Analysis and validation of space averaged drag model for numerical simulations of gas-solid flows in fluidized beds

This thesis presents an approach for formulating and validating a space averaged drag model for coarse mesh simulations of gas-solid flows in fluidized beds using the two-fluid model. Proper modeling for fluid dynamics is central in understanding any industrial multiphase flow. The gas-solid flows in fluidized beds are heterogeneous and usually simulated with the Eulerian description of phases. Such a description requires the usage of fine meshes and small time steps for the proper prediction of its hydrodynamics. Such constraint on the mesh and time step size results in a large number of control volumes and long computational times which are unaffordable for simulations of large scale fluidized beds. If proper closure models are not included, coarse mesh simulations for fluidized beds do not give reasonable results. The coarse mesh simulation fails to resolve the mesoscale structures and results in uniform solids concentration profiles. For a circulating fluidized bed riser, such predicted profiles result in a higher drag force between the gas and solid phase and also overestimated solids mass flux at the outlet. Thus, there is a need to formulate the closure correlations which can accurately predict the hydrodynamics using coarse meshes. This thesis uses the space averaging modeling approach in the formulation of closure models for coarse mesh simulations of the gas-solid flow in fluidized beds using Geldart group B particles. In the analysis of formulating the closure correlation for space averaged drag model, the main parameters for the modeling were found to be the averaging size, solid volume fraction, and distance from the wall. The closure model for the gas-solid drag force was formulated and validated for coarse mesh simulations of the riser, which showed the verification of this modeling approach. Coarse mesh simulations using the corrected drag model resulted in lowered values of solids mass flux. Such an approach is a promising tool in the formulation of appropriate closure models which can be used in coarse mesh simulations of large scale fluidized beds.

Modeling and Control of the Power Conversion Unit in a Solid Oxide Fuel Cell Environment

In this doctoral thesis, a power conversion unit for a 10 kWsolid oxide fuel cell is modeled, and a suitable control system is designed. The need for research was identified based on an observation that there was no information available about the characteristics of the solid oxide fuel cell from the perspective of power electronics and the control system, and suitable control methods had not previously been studied in the literature. In addition, because of the digital implementation of the control system, the inherent characteristics of the digital system had to be taken into account in the characteristics of the solid oxide fuel cell (SOFC). The characteristics of the solid oxide fuel cell as well the methods for the modeling and control of the DC/DC converter and the grid converter are studied by a literature survey. Based on the survey, the characteristics of the SOFC as an electrical power source are identified, and a solution to the interfacing of the SOFC in distributed generation is proposed. A mathematical model of the power conversion unit is provided, and the control design for the DC/DC converter and the grid converter is made based on the proposed interfacing solution. The limit cycling phenomenon is identified as a source of low-frequency current ripple, which is found to be insignificant when connected to a grid-tied converter. A method to mitigate a second harmonic originating from the grid interface is proposed, and practical considerations of the operation with the solid oxide fuel cell plant are presented. At the theoretical level, the thesis discusses and summarizes the methods to successfully derive a model for a DC/DC converter, a grid converter, and a power conversion unit. The results of this doctoral thesis can also be used in other applications, and the models and methods can be adopted to similar applications such as photovoltaic systems. When comparing the results with the objectives of the doctoral thesis, we may conclude that the objectives set for the work are met. In this doctoral thesis, theoretical and practical guidelines are presented for the successful control design to connect a SOFC-based distributed generation plant to the utility grid.

Determination of chlorophenols from water by solid phase microextraction - ion mobility spectrometry (SPME-IMS)

Chlorophenols have been classified as possible carcinogens for humans. Chlorophenols have been used as pesticides and wood preservatives. In Finland, during 1930 – 1980s, saw mills used KY-5 wood preservative that contained 2,4,6-TCP, 2,3,4,6-TeCP and PCP. Especially in Finland chlorophenols have entered the environment by leaking from contaminated grounds of old saw mills. Although chlorophenol concentrations found in environment do not cause acute concern, long term exposure can increase the risk of cancer. SPME is relatively cheap and simple sampling method, in which the sample extraction and concentration are performed in a single step. Solvents are not required in SPME. IMS is based on the detection of sample ion drift times. Based on the drift times, reduced mobilities are calculated, which are comparable despite the measurement conditions. SPME-IMS coupling has not been used earlier in the determination of chlorophenols from water samples. The scope of this work was to study, if SPME-IMS system is suitable for detecting chloro-phenols from water samples. The aim was to determine the most optimal extraction condi-tions, which were then applied to real water samples. Following detection limits were deter-mined: 2,4,6-TCP: 0.33 mg/l; 2,3,4,6-TeCP: 0.63 mg/l and PCP: 1.63 mg/l. Detection limits were high compared to the highest possible chlorophenol concentration that is allowed in Finnish drinking water, 10 μg/l. Detected concentrations from water sample differed from verified concentrations in the case of 2,3,4,6-TeCP by 4.6 % and in the case of 2,4,6-TCP by 48.4 %. Based on the results it can be said that SPME-IMS setup is suitable for preliminary analysis of mg/l chlorophenol concentrations from water samples.

Characterisation of Laser Beam and Paper Material Interaction

It is known already from 1970´s that laser beam is suitable for processing paper materials. In this thesis, term paper materials mean all wood-fibre based materials, like dried pulp, copy paper, newspaper, cardboard, corrugated board, tissue paper etc. Accordingly, laser processing in this thesis means all laser treatments resulting material removal, like cutting, partial cutting, marking, creasing, perforation etc. that can be used to process paper materials. Laser technology provides many advantages for processing of paper materials: non-contact method, freedom of processing geometry, reliable technology for non-stop production etc. Especially packaging industry is very promising area for laser processing applications. However, there are only few industrial laser processing applications worldwide even in beginning of 2010´s. One reason for small-scale use of lasers in paper material manufacturing is that there is a shortage of published research and scientific articles. Another problem, restraining the use of laser for processing of paper materials, is colouration of paper material i.e. the yellowish and/or greyish colour of cut edge appearing during cutting or after cutting. These are the main reasons for selecting the topic of this thesis to concern characterization of interaction of laser beam and paper materials. This study was carried out in Laboratory of Laser Processing at Lappeenranta University of Technology (Finland). Laser equipment used in this study was TRUMPF TLF 2700 carbon dioxide laser that produces a beam with wavelength of 10.6 μm with power range of 190-2500 W (laser power on work piece). Study of laser beam and paper material interaction was carried out by treating dried kraft pulp (grammage of 67 g m-2) with different laser power levels, focal plane postion settings and interaction times. Interaction between laser beam and dried kraft pulp was detected with different monitoring devices, i.e. spectrometer, pyrometer and active illumination imaging system. This way it was possible to create an input and output parameter diagram and to study the effects of input and output parameters in this thesis. When interaction phenomena are understood also process development can be carried out and even new innovations developed. Fulfilling the lack of information on interaction phenomena can assist in the way of lasers for wider use of technology in paper making and converting industry. It was concluded in this thesis that interaction of laser beam and paper material has two mechanisms that are dependent on focal plane position range. Assumed interaction mechanism B appears in range of average focal plane position of 3.4 mm and 2.4 mm and assumed interaction mechanism A in range of average focal plane position of 0.4 mm and -0.6 mm both in used experimental set up. Focal plane position 1.4 mm represents midzone of these two mechanisms. Holes during laser beam and paper material interaction are formed gradually: first small hole is formed to interaction area in the centre of laser beam cross-section and after that, as function of interaction time, hole expands, until interaction between laser beam and dried kraft pulp is ended. By the image analysis it can be seen that in beginning of laser beam and dried kraft pulp material interaction small holes off very good quality are formed. It is obvious that black colour and heat affected zone appear as function of interaction time. This reveals that there still are different interaction phases within interaction mechanisms A and B. These interaction phases appear as function of time and also as function of peak intensity of laser beam. Limit peak intensity is the value that divides interaction mechanism A and B from one-phase interaction into dual-phase interaction. So all peak intensity values under limit peak intensity belong to MAOM (interaction mechanism A one-phase mode) or to MBOM (interaction mechanism B onephase mode) and values over that belong to MADM (interaction mechanism A dual-phase mode) or to MBDM (interaction mechanism B dual-phase mode). Decomposition process of cellulose is evolution of hydrocarbons when temperature is between 380- 500°C. This means that long cellulose molecule is split into smaller volatile hydrocarbons in this temperature range. As temperature increases, decomposition process of cellulose molecule changes. In range of 700-900°C, cellulose molecule is mainly decomposed into H2 gas; this is why this range is called evolution of hydrogen. Interaction in this range starts (as in range of MAOM and MBOM), when a small good quality hole is formed. This is due to “direct evaporation” of pulp via decomposition process of evolution of hydrogen. And this can be seen can be seen in spectrometer as high intensity peak of yellow light (in range of 588-589 nm) which refers to temperature of ~1750ºC. Pyrometer does not detect this high intensity peak since it is not able to detect physical phase change from solid kraft pulp to gaseous compounds. As interaction time between laser beam and dried kraft pulp continues, hypothesis is that three auto ignition processes occurs. Auto ignition of substance is the lowest temperature in which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. Three auto ignition processes appears in range of MADM and MBDM, namely: 1. temperature of auto ignition of hydrogen atom (H2) is 500ºC, 2. temperature of auto ignition of carbon monoxide molecule (CO) is 609ºC and 3. temperature of auto ignition of carbon atom (C) is 700ºC. These three auto ignition processes leads to formation of plasma plume which has strong emission of radiation in range of visible light. Formation of this plasma plume can be seen as increase of intensity in wavelength range of ~475-652 nm. Pyrometer shows maximum temperature just after this ignition. This plasma plume is assumed to scatter laser beam so that it interacts with larger area of dried kraft pulp than what is actual area of beam cross-section. This assumed scattering reduces also peak intensity. So result shows that assumably scattered light with low peak intensity is interacting with large area of hole edges and due to low peak intensity this interaction happens in low temperature. So interaction between laser beam and dried kraft pulp turns from evolution of hydrogen to evolution of hydrocarbons. This leads to black colour of hole edges.

Side peaks in Nb and NbTi point-contact spectra

Superconductor – normal metal point contacts were investigated, using different combinations of Cu, brass, Nb and NbTi. The resulting spectra contained side peaks. The currents at which these side peaks appeared, depended on the radii of the contacts. For contacts with Nb this dependence was quadratic, while for contacts with NbTi it was linear. Based on this, we argue that the side peaks in the case of the Nb contacts are due to the critical current density being exceeded. In contrast, side peaks of the NbTi contacts are caused by the self-magnetic field exceeding the lower critical field of NbTi. The NbTi contacts did not show the expected contribution from the vanishing Maxwell resistance of the superconductor, a question which remained open.

Carbon Nanotubes as Material for Supercapacitor Electrodes

Electrochemical double-layer supercapacitors have an intermediate position between rechargeable batteries, which can store high amounts of energy, and dielectric capacitors, which have high output power. Supercapacitors are widely suggested to be used in automobiles (recuperation during braking, facilitate engine starting, electric stabilization of the system), industry (forklifts, elevators), hybrid off-road machinery and also in consumer electronics. Supercapacitor electrodes require highly porous material. Typically, activated carbon is used. Specific surface area of activated carbon is approximately 1000 m2 per gram. Carbon nanotubes represent one of prospective materials. According to numerous studies this material allows to improve the properties of supercapacitors. The task of this Master‘s Thesis was to test multiwalled carbon nanotubes and become confident with the testing methods.

Oxidation rates of carbon and nitrogen in char residues from solid fuels

Computational fluid dynamics (CFD) modeling is an important tool in designing new combustion systems. By using CFD modeling, entire combustion systems can be modeled and the emissions and the performance can be predicted. CFD modeling can also be used to develop new and better combustion systems from an economical and environmental point of view. In CFD modeling of solid fuel combustion, the combustible fuel is generally treated as single fuel particles. One of the limitations with the CFD modeling concerns the sub-models describing the combustion of single fuel particles. Available models in the scientific literature are in many cases not suitable as submodels for CFD modeling since they depend on a large number of input parameters and are computationally heavy. In this thesis CFD-applicable models are developed for the combustion of single fuel particles. The single particle models can be used to improve the combustion performance in various combustion devices or develop completely new technologies. The investigated fields are oxidation of carbon (C) and nitrogen (N) in char residues from solid fuels. Modeled char-C oxidation rates are compared to experimental oxidation rates for a large number of pulverized solid fuel chars under relevant combustion conditions. The experiments have been performed in an isothermal plug flow reactor operating at 1123-1673 K and 3-15 vol.% O2. In the single particle model, the char oxidation is based on apparent kinetics and depends on three fuel specific parameters: apparent pre-exponential factor, apparent activation energy, and apparent reaction order. The single particle model can be incorporated as a sub-model into a CFD code. The results show that the modeled char oxidation rates are in good agreement with experimental char oxidation rates up to around 70% of burnout. Moreover, the results show that the activation energy and the reaction order can be assumed to be constant for a large number of bituminous coal chars under conditions limited by the combined effects of chemical kinetics and pore diffusion. Based on this, a new model based on only one fuel specific parameter is developed (Paper III). The results also show that reaction orders of bituminous coal chars and anthracite chars differ under similar conditions (Paper I and Paper II); reaction orders of bituminous coal chars were found to be one, while reaction orders of anthracite chars were determined to be zero. This difference in reaction orders has not previously been observed in the literature and should be considered in future char oxidation models. One of the most frequently used comprehensive char oxidation models could not explain the difference in the reaction orders. In the thesis (Paper II), a modification to the model is suggested in order to explain the difference in reaction orders between anthracite chars and bituminous coal chars. Two single particle models are also developed for the NO formation and reduction during the oxidation of single biomass char particles. In the models the char-N is assumed to be oxidized to NO and the NO is partly reduced inside the particle. The first model (Paper IV) is based on the concentration gradients of NO inside and outside the particle and the second model is simplified to such an extent that it is based on apparent kinetics and can be incorporated as a sub-model into a CFD code (Paper V). Modeled NO release rates from both models were in good agreement with experimental measurements from a single particle reactor of quartz glass operating at 1173-1323 K and 3-19 vol.% O2. In the future, the models can be used to reduce NO emissions in new combustion systems.

A Soft Contact Collision Method For Real-Time Simulation of Triangularized Geometries in Multibody Dynamics

When modeling machines in their natural working environment collisions become a very important feature in terms of simulation accuracy. By expanding the simulation to include the operation environment, the need for a general collision model that is able to handle a wide variety of cases has become central in the development of simulation environments. With the addition of the operating environment the challenges for the collision modeling method also change. More simultaneous contacts with more objects occur in more complicated situations. This means that the real-time requirement becomes more difficult to meet. Common problems in current collision modeling methods include for example dependency on the geometry shape or mesh density, calculation need increasing exponentially in respect to the number of contacts, the lack of a proper friction model and failures due to certain configurations like closed kinematic loops. All these problems mean that the current modeling methods will fail in certain situations. A method that would not fail in any situation is not very realistic but improvements can be made over the current methods.

Probabilistic contact preferences in protein-ligand and protein-protein complexes

Modeller för intermolekulär växelvärkan utnyttjas brett inom biologin. Analys av kontakter mellan proteiner och läkemedelsforskning representerar typiska tillämpningsområden för dylika modeller. En modell som beskriver sådana molekylära växelverkningar kan utformas med hjälp av biofysisk teori, vilket tenderar att resultera i ytterst tung beräkningsbörda även för enkla tillämpningar. Ett alternativt sätt att formulera modeller är att utnyttja stora databaser som innehåller strukturmätningar gjorda med hjälp av till exempel röntgendiffraktion. Då man använder sig av empiriska mätdata direkt, möjliggör en statistisk modell att osäkerheten och inexaktheten i datat tas till hänsyn på ett adekvat sätt, samtidigt som beräkningsbördan håller sig på en rimligare nivå jämfört med kvantmekaniska metoder som i princip borde ge de optimala resultaten. I avhandlingen utvecklades en 3D modell för numerisk undersökning av intermolekulär växelverkan baserad på Bayesiansk statistik. Modellens syfte är att åstadkomma prognoser för det hurdana eller vilka molekylstrukturer prefereras i en given kontext, d.v.s. är mer sannolika inom ramen för interaktion. Modellen testades i essentiella molekyläromgivningar - en liten molekyl vid sin bindningsplats hos ett protein och en gränsyta mellan proteinerna i ett komplex. De erhållna numeriska resultaten motsvarar väl experimentella resultat som tidigare rapporterats i litteraturen, exempelvis kvalitativa bindningsaffiniteter och kemisk kännedom av vissa aminosyrors rumsliga förmågor att utgöra bindningar. I avhandlingen gjordes ytterligare preliminära tester av den statistiska ansatsen för modellering av den centrala molekylära strukturella anpassningsbarheten. I praktiken är den utvecklade modellen ämnad som ett led i en mer omfattande analysmetod, så som en s.k. farmakofor modell. Molekyylivuorovaikutusten mallintamista hyödynnetään laajasti biologisten kysymysten tarkastelussa. Tyypillisiä esimerkkejä sovelluskohteista ovat proteiinien väliset kontaktit ja lääkesuunnittelu. Vuorovaikutuksia kuvaavan mallin lähtökohta voi olla molekyyleihin liittyvä teoria, jolloin soveltamiseen liittyvä laskenta saattaa olla erityisen raskasta, tai suuri havaintojoukko joka on saatu aikaan esimerkiksi mittaamalla rakenteita röntgendiffraktio menetelmällä. Tilastollinen malli mahdollistaa havaintoaineistossa olevan epätarkkuuden ja epävarmuuden huomioimisen, samalla pitäen laskennallisen kuorman pienempänä verrattuna periaatteessa parhaan tuloksen antavaan kvanttimekaaniseen mallinnukseen. Väitöstyössä kehitettiin bayesiläiseen tilastotieteeseen perustuva 3D malli molekyylien välisten vuorovaikutusten laskennalliseen tarkasteluun. Mallin tehtävä on tuottaa ennusteita sen suhteen, minkä tai millaisten molekyylirakenteiden väliset kompleksit ovat etusijalla, toisin sanoen todennäköisempiä, vuorovaikutustilanteessa. Työssä kehitetyn menetelmän toimivuutta testattiin käyttötarkoituksen suhteen olennaisissa molekyyliympäristöissä - pieni molekyyli sitoutumiskohdassaan proteiinissa sekä rajapinta kahden proteiinin välilllä proteiinikompleksissa. Saadut laskennalliset tulokset vastasivat hyvin vertailuun käytettyjä kirjallisuudesta saatuja kokeellisia tuloksia, kuten laadullisia sitoutumisaffiniteetteja, sekä kemiallista tietoa esimerkiksi tiettyjen aminohappojen avaruudellisesta sidoksenmuodostuksesta. Väitöstyössä myös alustavasti testattiin tilastollista lähestymistapaa tärkeän molekyylien rakenteellisen mukautuvuuden mallintamiseen. Käytännössä malli on tarkoitettu osaksi jotakin laajempaa analyysimenetelmää, kuten farmakoforimallia.

Development of porous glass-fiber reinforced composite for bone implants. Evaluation of antimicrobial effect and implant fixation

Cranial bone reconstructions are necessary for correcting large skull bone defects due to trauma, tumors, infections and craniotomies. Traditional synthetic implant materials include solid or mesh titanium, various plastics and ceramics. Recently, biostable glass-fiber reinforced composites (FRC), which are based on bifunctional methacrylate resin, were introduced as novel implant solution. FRCs were originally developed and clinically used in dental applications. As a result of further in vitro and in vivo testing, these composites were also approved for clinical use in cranial surgery. To date, reconstructions of large bone defects were performed in 35 patients. This thesis is dedicated to the development of a novel FRC-based implant for cranial reconstructions. The proposed multi-component implant consists of three main parts: (i) porous FRC structure; (ii) bioactive glass granules embedded between FRC layers and (iii) a silver-polysaccharide nanocomposite coating. The porosity of the FRC structure should allow bone ingrowth. Bioactive glass as an osteopromotive material is expected to stimulate the formation of new bone. The polysaccharide coating is expected to prevent bacterial colonization of the implant. The FRC implants developed in this study are based on the porous network of randomly-oriented E-glass fibers bound together by non-resorbable photopolymerizable methacrylate resin. These structures had a total porosity of 10–70 volume %, of which > 70% were open pores. The pore sizes > 100 μm were in the biologically-relevant range (50-400 μm), which is essential for vascularization and bone ingrowth. Bone ingrowth into these structures was simulated by imbedding of porous FRC specimens in gypsum. Results of push-out tests indicated the increase in the shear strength and fracture toughness of the interface with the increase in the total porosity of FRC specimens. The osteopromotive effect of bioactive glass is based on its dissolution in the physiological environment. Here, calcium and phosphate ions, released from the glass, precipitated on the glass surface and its proximity (the FRC) and formed bone-like apatite. The biomineralization of the FRC structure, due to the bioactive glass reactions, was studied in Simulated Body Fluid (SBF) in static and dynamic conditions. An antimicrobial, non-cytotoxic polysaccharide coating, containing silver nanoparticles, was obtained through strong electrostatic interactions with the surface of FRC. In in vitro conditions the lactose-modified chitosan (chitlac) coating showed no signs of degradation within seven days of exposure to lysozyme or one day to hydrogen peroxide (H2O2). The antimicrobial efficacy of the coating was tested against Staphylococcus aureus and Pseudomonas aeruginosa. The contact-active coating had an excellent short time antimicrobial effect. The coating neither affected the initial adhesion of microorganisms to the implant surface nor the biofilm formation after 24 h and 72 h of incubation. Silver ions released to the aqueous environment led to a reduction of bacterial growth in the culture medium.

Contact with friction using the augmented Lagrangian Method: a conditional constrained minimization problem

This work presents a formulation of the contact with friction between elastic bodies. This is a non linear problem due to unilateral constraints (inter-penetration of bodies) and friction. The solution of this problem can be found using optimization concepts, modelling the problem as a constrained minimization problem. The Finite Element Method is used to construct approximation spaces. The minimization problem has the total potential energy of the elastic bodies as the objective function, the non-inter-penetration conditions are represented by inequality constraints, and equality constraints are used to deal with the friction. Due to the presence of two friction conditions (stick and slip), specific equality constraints are present or not according to the current condition. Since the Coulomb friction condition depends on the normal and tangential contact stresses related to the constraints of the problem, it is devised a conditional dependent constrained minimization problem. An Augmented Lagrangian Method for constrained minimization is employed to solve this problem. This method, when applied to a contact problem, presents Lagrange Multipliers which have the physical meaning of contact forces. This fact allows to check the friction condition at each iteration. These concepts make possible to devise a computational scheme which lead to good numerical results.

Development of a three-dimensional nonlinear viscoelastic constitutive model of solid propellant

A three dimensional nonlinear viscoelastic constitutive model for the solid propellant is developed. In their earlier work, the authors have developed an isotropic constitutive model and verified it for one dimensional case. In the present work, the validity of the model is extended to three-dimensional cases. Large deformation, dewetting and cyclic loading effects are treated as the main sources of nonlinear behavior of the solid propellant. Viscoelastic dewetting criteria is used and the softening of the solid propellant due to dewetting is treated by the modulus decrease. The nonlinearities during cyclic loading are accounted for by the functions of the octahedral shear strain measure. The constitutive equation is implemented into a finite element code for the analysis of propellant grains. A commercial finite element package ‘ABAQUS’ is used for the analysis and the model is introduced into the code through a user subroutine. The model is evaluated with different loading conditions and the predicted values are in good agreement with the measured ones. The resulting model applied to analyze a solid propellant grain for the thermal cycling load.

Gas-solid two-phase flow in the riser of circulating fluidized beds: mathematical modelling and numerical simulation

A mathematical model is developed for gas-solids flows in circulating fluidized beds. An Eulerian formulation is followed based on the two-fluids model approach where both the fluid and the particulate phases are treated as a continuum. The physical modelling is discussed, including the formulation of boundary conditions and the description of the numerical methodology. Results of numerical simulation are presented and discussed. The model is validated through comparison to experiment, and simulation is performed to investigate the effects on the flow hydrodynamics of the solids viscosity.

Two-Phase (Solid-Liquid) Flow in Inclined Pipes

This paper presents an experimental research about the behavior of two-phase flows in inclined pipes. The inclination angle varied from 5° to 45° and the slurry solid concentration varied up to 15%. It was concluded that the head losses of the downward sloping pipe flow are always lower than the head losses of the horizontal flow and these are always lower than the head losses of the upward sloping pipe flow, regardless the concentration and inclination angle. It was possible to develop empirical equations to calculate the head losses of the horizontal flow and the upward and downward sloping pipe flows.