Inaug.-Diss.--Bonn, 2000. / Includes bibliographical references (p. [119]-124).

Optimization of the performance of absorption systems during transient operations such as start-up and shut-down is particularly important for small-capacity chillers and heat pumps to minimize lifecycle costs. Dynamic models in the literature have been used to study responses to step changes in a single parameter, but more complex processes such as system start-up have not been studied in detail. A robust system-level model for simulating the transient behavior of an absorption chiller is developed here. Individual heat and mass exchangers are modeled using detailed segmental models. The UA-values and thermal masses of heat exchangers used in the model are representative of a practical operational chiller. Thermal masses of the heat exchangers and energy storage in the heat exchanging fluids are accounted for to achieve realistic transient simulation of the heat transfer processes in the chiller. The pressure drop due to fluid flow across the heat exchangers is considered negligible in comparison to the pressure difference between the high- and low-side components (~ 1.5 MPa). In components with significant mass transfer effects, reduced-order models are employed to decrease computational costs while also maintaining accurate system response. Mass and species storage in the cycle are modeled using storage devices. The storage devices account for expansion and contraction of the refrigerant and solution in the cycle as the system goes through start-up, shut-down, and other transient events. A counterflow falling film desorber model is employed to account for the heat and mass transfer interactions between the liquid and vapor phases, inside the desorber. The liquid film flows down counter to the rising vapor, thereby exchanging heat with the counterflowing heated coupling fluid. A segmented model is used to account for these processes, and a solver is developed for performing rapid iteration and quick estimation of unknown vapor and liquid states at the outlet of each segment of the desorber. Other components such as the rectifier, expansion valves and solution pump are modeled as quasi-steady devices. System start-up is simulated from ambient conditions, and the coupling fluid temperatures are assumed to start up to their steady-state values within the first 90 s of simulation. It is observed that the system attains steady-state in approximately 550 s. The evaporator cooling duty and COP of the chiller during steady-state are observed to be 3.41 kW and 0.60, respectively. Steady-state parameters such as flow rates, heat transfer rates and concentrations are found to match closely with results from simulations using corresponding steady-state models. Several control responses are investigated using this dynamic simulation model. System responses to step changes in the desorber coupling fluid temperature and flow rate, solution pumping rate, and valve setting are used to study the effects of several control strategies on system behavior. Results from this analysis can be used to optimize start-up and steady state performances. The model can also be used for devising and testing control strategies in commercial applications.

Efficient heat management in energy intensive applications such as server and data centers has become a national concern due to the magnitude of the energy consumed. In that matter, the absorption refrigeration system is an attractive solution because the abundant waste heat available in the data centers can be recycled to run the heat pump, which will bring about significant cooling cost savings. The use of absorption refrigeration has been limited due to the drawbacks related to the working fluids in commercially available equipment. Recently, ionic liquids (ILs) have been suggested as the absorbent in absorption heat pumps due to their tunable properties, negligible volatility and high thermal stability. The non-random-two-liquid-model was initially used to analyze the feasibility of the new IL based working fluid. Hydrofluorocarbons (HFCs) were paired with IL absorbents due to their good properties as refrigerants. The cooling-to-total-energy (CE) efficiency had a local maximum with respect to desorber temperature due to the solubility limit at lower temperatures and large heating requirements at higher temperatures. The waste heat recycling coefficient of performance (COP) continually increased with respect to desorber temperature and among the HFCs studied in this work, R134 gave the highest COP value, which is up to 40 times higher than that of typical vapor compression systems and 60 times higher than NH3/H2O and H2O/LiBr absorption refrigeration systems. A Redlich-Kwong equation of state (RK-EOS) was employed for accurate computation of mixture properties over a wide range of operating conditions. Analysis using the RK-EOS model showed that the CE trend in refrigerants followed the trend of solubility in the [bmim][PF6] IL. However, the trend in COP was different from that of CE as the operating pressure ranges became an important factor. Required pumping work of the working fluids has also been analyzed using a two phase pressure drop equation and the results show that the impact of viscous IL flow is insignificant compared to the total pumping work. The HFCs studied in this work have very similar structures. However, the extent of solubility and system efficiency in the same IL, [bmim][PF6], made a large difference. Most surprisingly, even when the refrigerant had the same chemical formula, the change in fluorine position in tetrafluoroethane showed significantly different system performance. The symmetrical tetrafluoroethane had superior CE and COP over the asymmetrical tetrafluoroethane most likely due to the higher probability to form hydrogen bonding with the absorbent. The computational results for various HFC/IL pairs show that in selecting the working fluid pairs, the refrigerant should have high overall solubility in the IL and a large gradient of solubility with respect to temperature. Also, refrigerants with small pressure ranges are preferred. In addition to the simulation study, a bench-top absorption refrigeration system was built and operated using IL based working fluids for the first time. The effect of cooling was observed by operating the test system. The experimental results were congruent with the predictions from the modeling work. In conclusion, an absorption refrigeration system based on the IL chemical compressor has been shown to be a promising solution in applications which need efficient cooling and generate abundant waste heat.

Layered metal sulfides MoS2 and WS2 exhibit highly anisotropic surface chemistry. Adsorption of molecules is stronger on the atomic layer edges than on atomic planes. The edges are catalytically active in the petroleum hydrodesulfurization, while the layer planes are inert. Dispersing MoS2 and WS2 on the nanometer scale can also lead to the onset of photocatalytic properties due to the bandgap tuning by quantum confinement. In this work, we aim at determining how the adsorption on surface sites is altered for the nanoparticles compared to the bulk sulfides (micron-sized particles). A comparative study of the MoS2 and WS2 nanoparticles and bulk materials is done by attempting the adsorption of small molecules (N2, CO, acetone, and acetonitrile) to probe the surface sites. MoS2 and WS2 nanoparticles were synthesized by thermal decomposition of the metal hexacarbonyls in presence of sulfur in high-boiling solvents. The size range is 5-30 nm from Transmission Electron Microscopy. Transmission Infrared Spectroscopy was used to monitor the spectra of the probe molecules. A dedicated experimental setup has been constructed that consists of a high-vacuum chamber with a base pressure of 5×10-7 Torr. At the lowest achievable temperature of the sample (-145°C), N2, CO, and acetone were found to not adsorb strongly enough to be retained in vacuum on these materials. Acetonitrile was found to adsorb on these materials at -145°C and to desorb between -90°C and -50°C. The nanomaterial samples adsorbed significantly more acetonitrile than the corresponding bulk sulfides, as judged by the infrared signals intensity. Qualitatively, adsorbed acetonitrile species on nanodispersed and bulk sulfides are the same. It is likely that most of the adsorbed acetonitrile observed is physisorbed as ice or adsorbed on the sulfur-terminated terraces. At the final stages of desorprtion, distinctly different adsorbed species are seen whose CN stretching IR bands are shifted to higher frequencies. It is likely that these minority species are at monolayer or submonolayer coverages. The exact nature of the species requires further studies. / Department of Chemistry

Access to abstract permanently restricted to Ball State community only / Construction of experimental setup -- Synthesis of MoS₂ and WS₂ fullerene like structures -- Synthesis of MoS₂ and WS₂ nanotube like structures -- Infrared spectroscopy of acetonitrile adsorption on synthesized MoS₂ materials. / Access to thesis permanently restricted to Ball State community only / Department of Chemistry

No description available.

There are high demands on flexographic print quality to be sufficiently high and consistent in order to create a competitive packaging. At the same time the production efficiency need to be high. Printers thus need to achieve the same quality every time and quickly start-up new printing jobs. To accomplish this, one needs to gain a thorough understanding of how the liquid packaging board interacts with the ink and impacts the print quality. This thesis focuses on water-based ink absorption of liquid packaging boards and particularly on a) how uniformity of ink absorption can be measured and b) to what extent the absorption characteristics contribute to print mottle in flexographic printing. The work encompasses two parts. First, an absorption non-uniformity test method has been developed using a staining technique. This method is unique as it measures how unevenly an aqueous solution is absorbed, in a short time period and without impact from surface roughness. Moreover, the contributions from white-top mottle and absorption non-uniformity can be quantified simultaneously from one single measurement. Second, a method to independently study the effects of absorption non-uniformity on print quality has been established. This is achieved by introducing artificial absorption non-uniformities with well-controlled barrier patterns. A barrier pattern may modify local pore structure and/or surface energy, hence lead to lateral absorption variations. By these means, it is possible to produce a substrate property-matrix; encompassing absorption non-uniformity and for example surface roughness. It was demonstrated that non-uniform absorption indeed has a negative impact on print quality, both on smoother and rougher boards. Low absorption made print density decrease and uneven absorption caused print mottle. This was the case when other properties of the samples were kept within a narrow range; otherwise surface roughness appeared to determine print mottle ranking.

Hydrogen-related defects in single-crystal ZnO and rutile TiO2 are investigated by means of infrared absorption, Raman scattering, photoluminescence and photoconductivity. Four different defect centers in ZnO are considered: bond-centered hydrogen (HBC ), hydrogen bound within the oxygen vacancy (HO), hydrogen molecules, and a defect, which gives rise to a local vibrational mode at 3326 cm−1 . The measurements identify HBC as a shallow donor with an ionization energy of 53 meV. The internal 1s → 2p transition of HBC is detected at 330 cm−1 in the Raman scattering and photoconductivity spectra. The decay of an exciton bound to HBC results in the photoluminescence line at 3360.1 ± 0.2 meV. The local vibrational mode of the O–H bond for bond-centered hydrogen has a frequency of 3611 cm−1 (H-I) and an effective charge of 0.28±0.03e. It is found that bond-centered hydrogen is unstable against annealing at 190 °C due to diffusion and trapping by other defects. The dominant sink is the hydrogen molecule. It is demonstrated that the well-known I4 photoluminescence line at 3362.8 meV is due to the recombination of excitons bound to the HO donor. The ionization energy of the HO donor is determined to be 47 meV. The 1s → 2pz (2pxy) electronic transition of HO is detected at 265 cm−1 in photoconductivity spectra. The formation of HO occurs via trapping of HBC at vacancies left by out-diffusing oxygen. It is shown that sub-band gap illumination leads to an intensity reduction of the O–H local vibrational mode at 3326 cm−1 and the appearance of a previously unreported infrared absorption line at 3358 cm−1. The signals are identified as stretch modes of an O–H bond associated with the same defect in different charge states. The measurements indicate that this defect has a deep level in the band gap of ZnO at roughly Ec − 1.7 eV. Additionally, results on the thermal stability, uniaxial stress response, and temperature dependence of the transition rates between the two charge states of this defect are presented. Interstitial hydrogen in rutile TiO2 is studied by infrared absorption. It is shown that the defect is a shallow donor with an ionization energy of 10 meV. The absorption lines at about 3290 cm−1 consists of local vibrational modes due to the neutral and the positive charge states of the donor with relative intensities depending on the measurement conditions. In the neutral charge state, the defect reveals two modes at 3288.3 and 3292 cm−1 (10 K), whereas the positive charge state has a vibrational mode at 3287.4 cm−1. An unknown hydrogen complex was found to contribute to the 3288 cm−1 feature.

Parmi les procédés de conversion de la biomasse, la gazéification à l’air présente l’avantage d’un rendement de conversion électrique attractif, en particulier pour les petites puissances. Cependant, l’élimination des goudrons générés reste un verrou à lever pour permettre l’émergence industrielle du procédé de gazéification. Le traitement par absorption est considérée comme technologiquement mature mais le choix du solvant de lavage, sa gestion et l’optimisation du laveur représentent un élément crucial. Afin de mettre en avant les solvants envisageables, une méthodologie rapide et robuste de choix de solvant basée sur une analyse multicritère a été développée. La comparaison de plus d’une centaine de solvants conventionnels a permis de mettre en avant deux solvants. Afin d’étudier des performances de ces solvants, un pilote de lavage de type Venturi (laveur compact présentant une aire interfaciale importante et peu de risque d’encrassement) a été dimensionné et caractérisé. L’étude expérimentale sur gaz réel a permis d’une part d’étudier l’évolution de la charge en goudron dans le solvant et d’autre part de mesurer les efficacités de traitement des solvants sur le benzène, le naphtalène, l’anthracène et le pyrène. Ces données expérimentales ont ensuite été utilisées afin de valider la modélisation du laveur Venturi effectuée dans l’optique d’une optimisation et d’une extrapolation à l’échelle industrielle de l’unité de lavage. Afin d’estimer le coût de traitement en €/MWh, le dimensionnement des équipements à l’échelle industrielle a été effectué pour les deux solvants dans différents cas de figure afin d’effectuer leur évaluation technico-économique. / Among biomass conversion processes, air gasification has the advantage of an attractive electrical conversion yield, especially for small powers. However, the elimination of the generated tar remains a lock to lift to allow the industrial emergence of the gasification process. The treatment by absorption is considered to be technologically mature but the choice of cleaning solvent, its management and the optimization of the scrubber represent a crucial element. In order to highlight possible solvents, a fast and robust solvents screening methodology based on a multi-criteria analysis was developed. Comparison of more than 100 of conventional solvents allowed to forward two solvents. To investigate performance of these solvents, a pilot scale Venturi scrubber (compact scrubber with a large Interfacial area and low risk fouling risk) has been designed and characterized. The experimental study on real raw gas allowed to study the evolution of the tar loading factor in the solvent and to measure treatment efficiencies of solvents on benzene, naphthalene, anthracene and pyrene. These experimental data were then used to validate the modelling of the Venturi scrubber in the context of optimization and extrapolation on the industrial scale of the washing unit. In order to estimate the cost of treatment in €/ MWh, the sizing of the equipment on an industrial scale has been made for two solvents in different cases in order to process to a technico-economical evaluation.

Au cours de leur propagation, les ondes sismiques sont atténuées par deux phénomènes : l'absorption causée par les propriétés anélastiques des matériaux, d'une part, et la diffusion ou " scattering " causée par la présence d'hétérogénéités de petite échelle dans le milieu d'autre part. L'objectif de cette thèse est de cartographier les propriétés de diffusion et d'absorption des ondes sismiques dans deux contextes géophysiques extrêmes présentant des échelles spatiales très différentes. La première partie du manuscrit est consacrée à la stratification d'hétérogénéité dans la Lune. À l'aide d'un modèle original de diffusion en géométrie sphérique, nous avons inversé les mesures de temps d'arrivée du maximum d'énergie et de décroissance de la coda sismique réalisée sur les données des missions Apollo. Nos inversions mettent en évidence un très fort contraste des propriétés de scattering entre le mégarégolithe très atténuant et le manteau lunaire profond transparent. L'atténuation est très largement dominée par le scattering et suggère la présence de fracturation jusqu'à environ 100 km de profondeur, affectant ainsi le manteau. Une nouvelle méthode d'estimation de la profondeur des séismes superficiels fondée sur les signaux diffus a été développée et permet de confirmer l'existence de failles actives autour de 50 km de profondeur. La deuxième partie de la thèse est consacrée à la structure d'atténuation de Taïwan, une région qui présente des structures géologiques très variées dans un contexte tectonique de double subduction. On utilise la MLTWA (Multiple Lapse Time Window Analysis) -une méthode fondée sur le rapport entre énergie cohérente et incohérente du signal sismique- pour imager les variations latérales d'atténuation. Dans un premier temps nous avons travaillé dans l'hypothèse classique de diffusion isotrope dans un demi-espace. Nos résultats mettent en évidence un niveau d'atténuation globale très élevé ainsi que de forts contrastes des propriétés de scattering sur des échelles spatiales fines, de l'ordre de 10-20 km. La diffusion est particulièrement marquée dans les bassins de la côte ouest, le sud et la chaîne côtière associée à la collision avec l'arc volcanique de Luçon à l'est. L'absorption augmente graduellement vers l'est et atteint son maximum sous l'arc volcanique. L'examen de l'accord entre données et modèles a posteriori montre sans ambiguïté les limites de l'hypothèse de diffusion isotrope dans un demi-espace sur un ensemble de stations situées le long des côtes. Ceci nous conduit à explorer les effets de la diffusion anisotrope dans un guide d'onde modélisant la croûte. La prise en compte de l'anisotropie améliore significativement l'accord du modèle aux données. En particulier, à basse fréquence (1-2 Hz), notre étude démontre la prédominance de rétro-diffusion. Ce résultat est compatible avec la présence de forts contrastes d'impédance dans la croûte et suggère la présence massive de fluides dans les zones de failles et de volcans à Taïwan. La mesure de l'anisotropie de la diffusion ouvre des perspectives nouvelles de caractérisation des hétérogénéités géophysiques de petite échelle. / During their propagation, seismic waves are attenuated by two phenomena: on one hand, absorption caused by the anelastic properties of the materials, and on the other hand, scattering caused by the presence of small-scale heterogeneities in the medium. The aim of this thesis is to map the properties of scattering and absorption of seismic waves in two extreme geophysical contexts with very different spatial scales. The first part of this memoir is devoted to the stratification of heterogeneities in the Moon. We use a new diffusion model in spherical geometry to invert measurements of the time of arrival of the maximum of energy and the seismic coda decay on data from the Apollo missions. Our inversions provide evidence of a very sharp contrast of scattering properties between the highly attenuating megaregolith and the transparent deep lunar mantle. Attenuation is largerly dominated by scattering and suggests the presence of fractures down to about 100 km depth, into the mantle. A new method for estimating the depth of shallow moonquakes based on diffusive signals was developped and confirms the existence of active faults around 50 km deep. The second part of this thesis is devoted to the attenuation structure of Taiwan, a region with a wide variety of geological structures in the context of two subduction zones. We use the MLTWA (Multiple Lapse Time Window Analysis) -a method based on the ratio between the coherent and incoherent energy of the seismic signal- to image the lateral variations of attenuation. We worked first with the classical hypothesis of isotropic scattering in a half-space. Our results provide evidence for a globally high level of attenuation with sharp contrasts of scattering properties across small spatial scales, of the order of 10-20 km. Scattering is particularly strong in the basins of the west coast, southern Taiwan and the eastern Coastal Range associated with the collision with the Luzon volcanic arc. Absorption increases gradually eastwards and reaches a maximum below the volcanic arc. A posteriori examination of the fit between data and model shows unambiguously the limits of the hypothesis of isotropic scattering in a half-space for a number of stations located along the coasts. This leads us to explore the effects of anisotropic scattering in a guide for seismic waves representing the crust. Taking anisotropy into account significantly improves the fitness of the model to the data. In particular, at low frequency (1-2 Hz), our study shows the prevalence of backscattering. This result is compatible with the presence of sharp contrasts of impedance in the crust and suggests the strong presence of fluids in fault zones and volcanoes in Taiwan. The measurement of scattering anisotropy opens new perspectives for characterizing small-scale geophysical heterogeneities.