This work analytically solves the governing equations of the laminar film condensation from pure vapors in vertical tubes to find the condensate film thickness. The solution is then extended to turbulent flow conditions for steam. All other relevant quantities are derived from the film thickness solution. For laminar film condensation of quiescent vapors, an exact explicit solution and an approximate closed-form solution were found by using a new definition of the dimensionless film thickness, the Lambert W-function, and a logarithmic function approximation. For laminar mixed-convection film condensation with interfacial shear stress, an approximate closed-form solution was found by using a new definition of the pressure gradient. For turbulent film condensation of steam, correlations of the turbulent vapor and liquid viscosities were formed by asymptotically comparing this approximate laminar closed-form solution to a turbulent flow numerical solution. The present solutions compared very well to published numerical and experimental data.

Thesis (M. Eng.(Mechanical and Aeronautical Engineering))--Universiteit van Pretoria, 2007. / Abstract in English. Includes bibliographical references.

M. Tech. Mechanical Engineering.

Orientador: Luiz Felipe Mendes de Moura / Dissertação (mestrado profissional) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-03T22:37:08Z (GMT). No. of bitstreams: 1 Martins_PaulinoCutrim_M.pdf: 1004109 bytes, checksum: 4a5643b8de366648397d80e7bfc388da (MD5) Previous issue date: 2004 / Mestrado / Refrigeração e Condicionamento Ambiental / Mestre Profissional em Engenharia Mecanica

Intracavity Laser Absorption Spectroscopy (ICLAS) at IR wavelengths offers an opportunity for spectral sensing of low vapor pressure compounds. We report here an ICLAS system design based on a quantum cascade laser (QCL) at THz (69.9 m) and IR wavelengths (9.38 and 8.1 m) with an open external cavity. The sensitivity of such a system is potentially very high due to extraordinarily long effective optical paths that can be achieved in an active cavity. Sensitivity estimation by numerical solution of the laser rate equations for the THz QCL ICLAS system is determined. Experimental development of the external cavity QCL is demonstrated for the two IR wavelengths, as supported by appearance of fine mode structure in the laser spectrum. The 8.1 m wavelength exhibits a dramatic change in the output spectrum caused by the weak intracavity absorption of acetone. Numerical solution of the laser rate equations yields a sensitivity estimation of acetone partial pressure of 165 mTorr corresponding to ~ 200 ppm. The system is also found sensitive to the humidity in the laboratory air with an absorption coefficient of just 3 x 10-7 cm -1 indicating a sensitivity of 111 ppm. Reported also is the design of a compact integrated data acquisition and control system. Potential applications include military and commercial sensing for threat compounds such as explosives, chemical gases, biological aerosols, drugs, banned or invasive organisms, bio-medical breath analysis, and terrestrial or planetary atmospheric science.

Numerous findings have indicated that the distance between the evaporator and condenser of the molecular still is not as critical, as far as mean free-path design criteria are concerned, as formerly believed. It had been recommended that studies be made on the present apparatus at Virginia Polytechnic Institute to determine exactly how critical this factor is. The purpose of this investigation was to study the effect of condenser temperature and location on the molecular distillation characteristics of stearic acid, and from this study, determine this relationship of condenser temperature and location to be used in centrifugal molecular still operation. This investigation was accomplished using a magnetically driven centrifugal molecular still with a five-inch rotor and employing the following operating conditions: operating pressure, 24 ± 2 microns of mercury, absolute; feed rate, 60 to 65 milliliters per minute; rotor speed, 1000 ± 50 revolutions per minute; feed-residue temperature differential, 24 ± 1 degrees centigrade; condenser water temperature, 25, 35, and 45 degrees centigrade; condenser locations, 3/4 2-1/2, and 5-3/8 inches from the evaporator; and number of passes of the feed over the rotor per fraction, one. lt was concluded from this investigation that if the Iocation of the condenser did not exceed a mean free-path multiple of three, that the location of the condenser had no effect on the elimination maximum of stearic acid, and the elimination maximum was increased approximately one degree centigrade for every four degrees centigrade decrease in condenser temperature. The elimination maximum was not affected by condenser temperature when the condenser was located in a position with a mean free path multiple of six. / Master of Science

Steam distillation is the traditional method used for the extraction of peppermint oil. This process is able to remove approximately 20% of the oils from the leaves of the plant. It is a very costly and un-sustainable process due to the release of carbon emissions. Solvent free microwave extraction promises yields of up to 65% of the "available" oils from the peppermint at 3% less cost (Velasco 2007). It can also reduce carbon emissions because it will be using electricity as a power source instead of fossil fuels, however not all electric companies use renewable energies. In 2009 a SFME pilot plant was assembled in North Carolina to test the efficiency of the microwave process on a larger than lab scale. Results from the experiments showed that the tube and shell condenser was unable to effectively condense the mint oil. The problem was determined to be the addition of air to the mixture due to the open ends of the microwave. However it was discovered that the spray scrubber after the condenser was able to collect a visible amount of oil. This inspired the design of a direct contact condenser (Pommerenck 2012). The direct contact condenser they designed, built, and tested showed vast improvements in steam capturing efficiency when compared to a tube and shell condenser. However due to the materials used for its construction it could not sustain operating temperatures seen in the microwave pilot plant. Using their design a new direct contact condenser was built using materials that would be able to withstand heavy temperatures. The condenser was constructed out of aluminum and contained stainless steel spray nozzles, both for their non-corrosive properties. Tests were conducted using 8 and 16 nozzles and tested over a range of 20-100% steam by mass. Additional tests were completed using the full 24 nozzles but due to the location of some of the nozzles coolant was lost as an aerosol with no way to quantify the loss. Comparing the data to research completed by Pommerenck et al. on efficiency of a tube and shell condenser used for the mint distillation process found that with increasing amounts of air there is a greater loss of heat transfer. This is believed to be the effects of a boundary layer of the noncondensable fluid, air, which forms along the tube and resists condensation from forming (Seunguim 2006). Pommerenck's tube and shell condenser used a coolant flow rate of 24 L/min while the flow rates tested in this research were 18 L/min and 36 L/min. The direct contact condenser showed a considerable increase in performance even with the smaller flow rate compared to the tube and shell unit, indicating removal of the boundary layer. The efficiency tends to follow the maximum theoretical efficiency while the tube and shell condenser lowers in efficiency. The overall goal of this project is to determine the feasibility of the use of a direct contact condenser for implementation in the solvent free microwave extraction of peppermint oil when air is present. / Graduation date: 2012

Cette thèse présente l'étude de l'interaction de lumière cohérente avec une couche sub-longueur d'onde de vapeur alcaline atomique confinée en nano-cellule et applications pour la formation de résonances optiques étroites.Nous développons un modèle théorique décrivant l'interaction résonante de lumière laser avec la couche mince de vapeur alcaline en présence d'un champ magnétique. Nous montrons qu'en raison d'un régime transitoire d'interaction, seuls les atomes lents contribuent au signal et leur spectre de transmission est essentiellement sans effet Doppler. La nature des spectres obtenus fait de la spectroscopie de transmission, en nano-cellule, une technique pratique pour l'étude de transitions très rapprochées et l'évolution de leur comportement dans un champ magnétique. Des expériences réalisées pour des champs magnétiques jusqu'à 7000 G montrent un excellent accord entre théorie et expérience.Nous explorons aussi la rotation Faraday du plan de polarisation de la lumière lors de sa propagation dans la couche mince atomique. Bien que l'angle de rotation soit très faible, nous observons que les résonances des spectres de rotation Faraday sont plus étroites que celles de transmission. Enfin, nous étudions de nouvelles possibilités pour former des résonances optiques étroites et montrons qu'un traitement par deuxième dérivée des spectres de transmission donne le meilleur rétrécissement de raies parmi toutes les méthodes étudiées dans cette thèse. / This thesis presents the study of coherent light interaction with a sub-wavelength atomic alkali vapor layer confined in a nano-cell and applications for the formation of narrow optical resonances.We develop a theoretical model describing the resonant interaction of the laser light with the thin alkali vapor layer in the presence of an external static magnetic field. We show that due to a transient regime of interaction, only slow atoms contribute to the signal and their transmission spectrum is essentially Doppler-free. The nature of the obtained spectra makes the transmission spectroscopy from a nano-cell a convenient technique to perform studies of closely-spaced atomic transitions and investigate their behavior in magnetic fields. Experimental realizations for magnetic field up to 7000~G show an excellent agreement between theory and experiment.We also explore the Faraday rotation of the plane polarization of light with the propagation through the thin atomic slab. We see that despite a small angle of rotation, Faraday rotation spectra exhibit resonances narrower than that for transmission. At last, we investigate new possibilities to form narrow optical resonances in nano-cells and show that second derivation processing of transmission spectra yields the strongest line narrowing among all methods studied in this thesis.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The steam-side side operation of a practical air-cooled steam condenser is investigated using a combination of CFD, numerical, analytical and experimental methods. Particular attention is directed towards the vapor flow distribution in the primary condensers and dephlegmator performance. Analysis of the vapor flow in the distributing manifold, connecting the steam turbine exhaust to the air-cooled heat exchangers, highlights the importance of careful design of the guide vanes in the manifold bends and junctions. Improved guide vane design and configuration can reduce the steam-side pressure drop over the manifold and improve the vapor flow distribution, which may be beneficial to condenser operation. The vapor flow in the primary condensers is shown to exhibit a non-uniform distribution amongst the heat exchanger tubes. The vapor flow distribution is strongly linked to the distribution of tube inlet loss coefficients through the heat exchanger bundles. The non-uniform flow distribution places an additional demand on dephlegmator performance, over and above the demands of row effects in the case of multi-row primary condenser bundles. Row effects are shown to account for as much as 70 % of available dephlegmator capacity in this case. Simultaneously, inlet loss coefficient distributions can account for up to 30 % of dephlegmator capacity. In some situations then, the dephlegmator is fully utilized under ideal operating conditions and there is no margin of safety to cope with non-ideal operation of the primary condensers. The upstream regions of the primary condensers are therefore exposed to a high risk of undesirable noncondensable gas accumulation. Reduced dephlegmator capacity due to insufficient ejector performance may further compound this problem. Single-row primary condenser bundles eliminate row effects and thereby significantly reduce the demands on dephlegmator performance. The use of such bundles in the dephlegmator would also measurably reduce ejector loading. In light of the findings of this study, it is recommended that single-row bundles be considered as the primary option for future air-cooled condenser applications. A hybrid (dry/wet) dephlegmator concept is analysed and shown to be able to provide measurably enhanced dephlegmator performance when operating in wet mode, while consuming only a small amount of water. The enhanced dephlegmator cooling translates to an increase in total air-cooled condenser capacity of up to 30 % at high ambient temperatures in this case. The benefit of this enhanced cooling capacity to steam turbine output may be significant. The hybrid dephlegmator concept therefore offers a simple, cost-effective and sustainable solution to the issue of reduced air-cooled condenser performance during hot periods. Careful design of the first and second stage bundle configurations in the hybrid dephlegmator is necessary to avoid flooding in the first stage during wet operation of the second. Furthermore, the slightly poorer dry-operation performance of the hybrid dephlegmator results in increased risk of non-condensable gas accumulation in multi-row primary condensers. Again, single-row primary condenser bundles would lay rest to such concerns. / AFRIKAANSE OPSOMMING: Die bedryf aan die stoom-kant van ʼn praktiese lugverkoelde-stoomkondensor word ondersoek met behulp van 'n kombinasie van berekeningsvloeimeganika, numeriese, analitiese en eksperimentele metodes. ʼn Spesifieke fokus word geplaas op die dampvloeiverspreiding in die primêre kondensors asook die deflegmatorwerksverrigting. Ontleding van die damp vloei in die verdeelspruitstuk, wat die uitlaat van die stoomturbine aan die lugverkoelde-stoomkondensor koppel, beklemtoon die belangrikheid van noukeurige ontwerp van die leilemme in die spruitstukdraaie en aansluitings. Verbeterde leilemontwerp en opstelling kan die drukval aan die stoom-kant van die draaie en aansluitings verminder en die dampvloeiverspreiding verbeter. Dit kan gevolglik lei tot verbeterde werksverrigting van die kondensor. Die studie toon dat ʼn nie-eenvormige dampvloeiverspreiding in die warmteruilerbuise van die primêre kondensors bestaan. Die verspreiding van buisinlaat-verlieskoëffisiënte deur die bundels van die warmteruiler is sterk afhanklik van die voorgenome dampvloeiverspreiding. Die nie-eenvormige vloeiverspreiding veroorsaak 'n groter aanvraag na deflegmator-werksverrigting, bo-en-behalwe nog vereistes van ry-effekte in die geval waar multi-ry-bundels vir primêre kondensors gebruik word. Ry-effekte is verantwoordelik vir so veel as 70 % van die beskikbare deflegmator kapasiteit. Terselfdertyd kan die verspreiding van inlaat-verlieskoëffisiënte verantwoordelik wees vir tot 30 % van die deflegmator kapasiteit. In sommige gevalle is die deflegmator dus ten volle aangewend onder ideale bedryfstoestande, en bestaan daar geen band van veiligheid om nie-ideale werksverrigting van die primêre kondensor te hanteer nie. Sekere dele van die stroom-op primêre kondensors word dus blootgestel aan 'n hoë risiko vir die opbou van ongewenste nie-kondenseerbare gasse. Verder kan ‘n vermindering in deflegmator kapasiteit, weens onvoldoende werksverrigting van die vakuumpompe, dié probleem vererger. Enkel-ry-bundels vir primêre kondensors vermy ry-effekte en lei sodoende tot ʼn aansienlike vermindering in die aanvraag na deflegmator-werksverrigting. Die gebruik van sulke bundels in die deflegmator sou die vakuumpomplas ook meetbaar verminder. Uit die bevindinge van hierdie studie word dit aanbeveel dat enkel-ry bundels beskou word as die primêre opsie vir toekomstige lugverkoelde-kondensor aansoeke. ’n Konsep vir ’n hibriede-deflegmator (droog/nat) word ontleed. Die studie toon dat, deur hierdie konsep in die nat-modus te gebruik, ’n meetbare verbetering in deflegmator-werksverrigting gesien kan word, ten koste van net ʼn klein hoeveelheid waterverbruik. Die verbetering in verkoelingsvermoë van die deflegmator beteken ʼn toename van tot 30 % in die totale verkoelingsvermoë van die lugverkoelde-kondensor gedurende periodes wanneer hoë omgewingstemperature heersend is. Die voordeel van hierdie verbeterde verkoelingsvermoë op die werksuitset van die stoomturbine kan beduidend wees. Die konsep vir ’n hibriede-deflegmator bied dus 'n eenvoudige, koste-effektiewe en volhoubare oplossing vir warm atmosferiese periodes, wanneer die lugverkoelde-kondensor se verkoelingsvermoë afneem. Noukeurige ontwerp van die eerste en tweede fase bundelkonfigurasies in die hibriede-deflegmator is nodig om oorstroming in die eerste fase, tydens nat werking van die tweede fase, te verhoed. Verder veroorsaak die effens swakker werksverrigting, gedurende die bedryf van die hibriede-deflegmator in die droog-modus, ʼn verhoogde risiko vir die opbou van nie-kondenseerbare gasse in multi-ry primêre kondensors. Weereens sal enkel-ry-bundels in primêre kondensors hierdie probleem oplos.

Les disjoncteurs à haute tension, présents dans le réseau électrique, permettent d'assurer en toute sécurité la distribution de l'électricité. Lorsqu'un défaut est détecté, ou lors d'une intervention de maintenance, l'ouverture des contacts électriques au sein du disjoncteur entraine l'apparition d'un arc électrique à ses bornes. La protection et la coupure ne seront effectives que lorsque l'arc électrique aura été coupé. De nombreux paramètres, géométriques et physiques entrent en jeu dans la capacité de coupure d'un disjoncteur. L'objectif de ces travaux de thèse consistait à analyser l'impact du plasma sur les différents matériaux constituant le disjoncteur. Une démarche purement théorique est abordée à travers l'utilisation du logiciel commercial ANSYS Fluent. Ces travaux sont menés en collaboration avec la société Siemens qui fournit les données expérimentales indispensables à la discussion et à la validation du modèle. Dans ce travail nous considérons dans un premier temps l'ablation des tuyères en téflon. Ce phénomène est pris en compte dans notre étude, au travers d'un modèle d'ablation basé sur la théorie de T. Christen. L'ablation des parois joue un rôle fondamental sur la montée en pression dans les volumes de chauffage, et a une incidence directe sur la réalisation de la coupure. Ainsi le rôle et la quantification des vapeurs de C2F4 sont discutés et détaillés. La deuxième interaction plasma/matériau qu'il convenait d'étudier est celle avec l'électrode mobile constituée d'un mélange tungstène cuivre. Cette interaction est rarement étudiée au niveau des travaux de la littérature. La mise en place nécessite le développement de modèles de sous couches, en proche voisinage de l'électrode, basés sur des balances d'énergie et de flux qui permettent de déterminer par des approches hors équilibre la température des électrodes et le taux de production de vapeurs. Sur une configuration réelle de disjoncteur les caractéristiques temporelles des grandeurs physiques (température, vitesses, pression) et électriques (courant, tension) sont présentées et discutées dans la phase fort courant. Nous concluons sur la nécessité de considérer l'ablation du C2F4 pour une bonne description de la montée en pression dans les volumes de chauffage car ils conditionnent le soufflage au moment du passage par le zéro du courant, et sur la nécessité de prendre en compte les vapeurs de cuivre car celles-ci sont présentes au passage par zéro du courant et conditionnent alors le pouvoir de coupure du disjoncteur. / High-voltage circuit breakers, present in the power grid, ensure in safety the electricity distribution. When an error is detected, or for a maintenance operation, the opening of the electrical contacts within the circuit breaker causes the appearance of an electric arc at its terminals. Protection and cut-off will only be effective if the electrical arc cut off. Many parameters, geometric and physical, are involved in the breaking capacity of a circuit breaker. The aim of this work is to analyze the impact of plasma on the various materials making up the circuit breaker. A purely theoretical approach is tackled through the use of the commercial software ANSYS Fluent. This work is carried out in collaboration with Siemens, which provides the experimental data necessary for the discussion and validation of the model. In this work, we first consider the ablation of teflon nozzles. This phenomenon is taken into account in our study, using an ablation model based on the theory of T. Christen. The ablation of the walls plays a fundamental role on the rise in pressure in the heating volumes, and has a direct effect on the cut-off realization. Thus, the role and quantification of C2F4 vapors are discussed and detailed. The second plasma / material interaction that should be studied is that with the mobile electrode made of a tungsten copper mixture. This interaction is rarely studied in the literature. The implementation requires the development of sub-layers models in the vicinity of the electrode, based on energy and flux balances, which make it possible to determine by non-equilibrium approaches the temperature of the electrodes and the rate of vapor production. On a real circuit breaker configuration, the temporal characteristics of the physical (temperature, velocities, pressure) and electrical (current, voltage) quantities are presented and discussed in the high current phase. We conclude on the need to consider the ablation of C2F4 for a good description of the rise in pressure in the heating volumes because they condition the blowing at the moment of current-zero and on the need to take into account the copper vapors because these are present at the zero crossing of the current and then condition the breaking capacity of the circuit breaker.