés via collision par un atome provenant de la phase gazeuse sur des surfacesde Tungstène - W(100) et W(110) - à taux de couverture non nul. Ces processussont ici étudiés pour leur intérêt en physique des interactions plasma-paroi. Dessurfaces d'énergie potentielle, construites à partir de calculs de structure électroniquebasés sur la théorie de la fonctionnelle densité (DFT), sont utilisées pour simuler,dans le cadre de la mécanique classique - incluant les corrections semi-classiquespertinentes - les processus ultrarapides dit de "Eley-Rideal" et par "atomes-chauds"(sub-picoseconde). La mise en place de modèle effectifs, pour tenir compte de ladissipation de l'énergie aux phonons de la surface et aux excitations électroniques(paires électron-trou), permet de rationaliser la dynamique non-adiabatique del'abstraction atomique aux surfaces métalliques
The rationalization of elementary processes at surfaces is of prime importance for numerous natural and technological areas. From a fundamental pointof view, the way the energy concomitant to any chemical reaction is distributed among the desorbing molecules degrees-of-freedom and the surface is far frombeing fully pictured. In this work, quasiclassical molecular dynamics (QCT)simulations have been carried out to investigate this issue for the recombination ofH2 and N2 resulting from atomic adsorbate abstraction by atom scattering off theW(100) and W(110) covered surfaces, these processes being of relevance inplasma-wall interactions. Potential energy surfaces, built from density functional(DFT) theory calculations, have been used to simulate, within the framework ofclassical dynamics (including semi-classical corrections), the subpicosecond Eley-Rideal and Hot-Atom processes. The implementation of effective models to accountfor energy dissipation to surface phonons and electron-hole pair excitations, have allowed to rationalize the non-adidabatic dynamics of atom abstraction at metalsurfaces

碩士
國立中央大學
天文研究所
92
We simulated the drift of particles in radial direction and surface density of accretion disk with considering the mass infall. The surface density of accretion disk will pass through growth phase, peak phase (about one hundred thousand years to million years), and decay phase. The drift of particle in the accretion disk with mass infall is different from the accretion disk without mass infall. There are two transient phenomena in the life time of accretion disks: FU-Orionis phenomenon and rapid disk dispersal. We adopted the ideas of large perturbation of surface density (Clarke et al, 1990) and photoevaporation model (Hollenbach et al, 1994) to explain these two transient phenomena. We also study the interaction between solid particles and gas in the accretion disk, and followed the results of Whipple (1972) and Weidenschilling (1977) to simulate the solid particle drift in the radial quiescent gas disks.

The objective of this investigation is to evaluate the ability of analytical and computational models to describe the momentum and heat transfer between the gas and particles in a falling-bed heat exchanger. Experimental data are presented for a test falling-bed heat exchanger. Measured temperatures, pressures, and overall heat transfer rates are compared to predicted values from analytical and computational models, and the capabilities and deficiencies of these modeling methods are discussed. In addition, the effect of the addition of a particle distributor on the performance of the falling-bed heat exchanger is measured. In the falling-bed heat exchanger, solid particles fall through a vertical column against a counterflowing gas stream flowing upward with a velocity less than the terminal velocity of the particle. Heat is exchanged between the falling particles and rising gas. This arrangement has been proposed for heat recovery and regeneration in power plants and other process applications. The ability to model and predict the heat transfer rate between the gas and particles is critical to the design of the falling-bed heat exchanger. The heat transfer between the gas and solid particles in these devices has typically been modeled by assuming steady-state and ideal, uniform, one-dimensional flow of the continuous fluid and the particle or droplets. This model, termed the uniform mixing model in this study, has been used in many instances to estimate the effective heat transfer coefficient and Nusselt number of the falling droplets and particles as a function of effective Reynolds number from experimental data. The addition of a particle distributor has been shown to increase the heat transfer effectiveness of the falling-bed heat exchanger in experiments. It has been determined that the uniform mixing model generally does not provide an accurate representation of the falling-bed heat exchanger, as it cannot account for gas and particle maldistributions such as those created by a particular particle distributor design. Computational fluid dynamics, which can permit the modeling of these spatial maldistributions, has been used to model the falling-bed heat exchanger. The predictions of the overall heat transfer rate from computational fluid dynamics are in better agreement with the measured values. However, discrepancies between the predicted and measured pressures and local temperatures indicate that the modeling of the turbulent mixing of momentum and energy is inadequate.

It is well established that rarefied flows cannot be properly described by traditional hydrodynamics, namely the Navier-Stokes equations for gas flows, and the Fourier’s law for heat transfer. Considering the significant advancement in miniaturization of electronic devices, where dimensions become comparable with the mean free path of the flow, the It is well established that rarefied flows cannot be properly described by traditional hydrodynamics, namely the Navier-Stokes equations for gas flows, and the Fourier's law for heat transfer. Considering the significant advancement in miniaturization of electronic devices, where dimensions become comparable with the mean free path of the flow, the study of rarefied flows is extremely important. This dissertation includes two main parts. First, we look into the heat transport in solids when the mean free path for phonons are comparable with the length scale of the flow. A set of macroscopic moment equations for heat transport in solids are derived to extend the validity of Fourier's law beyond the hydrodynamics regime. These equations are derived such that they remain valid at room temperature, where the MEMS devices usually work. The system of moment equations for heat transport is then employed to model the thermal grating experiment, recently conducted on a silicon wafer. It turns out that at room temperature, where the experiment was conducted, phonons with high mean free path significantly contribute to the heat transport. These low frequency phonons are not considered in the classical theory, which leads to failure of the Fourier's law in describing the thermal grating experiment. In contrast, the system of moment equations successfully predict the deviation from the classical theory in the experiment, and suggest the importance of considering both low and high frequency phonons at room temperature to capture the experimental results. In the second part of this study, we look into the gas-surface interactions for conventional gas dynamics when the gas flow is rarefied. An extension to the well-known Maxwell boundary conditions for gas-surface interactions are obtained by considering velocity dependency in the reflection kernel from the surface. This extension improves the Maxwell boundary conditions by providing an extra free parameter that can be fitted to the experimental data for thermal transpiration effect in non-equilibrium flows. The velocity dependent Maxwell boundary conditions are derived for the Direct Simulation Monte Carlo (DSMC) method and the regularized 13-moment (R13) equations for conventional gas dynamics. Then, a thermal cavity is considered to test and study the effect of these boundary conditions on the flow formation in the slip and early transition regime. It turns out that using velocity dependent boundary conditions allows us to change the size and direction of the thermal transpiration force, which leads to marked changes in the balance of transpiration forces and thermal stresses in the flow.
Graduate

Evaluation of host geologic sediment interactions with carbon dioxide is very important in sequestration strategies. The objective of the study is to experimentally investigate the effects of different soil mineral types on carbon dioxide hydrate formation. At isothermal, isochoric, and isobaric conditions, batch experiments were conducted with different types of solids (bentonite, kaolinite, nontronite, pyrite, and soil) and electrolytes (NaCl, KCl, CaCl2, and MgCl2) to measure carbon dioxide hydrate formation times. A 50 mL pressurized vessel was used for the experiment by bubbling gaseous CO2 into the solid suspension. We observed that the formation time of carbon dioxide hydrate was dependent on the reactor temperature (273.4 K and 277.1 K) and types of solid and electrolyte. A clear peak was observed in the temperature profile of each experimental run and determined as the hydrate formation time. This is due to the initiation of the hydrate crystallization and latent heat release at the hydrate formation time. The temperature profiles vary significantly with respect to the types of solids and electrolytes. As crystallization initiates, peaks were observed at higher temperatures in pyrite and soil suspensions. The results showed that hydrate formation times for clay minerals in water were approximately twice and 10 times faster than that for pyrite and soil, respectively. The rates of gas consumption were able to be determined by the pressure monitoring. The kaolinite appeared to have the fastest gas consumption rate among the clay mineral suspensions, which was 2.4 times and 7.4 times faster than nontronite and bentonite, respectively. Results from these experiments seem to provide an insight on the formation and growth of carbon dioxide hydrate, once sequestered into the sea bed sediments under the deep sea environment.

A raingauge network made of six stations was installed in the Hyblean region. Stations were located at different altitudes (from 5 m to 986 m a.s.l.) and along two directions (E-W and SW-NE). Rainwater samples were monthly collected for stable isotope measurements. Spatial distribution of rainwater isotope composition has confirmed the wet air masses move from South-East/South-West toward North. Water balance has highlighted that the annual volume of infiltrating waters is in the range of 1-1.5 *105 m3 Km-2. 82 well waters and 12 spring waters located within the Hyblean Plateau (South-Estern Sicily), were also collected from 1999 to 2001 during several surveys for chemical (major,minor and trace elements) analyses. Water chemistry allowed to identify two main aquifers: the first aquifer hosted within sedimentary rocks is characterized by earthalkaline bicarbonate waters, while the second aquifer, located within the volcanic deposits (mainly towards North- North-East) is characterized by groundwaters evolving from earthalkaline bicarbonate water-type towards a Na-HCO3-type. A slightly anomaly in water temperature (24-28°C) have been identified along the northern margin, while the lower Eh values have been recorded along the M.Lauro-Scicli and the Hyblean Malta Escarpment fault systems. Isotope composition of groundwaters has suggested the occurrence of evaporative processes during soil infiltration having a dD/d18O slope close to 4.5. Chemical and isotope composition of dissolved gases (d13CTDIC, d13CCH4, 3He/4He) have revealed, as expected, that deeply-derived gases rise along the main tectonic discontinuities. Chemical and isotope analyses of dissolved carbon have revealed the existence of two sampling sites (NA and FE samples) attesting the interaction between groundwaters and a consistent amount of deep inorganic carbon dioxide. He isotope ratios (from 0.81Ra to 6.19 Ra) have revealed the occurrence of mixing process, in different proportions, between crustal and mantle components. On the base of the obtained results, a clear picture of the groundwaters circulation within the Hyblean aquifers has been drawn. In framework of projecting of a geochemical network for the continuous monitoring of the local seismic activity the most suitable geochemical parameters and the sites of great interest have been identified.
- Unione Europea Fondo Sociale Europeo; - Ministero dell’Università e della Ricerca Scientifica e Tecnologica; - Università degli studi di Palermo
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