Rozprawy doktorskie na temat „Solid-gas 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).

Chapter 1 and 2, a brief introduction role of surface science, photoelectron spectroscopy and gas solid interaction in catalysis has been given. Photoelectron spectroscopy is a versatile technique that can be advantageously used for characterization of a number of surface properties of the solid surface and interfaces, like chemical composition, oxidation state, element mapping, solid-gas interactions etc. Conventionally, the photoelectron spectroscopy is an ultra-high vacuum technique. However, the photoelectrons cannot reach the detector due to inelastic collision with gas-phase molecules at high-pressure (>10-5 mbar) conditions. This obstacle has been overcome in the development of photoelectron spectrometers which can function at near ambient pressures. A new term ambient pressure (AP) or near ambient pressure (NAP) or high pressure (HP) has been introduced with photoelectron spectroscopy (PES) to distinguish it from the traditional UHV set ups. The NAP-PES can operate at near ambient pressure, by using the sophisticated electron energy analyzer and differentially pumped electro static lenses. Thus the NAP-PES, can bridge the “pressure gap” between the real world and ideal surface science studies. This thesis mainly focused on different studies to bridge the pressure gap between a real-world condition where the actual surface reaction happens and ideal surface science condition. All experiments are carried out in a custom built laboratory ambient pressure photoelectron spectrophotometer unit installed in our laboratory at CSIR-National Chemical Laboratory, Pune. The system is equipped with the differentially pumped Scienta R3000HP analyzer. Two sets of differential pumping are available in the electrostatic lens regime (ELR), and the third one is available in the electron energy analyzer (EEA). The distance between sample surface and aperture (of the cone, R = 0.4 mm) attached to ELR was maintained at 1.2 mm for all of the experiments reported. The main advantage with this design is a fast decrease in pressure with a steep pressure gradient from the aperture to the EEA. It is to be underscored that the first differential pumping records 2 × 10−4 mbar when the analysis chamber is at 1 mbar. This helps to minimize the inelastic scattering of low KE electrons. Further, R3000HP employs the advanced concept of electron converging with an aperture free ELR. In contrast to the conventional ELR, electrostatic voltages in the R3000HP model analyzer are applied in such a way that they converge all the electrons. The system also equipped Al-Mg dual anode, Al- monochromatic X-ray source, differentially pumped xvi discharge lamp to generate He I and He II UV radiation source The polycrystalline foils (Ni, Co, Si, Ag, Au, 99.999 pure) which are used in the experiments are purchased from MaTeck, Germany. These foils are cleaned by the several cycles of Ar sputtering and annealing in UHV up to 1000 K. A series of sputter-anneal cycles produced clean metal foil surfaces which is confirmed by the XPS as well as UVPES. In chapter 3a, Silver valence band was probed by PES at near ambient pressure of oxygen (up to 0.2 mbar) with He I radiation. Three distinct regimes have been identified in silver-oxygen interaction between 300 and 500 K, which are, (a) oxygen chemisorption between ambient and 390 K, (b) O-diffusion into the subsurface layers of Ag from 390 to 450 K, and (c) formation of metastable oxide on the silver surface above 450 K; the latter two regimes are dynamic in nature.. The trend in oxygen coverage on Ag 390 K and above 475 K is similar, but it decreases to the lowest in between 390 and 450 K, in the presence of large excess of molecular oxygen. Interaction with oxygen changes the work function of Ag from 4.95 (≤390 K) to 5.30 eV (400-450 K), and then to 5.7 eV (≥450 K). It is attributed to oxygen diffusion into the subsurface layers of Ag between 400 and 450 K and plays a key role for ethylene epoxidation reaction on Ag surfaces. Subsurface oxygen influences in two significant ways; it converts the Ag surface from metallic to electron deficient in nature, and facilitates the formation of space charge layer above the Ag surface. Oxygen when adsorbed on this electron deficient Ag surface, acts as electrophilic oxygen. The electrophilic oxygen can insert into the C=C double bond of an alkene, and hence forms an epoxide. Above 450 K, oxygen binds strongly and acts as nucleophilic oxygen. The nucleophilic oxygen favors complete combustion of alkene to carbon dioxide. Changes in the Ag-oxygen system are dynamic. The metallic surface reappears if oxygen supply is removed above 400 K. This emphasizes in situ and operando investigations are essential to understand the active structure of a catalyst. In chapter 3b, we have synthesized 5 wt % Fe2O3/support (support=Al2O3, CeO2, MgO, ZSM-5 and Nb2O5) catalysts by wet impregnation method. The synthesized catalysts subjected to different physico- chemical characterization techniques to understand the structure and morphology of the catalysts. These catalysts were screened for butane oxidative dehydrogenation (ODH) reaction in fixed bed reactor at different temperature between (450°C to 600°C) with varying butane: oxygen ratio (1:1, 1:0.5 and 1:0.25). Among all these catalyst xvii Fe2O3/Al2O3 shows best activity in terms of 1,3-butadiene yield (higher selectivity towards 1,3-butadiene) at all different temperatures. In order to understand the active site of the compared the results with another average catalyst i.e., Fe2O3/Nb2O5. The catalyst was screened at 0.2 mbar pressure (Argon: Butane: Oxygen is 2:1:0.5 respectively) under in situ condition from 298 to 500 K. We have concluded that the Fe on the Al2O3 support reduced to Fe+2 from Fe+3, whereas Fe on the Nb2O5 support remains in Fe+3 states and the reduced Fe+2 is responsible for the higher selectivity towards 1,3-butadiene Chapter 4 shows Valence band and core level photoelectron spectral measurements at near-ambient pressures (up to 0.5 mbar) were made in the presence of molecular oxygen to explore the various stages of silicon oxidation. Dangling bonds feature observed in NAP-UPS on clean Si-surfaces decreases due to adsorption of molecular oxygen between ambient temperature and up to 400 K at 0.1 mbar O2 pressure. The adsorption of oxygen on dangling bonds seems to be localized as islands and the same reflects as heterogeneous surface and responsible for the broadening in the oxygen gas phase vibrational features. This is further supported by an increase in the work function and can be correlated with the presence of Höfer (molecular) precursor. When the temperature increased to 500 K, molecular precursor species dissociates to –Si=O species and further supported by the change in the work function as well as by the oxidized silicon species from Si 2p core level. At 600 K the –Si=O species dissociates to form a uniform 2D oxide layer on the silicon surface, which is characterized by the sharp vibration features of gas-phase oxygen molecules. This layer is also quite stable up 800 K and without any further oxidation in bulk. However, when the temperature increased to 850 K at 0.2 mbar oxygen pressure, bulk Si oxidation begins and the work function increases drastically by 1 eV. An angle-dependent Si 2p core level spectra recorded map out the presence of all possible oxidation states (elemental Si0 to Si4+) from bulk to the surface. A continuous change in work function and electronic states observed due to gas-solid (O2-Si) interaction indicates the implications of heterogeneous catalysis and electrochemistry. In chapter 5 the gas phase vibrational spectra of reactive and inert gases have been studied by the in situ ultraviolet photoelectron spectroscopy up to 0.3 mbar. Results obtained is divided into two parts and discussed. In the first part, we have studied the molecular photoelectron spectra of monoatomic Argon gas and some homonuclear diatomic molecular gases like H2, O2, and N2 by using NAPUPS and the effect of pressure on their energy xviii position. In this study, we have demonstrated that NAPUPS can be an essential tool to determine the gaseous composition and their electronic configuration. In the second part, we have studied the influence of surface nature on the binding energy position and pattern of the vibrational features of Nitrogen and Argon gas. It has been observed that with changing the electronic nature of the surface, the binding energy of vibrational spectra also changes which reflects the change in the work function of the material. Further, if the solid surface undergoes any chemical/electronic changes due to gas-solid interaction, such as oxidation, the work function of the surface changes again and underscores the identification of in-situ changes. Therefore, the change in the binding energy of the gas phase can be used to determine the actual work function change of material during the the chemical reaction
AcSIR

Chemistry at a catalyst surface i.e. response of the catalyst for structure and electronic change during catalysis, and gas-solid interaction of catalyst and reactant is very crucial for the fundamental understanding of a catalytic process. Tracking surface phenomenon during catalysis can shed light on the surface electronic structure, catalyst nature and provides a fundamental correlation between catalytic performance and possible pathways, which helps to design a catalyst in a better way. NAPPES is a powerful tool that is inherently surface sensitive, and chemical-specific, with the ability to probe sample surfaces as well as reactants at pressures closer to ambient pressure. Hence it has potential to provide remarkable outputs in the field of environmental chemistry and catalysis. The present thesis work has been devoted to bridging the material and pressure gap in between surface science under ideal conditions and real-world catalysis conditions. A sincere attempt has been made to understand the solid-gas interaction and its application to heterogeneous catalysis by utilizing NAPPES.
CSIR
AcSIR

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.

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.

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 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.

In this thesis, a non-local continuum approach coupled with the phase-field theory and Jones-Wilkins-Lee (JWL) equation of state (EOS) is proposed to model and simulate blast-induced fracture in brittle materials. Also, we study the evolution of field variables in the underground explosion of steel fiber reinforced concrete (SFRC). For numerical implementation, we have used a non-ordinary state-based peridynamic theory coupled with a diffusive phase-field damage approach. Moreover, JWL EOS from the family of isentropes has been formed, and finally, the constitutive relations have been arrived with the help of the JWL EOS. As the conventional equation of motion cannot capture the discontinuities in the damaged portion of the material, derivative-free integro-differential equations of motion have been used in line with the peridynamics approach to overcome the limitation of the conventional equation of motion. The motivation for using non-ordinary state-based (NOSB) peridynamics (PD) over the bond-stretch-based (BSB) or bond-energy-based (BEB) models comes from the fact that the applicability of BSB and BEB models are only limited to materials with Poisson's ratio 1/4. We have introduced a dynamic failure mechanism due to blast-induced stress wave propagation in rock media, generated due to the pressure of a high-velocity gaseous detonator. Furthermore, an expansion of stress wave in the SFRC mass due to the underground explosion of highly pressurized detonators (Iregel 1175U) has been studied in this thesis. A program-burn algorithm inside the JWL EOS has been implemented to model rock fragmentation. A predictor-corrector explicit time integration scheme has been used to update the field variables at every time step. The phase-field damage can capture well the explosive-induced fracture in rock media and damage in the SFRC medium for the underground explosion. The current numerical approach suggests a versatile physics-oriented future model for this class of problems. The formulation proposed in the thesis has been validated against two numerical benchmark tests and has shown good predictive quality.

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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