Thèses sur le sujet « Gas/particle »

An Eulerian-Eulerian model for dilute gas-particle turbulent flows is

developed for engineering applications. The aim is to understand the effect of particles on turbulent flows. The model is implemented in a finite element code which is used to perform numerical simulations. The feedback from the particles on the turbulence and the mean flow of the gas in a vertical channel flow is studied. In particular, the influence of the particle response time and particle volume fraction on the preferential concentration of the particles near the walls, caused by the turbophoretic effect is explored. The study shows that the particle feedback decreases the accumulation of particles on the walls. It is also found that even a low particle volume fraction can have a significant impact on the turbulence and the mean flow of the gas. A model for the particle fluctuating velocity in turbulent gas-particle flow is derived using a set of stochastic differential

equations. Particle-particle collisions were taken into account. The model shows that the particle fluctuating velocity increases with increasing particle-particle collisions and that increasing particle response times decrease the fluctuating velocity.

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.

A turbulent two-phase flow model using kinetic theory of granularflows for the particle phase is developed and implmented in afinite element code. The model can be used for engineeringapplications. However, in this thesis it is used to investigateturbulent gas-particle flows through numerical simulations.  The feedback from the particles on the turbulence and the meanflow of the gas in a vertical channel flow is studied. In particular,the influence of the particle response time, particle volumefraction and particle diameter on the preferential concentration ofthe particles near the walls, caused by the turbophoretic effect isexplored. The study shows that when particle feedback is includedthe accumulation of particles near the walls decreases. It is also foundthat even at low volume fractions particles can have a significant impacton the turbulence and the mean flow of the gas. The effect of particles on a developing turbulent vertical upward pipeflow is also studied. The development length is found to substantiallyincrease compared to an unladen flow. To understand what governs thedevelopment length a simple estimation was derived, showing that itincreases with decreasing particle diameters in accordance with themodel simulations. A model for the fluctuating particle velocity in turbulentgas-particle flow is derived using a set of stochastic differentialequations taking into account particle-particle collisions. Themodel shows that the particle fluctuating velocity increases whenparticle-particle collisions become more important and that increasingparticle response times reduces the fluctuating velocity. The modelcan also be used for an expansion of the deterministic model for theparticle kinetic energy.
QC20100726

This thesis describes validated improvements in the modelling of micron-sized particle deposition within gas turbine engine secondary air systems. The initial aim of the research was to employ appropriate models of instantaneous turbulent flow behaviour to RANS CFD simulations, allowing the trajectory of solid particulates in the flow to be accurately predicted. Following critical assessment of turbophoretic models, the continuous random walk (CRW) model was chosen to predict instantaneous fluid fluctuating velocities. Particle flow, characterised by non-dimensional deposition velocity and particle relaxation time, was observed to match published experimental vertical pipe flow data. This was possible due to redefining the integration time step in terms of Kolmagorov and Lagrangian time scales, reducing the disparity between simulations and experimental data by an order of magnitude. As no high temperature validation data for the CRW model were available, an experimental rig was developed to conduct horizontal pipe flow experiments under engine realistic conditions. Both the experimental rig, and a new particulate concentration measurement technique, based on post test aqueous solution electrical conductivity, were qualified at ambient conditions. These new experimental data compare well to published data at non-dimensional particle relaxation times below 7. Above, a tail off in the deposition rate is observed, potentially caused by a bounce or shear removal mechanism at higher particle kinetic energy. At elevated temperatures and isothermal conditions, similar behaviour is observed to the ambient data. Under engine representative thermophoretic conditions, a negative gas to wall temperature gradient is seen to increase deposition by up to 4.8 times, the reverse decreasing deposition by a factor of up to 560 relative to the isothermal data. Numerical simulations using the CRW model under-predict isothermal deposition, though capturing relative thermophoretic effects well. By applying an anisotropic Lagrangian time scale, and cross trajectory effects of the external gravitational force, good agreement was observed, the first inclusion of the effect within the CRW model. A dynamic mesh morphing method was then developed, enabling the effect of large scale particle deposition to be included in simulations, without continual remeshing of the fluid domain. Simulation of an impingement jet array showed deposition of characteristic mounds up to 30% of the hole diameter in height. Simulation of a passage with film-cooling hole off-takes generated hole blockage of up to 40%. These cases confirmed that the use of the CRW generated deposition locations in line with scant available experimental data, but widespread airline fleet experience. Changing rates of deposition were observed with the evolution of the deposits in both cases, highlighting the importance of capturing changing passage geometry through dynamic mesh morphing. The level of deposition observed, was however, greater than expected in a real engine environment and identifies a need to further refine bounce-stick and erosion modelling to complement the improved prediction of impact location identified in this thesis.

Doutoramento em Ciências e Engenharia do Ambiente
Os incêndios florestais são uma importante fonte de emissão de compostos gasosos e de aerossóis. Em Portugal, onde a maioria dos incêndios ocorre no norte e centro do país, os incêndios destroem todos os anos milhares de hectares, com importantes perdas em termos económicos, de vidas humanas e qualidade ambiental. As emissões podem alterar consideravelmente a química da atmosfera, degradar a qualidade do ar e alterar o clima. Contudo, a informação sobre as caraterísticas das emissões dos incêndios florestais nos países do Mediterrâneo é limitada. Tanto a nível nacional como internacional, existe um interesse crescente na elaboração de inventários de emissões e de regulamentos sobre as emissões de carbono para a atmosfera. Do ponto de vista atmosférico da monitorização atmosférica, os incêndios são considerados um desafio, dada a sua variabilidade temporal e espacial, sendo de esperar um aumento da sua frequência, dimensão e severidade, e também porque as estimativas de emissões dependem das caraterísticas dos biocombustíveis e da fase de combustão. O objetivo deste estudo foi quantificar e caraterizar as emissões de gases e aerossóis de alguns dos mais representativos incêndios florestais que ocorreram no centro de Portugal nos verões de 2009 e de 2010. Efetuou-se a colheita de amostras de gases e de duas frações de partículas (PM2.5 e PM2.5-10) nas plumas de fumo em sacos Tedlar e em filtros de quartzo acoplados a um amostrador de elevado volume, respetivamente. Os hidrocarbonetos totais (THC) e óxidos de carbono (CO e CO2) nas amostras gasosas foram analisados em instrumentos automáticos de ionização de chama e detetores não dispersivos de infravermelhos, respetivamente. Para algumas amostras, foram também quantificados alguns compostos de carbonilo após reamostragem do gás dos sacos Tedlar em cartuchos de sílica gel revestidos com 2,4-dinitrofenilhidrazina (DNPH), seguida de análise por cromatografia líquida de alta resolução. Nas partículas, analisou-se o carbono orgânico e elementar (técnica termo-óptica), iões solúveis em água (cromatografia iónica) e elementos (espectrometria de massa com plasma acoplado por indução ou análise instrumental por ativação com neutrões). A especiação orgânica foi obtida por cromatografia gasosa acoplada a espectrometria de massa após extração com recurso a vários solventes e separação dos extratos orgânicos em diversas classes de diferentes polaridades através do fracionamento com sílica gel. Os fatores de emissão do CO e do CO2 situaram-se nas gamas 52-482 e 822-1690 g kg-1 (base seca), mostrando, respetivamente, correlação negativa e positiva com a eficiência de combustão. Os fatores de emissão dos THC apresentaram valores mais elevados durante a fase de combustão latente sem chama, oscilando entre 0.33 e 334 g kg-1 (base seca). O composto orgânico volátil oxigenado mais abundante foi o acetaldeído com fatores de emissão que variaram desde 1.0 até 3.2 g kg-1 (base seca), seguido pelo formaldeído e o propionaldeído. Observou-se que as emissões destes compostos são promovidas durante a fase de combustão latente sem chama. Os fatores de emissão de PM2.5 e PM10 registaram valores entre 0.50-68 e 0.86-72 g kg-1 (base seca), respetivamente. A emissão de partículas finas e grosseiras é também promovida em condições de combustão lenta. As PM2.5 representaram cerca de 90% da massa de partículas PM10. A fração carbonosa das partículas amostradas em qualquer dos incêndios foi claramente dominada pelo carbono orgânico. Foi obtida uma ampla gama de rácios entre o carbono orgânico e o carbono elementar, dependendo das condições de combustão. Contudo, todos os rácios refletiram uma maior proporção de carbono orgânico em relação ao carbono elementar, típica das emissões de queima de biomassa. Os iões solúveis em água obtidos nas partículas da pluma de fumo contribuíram com valores até 3.9% da massa de partículas PM2.5 e 2.8% da massa de partículas de PM2.5-10. O potássio contribuiu com valores até 15 g mg-1 PM2.5 e 22 g mg-1 PM2.5-10, embora em massa absoluta estivesse maioritariamente presente nas partículas finas. Os rácios entre potássio e carbono elementar e entre potássio e carbono orgânico obtidos nas partículas da pluma de fumo enquadram-se na gama de valores relatados na literatura para emissões de queima de biomassa. Os elementos detetados nas amostras representaram, em média, valores até 1.2% e 12% da massa de PM2.5 e PM2.5-10, respetivamente. Partículas resultantes de uma combustão mais completa (valores elevados de CO2 e baixos de CO) foram caraterizadas por um elevado teor de constituintes inorgânicos e um menor conteúdo de matéria orgânica. Observou-se que a matéria orgânica particulada é composta principalmente por componentes fenólicos e produtos derivados, séries de compostos homólogos (alcanos, alcenos, ácidos alcanóicos e alcanóis), açúcares, biomarcadores esteróides e terpenóides, e hidrocarbonetos aromáticos policíclicos. O reteno, um biomarcador das emissões da queima de coníferas, foi o hidrocarboneto aromático dominante nas amostras das plumas de fumo amostradas durante a campanha que decorreu em 2009, devido ao predomínio de amostras colhidas em incêndios em florestas de pinheiros. O principal açúcar anidro, e sempre um dos compostos mais abundantes, foi o levoglucosano. O rácio levoglucosano/OC obtido nas partículas das plumas de fumo, em média, registaram valores desde 5.8 a 23 mg g-1 OC. Os rácios levoglucosano/manosano e levoglucosano/(manosano+galactosano) revelaram o predomínio de amostras provenientes da queima de coníferas. Tendo em conta que a estimativa das emissões dos incêndios florestais requer um conhecimento de fatores de emissão apropriados para cada biocombustível, a base de dados abrangente obtida neste estudo é potencialmente útil para atualizar os inventários de emissões. Tem vindo a ser observado que a fase de combustão latente sem chama, a qual pode ocorrer simultaneamente com a fase de chama e durar várias horas ou dias, pode contribuir para uma quantidade considerável de poluentes atmosféricos, pelo que os fatores de emissão correspondentes devem ser considerados no cálculo das emissões globais de incêndios florestais. Devido à falta de informação detalhada sobre perfis químicos de emissão, a base de dados obtida neste estudo pode também ser útil para a aplicação de modelos no recetor no sul da Europa.
Wildfires are an importante emission source of gaseous compounds and aerosol particles. In Portugal, where most fire events occur in northern and central areas of the country, wildfires destroy every year thousands of hectares, with important losses in terms of economic disruptions, human lives and environmental quality. Emissions can substantially perturb atmospheric chemistry, degrade air quality and alter weather and climate. However, limited data exist on the emission characteristics from this source in Mediterranean countries. At both national and international levels, there is an increasing focus on the establishment of emission inventories and regulations of regional carbon emissions to the atmosphere. From the standpoint of atmospherically-based carbon monitoring programs, fires are challenging because they tend to be extremely variable in intensity, space and time, they are expected to increase in number and severity in the future, and because emission estimates depend on biofuel characteristics and combustion phase. The aim of this study was to quantify and characterise the emissions of trace gases and aerosol particles from some of the most representative wildfires that occurred in central Portugal during the summers of 2009 and 2010. Gases and particles of two size fractions (PM2.5 and PM2.5-10) were collected from the smoke plumes in Tedlar bags and on quartz filters mounted on a high volume sampler, respectively. The gaseous compounds were subsequently analised for total hydrocarbons (THC) and carbon oxides (CO and CO2) in automatic instruments with flame ionisation and non-dispersive infrared detectors, respectively. For some smoke samples, carbonyls were also quantified after drawing air from the Tedlar bags through cartridges containing silica gel coated with 2,4-dinitrophenylhydrazine (DNPH) reagent and followed by analysis by high performace liquid chromatography. Particles were analysed for organic and elemental carbon (thermal-optical technique), water-soluble ions (ion chromatography) and trace elements (inductively coupled plasma mass spectrometry or instrumental neutron activation analysis). The organic speciation was obtained by gas chromatography coupled to mass spectrometry after multi-solvent extraction and separation of the organic extracts into several classes of different polarities by flash chromatography on silica gel. The CO and CO2 emission factors were in the ranges 52-482 and 822-1690 g kg-1 (dry basis), showing, respectively, negative and positive correlations with the combustion efficiency. The THC emission factors were higher during smouldering conditions with values ranging between 0.33 and 334 g kg-1 (dry basis). The most abundant oxygenated volatile organic compound measured was acetaldehyde with emission factors ranging from 1.0 to 3.2 g kg-1 (dry basis), followed by formaldehyde and propionaldehyde. The emission of these compounds were enhanced during the smouldering phase. PM2.5 and PM10 emission factors were in the ranges 0.50-68 and 0.86-72 g kg-1 (dry basis), respectively. The emission of fine and coarse particles was promoted by smouldering combustion conditions. PM2.5 particles contributed to around 90% of the PM10 mass. The carbonaceous fraction of smoke particulate samples from any of the fires was clearly dominated by organic carbon. A wide range of organic carbon-to-elemental carbon concentration ratios was obtained, depending on the combustion conditions. However, all the ratios reflected a much higher proportion of organic carbon in relation to elemental carbon, typical of biomass burning emissions. The water-soluble ions obtained in smoke particles contributed with values up to 3.9% of the PM2.5 and 2.8% of the PM2.5-10 particles. Potassium contributed up to 15 g mg-1 PM2.5 and 22 g mg-1 PM2.5-10, although in absolute mass it was overwhelmingly present in fine particles. The potassium-to-elemental carbon and potassium-to-organic carbon ratios obtained in smoke particles were in accordance with those reported in the literature for biomass burning sources. Trace elements detected in smoke samples represented, on average, up to 1.2% and 12% of the PM2.5 and PM2.5-10 mass, respectively. Particles from a more complete combustion (higher CO2 and lower CO values) were characterised by a higher content of inorganic constituents and a lower organic content. The particulate organic matter was mainly composed of phenolic compounds and their alteration products, homologous series (n-alkanes, n-alkenes, n-alkanoic acids and n-alkanols), sugar constituents, steroid and terpenoid biomarkers, and polycyclic aromatic hydrocarbons. Retene, a biomarker of softwood smoke, was the dominant aromatic hydrocarbon in smoke samples collected during the 2009 campaign, due to a predominance of samples from wildfires in pine forests. The major anhydrosugar, and always one of the most abundant compounds, was levoglucosan. The levoglucosan/OC ratio obtained in the smoke particles, on average, ranged from 5.8 to 23 mg g-1 OC. The levoglucosan-to-mannosan and the levoglucosan-to-mannosan plus galactosan ratios determined reveal a predominance of samples from softwood combustion. Since estimation of wildfire emissions requires knowledge of fuel-appropriate emissions factors, the comprehensive database obtained in this study is potentially useful to update the current emission inventories. It has been observed that the smouldering phase, which can occur simultaneously with the flaming front and continue for several hours to days, may contribute to significant amounts of atmospheric pollutants and the corresponding emission factors should be considered when calculating the global wildfire emissions. Due to the lack of detailed emission profiles, the databases obtained in this study can also be very helpful for receptor modelling in southern Europe.

Pulmonary and oral drug administrations are usually the preferred methods of delivery of active pharmaceutical ingredients.Generally,pulmonary drug formulations are more attractive compared to oral formulations since they consist of micron-sized powders with high surface area thus having faster onset of action,as well as minimizing the drug dosage and side effects.Oral insulin formulations,if achievable,would provide an alternative to injectable insulin,as the common drawbacks of injectable insulin are the multiple daily injections and the possibility of skin infections at the injection site. In this study,the feasibility of using dense gas particle processing techniques known as the Aerosol Solvent Extraction System (ASES),Gas Anti-Solvent (GAS)and High-Pressure Media Milling (HPMM)for pharmaceutical processing was assessed.The ASEStechnique,utilizing dense ethane,was employed to prepare insulin-lactose formulations for pulmonary administration whilst the GAS and ASES techniques,utilizing dense CO2,were employed to prepare microencapsulated formulations containing insulin and Eudragit?? S100 for oral administration.Furthermore,the HPMM technique,utilizing dense hydrofluocarbon (HFC)134a/227ea,was employed to prepare suspension Metered Dose Inhaler (MDI)formulations containing budesonide and various surfactants. The Fine Particle Fraction (FPF)of processed insulin without the presence of lactose was found to be 44%.In other words,44% of processed insulin delivered to the impactor stages (excluding the throat and neck)has aerodynamic diameter of less than 5??m.With the addition of lactose as carrier,the FPFof the insulin-lactose (1:1w/w)formulation increased to 64%.The increase in FPFwas attributed to the lower density of lactose particles compared to that of insulin particles to produce an intimate mixture with enhanced powder flowability and aerodynamic performance. Proteins for oral delivery should ideally be formulated with acid-resistant polymer as a protective coating to protect against enzymatic degradation in the stomach.Eudragit?? S100,which is insoluble or almost impermeable at pH 1-4and soluble at pH 5-7,was used to prepare oral insulin formulations.The insulin release at pH 3was sustained by the Eudragit?? S100coating and the encapsulation efficiency of insulin??Eudragit?? S100formulations varied between 6% and 24% depending on the initial drug to polymer ratio. One of the major therapies utilizing metered dose inhaler formulations in the treatment of asthma has been studied using the HPMM process.The HPMM process has been demonstrated to be an efficient milling process for the enhancement of the physical stability and aerodynamic performance of budesonide in HFC-134a/227ea propellant formulations.No significant change in physical stability was observed in the formulations for 2 weeks.

Current and future legislation demands ever decreasing levels of pollution from gas turbine engines, and with combustor performance playing a critical role in resultant emissions, a need exists to develop a greater appreciation of the fundamental causes of unsteadiness. Particle Image Velocimetry (PIV) provides a platform to enable such investigations. This thesis presents the development of PIV measurement methodologies for highly turbulent flows. An appraisal of these techniques applied to gas turbine combustors is then given, finally allowing a description of the increased understanding of the underlying fluid dynamic processes within combustors to be provided. Through the development of best practice optimisation procedures and correction techniques for the effects of sub-grid filtering, high quality PN data has been obtained. Time average statistical data at high spatial resolution has been collected and presented for generic and actual combustor geometry providing detailed validation of the turbulence correction methods developed, validation data for computational studies, and increased understanding of flow mechanisms. These data include information not previously available such as turbulent length scales. Methodologies developed for the analysis of instantaneous PIV data have also allowed the identification of transient flow structures not seen previously because they are invisible in the time average. Application of a new `PDF conditioning' technique has aided the explanation of calculated correlation functions: for example, bimodal primary zone recirculation behaviour and jet misalignments were explained using these techniques. Decomposition of the velocity fields has also identified structures present such as jet shear layer vortices, and through-port swirling motion. All of these phenomena are potentially degrading to combustor performance and may result in flame instability, incomplete combustion, increased noise and increased emissions.

Transdermal powdered drug delivery is an emerging technology for the injection of drugs through human skin, in which particles of solid drug are entrained in a high-speed gas flow and directed towards the skin at a high enough velocity to penetrate the outer layer of dead cells. Hand-held devices based on this idea offer a means of safe, painless and effective delivery of many drugs and vaccines. This thesis describes a programme of research into the fluid dynamics which determine the particle velocity distribution, the most important mechanical characteristic of the system, in prototype drug delivery devices. Pressure measurements are described which enable characterisation of the gas flow in the drug delivery devices. These are complemented by optical particle detection experiments, which provide a record of the timing of drug particle delivery with respect to the gas flow. Doppler Global Velocimetry (DGV) has been used to measure the velocity field of drug particles. Various tasks involved in the application of DGV to these flow-fields are described. In particular, the use of time-integrated DGV for measurements of unsteady, short-duration flows is discussed. Time-integrated DGV, applied at a range of operating conditions, has provided information on the variation of particle delivery velocity with particle size and with total mass of particles. Time-resolved DGV measurements reveal that particles first emerge in a slow-moving cloud which is driven by transient starting process in the gas flow, followed by a faster stream of particles entrained in a quasi-steady gas flow. The experimental results are complemented by numerical computations of certain aspects of the drug delivery flows. These computations are compared with experimental results, and used to gain additional information on the functioning of the system as a whole.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 251-265).
Accurate spectroscopy has driven advances in chemistry, materials science, and physics. However, despite their importance in the study of highly correlated systems, two-dimensional systems (2DES) have proven difficult to probe spectroscopically. Typical energy scales are on the order of a millielectron volt (meV), requiring high resolution, while correlated states of interest, such as those found in the integer and fractional quantum Hall effect, are destroyed by excessive electron heating. Approaches based on tunneling have been hampered by problems such as ohmic heating and low in-plane conductivity, while optical approaches probe long-wavelength excitations which can be difficult to interpret. Here we present a refined spectroscopic technique, time domain capacitance spectroscopy (TDCS), with which we measure the single particle density of states (DOS) of a 2DES with temperature-limited resolution. In TDCS, sharp voltage pulses disequilibrate a metallic contact from a nearby 2DES, inducing a tunnel current. We detect this current by monitoring the image charge of the tunneled electrons on a distant electrode. No ohmic contact to the 2DES is required. The technique works when the 2DES is empty or has vanishing in-plane conductivity, as frequently occurs in studying the quantum Hall effect. Using TDCS, we perform unprecedentedly high resolution measurements of the DOS of a cold 2DES in GaAs over a range from 15 meV above to 15 meV below the Fermi surface. We provide the first direct measurements of the width of the single-particle exchange gap and single particle lifetimes in the quantum Hall system. At higher energies, we observe the splitting of highly excited Landau levels by spin polarization at the Fermi surface, demonstrating that the high energy spectrum reflects the low temperature ground state in these highly correlated systems. These measurements bring to light the difficult to reach and beautiful structure present far from the Fermi surface.
by Oliver Eugene Dial, III.
Ph.D.

In this work the methods of exact quantum number conservation in statistical mechanics are discussed and applied to the field of high energy nucleus-nucleus collisions. Various types of hadronic gas models are discussed as well as their merits and restrictions. Attempts to construct a phenomenological equation of state for nuclear matter are discussed in the context of the phase transition from hadronic matter to the quark-gluon plasma (QGP).

This PhD thesis focuses on numerical and analytical methods for simulating the dynamics of volcanic ash plumes. The study starts from the fundamental balance laws for a multiphase gas– particle mixture, reviewing the existing models and developing a new set of Partial Differential Equations (PDEs), well suited for modeling multiphase dispersed turbulence. In particular, a new model generalizing the equilibrium–Eulerian model to two-way coupled compressible flows is developed. The PDEs associated to the four-way Eulerian-Eulerian model is studied, investigating the existence of weak solutions fulfilling the energy inequalities of the PDEs. In particular, the convergence of sequences of smooth solutions to such a set of weak solutions is showed. Having explored the well-posedness of multiphase systems, the three-dimensional compressible equilibrium–Eulerian model is discretized and numerically solved by using the OpenFOAM® numerical infrastructure. The new solver is called ASHEE, and it is verified and validated against a number of well understood benchmarks and experiments. It demonstrates to be capable to capture the key phenomena involved in the dynamics of volcanic ash plumes. Those are: turbulence, mixing, heat transfer, compressibility, preferential concentration of particles, plume entrainment. The numerical solver is tested by taking advantage of the newest High Performance Computing infrastructure currently available. Thus, ASHEE is used to simulate two volcanic plumes in realistic volcanological conditions. The influence of model configuration on the numerical solution is analyzed. In particular, a parametric analysis is performed, based on: 1) the kinematic decoupling model; 2) the subgrid scale model for turbulence; 3) the discretization resolution. In a one-dimensional and steady-state approximation, the multiphase flow model is used to derive a model for volcanic plumes in a calm, stratified atmosphere. The corresponding Ordinary Differential Equations (ODEs) are written in a compact, dimensionless formulation. The six non-dimensional parameters characterizing a multiphase plume are then written. The ODEs is studied both numerically and analytically. Different regimes are analyzed, extracting the first integral of motion and asymptotic solutions. An asymptotic analytical solution approximating the model in the general regime is derived and compared with numerical results. Such a solution is coupled with an electromagnetic model providing the infrared intensity emitted by a volcanic ash plume. Key vent parameters are then retrieved by means of inversion techniques applied to infrared images measured during a real volcanic eruption.

The aim of this thesis is three-fold: i) to investigate the performance of both the Eulerian-Lagrangian model and the Eulerian-Eulerian model to simulate the turbulent gas-particle flow; ii) to investigate the indoor airflows and contaminant particle flows using the Eulerian-Lagrangian model; iii) to develop and validate particle-wall collision models and a wall roughness model for the Eulerian-Lagrangian model and to utilize these models to investigate the effects of wall roughness on the particle flows. Firstly, the Eulerian-Lagrangian model in the software package FLUENT (FLUENT Inc.) and the Eulerian-Eulerian model in an in-house research code were employed to simulate the gas-particle flows. The validation against the measurement for two-phase flow over backward facing step and in a 90-degree bend revealed that both CFD approaches provide reasonably good prediction for both the gas and particle phases. Then, the Eulerian-Lagrangian model was employed to investigate the indoor airflows and contaminant particle concentration in two geometrically different rooms. For the first room configuration, the performances of three turbulence models for simulating indoor airflow were evaluated and validated against the measured air phase velocity data. All the three turbulence models provided good prediction of the air phase velocity, while the Large Eddy Simulation (LES) model base on the Renormalization Group theory (RNG) provided the best agreement with the measurements. As well, the RNG LES model is able to provide the instantaneous air velocity and turbulence that are required for the evaluation and design of the ventilation system. In the other two-zone ventilated room configuration, contaminant particle concentration decay within the room was simulated and validated against the experimental data using the RNG LES model together with the Lagrangian model. The numerical results revealed that the particle-wall coll ision model has a considerable effect on the particle concentration prediction in the room. This research culminates with the development and implementation of particle-wall collision models and a stochastic wall roughness model in the Eulerian-Lagrangian model. This Eulerian-Lagrangian model was therefore used to simulate the gas-particle flow over an in-line tube bank. The numerical predictions showed that the wall roughness has a considerable effect by altering the rebounding behaviours of the large particles and consequently affecting the particles motion downstream along the in-line tube bank and particle impact frequency on the tubes. Also, the results demonstrated that for the large particles the particle phase velocity fluctuations are not influenced by the gas-phase fluctuations, but are predominantly determined by the particle-wall collision. For small particles, the influence of particle-wall collisions on the particle fluctuations can be neglected. Then, the effects of wall roughness on the gas-particle flow in a two-dimensional 90-degree bend were investigated. It was found that the wa ll roughness considerably altered the rebounding behaviours of particles by significantly reducing the 'particle free zone' and smoothing the particle number density profiles. The particle mean velocities were reduced and the particle fluctuating velocities were increased when taking into consideration the wall roughness, since the wall roughness produced greater randomness in the particle rebound velocities and trajectories.

An experimental and theoretical study of the particle trajectories in a gas turbine intake has been presented. computer model was written to simulate a particle behaviour flight in a theoretical flow which was assumed to inviscid, irrotational and incompressible. The model is also on other assumptions which imposes several limitations the accuracy of the predicted results. These limitations the objectives of the experimental investigation of particle trajectories which was carried out in a 30.0 section of an axisymmetric helicopter inertial separator. The separator section was fully instrumented with pressure tappings to determine the near-wall flow condition. The flowfield at the central (vertical) plane of separator was also measured with a two spot laser anemometer. The dust particles used in the tests were the spherical ballotini and irregular quartz particles with diameter ranging f-rom 15.0 to 150.0 microns. These particles seeded locally into the separator at three initial positions. The restitution ratios for the quartz particle based on experimental data and the ballotini particle's were based on a simple relation, which was derived by and error matching of predicted and experimental results. The particle trajectories, velocities and angles in separator were measured at several stations using the anemometer. The measured results were compared with predicted values from the model which has been modified accept both the experimentally measured and inviscid flowfield. The particle shape factor was also included to account for the higher drag on the non-spherical particle. Further modification was also made to include the restitution ratios of the ballotini particle. Good agreement found between measured and predicted particle trajecto- velocities and angles for both the spherical and non- spherical particle. The trajectories of the large particles (>100. Oum) are ballistic' in nature which are governed by the inertia forces. The trajectories of the smaller particles are influenced by the both aerodynamic and inertia forces.

Physics
Ph.D.
This work presents results on the study of the scintillation of high-pressure Xenon gas irradiated by various sources. Noble gases such as Xenon give off characteristic scintillation light when irradiated. The goal of the study was to develop a characteristic based on the scintillation time response of Xenon gas that would reliably discriminate between events from different types of primary radiation (neutron or gamma). A reliable discrimination characteristic would enable the development of room temperature, gas phase detectors for use in the search for Galactic Dark Matter. The surprising result of the present work was that a reliable discrimination characteristic existed for distinguishing x-ray, gamma ray, and alpha particle events. Results for neutrons were negative. This was due to several factors: Ionization tracks in xenon generally form two roughly cylindrical regions. A region near the center of the track, called the core, has very dense ionization. An outer region, called the penumbra, has sparse ionization. In Xenon, recombination of ions and the subsequent scintillation from the penumbra region happens slowly and can be easily distinguished from scintillation that happens in the core region. Nuclear recoils resulting from neutron collisions that give recoil energies in the same range as that predicted for WIMP-nuclear collisions are of such low energy that they do not produce a significant penumbra region in Xenon gas. As such, the scintillation time response for these events is similar to that of high-energy gamma rays. Other results of the present work include: The amount of energy deposited in the gas needed to produce a scintillation photon was measured for gamma rays and was found to be in agreement with results from other experiments. Low-energy gamma rays appeared to produce more scintillation photons for an equal amount of energy deposited than high-energy gamma rays. The decay of the singlet and triplet molecular states of xenon was observed and the lifetimes of these states were measured. The singlet state lifetime was found to be independent of pressure while the triplet state lifetime was dependent on pressure. The lifetimes were measured and compared to previous results. A better understanding of the ionization, recombination, and scintillation processes of gaseous Xenon was achieved. Argon gas has been proposed as an alternative to Xenon gas for use in a high-pressure gas scintillation detector due to its lower mass and its property of forming a core ionization region that is much less dense than the core region of xenon. This substitution may allow for a reliable discrimination characteristic to be developed.
Temple University--Theses

The onset of a new state of matter in the nucleus, whereby the system can be described as a dilute gas of α-particles, represents a fantastic probe of the nuclear force. The observables from such an α-gas state are discussed here in tandem with a discussion of traditional α-cluster structure. An experiment, utilising a high energy compound nucleus reaction to study the ¹²C(¹⁶O,7α) reaction was performed at LNS Catania to search for the signatures of such a state of matter. Direct evidence of near-threshold α-gas states was found to be impeded by the effect of the Coulomb barrier. The break-up of the system into α-particles was seen to exceed the predictions from a statistical decay model of the compound nucleus which was performed. These results were instead compared with the Fermi break-up model which showed a greater agreement with the data but the limitations of the model meant an exact agreement was not achieved. No evidence of α-condensation was found in this experiment however an improved experiment where the effect of the Coulomb barrier is no longer important is presented as a future probe of this structure.

Theoretical and experimental studies of particle deposition in turbulent pipe flow have been carried out for over forty years, but some of the most important transport mechanisms are still not well understood. The first part of this thesis is concerned with the calculation of particle density when using Lagrangian methods to predict inertial particle transport in two-dimensional laminar fluid flows. Traditionally, Lagrangian calculations involve integrating the particle equations of motion along particle pathlines, and the particle density is obtained by applying a statistical averaging procedure to those pathlines which intersect a particular computational grid cell. Unfortunately, extremely large numbers of particles are required to reduce the statistical errors to acceptable levels, and this makes the method computationally expensive. Recently, the Full Lagrangian approach has been developed, which allows the direct calculation of the particle density along particle pathlines. This method had previously been applied only to simple analytical flow fields. The application of the method to CFD generated fluid velocity fields was shown to be possible, and the results obtained using the Full Lagrangian approach were compared to those from a traditional Lagrangian approach. It was found that better quality solutions could be obtained with the use of far fewer particle pathlines. An analysis of the manner in which the Full Lagrangian approach deals with particles whose paths cross each other (and the resulting discontinuity in particle density) was also undertaken, and this illustrates the sophistication of the method. The second part of the thesis comprises an experimental and theoretical study of the deposition of small particles in turbulent flows by thermophoresis. Thermophoresis is the phenomenon whereby small particles suspended in a gas in which there exists a temperature gradient experience a force in the direction opposite from that of the temperature gradient. Previous researchers have attempted to impose a radial temperature difference in pipe flow experiments, but have not yet succeeded in attaining a constant thermophoretic force along the length of the pipe. This limits the accuracy and usefulness of the data for the validation of theoretical expressions for the thermophoretic fluxes. An experimental rig has been designed to achieve a constant thermophoretic force. This was done by using an annular geometry with a cold inner wall and hot outer wall. The particle size was varied and the deposition flux was measured for turbulent flow with three temperature differences. The deposition fluxes for small particles were found to be independent of dimensionless particle size, with each increase in temperature difference resulting in an increase in magnitude of the flux. Evidence of a thermophoresis-turbulence coupling was found for intermediate-sized particles, and large particles were not influenced by thermophoresis. A theory of particle deposition, developed for the case of turbulent pipe flow, was modified to study flow in a turbulent annulus, so that theoretical expressions for the thermophoretic fluxes could be included and compared with the experimental results. Agreement with experimental data was quite good, but some deficiencies in a widely used theoretical expression for the thermophoretic flux were exposed. An alternative expression was used, which gave much better agreement with the experimental data, and the mechanisms behind the thermophoresis-turbulence coupling were also investigated. The validation of this expression for the thermophoretic force will allow its inclusion in numerical studies of particle deposition in more complex geometries.

In Victoria, Australia, brown coal is utilised as a major source of energy for the power generation industry. Victorian and South Australian brown coals have a very high moisture content and therefore, the efficiencies of power generation in traditional pulverised fuel fired furnaces are low. Fluidised beds offer a number of advantages over conventional furnaces, leading to improvements in efficiency and environmental impact. A disadvantage with implementing fluidised bed technology is the issue of scale-up. Fluidised bed behaviour can alter significantly with changes in scale, because of their strong dependence on the bed hydrodynamics. Hence, there is a need to accurately model bed behaviour to ensure that the effect of changes in scale are well understood and will not become costly and time consuming. Computational Fluid Dynamics (CFD) techniques can be applied to fluidised bed systems to gain a better understanding of the hydrodynamic behaviour involved. In the past, numerical models have considered only single particle sizes due to the added complexity of interaction between particles of differing sizes and densities. Industrial fluidised beds typically contain more than one particle size and density, therefore there is a need to develop a numerical model which takes this into account. The aim of this thesis is to develop and validate CFD techniques for modelling the behavior of a gas-solid fluidised bed containing more than one particle size and density. To provide validation data for the numerical model, physical experiments are undertaken on a small two-dimensional bubbling gas-solid fluidised bed. Mixing and segregation behaviour of different materials are investigated. The experiments demonstrate that whilst only a small proportion of the bed consists of different size/density particles, significant changes in bed behaviour are apparent. Changes in bubble rise velocity, bubble size and bubble shape are observed. A number of constitutive equations must be included in the numerical model, including relationships for the momentum transfer between various phases and solids pressure. Different combinations of these constitutive equations are investigated. A new equation for particle-particle interactions is derived and included in a CFD model. The CFD model is validated against both data in the literature and physical experiments. From the validation studies, an optimum equation set is identified. This optimum equation set produces numerical results that closely resemble experimental bed behaviour, thus bringing the goal of solving scale-up problems one step closer. The use of this type of CFD model will ultimately result in timely and cost effective solutions for both the power generation and chemical processing industries.

An experimental study of the particle-to-metal interaction during high temperatures and velocity impact conditions is presented. A novel continuous erosion testing facility have been used to study the effect of particle and metal target temperatures as well as impact particle velocity on the erosion/deposition behaviour of the stainless steel 321, Nimonic 75, and aluminium target materials. The study was carried out to provide database information on the behaviour of those metals under simulated gas turbine conditions. The erosive particles used were quartz sand with diameters ranging from 20-30 μm. The erosion characteristics of stainless steel 321 were recorded at target surface temperature of 285°C, 415°C, 570°C and 715°C. The tests were carried out at two different impingement angles of 30° and 60° and at particle impact velocities of up to 300m/s. The effects of particle temperatures of 550°C, 750°C and 950°C on erosion/deposition rates were examined. The Nimonic 75 target temperatures were slightly modified to give a similar surface to melting point ratio as the stainless steel. The Nimonic 75 was tested at 545°C, 685°C, 825°C and 965°C surface temperatures and at the same particle velocities and temperature used for the stainless steel tests. The Nimonic targets were only tested at one impact angle of 30°. The aluminium targets were only tested at an impact angle of 60° and particle impact velocity of 100 m/s. The surface temperature was modified to give a ratio up to 0.8 of the melting point temperature, where the particle temperature was set to be 350°C, 550°C and 750°C. It was found that particle and target temperatures, impact velocity and angle have a significant effect on the erosion/deposition characteristics. There is a threshold target and particle temperature for which deposition begins, and it depends on impact velocity and angle. The Nimonic 75 targets exhibit a better resistance to particle deposition over the stainless steel 321 at high impact velocity and temperatures. Simple models of the erosion/deposition were established to describe the conditions of particle deposition on the stainless steel and Nimonic targets. The aluminium targets show an increase in the erosion rate as target temperature reaches certain level, which then drops as target temperature continues to increase beyond this point.

Propulsion systems are exposed to a variety of foreign objects that can significantly damage or impact their performance. These threats can range from severe dangers such as sandstorms and volcanic eruptions, which can induce engine failure in minutes, to condensation and moisture during ground tests that can negatively impact the engine's fuel efficiency. While numerous computational and experimental studies have investigated the effects of particle ingestion on the component level, an accurate in-situ measurement technique is needed for a systematic understanding of the effects and real-time engine health monitoring. Optical measurement techniques are attractive for this application due to their non-intrusive nature. However, conventional optical particle measurement methods assume the particle to be spherical, which introduces large errors for measuring particles with complex and irregular shapes, such as sand, volcanic ash, and ice crystals. The light-particle interaction contains information on the desired parameters, such as particle shape and size. The research presented in this dissertation uses this idea for a novel particle sensor concept. Scattering and extinction of light by particles are chosen as crucial features that can identify the particle as its unique signature. Numerical tools are used to simulate the scattering and extinction for particles the sensor is expected to encounter. Machine learning models are trained using the data to use scattering and extinction as inputs and estimate the particle parameters. Different types and applications of supervised machine learning models were investigated, including a layered approach with numerous models and a generalized approach with a single neural network. The particle sensor is first demonstrated using data found in the literature. This study confirmed the importance of non-spherical particles in the library to guide the machine learning models. Further demonstrations are made at a full engine and wind tunnel scale to measure injected condensation and sand sprays, respectively. The mass flow rates of the ingested material were calculated using the model outputs and validated.
Doctor of Philosophy
Foreign objects ingested into gas turbines can cause serious damage and degrade their performance. Threats can range from sand, dust, and volcanic ash to condensation on ground and high altitude ice crystals. On the component level, experiments and simulations have been performed to establish the performance decrease and risks to continued operations. However, there is a need for a real-time and non-intrusive measurement technique for the ingested mass. While there are established optical methods applicable for this use, these existing methods assume the particle shape to be spherical. The light-particle interaction contains information on the desired parameters, such as particle shape and size. Optical measurements of these interactions, such as scattering and extinction, can serve as "fingerprints" that can be used to estimate particle parameters. A novel particle measurement technique utilizing supervised machine learning models is presented. The models are trained using a library containing numerically calculated scattering and extinction data. Laser scattering and extinction measurements are used as inputs for the models. This new technique is first demonstrated by sizing particles found in a particle scattering database in the literature. The method's versatility and ruggedness are then demonstrated by accurately estimating the volume flow rate of a spray nozzle spraying water into a research engine. Additionally, the mass flow of sand particles is measured using this technique in a high-speed wind tunnel, in a similar environment to an engine inlet.

Thesis (M.S.)--West Virginia University, 2004.
Title from document title page. Document formatted into pages; contains x, 98 p. : ill. (some col.) + 1 video file. Includes a video file (29 sec.). Includes abstract. Includes bibliographical references (p. 91-92).

Experimental and computational investigations of dilute gas/particle flow in a vertical lifter are performed. The effect of superficial gas velocity, particle density, particle size distribution and particle loading on particle velocities, particle fluctuations and particle cross-moment have been studied experimentally using laser Doppler anemometry (LDA) and particle image velocimetry (PIV). The results from the experimental investigation is compared with the computational investigation using FluentR. The experimental measurements are performed on a lab-scale vertical lifter, consisting of a fluidizing silo and a receiving tank with a glass pipe in which the solids phase is transported. The particles are placed in the fluidization tank and transport air enters at the bottom of the silo. The transport pipe is suspended above the inlet and as the transport air passes the opening, the particles are dragged into the air flow and transported upwards to the receiving tank. Fluidizing air is used to control the particle loading in the system and supplied through a distribution plate. The test section of the transport pipe is made of glass to enable the use of the optical laser based investigation techniques, LDA and PIV. Two types of powders are used, ZrO2 and glass, each with two different particle size distributions, average diameter of 260 and 530 micron and 120 and 518 micron, respectively. The experimental techniques LDA and PIV are used to investigate a dilute gas/particle vertical flow. The two techniques are also evaluated for use on this type of flow. LDA is a single point measurement technique, which means that one point is measured at a time. The acquisition stops when a pre-set criteria is reached, this can either be based on sample number or time. A measurement spanning over the whole cross-section of the pipe consists of several points. These points makes up a cross-sectional profile. PIV on the other hand is a whole field technique and consequently the whole cross-section of the pipe is measured simultaneously. Within a given time interval two laser pulses light up the flow and the reflection of the particles is captured by a camera. Satisfactory measurements of all the particle types are performed using LDA. The mean axial and normal particle velocities and fluctuations as well as the cross-moment, are measured at varying particle volume fraction and superficial gas velocity. The value of the measured quantities will vary depending on the particle size, particle density, particle volume fraction and superficial gas velocity. A comparison between the particle types show that the mean axial particle velocity is highest for the lighter and smaller particles, but the fluctuations are greatest for the larger and heavier particles. For smaller particles LDA is a very efficient measurement tool. For the largest particles the acquisition can be time consuming due to relatively few particles in the system. PIV measurements are generally performed satisfactory on all of the particle types. The exception is measurements performed on the smaller particles at the higher particle volume fractions. The mean axial and normal particle velocities and fluctuations including the crossmoments are measured at varying particle volume fraction and superficial gas velocity. The results from the measurements show that the measured quantities will vary depending on the particle size, particle density, particle volume fraction and superficial gas velocity. When comparing the particle types, it is observed that the mean axial particle velocity is highest for the smaller and lighter particles while the fluctuations are lower than for the larger and heavier ones. The combination of particles larger than commonly used tracer particles and higher particle volume fractions is challenging, but overall the PIV technique works well. Comparison between LDA and PIV results show generally a good agreement for the mean axial particle velocity. The mean axial and normal particle fluctuations and the particle crossmoment are generally measured lower when using PIV. Simulations are performed using FluentR and the model Euler-Euler, where both phases are  regarded as being continua. The kinetic theory of granular flow (KTGF) is included for the solids phase. Initially only the single transport pipe is simulated in 2d and 3d. The flow in this pipe is dilute and therefore the simulations which included KTGF and the Gidaspow drag model are compared to simulations enabling the constant viscosity model (CVM) and the Schiller-Naumann drag model. The results from the simulations show very little difference between the two simulations. Euler-Euler with KTGF 2d and 3d simulations are performed for all of the particle types. Little difference between 2d and 3d simulations are observed. A comparison between simulations and experimental results, LDA and PIV, showed good agreement for axial particle velocity for all of the particle types. The upward transport of particles in a vertical pipe is also simulated using Euler-Lagrange. Here a number of particles are tracked and compared to the experiments with good agreement. Simulations of the vertical lifter, a silo containing the particles and a transport pipe, show that simulations using Euler-Euler including KTGF and the Gidaspow drag model over-predicted the particle volume fraction in the pipe compared to the experiments. The reason for this discrepancy is that the experimental set-up is modified to give low particle volume fractions in the transport pipe to enable the use of lasers to investigate the flow.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2004.
Includes bibliographical references (p. 290-300).
The performance of coated fuel particles is essential for the development and deployment of High Temperature Gas Reactor (HTGR) systems for future power generation. Fuel performance modeling is indispensable for understanding the physical behavior of fuel particles and achieving their high reliability during operations and accidents through a guided design process. This thesis develops an integrated fuel performance model of coated particle fuel to comprehensively study its mechanical behavior and define an optimum fuel design strategy with the aid of the model. Key contributions of the thesis include a pyrocarbon layer crack induced particle failure model with a fracture mechanics approach, mechanical analysis of particles with better representation of irradiation induced creep, a proposed fuel optimization procedure, the capability to simulate arbitrary irradiation histories, and the incorporation of Monte Carlo sampling to account for the statistical variation of particle properties.
(cont.) Stress calculations in this model were benchmarked with the FUEL code and finite element calculations of Idaho National Engineering and Environmental Laboratory (INEEL) and with model predictions for High Temperature Test Reactor (HTTR) first- loading fuel. Fuel failure predictions were made for New Production-Modular HTGR irradiated fuel capsules, which show good agreements with experiments. Based on the simulations, it is suggested that in most cases the pyrocarbon crack induced fuel failure mechanism plays a much more important role in fuel performance than the more widely accepted pressure vessel fuel failure mechanism. After the establishment of the model, parametric study was conducted to find out the effects of various input variables on fuel performance, and fuel design and optimization procedure was proposed accordingly. Simulations with optimized fuel configurations demonstrate that superior fuel performance can be achieved with model analysis. The model also prepares interfaces for further improvements on various modules upon arrival of new information.
by Jin Wang.
Ph.D.

This thesis investigates single particle models to describe non catalytic gas-solid reactions. A comparative study was made between the traditional shrinking core model and more detailed continuous models, involving the solution of microscopic balances for the solid and gas phases inside a single porous particle. Such a study proved that in some cases the use of the shrinking core model can lead to severe errors in the prediction of conversion, and that kinetic parameters in SCM are affected by the particle size. Different diffusion models were tested for the continuous model, and the inaccuracy of the Fick law compared to multicomponent Stefan-Maxwell was evaluated, depending on the concentration of the reaction gas in the mixture. The thesis also proved that natural convection inside the particle can be neglected by changing the balance from mass to molar basis or vice versa, depending on the type of reaction considered. An equation for the local particle porosity was also included, to account for the local changes of gas diffusivity as effect of the reaction. The effect of the pore size distribution was studied, by writing the particle model as a population balance, including different diffusive resistances for different pore sizes, for the cases when Knudsen or solid state diffusion can be important. Sintering phenomena were included, by extending the grain model with an empiric equation. Simulations with simultaneous gas solid reactions were performed, also considering non uniform initial distributions of the solid phases inside the particle: sensitivity studies proved that the position of the solid reagents in the particle may have a great influence on the model results, even when intra particle diffusion is fast compared to the chemical reactions. Gas-solid models were also used to simulate real processes. In particular, thanks to collaboration with an industrial research project, a kinetic study with a CFD model was developed, applying a shrinking core model to simulate real reactors for the direct reduction of iron ores with syn gas at high temperature and pressure. Finally, thanks to the collaboration with the Technical University of Eindhoven, a continuous model was used to simulate reactions of reduction of iron-titanium oxides in chemical looping combustion processes, comparing the results with experimental data.
Questa tesi investiga modelli di singola particella per descrivere reazioni gas-solido non catalitiche. E’ stato fatto uno studio comparativo fra il tradizionale shrinking core model e modelli continui più dettagliati che comprendono la risoluzione dei bilanci microscopici per le fasi gas e solida dentro una singola particella porosa. Tale studio ha provato che in alcuni casi lo shrinking core model può condurre ad errori importanti nella predizione della conversione, e che i parametri cinetici nel SCM dipendono dalla dimensione della particella. Sono stati testati diversi modelli di diffusione all’interno del modello continuo, e la non accuratezza della legge di Fick rispetto alla Stefan-Maxwell multicomponente è stata valutata, a seconda della concentrazione del gas reagente nella miscela. La tesi prova anche che la convezione naturale all’interno della particella può essere trascurata cambiando i bilanci da massivi a molari o vice versa, a seconda del tipo di reazione considerata. Un’equazione che descrive la porosità locale della particella è stata inclusa al modello, per tener conto dei cambiamenti della diffusività effettiva del gas per effetto della reazione. L’effetto della distribuzione della dimensione dei pori è stato investigato, riscrivendo il modello di particella come bilancio di popolazione, includendo diverse resistenze diffusive per diverse dimensioni dei pori, per i casi in cui La diffusione di Knudsen o la diffusione in stato solido possono essere importanti. Fenomeni di sinterizzazione sono stati inclusi, estendendo il tradizionale grain model con un’equazione empirica. Sono state fatte simulazioni di reazioni gas solido con più reazioni, considerando anche distribuzioni disomogenee delle fasi solide all’interno della particella: studi di sensitività hanno dimostrato che la posizione dei reagenti solidi nella particella possono avere un effetto importante sui risultati del modello, anche nel caso in cui la diffusione all’interno della particella è veloce rispetto alle reazioni chimiche. Modelli di reazione gas-solido sono stati usati anche per simulare processi reali. In particolare, grazie alla collaborazione con un progetto di ricerca industriale, uno studio cinetico con modelli CFD è stato sviluppato, applicando lo shrinking core model per simulare reattori reali per la riduzione diretta di ossidi di ferro con gas di sintesi ad alte temperature e pressioni. Infine, grazie alla collaborazione con l’Università Tecnica di Eindhoven, un modello continuo è stato usato per simulare reazioni di riduzione di ossidi di ferro-titanio in processi di chemical looping combustion, confrontando i risultati con i dati sperimentali.

This thesis focuses on the development and implementation of an advanced numerical model to investigate complex fluid flow behaviour through novel metal foam heat exchangers used in various industrial applications such as computer heat sinks and air-conditioners. The developed numerical model permits engineers to better optimize heat exchanger designs. Moreover, the project delves into heat exchanger fouling which is a multifaceted issue in the industry. In this regard, a non-toxic and cost-effective anti-fouling heat exchanger fouling is proposed.