Thèses sur le sujet « Flash-heat »

SummaryIn this study, the Trilateral Flash Cycle (TFC) and the Partially Evaporating Cycle (PEC) have been analyzed and compared to the Organic Rankine Cycle (ORC) for power production from low temperature heat sources. This study is a continuation of the work done in my project thesis fall 2013.The ORC is a well-known technology that is in use in several plants today. The TFC and PEC on the other hand are still in a state of technical development. The biggest challenge for the TFC and PEC is the required two-phase expansion. Lately, two-phase expanders with high efficiencies have been developed, which makes the TFC and PEC economically interesting.Currently, only a few studies on the TFC and PEC can be found, and most of them are theoretical considerations. All of these studies finds the TFC promising for low temperature heat sources, which was also the findings of my project thesis. The PEC is found to be promising for smaller systems where the working fluid pump efficiency is low.The TFCs main difference from the ORC is that the heating process ends at the boiling point of the working fluid, i.e. there is no evaporation and superheating. This leads to a better temperature match between the working fluid and the heat source, such that more heat can be transferred to the working fluid. Power is produced in a two-phase expander after the heating process. The cost pr. kWh for TFC systems have been estimated to be lower than for ORC systems due to the elimination of the evaporator, separator drum, gear box, lube oil system and the fact that simpler heat exchangers can be used.In the PEC, the working fluid is allowed to be partially evaporated during the heating process. This is done in an attempt to combine the advantages of the TFC and the ORC.The ORC, TFC and PEC have been simulated in a Microsoft Excel calculation tool, using Visual Basic for Applications. The simulations include detailed heat exchanger models to calculate heat transfer coefficients and pressure losses, and two-phase expander efficiency models for the TFC and PEC. The three cycles have been simulated and optimized for maximum net power production for three cases using different heat source temperatures. Air with a mass flow of 10 kg/s and temperatures of 100, 150 and 200 °C are used for Case I, Case II and Case III respectively. Water at 20 °C is used as the heat sink. The three cases are simulated with eight different working fluids, R123, R134a, R245fa, R1234ze(E), butane, pentane, isopentane and propane with maximum heat exchanger areas of 1000, 1500, 2000, 2500, 3000, 3500 and 4000 m2. Different performance parameters are calculated and used to compare the performance of the ORC, TFC and PEC, and the different working fluids. The results show that the TFC has the lowest power production for all cases, and the largest estimated system size. Both the total heat exchanger area and expander outlet volume flow are generally higher for the TFC systems, especially for the lower heat source temperature cases. For the 100 °C and 150 °C cases the power production for the TFC and ORC is in the same range. Since TFC systems are estimated to have a lower cost than ORC systems, they can be suitable for systems with heat sources in this range when system size is not a critical factor. The PEC does not show any advantage over the ORC for the cases analyzed here. This study shows less promising results for the TFC than my project thesis and other published studies. This is mainly due to the variable two-phase expander efficiency used here, and that none of the other studies considers pressure losses in the system or calculation of heat transfer coefficients for each working fluid.A scientific paper on the main results from the study before the simulation of the PEC and inclusion of the heat exchanger models is given in Appendix C. This paper has been submitted to the journal Energy. A scientific paper on the final results of the study is given in Appendix D. This paper has been submitted to the Gustav Lorentzen Conference.

There has been renewed significance for innovative energy conversion technologies, particularly the heat recovery-to-power technologies for sustainable power generation from renewable energies and waste heat. This is due to the increasing concern over high demand for electricity, energy shortage, global warming and thermal pollution. Among the innovative heat recovery-to- power technologies, the proposed trilateral flash cycle (TFC) is a promising option, which presents a great potential for development. Unlike the Rankine cycles, the TFC starts the working fluid expansion from the saturated liquid condition rather than the saturated, superheated or supercritical vapour phase, bypassing the isothermal boiling phase. The challenges associated with the need to establish system design basis and facilitate system configuration design-supporting analysis from proof-of-concept towards a market-ready TFC technology are significant. Thus, there is a great need for research to improve the understanding of its operation, behaviour and performance. The objective of this study is to develop and establish simulation tools of the TFCs for improving the understanding of their operation, physics of performance metrics and to evaluate novel system configurations for low-grade heat recovery-to-power generation. This study examined modelling and process simulation of the TFC engines in order to evaluate their performance metrics, predictions for guiding system design and parameters estimations. A detailed thermodynamic analysis, performance optimization and parametric analysis of the cycles were conducted, and their optimized performance metrics compared. These were aimed at evaluating the effects of the key parameters on system performances and to improve the understanding of the performance behaviour. Four distinct system configurations of the TFC, comprising the simple TFC, TFC with IHE, reheat TFC and TFC with feed fluid-heating (or regenerative TFC) were examined. Steady-state steady-flow models of the TFC power plants, corresponding to their thermodynamic processes were thermodynamically modelled and implemented using engineering equation solver (ESS). These models were used to determine the optimum synthesis/ design parameters of the cycles and to evaluate their performance metrics, at the subcritical operating conditions and design criteria. Thus, they can be valuable tools in the preliminary prototype system design of the power plants. The results depict that the thermal efficiencies of the simple TFC, TFC with IHE, reheat TFC and regenerative TFC employing n-pentane are 11.85 - 21.97%, 12.32 - 23.91%, 11.86 - 22.07% and 12.01 - 22.9% respectively over the cycle high temperature limit of 393 - 473 K. These suggest that the integration of an IHE, fluid-feed heating and reheating in optimized design of the TFC engine enhanced the heat exchange efficiencies and system performances. The effects of varying the expander inlet pressure at the cycle high temperature and expander isentropic efficiency on performance metrics of the cycles were significant. They have assisted in selecting the optimum-operating limits for the maximum performance metrics. The thermal efficiencies of all the cycles increased as the inlet pressures increased from 2 - 3 MPa and increased as the expander isentropic efficiencies increased from 50 - 100%, while their exergy efficiencies increased. This is due to increased net work outputs that suggest optimal value of pressure ratios between the expander inlets and their outlets. A comprehensive evaluation depicted that the TFC with IHE attained the best performance metrics among the cycles. This is followed by the regenerative TFC whereas the simple TFC and reheat TFC have the lowest at the same subcritical operating conditions. The results presented show that the performance metrics of the cycles depend on the system configuration, and the operating conditions of the cycles, heat source and heat sink. The results also illustrate how system configuration design and sizing might be altered for improved performance and experimental measurements for preliminary prototype development.

Low grade heat (LGH) sources, here defined as below 80ºC, are one group of abundant energy sources that are under-utilised in the production of electricity. Industrial waste heat provides a convenient source of concentrated LGH, while solar ponds and geothermal resources are examples of sustainable sources of this energy. For a number of years RMIT has had two ongoing, parallel heat engine research projects aimed at the conversion of LGH into electricity. The Thermosyphon Rankine Engine (TSR) is a heat engine that uses water under considerable vacuum. The other research stream uses a hydrocarbon based working fluid in a heat engine employing the Trilateral Flash Cycle (TFC). The TSR Mk V was designed and built as a low cost heat engine for the conversion of LGH into electricity. Its main design advantages are its cost and the employment of only one moving part. Using the data gained from the experimental rig, deviations from the expected results (those derived theoretically) were explored to gain insight for further development. The results from the TSR rig were well below those expected from the design specifications. Although the experimental apparatus was able to process the required heat energy, the efficiency of conversion fell well below the expected 3% and was approximately 0.2%. The inefficiency was explained by a number of contributing factors, the major being form drag upon the rotor that contributed around 2/3 of the losses. Although this was the major cause of the power loss, other factors such as the interference with the rotor by the condensate on its return path contributed to the overall poor performance of the TSR Mk V. The RMIT TFC project came about from exploration of the available academic literature on the subject of LGH conversion. Early work by researchers into applying Carnot's theory to finite heat sources led them to explore the merits of sensible heat transfer combined with a cycle that passes a liquid (instead of a gas) though an expander. The results showed that it was theoretically possible to extract and convert more energy from a heat source of this type using this method than using any other alternative. This previous research was targeted at heat sources above 80ºC and so exploration of the theoretical and empirical results for sources below this temperature was needed. Computer models and an experimental rig using isopentane (with a 28ºC boiling point at atmospheric pressure) were produced to assess the outcomes of employing low temperature heat sources using a TFC. The experimental results from the TFC research proved promising with the efficiency of conversion ranging from 0.8% to 2.4%. Although s uch figures seem poor in isolation, it should be noted that the 2.4% efficiency represents an achievement of 47% of the theoretical ideal conversion efficiency in a rig that uses mainly off-the-shelf components. It also confirms that the TFC shows promise when applied to heat sources less than 80ºC.

Ethyl alcohol (ethanol) as secondary fluids is very popular as heat transfer fluid for indirect refrigeration system with ground source heat pump systems (GSHP) in several countries such as Sweden, Norway, Switzerland, Finland and other European countries. There have been several researches about the future ofthe refrigeration sector, refrigerants and refrigeration systems. Moreover, strict regulations such as F-gasregulation and Kigali Amendment forcing a phase down of many current widely used high global warming potential (GWP) refrigerants, i.e. R134a or R410A. Therefore, secondary refrigeration systems and their working fluids are expected to play a key role in order to minimize the refrigerant charge in the systems, reduce the indirect refrigerant leakages as well as increase the safety during operation. The aim of this thesis is to investigate the effect different additives to increase the flame point together with ethanol-based secondary fluids and validate their thermophysical properties by comparing them with reference values for pure ethanol water solutions. The study aims to design a new commercial ethyl alcohol-based product for GSHP system that could replace existing ones in the Swedish market and could workwith natural or flammable low GWP refrigerants. Different high flash point additives were tested such as 1-propyl alcohol, n-butyl alcohol, glycerol andpropylene carbonate. Thermophysical properties were investigated and a GSHP model in Excel was created in order to assess the energy performance of the resulted blends. After screening different blends and assessing the energy performance, glycerol as additive in low concentration seems to be the future for the ethyl alcohol-based secondary fluids because of its high flashpoint (160ºC) that will reduce the flammability risk associated to ethyl alcohol blends, the low viscosity (by 12% lower compared to pure ethyl alcohol blends) that help reduce pumping power by 4.5% compared topure ethyl alcohol blends. Moreover, ethyl alcohol and glycerol blend showed the lost in heat transfer coefficient by 4% lower compared to pure ethyl alcohol blends due to lower thermal conductivity compared to pure ethyl alcohol blends. Finally, it is a rather cheap and natural product which has no problem related to corrosion since ethyl alcohol and glycerol are less corrosive than water. Although, flash point test was not conducted so there is no data regarding the flash point, it is expected the flash point is increased due to the high flash point of glycerol compared to ethyl alcohol or other possible additives. Therefore, it is expected that the flammability risk associated to ethyl alcohol-based secondary fluids is reduced.
Etylalkohol (etanol) som köldbärare är mycket populärt som värmeöverföringsvätska för indirekt kylsystemmed bergvärmepumpsystem (BVP) i Sverige, Norge, Schweiz, Finland och andra europeiska länder. Fleraundersökningar har gjorts om kylsektorns framtid, köldmedier och kylsystem. Dessutom strängaförordningar som F-gas förordning och Kigali- förordning tvingar en utfasning av många nuvarande allmäntanvända köldmedier med den höga globala uppvärmningspotentialen (GWP), dvs. R134a eller R410A. Därför förväntas det att kylsystem och deras köldbärare spela en nyckelroll för att minimera köldmediumsmängd i systemen, minska de indirekta köldmedieläckage och öka säkerheten under drift. Syftet med detta examensarbete är att undersöka effekten av olika tillsatser för att öka flammanpunkten tillsammans med etanolbaserade köldbärare och validera deras termofysikaliska egenskaper genom att jämföra dem med referensvärden för rena etanolvattenlösningar. Studien syftar till att utforma en nykommersiell etylalkoholbaserad produkt för BVP-system som skulle kunna ersätta befintliga produkter på den svenska marknaden och kan arbeta med naturliga eller brandfarliga köldmedier med låg GWP. Olika tillsatser med hög flampunkt testades såsom 1-propylalkohol, n-butylalkohol, glycerol och propylenkarbonat. Termofysikaliska egenskaper undersöktes och en BVP-modell i Excel skapades för att bedöma energiprestanda för olika blandningarna. De erhållna resultaten för olika blandningar visar att glycerol i en låg koncentration som tillsats kan vara framtidens additiv för de etylalkoholbaserade köldbärare på grund av dess höga flampunkt (160 ºC) som förmodligen kan minska brandrisken för etylalkoholblandningar. Dessutom hade glycerol och etanolblandningar den lägsta viskositeten (c.a.12% lägre jämfört med ren etylalkoholblandningar) som bidrar tillen minskning av pumpeffekten med c.a. 4,5% jämfört med rena etylalkoholblandningar. Däremot visade etylalkohol och glycerol blandningen c.a. 4% lägre värmeöverövergångstal jämfört med de rena etylalkoholblandningar på grund av lägre värmeledningsförmåga jämfört med ren etylalkoholblandningar. Slutligen är glycerol en ganska billig och naturlig produkt som inte har några korrosionsproblem eftersom etylalkohol och glycerol är mindre frätande än vatten. Även om flampunkttest inte genomfördes i projektet, förväntas det att flampunkten ökas lite på grund av den höga flampunkten av glycerol jämfört med etylalkohol och andra tillsatser. Därför förväntas det att brännbarhetsrisken förknippad med etylalkoholbaserade köldbärare reduceras.

L'objectif de cette étude consiste à développer une approche permettant d'économiser de l'énergie de haut niveau thermique dans le procédé de la distillation atmosphérique du pétrole. Cette approche repose sur le fait que l'installation de plusieurs flashs sur le train de préchauffage de ce procédé associée à un choix approprié du plateau d'alimentation des vapeurs résultantes à la colonne de distillation, provoque une diminution de la chaleur apportée à ce train de préchauffage. Le déficit de chaleur de haut niveau thermique ainsi créé est causé par la réduction des débits des reflux latéraux de la colonne de distillation et il est compensé par de la chaleur de bas niveau thermique apportée par les effluents résiduaires disponibles dans toute la raffinerie. Cette approche peut être appliquée aux nouveaux procédés comme à l'expansion des procédés existants et l'économie d'énergie de haut niveau thermique dans le four peut atteindre les 21%. Une réduction équivalente des gaz à effet de serre est aussi observée
The objective of this thesis is to demonstrate, on thermodynamic grounds, that introducing a flash in the preheating train of an atmospheric oil distillation process , together with an appropriate feeding of the resulting vapors into the column, could potentially bring substantial energy savings by reducing the duty of the preheating furnace and by reducing the distillation column irreversibilities. This idea has been expended by showing how this can be done while keeping the throughput and the product characteristics unchanged. The outcome is that placing several flashes after the heat exchangers and feeding the corresponding vapor streams to the appropriate trays of the column, reduces the pumparound flows and then the heat brought to the preheating train. The resulting heat deficit may then be compensated in additional heat exchangers by using low level heat recuperated from the products of the distillation and/or imported from other processes. The use of this residual heat reduces the furnace duty by approximately an equivalent amount and could be as high as 21%. The approach can be applied in the design of news processes or in the revamping of existing ones

Thesis(M.S.)--Case Western Reserve University, 2009
Title from PDF (viewed on 2010-01-28) Department of EMC - Mechanical Engineering Includes abstract Includes bibliographical references and appendices Available online via the OhioLINK ETD Center

The "classical" laser-flash method is today the most used technique to measure the thermal diffusivity of a wide range of materials. This work describes the development of a new technique, based on the laser-flash method, which measures simultaneously on the same sample with an absolute method the thermal diffusivity and the specific heat, and its application to a number of high melting-point refractory materials. In this work, a new data processing procedure, which takes radiative and conductive heat losses into consideration, is introduced, and the thermal diffusivity, a, and specific heat, cp are determined by fitting the entire experimental transient temperature curve. The thermal conductivity is then calculated from the measured a and cp values via the relationship = apcP, where p is the density of the material. For the calculation the measured room temperature values of p corrected to the temperature of interest via literature data on thermal expansion are used. The new technique is applied to measure the specific heat, thermal diffusivity of POCO AXM 5Q graphite, zirconium dioxide and uranium dioxide (materials of scientific and technological interest) at very high temperatures (above 1800K) from which thermal conductivity values can be calculated. The values obtained, having a precision of ~2% in the case of the thermal diffusivity, and ~7% for the specific heat and the thermal conductivity, are discussed and compared with literature data. The results obtained for uranium dioxide are used for a critical analysis of the physical mechanisms underlying the heat transport in this material.

As amarras de correntes de aço para aplicação naval utilizadas na indústria de óleo e gás são fabricadas para atender elevadas exigências operacionais. Com a prospecção de petróleo em águas profundas e ultraprofundas o conhecimento de suas propriedades torna-se relevante. Frente a essa necessidade foi realizado o estudo das propriedades mecânicas e metalúrgicas de um elo de corrente de amarra naval na indústria do petróleo proveniente de operação. Esse estudo teve como objetivo principal identificar e avaliar as propriedades mecânicas e metalúrgicas das regiões resultante do processo de soldagem por centelhamento frente ao material de base. Como objetivo secundário foi classificado o material de acordo com algumas propriedades mecânicas recomendadas pela IACS UR W22. Para isso foram realizados ensaios de tração, microdureza, tenacidade ao impacto Charpy e à fratura CTOD, com avaliações metalúrgicas no material de base e região de união por centelhamento. Como resultado foi verificado que o processo de soldagem por centelhamento produziu zonas de extensões reduzidas e com menores valores de tenacidade à fratura e ao impacto. O processo de centelhamento produziu união com espessura de 0,375 mm, com uma zona de solda revelada por ataque químico e duas zonas afetadas pelo calor caracterizadas apenas por perfil de microdureza, com 0,150 mm e 0,300 mm, respectivamente. Os menores valores de tenacidade ao impacto Charpy e à fratura CTOD foram medidos nos corpos de prova posicionados na zona afetada pelo calor. Os resultados obtidos em todos os ensaios foram comparados com as recomendações técnicas IACS UR W22, onde caracterizou o elo como grau R4.
Mooring chains of offshore systems anchors are design to be used under extreme environment operation. New fields of exploration have been operated in deep water and ultra-deep water, wherefore, require more knowledge of the mechanical and metallurgical properties. Hence a study of a studless link of offshore chain from the operating service component was carried out. The main goal of this work was to determine and evaluate the properties of the base material and the zones produced by Flash Butt Welding Process. Furthermore, the material was characterized by the IACS UR W22 standard recommendation. In this study tests of tensile test, microhardness, Charpy and CTOD toughness, mechanical and metallurgical evaluation were performed in the base material and welding zones. The results showed small welding zones with reduced impact and fracture toughness. The lowest values were seen in specimens from the heat affect zone. The weld zone thickness observed by chemical etching extend 0,375 mm and the microhardness profile indicated a Heat Affected Zone (HAZ) with 0,150 and 0,300 mm. The lowest Charpy and CTOD toughness tests were measured in the HAZ specimens. These results were compared with IACS UR W22 standard recommendation and the link chain was characterized as R4 grade.

The aim of this study is to synthesize a material with high thermal conductivity and a low coefficient of thermal expansion (CTE), useful as a heat sink. Carbon nanofibres (CNF) are first coated with copper by an electroless plating technique and then sintered to a solid sample by either spark plasma sintering (SPS) or hot pressing (HP). The final product is a carbon nanofibre reinforced copper composite. Two different fibre structures are considered: platlet (PL) and herringbone (HB). The influence of the amount of CNF reinforcement (6-24 %wt), on the thermal conductivity and CTE is studied. CNF has an excellent thermal conductivity in the direction along the fibre while it is poor in the transverse direction. The CTE is close to zero in the temperature range of interest. The adhesion of Cu to the CNF surface is in general poor and thus improving the the wetting of the copper by surface modifications of the fibres are of interest such that thermal gaps in the microstructure can be avoided. The poor wetting results in CNF agglomerates, resulting in an inhomogeneous microstructure. In this report a combination of three different types of surface modifications has been tested: (1) electroless deposition of copper was used to improve Cu impregnation of CNF; (2) heat treatment of CNF to improve wetting; and (3) introduction of a Cr buffer layer to further enhance wetting. The obtained composite microstructures are characterized in terms of chemical composition, grain size and degree of agglomeration. In addition their densities are also reported. The thermal properties were evaluated in terms of thermal diffusivity, thermal conductivity and CTE. Cr/Cu coated platelet fibres (6wt% of CNF reinforcement) sintered by SPS is the sample with the highest thermal conductivity, ~200 W/Km. The thermal conductivity is found to decrease with increasing content of CNFs.

Thesis (M.S.)--State University of New York at Binghamton, Thomas J. Watson School of Engineering and Applied Science, Department of Mechanical Engineering, 2009.
Includes bibliographical references.

[ES] Esta tesis doctoral presenta un estudio de compresores scroll con inyección de vapor (SCVI) para bombas de calor que operan en climas fríos o para aplicaciones de calentamiento de agua a alta temperatura. Para ello, se comparó experimentalmente un SCVI con un compresor de dos etapas de pistones (TSRC) trabajando con R-407C en condiciones extremas. La comparación se realizó en términos de eficiencias del compresor, capacidad, COP y rendimientos estacionales tanto para el modo calefacción como para el modo refrigeración. Los resultados proporcionan una idea general sobre el rango de aplicación de los compresores estudiados y sobre las diferencias en los rendimientos de los compresores. Sin embargo, se identificaron varias limitaciones en la caracterización de los compresores y en el análisis del ciclo. Esto motivó a profundizar en el estudio del ciclo de compresión de dos etapas y sus componentes. El siguiente paso fue realizar un análisis teórico de los ciclos de compresión de dos etapas para aplicaciones de calefacción, en donde se identificó a la presión intermedia y a la relación de inyección como los parámetros del sistema más influyentes sobre el COP. La presión intermedia se optimizó para dos configuraciones de inyección (tanque de separación y economizador) utilizando varios refrigerantes. Basándose en los resultados de la optimización, se propuso una correlación que permite obtener la presión intermedia óptima del ciclo, considerando la influencia del subenfriamiento a la salida del condensador. Además, se analizó la influencia del diseño de los componentes del sistema sobre el COP del ciclo. Posteriormente, el estudio se profundizó a nivel de componentes. El factor más crítico en el sistema es el rendimiento del compresor. Por lo tanto, el siguiente paso fue evaluar la influencia de varios sistemas de compresión con inyección de vapor sobre el COP. Se tomaron en cuenta tres tecnologías de compresores, un SCVI, un TSRC y un compresor scroll de dos etapas (TSSC). Estas tecnologías de compresores fueron caracterizadas y modeladas para estudiar su rendimiento. Para ello, se propuso una nueva metodología para caracterizar compresores scroll con inyección de vapor. Esta metodología permite evaluar el rendimiento del compresor independientemente del mecanismo de inyección que se utiliza en el ciclo. Se identificó una correlación lineal entre la relación de inyección de refrigerante y la relación de compresión intermedia. Esta correlación se utiliza para determinar el flujo másico de inyección en función de la presión intermedia. Posteriormente, se propuso un modelo semi-empírico de compresores scroll y una metodología para extender dicho modelo para compresores scroll con inyección de vapor. Los modelos fueron ajustados y validados usando datos experimentales de cuatro compresores scroll trabajando con R-290 y un SCVI trabajando con R-407C. Finalmente, se comparó un SCVI con dos compresores de dos etapas, un TSSC y un TSRC, trabajando en condiciones extremas. Se optimizó la relación de volúmenes de los compresores de dos etapas. Los resultados muestran que, en las condiciones nominales de funcionamiento (Te=-15 °C, Tc=50 °C), la relación de volúmenes óptima del TSSC es 0.58, y del TSRC es 0.57. El TSSC consigue un COP 6% mayor que el SCVI y un COP 11.7% mayor que el TSRC. Bajo un amplio rango de condiciones de operación, el SCVI presenta una mejor eficiencia y COP para relaciones de presión inferiores a 5. Para relaciones de presión más altas, el TSSC presenta mejor rendimiento y consigue una temperatura de descarga más baja. Se concluye que el SCVI es una solución fácil de implementar, desde el punto de vista del mecanizado, y que permite extender el mapa de trabajo de los compresores de una etapa. Sin embargo, los resultados muestran que la compresión en dos etapas consigue mejorar en mayor medida el COP del ciclo y la capacidad, con una mayor redu
[CA] Aquesta tesi doctoral presenta un estudi de compressors scroll amb injecció de vapor (SCVI) per a bombes de calor que operen en climes freds o per a aplicacions d'escalfament d'aigua a alta temperatura. Per a això, es va comparar experimentalment un SCVI amb un compressor de dues etapes de pistons (TSRC) treballant amb R-407C en condicions extremes. La comparació es va realitzar en termes d'eficiències del compressor, capacitat, COP i rendiments estacionals tant per al mode calefacció com per al mode refrigeració. Els resultats proporcionen una idea general sobre el rang d'aplicació dels compressors estudiats i sobre les diferències en els rendiments dels compressors. No obstant això, es van identificar diverses limitacions en la caracterització dels compressors i en l'anàlisi del cicle. Això va motivar a aprofundir en l'estudi del cicle de compressió de dues etapes i els seus components. El següent pas va ser realitzar una anàlisi teòrica dels cicles de compressió de dues etapes per a aplicacions de calefacció, on es va identificar la pressió intermèdia i la relació d'injecció com els paràmetres del sistema més influents sobre el COP. La pressió intermèdia es va optimitzar per a dues configuracions d'injecció (tanc de separació i economitzador) utilitzant diversos refrigerants. Basant-se en els resultats de l'optimització, es va proposar una correlació que permet obtindre la pressió intermèdia òptima del cicle, considerant la influència del subrefredament a l'eixida del condensador. A més, es va analitzar la influència del disseny dels components del sistema sobre el COP del cicle. Posteriorment, l'estudi es va aprofundir a nivell de components. El factor més crític en el sistema és el rendiment del compressor. Per tant, el següent pas va ser avaluar la influència de diversos sistemes de compressió amb injecció de vapor sobre el COP. Es van prendre en compte tres tecnologies de compressors, un SCVI, un TSRC i un compressor scroll de dues etapes (TSSC). Aquestes tecnologies de compressors van ser caracteritzades i modelades per a estudiar el seu rendiment. Per a això, es va proposar una nova metodologia per a caracteritzar compressors scroll amb injecció de vapor. Aquesta metodologia permet avaluar el rendiment del compressor independentment del mecanisme d'injecció que s'utilitza en el cicle. Es va identificar una correlació lineal entre la relació d'injecció de refrigerant i la relació de compressió intermèdia. Aquesta correlació s'utilitza per a determinar el flux màssic d'injecció en funció de la pressió intermèdia. Posteriorment, es va proposar un model semi-empíric de compressors scroll i una metodologia per a estendre aquest model per a compressors scroll amb injecció de vapor. Els models van ser ajustats i validats utilitzant dades experimentals de quatre compressors scroll treballant amb R-290 i un SCVI treballant amb R-407C. Finalment, es va comparar un SCVI amb dos compressors de dues etapes, un TSSC i un TSRC, treballant en condicions extremes. Es va optimitzar la relació de volums dels compressors de dues etapes. Els resultats mostren que, en les condicions nominals de funcionament (Te=-15 °C, Tc=50 °C), la relació de volums òptima del TSSC és 0.58, i del TSRC és 0.57. El TSSC aconsegueix un COP 6% major que el SCVI i un COP 11.7% major que el TSRC. Sota un ampli rang de condicions d'operació, el SCVI presenta una millor eficiència i COP per a relacions de pressió inferiors a 5. Per a relacions de pressió més altes, el TSSC presenta millor rendiment i aconsegueix una temperatura de descàrrega més baixa. Es conclou que el SCVI és una solució fàcil d'implementar, des del punt de vista del mecanitzat, i que permet estendre el mapa de treball dels compressors d'una etapa. No obstant això, els resultats mostren que la compressió en dues etapes aconsegueix millorar en major mesura el COP del cicle i la capacitat, amb una major reducció de la
[EN] This Ph.D. thesis presents a study of scroll compressors with vapor-injection (SCVI) for heat pumps operating in cold climates or in high-temperature water heating applications. To do so, firstly, an SCVI was experimentally compared with a two-stage reciprocating compressor (TSRC) working with R-407C under extreme conditions. The comparison was made in terms of compressor efficiencies, capacity, COP, and seasonal COP, both for heating and cooling modes. The results give a general idea about the application range of the studied compressors and the differences in the compressors' performance. Nevertheless, several restrictions in the compressors' characterization and the cycle analysis were identified. This motivated us to deepen in the study of the two-stage compression cycle and its components. The next step was performing a theoretical analysis of two-stage compression cycles for heating applications, where the intermediate pressure and the injection ratio were identified as the most influential system parameters on the COP. The intermediate pressure was optimized for two vapor-injection configurations (flash tank and economizer) using several refrigerants. Based on the optimization results, a correlation was proposed that allows obtaining the optimal intermediate pressure of the cycle, considering the influence of the subcooling at the condenser outlet. In addition, a theoretical analysis of the influence of the design of the system components on the COP of the cycle was performed. Once the thermodynamic analysis of the two-stage cycle was carried out, the study was deepened at the component level. The most critical factor in the system is the compressor performance. Hence, the next step was evaluating the influence of several compression systems with vapor-injection on the COP. Three compressor technologies were taken into account, an SCVI, a TSRC and a two-stage scroll compressor (TSSC). These compressor technologies were characterized and modeled in order to study their performance. To do so, a new methodology to characterize SCVI was proposed. This methodology allows evaluating the compressor performance independently of the injection mechanism used in the cycle. A linear correlation was identified between the refrigerant injection ratio and the intermediate compression ratio. This correlation is used to determine the injection mass flow as a function of the intermediate pressure. Then, a semi-empirical model of scroll compressors and a methodology to extend the model for scroll compressors with vapor-injection was proposed. The models were adjusted and validated using experimental data from four scroll compressors working with R-290 and an SCVI compressor working with R-407C. Finally, an SCVI was compared with two two-stage compressors, a TSSC, and a TSRC, working in extreme conditions. The displacement ratio of the two-stage compressors was optimized. Results show that, at the nominal operating conditions (Te=-15 °C, Tc=50 °C), the optimal displacement ratio of the TSSC is 0.58, and of the TSRC is 0.57. The TSSC achieves 6% larger COP than the SCVI and 11.7% larger COP than the TSRC. Under a wide range of operating conditions, the SCVI presents a better efficiency and COP for pressure ratios below 5. For higher-pressure ratios, the TSSC presents better performance and achieves lower discharge temperature. It is concluded that the SCVI is an easy solution to implement from the point of view of machining, which allows extending the working map of the single-stage compressors. However, the results show that the two-stage compression technology gets further improve the COP of the cycle and the capacity, with a greater reduction of the discharge temperature operating under extreme conditions.
I thank the financial support provided by the Secretaría de Educación Superior, Ciencia, Tecnología e Innovación (SENESCYT) of Ecuador, through the international scholarship program for postgraduate studies “Convocatoria Abierta 2013 Segunda Fase, Grant No 2015-AR37665”.
Tello Oquendo, FM. (2019). Study of scroll compressors with vapor-injection for heat pumps operating in cold climates or in high-temperature water heating applications [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/120473
TESIS

Depuis la fin du vingtième siècle, la recherche et le développement s'intéresse de plus en plus à l'utilisation des mousses céramiques solides pour une myriade d'applications hautes températures en raison d'avantageuses propriétés thermiques, mécaniques, chimiques ou optiques. Dans un contexte énergétique mondial qui tend vers une réduction de la consommation d'énergies fossiles et d'émissions de gaz à effet de serre, elles apparaissent comme une solution prometteuse par leur capacité à absorber, récupérer et convertir un flux radiatif important. Elles font notamment l'objet d'un intérêt grandissant dans le domaine du solaire thermique tandis que le LEMTA s'intéresse à ces matériaux comme récupérateur des chaleurs fatales liés aux procédés industriels à haute température (verrerie, cimenterie, métallurgie...). Cependant, certaines études ont montré que l'amélioration des systèmes actuels et futurs passe nécessairement par une meilleure compréhension du lien entre les propriétés structurales/thermiques/optiques associées à ces milieux hétérogènes (géométrie, porosité, diamètre de pores, conductivité, émissivité...) et les transferts de chaleur. Les objectifs de ce travail sont de fournir des outils de modélisation et de caractérisation pertinents qui permettent la description des transferts thermiques conductif, convectif (forcé) et radiatif. Trois méthodes de caractérisations proposées et sont détaillées dans ce manuscrit. 1. Dans le cas de transferts conducto-radiatif, la principale difficulté réside dans la capacité à séparer la contribution de chaque mode de transfert de chaleur. Afin de dépasser ces limites, une méthode transitoire sur le principe de la méthode flash a été utilisée. La mousse céramique, placée entre deux semelles céramiques puis au centre d'un four tubulaire, subit une excitation thermique tandis que l'élévation thermique est mesurée sur la face opposée à cette dernière. En parallèle, un modèle direct résolvant l'Equation de la Chaleur par la méthode des Volumes Finis et l'Equation du Transfert Radiatif par une méthode de Monte Carlo réciproque optimisée a été implémentée. Cette dernière permet, tout en conservant des temps de calcul raisonnable, de résoudre plus finement les transferts par rayonnement par comparaison avec les méthodes utilisées habituellement. Une estimation simultanée d'une conductivité phonique équivalente et d'une épaisseur optique équivalente a été réalisée sur une grande variété de mousses structurées et stochastiques composées de SiC ou de SiSiC jusqu'à 800 °C. Par comparaison avec des modèles simplifiés comme celui de Rosseland, cette étude permet de mieux appréhender la validité de l'approximation de la diffusion et d'orienter le choix de la modélisation et des paramètres à utiliser. 2. Une autre difficulté réside dans la capacité à résoudre numériquement un problème couplé directement sur la géométrie hétérogène 3D du milieu poreux. Une méthode de Monte Carlo permettant la résolution des deux modes de transfert en un seul algorithme a été étudiée. La méthode flash a été réalisée grâce à cette dernière de façon entièrement numérique. Elle permet une obtention plus rapide de la solution avec l'augmentation des transferts par rayonnement. La procédure proposée a permis l'estimation des propriétés équivalentes de mousses structurées (cellules de Kelvin). 3. Une méthode de caractérisation des coefficients de dispersion thermique axial et radial a été développée. Le banc expérimental est composé de 5 sections de 20 cm de mousses SiC séparées par des brides assurant la mesure de la température. Un écoulement d'air chaud est appliqué sur le milieu poreux à l'équilibre avec la température ambiante. En parallèle, un modèle à une température permet une résolution 2D axisymmétrique et multicouche du problème thermique. L'application d'un algorithme d'inversion permet l'estimation des deux coefficients pour différentes vitesses d'écoulement
Since the end of the twentieth century, research and development has increasingly focused on the use of solid ceramic foams for a myriad of high-temperature applications because of their advantageous thermal, mechanical, chemical or optical properties. In a global energy context that is tending to reduce the consumption of fossil fuels and greenhouse gas emissions, they appear to be a promising solution due to their ability to absorb, recover and convert a large radiative flux. In particular, they are the subject of growing interest in the field of solar thermal energy, while LEMTA is interested in these materials as a recover of fatal heat present in high-temperature industrial processes (glassware, cement, metallurgy, etc.). However, some studies have shown that improving current and future systems requires a better understanding of the relationship between the structural/thermal/optical properties associated with these heterogeneous media (geometry, porosity, pore diameter, conductivity, emissivity, etc.) and heat transfers. The objectives of this work are to provide relevant modelling and characterization tools that allow the description of conductive, convective (forced) and radiative heat transfers. Three characterization methods are studied and detailed in this manuscript. 1. In the case of conductive and radiative transfer, the main difficulty lies in the ability to separate the contribution of each heat transfer mode. In order to exceed these limits, a transitional method based on the principle of the flash method was used. The ceramic foam, placed between two ceramic soleplates and in the centre of a tubular furnace, is thermally excited while the thermal elevation is measured on the side opposite the latter. In parallel, a direct model resolving the Energy Balance Equation by the Finite Volume Method while the Radiative Transfer Equation by an optimized reciprocal Monte Carlo method was implemented. The latter method allows, while maintaining reasonable calculation time, to solve radiation transfers more generally than the method usually used. A simultaneous identification of equivalent phononic conductivity and optical thickness was performed on a wide variety of structured and stochastic foams composed of SiC or SiSiC up to 800 °C. Compared to simplified models such as Rosseland's, this study provides a better understanding of the validity of the diffusion approximation and guides the choice of modeling and parameters to be used. 2. Another difficulty lies in the ability to numerically solve a coupled problem directly on the 3D heterogeneous geometry of the porous medium. A Monte Carlo method allowing the resolution of both transfer modes in a single algorithm has been employed. The flash method was performed thanks to the latter in a entirely numerical manner. It allows the solution to be obtained quickly with the increase in radiation transfer. The proposed procedure allowed the identification of the equivalent properties of structured foams (Kelvin's cells). 3. A method for characterizing axial and radial thermal dispersion coefficients has been implemented. the experimental bench is composed of 5 sections of 20 cm SiC foams separated by flanges for temperature measurement. A hot air flow is applied to the porous medium at equilibrium with the ambient temperature. In parallel, a one-temperature model allows a 2D axisymmetric and multilayer resolution of the thermal problem. The application of an inversion algorithm allows the estimation of the two coefficients for different flow rates

This dissertation thesis deals with the development of technology which could tackle two major issues related to biogas plants. These issues concern the insufficient use of waste heat from biogas combustion and its subsequent processing. It also concerns the use of the fermentation residues which are formed in large quantities and whose use is restricted by law. Based on a literary search of separation methods, a vacuum evaporator was selected as the most suitable technology. Its advantages include its simple construction, operational reliability and robustness, low costs of thickening medium pre-treatment, potential for a quick commercial application and, especially, the chance to use a low-potential waste heat. A primary purpose of this technological unit is the reduction in the volume of fermentation residues. Other benefits include the efficient use of waste heat from a biogas plant, which would otherwise be wasted. Evaporators with a low consumption of electrical energy (which is a main product of a biogas plant) seem to be the best option for applications in the biogas plants. Three of these technologies were subjected to a more thorough analysis, which included the development of computational models and their quantification for conditions in a sample biogas plant. A one-stage evaporator with a forced circulation (680 – 712 kWhth/m3, 25.9 – 30.5 kWhel/m3) was evaluated as the least suitable option in terms of energy demands. The energy intensity of a three-stage evaporator with a falling film (241 – 319 kWhth/m3, 12.0 – 23.6 kWhel/m3) and a nine-stage flash evaporator (236 – 268 kWhth/m3, 13.6 – 18.4 kWhel/m3) is significantly lower. A multi-stage flash evaporator (MSF) was then chosen for development and will form the central focus of this thesis. The reasons for the choice are as follows: the low requirements on the heat transfer surface, good operational experience in the field of desalination, its simple construction, modularity and evaporation outside the heat transfer surface. A thorough technical-economic evaluation was also performed on the integration of the evaporator into the biogas plant. The main part of the work included the experimental development of a MSF evaporator prototype. The main objective of this development was to achieve a stable flow rate of the thickening liquid digestate fraction and the continuous formation of the distillate. This was not an easy objective to achieve, especially due to the properties of the liquid digestate, which has a non-newtonian characteristic and increased density and viscosity compared to water. The tendency of the liquid digestate to form foam was also the subject of analysis. The development of the evaporator and first successful operational test are described in the thesis in detail. This required the use of an anti-foaming product. A fully-developed prototype of the MSF evaporator allowed us to achieve continuous operation with a distillate production, reaching from 5 to 10 kg/h at a liquid digestate flow rate of 0.4–0.5 m3/h. The main drawback of this technology is the pollution of the distillate with ammonia nitrogen, and it is for this reason that the basic procedures of its subsequent elimination was selected for further analysis.

Thesis (Ph. D.)--University of California, Riverside, 2010.
Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed May 18, 2010). Includes bibliographical references. Also issued in print.

Ce travail porte sur la modélisation par marcheurs browniens du transfert conductif instationnaire au sein d’un milieu hétérogène. Le milieu est représenté par une structure voxélisée 3D. Chaque marcheur transporte une enthalpie élémentaire au cours de son déplacement. Ce mouvement d’enthalpie représente le flux conductif et permet de simuler la conduction en régime transitoire de façon quantitative. Une étude a montré l’importance du choix du pas de temps du calcul. Une valeur empirique de ce paramètre, dépendant du pas spatial et de la diffusivité des constituants, a été établie, permettant de modéliser correctement la conduction dans chacun des régimes de transfert. Plusieurs problèmes liés au comportement des marcheurs ont dû être résolus pour pouvoir modéliser deux techniques expérimentales de caractérisation thermique bien connues : la technique dite de la plaque chaude gardée et la méthode flash. Une condition de température imposée est modélisée par un réservoir dont le nombre de marcheurs est régulé. Une condition de paroi adiabatique impose une réflexion spéculaire aux marcheurs. Un critère stochastique de transmission basé sur les effusivités a été établi pour traiter la rencontre d’un marcheur avec une interface entre deux constituants d’un milieu hétérogène. La gestion des pertes convectives aux frontières du domaine de calcul se base également sur un critère de transmission faisant intervenir l’effusivité du constituant, le pas de temps et le coefficient d’échange convectif. Une condition de flux imposé, de profil temporel quelconque, se traduit par une injection de marcheurs à travers la frontière concernée. L’association complexe de ces briques a permis de modéliser le transfert thermique instationnaire dans des structures hétérogènes voxélisées. La comparaison de nos résultats à ceux issus d’approches plus classiques a permis de valider la capacité de notre modèle à caractériser les propriétés thermiques phoniques de structures virtuelles ou réelles. On conclut ce travail en posant les bases d’une stratégie de simulation du couplage conducto-radiatif à l’échelle locale de la structure voxélisée : l’apport énergétique du rayonnement est modélisé par un terme source de puissance volumique dans les voxels
This work deals with the modeling of transient conduction heat transfer by brownian walkers within a heterogenous medium. The medium is described by a 3D voxelised structure. Each walker carries an elementary enthalpy during its movement. This enthalpy motion represents the thermal flux and allows to solve quantitatively the transient thermal conduction. A study demonstrates the importance of the time step chosen in the simulations. An empirical value of this parameter, based on the spatial resolution and the thermal diffusivities of the constituents, has been established to model accurately the conduction at each step of the heat transfer. Several problems related to the behavior of the walkers had to be solved to model two well-known experimental thermal characterization techniques: the hot guarded plate technique and the flash method. An imposed temperature condition is modeled by a reservoir where the number of walkers is regulated. An adiabatic wall condition imposes a specular reflection to the walkers. A stochastic transmission criterion, based on the thermal effusivities, was established to treat the behavior of a walker at voxel-voxel interface between two different constituents. Convective losses at a boundary of the numerical structure are represented by an analogous stochastic transmission criterion involving the effusivity, the convective exchange coefficient and the time step. An imposed thermal flux condition, of any temporal profile, is modeled by an injection of walkers through the boundary. The complex combination of these computing blocks allowed to model transient heat transfer within voxelised heterogeneous structures. The comparison between our results and those issued from more classical approaches allowed to validate the ability of our model to characterize the phonic thermal properties of virtual and real structures. We conclude this PhD work with preliminary activities aiming at the simulation of conduction-radiation coupling at the voxel scale of the structure: the radiative contribution is modeled by an internal power source term within the voxels

This diploma thesis is mainly focused on the digestate thickening in a biogas plant. First, overview of the biogas technology in the Czech Republic is presented. Furthermore, problems with waste heat utilization and processing of fermentation residues (digestate) are described in more detail. Based on the research, multi-stage flash (MSF) evaporation was chosen as the technology for the digestate thickening. The main part deals with the integration of the chosen technology into a biogas plant process. The programming language Python was used to simplify the given task. In the preliminary stage, a complex mathematical model of a biogas plant was created, focusing particularly on the mass and energy balances. Subsequently, a computational model of the MSF evaporator was programmed. A procedure for the integration was suggested using the sensitivity analyses. Additionally, the model of a biogas plant was extended with the MSF evaporator. The key outcome of the thesis is a technical-economic analysis in which the impact of digestate transport price and electricity feed-in tariff on payback period is investigated. The results suggest profitability of MSF evaporator for biogas plants without subsidized feed-in tariff. Contrastingly, installation of MSF evaporator in older biogas plants with subsidized feed-in tariff can be economically viable only in cases of significantly longer transport distances.

Le CEA travaille sur des matériaux poreux - alvéolaires, composites, céramiques, etc. - et cherche à optimiser leurs propriétés pour des utilisations spécifiques. Ces matériaux, souvent composés de plusieurs constituants, ont en général une structure complexe avec une taille de pores de quelques dizaines de microns. Ils sont mis en oeuvre dans des systèmes de grande échelle, supérieure à leurs propres échelles caractéristiques, dans lesquels on les considère comme équivalents à des milieux homogènes, sans prendre en compte sa microstructure locale, pour simuler leur comportement dans leur environnement d'utilisation.Nous nous intéressons donc à la caractérisation des propriétés thermiques effectives de matériaux à microstructure hétérogène en cherchant à déterminer par méthode inverse en fonction de la température la diffusivité thermique qu'ils auraient s'ils étaient homogènes.L'identification de la diffusivité de matériaux poreux et/ou semi-transparents est rendue difficile par le couplage conducto-radiatif fort qui peut se développer rapidement dans ces milieux avec une augmentation de la température. Nous avons donc modélisé le transfert de chaleur couplé conducto-radiatif en fonction de la température au sein de matériaux poreux multiconstituants à partir de leur microstructure numérisée en voxels. Notre démarche consiste à nous appuyer sur la microstructure 3D obtenue par tomographie. Ces microstructures servent de support numérique à cette modélisation qui permet d'une part de simuler tout type d'expériences thermiques numériques - en particulier la méthode flash dont les résultats nous permettent de déduire la diffusivité thermique -, et d'autre part de reproduire le comportement thermique de ces échantillons dans leur condition d'utilisation.

In this study the data taken from Outokumpu type Flash smelter of Eti-Bakir Plant (Samsun, Turkey) was used to write a computer program in Visual Basic with interface to Excel. Flash smelting is the pyrometallurgical process for smelting metal sulfide concentrates, used in Eti-Bakir plant. In this plant, copper flash smelting consists of blowing fine, dried copper sulfide concentrate mixtures, silica flux, lignite with air into the furnace and natural gas as main fuel. The molten matte is the principal product of the furnace and slag contains 0.5-2% Cu. It is sent to a slag treatment (flotation) process for Cu recovery. This flash furnace off-gas contains from 8-12 volume % SO2 which is fixed as H2SO4. Written program was used to optimize the consumption of oxygen enriched air, fuel and lignite in this Flash Smelter by making material and heat balance of the plant.

Thesis (MEng) -- Stellenbosch University, 2014.
ENGLISH ABSTRACT: This project investigates aspects of a novel inherently safe nuclear power steam supply system as safety is of paramount importance. The system envisaged has unique features namely: a) a two-phase flow flash-tube type natural circulating primary loop (also the secondary radioactive particle containment); b) a twophase flow thermosyphon heat pipe type heat exchanger secondary loop is used to transfer heat from the primary loop to the steam generators, thereby physically separating the two flow streams from one another; c) a natural convection air cooled condenser for the removal of the reactor’s residual heat; d) a unique core using TRISO type fuel (acting as the primary radioactive particle containment) with life of at least 8.9 years; e) a steel containment vessel acting as a tertiary radioactive product containment; f) a concrete containing structure with air vents to allow air to pass over the main steel containment vessel for cooling purposes in the case of an emergency, and for the removal of parasitic heat during operation. In particular the primary and secondary loops of the proposed system are investigated. This is done by design, construction and testing of a small scale experimental set-up of the primary and secondary loops as well as the development of theoretical models for the two loops. A literature survey focusing on nuclear technology, thermosyphon loops, natural circulating loop instabilities, heat pipes, and two-phase flow modelling is presented to give a brief overview of the technologies as well as tools used in the work undertaken. Observations of the inside flow behaviour of the primary loop experimental set-up were made possible by windows providing many insights into the inner workings, such as plume formation and geysering. The transient response of the secondary heat pipe loop start-up is also investigated. A thermal resistance theoretical model was developed for the secondary loop using heat transfer formulae from theory as well as experimentally semiempirical correlated formula. Different states of operation of the secondary loop were observed during testing with the theoretical model of the condensing regime correlating well, two-phase regime correlating acceptably and liquid regime correlating poorly to experimental results and thus were modelled using an experimentally determined overall heat transfer coefficient. The secondary loop model of the liquid regime is coupled with the primary loop theoretical model to predict the system’s performance. A homogeneous, one-dimensional, simple theoretical model for the primary loop was derived and computer simulated. The results did not compare well with experimental results for single phase flow and failed to capture the onset of two-phase flow. The assumptions of one dimensional model with a unidirectional flow, a hydrostatic pressure problem, a constant volumetric flow rate and the inability of the implementation of the code to handle expansion are noted as some of the flaws in the theoretical model. The following recommendations are made: a more advanced design of the pressuriser should be incorporated into the experiment; the secondary loop’s theoretical model should be characterised under a broader set of operating conditions; the computer program can be used as the basis for further research and implementation of alternative solution algorithms and models.
AFRIKKANSE OPSOMMING: Hierdie projek ondersoek aspekte van ’n ongewone, essensieel veilige kernkrag stoomtoevoer-stelsel, omdat veiligheid van kardinale belang is. Die stelsel wat voorgestel is, het unieke eienskappe, naamlik: a) ’n twee-fasevloei flits-buistipe natuurlik sirkulerende primêre lus (wat ook die sekondêre inperking van radioaktiewe materiaal bevat); b) ’n twee-fasevloei termo-heweleffek sekondêre lus hitte-pyp hitte-uitruiler word gebruik om die hitte vanaf die primêre lus oor te dra na die stoomkragopwekkers en daardeur word die twee strome se vloei fisies geskei van mekaar; c) ’n natuurlike konveksie lugverkoelde kondensor word gebruik vir die verwydering van die reaktors se oortollige hitte; d) ’n unieke kern gebruik TRISO-tipe brandstof (wat as die primêre inperking van radioaktiewe materiaal optree) met ’n lewe van minstens 8.9 jaar; e) ’n inperkingshouer van staal wat optree as ’n tersiêre radioaktiewe produkhouer; f) ’n betonstruktuur met lugventilasie om toe te laat dat lug oor die hoof staalhouer vloei vir verkoeling in ’n noodgeval, en vir die verwydering van parasitiese hitte tydens werking. Hoofsaaklik word die primêre en sekondêre lusse van die voorgestelde stelsel ondersoek. Dit word gedoen deur die ontwerp, konstruksie en die toets van ’n eksperimentele opstelling van die primêre en sekondêre lusse op klein skaal, sowel as die ontwikkeling van teoretiese modelle vir die twee lusse. ’n Literatuurstudie wat fokus op kerntegnologie, termo-heweleffeklusse, natuurlik sirkulerende lus instabiliteit, hitte-pype, en twee-fase vloeimodellering word aangebied om ’n kort oorsig te gee van die tegnologie, sowel as gereedskap gebruik in die werk wat onderneem is. Om die interne vloeigedrag van die primêre lus se eksperimentele opstelling waar te neem, word daar gebruik gemaak van vensters wat dien as ’n manier om die innerlike werking van die proses soos pluimvorming en die kook van die water in die warmwaterkolom te toon. Die oorgangsreaksie van die sekondêre hittepyplus aanvangs is ook ondersoek. ’n Teoretiese termiese weerstandmodel is ontwikkel vir die sekondêre lus met behulp van hitte-oordragformules waarvoor hitte-oordragteorie gebruik is, wat met eksperimentele semi-empiriese formules gekorreleer is. Verskillende toestande van die sekondêre lus se werking is waargeneem gedurende die toetse. Die teoretiese model het goed met die kondensasiestaat gekorreleer, terwyl by die twee-fasewerkswyse aanvaarbare korrellasies aangetref is en die uiteindelike vloeitoestand swakker gekorrelleer het met eksperimentele resultate en dus gemodelleer is met behulp van die NTU-effektiwiteitsmetode. Die sekondêre lusmodel van die vloeistoftoestand is gekoppel met die primêre lus teoretiese model om die werking van die stelsels te voorspel. ’n Homogene een-dimensionele eenvoudige teoretiese model van die primêre lus is afgelei en ’n rekenaar simulasie is uitgevoer. Die resultate vergelyk nie goed met die eksperimentele resultate vir enkelfasevloei en kon nie die aanvang van twee-fasevloei beskryf nie. Die aannemings van ’n een-dimensionele model met eenrigting vloei, ’n hidrostatiese druk probleem, ’n konstant volumetries vloeitempo en die onvermoë van die implementering van die kode om uitbreiding te hanteer is bekend as ’n paar van die foute in die teoretiese model. Die volgende aanbevelings word gemaak: ’n meer gevorderde ontwerp van drukreëlaar moet in die eksperiment ingesluit word; die sekondêre lus se teoretiese model moet gekenmerk word onder ’n wyer stel bedryfsomstandighede, en die rekenaar program kan gebruik word as die basis vir verdere navorsing en die implementering van alternatiewe algoritmes en modelle.

La thermographie infrarouge est une méthode largement employée pour la caractérisation des propriétés thermophysiques des matériaux. L’avènement des diodes laser pratiques, peu onéreuses et aux multiples caractéristiques, étendent les possibilités métrologiques des caméras infrarouges et mettent à disposition un ensemble de nouveaux outils puissants pour la caractérisation thermique et le contrôle non desturctif. Cependant, un lot de nouvelles difficultés doit être surmonté, comme le traitement d’une grande quantité de données bruitées et la faible sensibilité de ces données aux paramètres recherchés. Cela oblige de revisiter les méthodes de traitement du signal existantes, d’adopter de nouveaux outils mathématiques sophistiqués pour la compression de données et le traitement d’informations pertinentes. Les nouvelles stratégies consistent à utiliser des transformations orthogonales du signal comme outils de compression préalable de données, de réduction et maîtrise du bruit de mesure. L’analyse de sensibilité, basée sur l’étude locale des corrélations entre les dérivées partielles du signal expérimental, complète ces nouvelles approches. L'analogie avec la théorie dans l'espace de Fourier a permis d'apporter de nouveaux éléments de réponse pour mieux cerner la «physique» des approches modales.La réponse au point source impulsionnel a été revisitée de manière numérique et expérimentale. En utilisant la séparabilité des champs de température nous avons proposé une nouvelle méthode d'inversion basée sur une double décomposition en valeurs singulières du signal expérimental. Cette méthode par rapport aux précédentes, permet de tenir compte de la diffusion bi ou tridimensionnelle et offre ainsi une meilleure exploitation du contenu spatial des images infrarouges. Des exemples numériques et expérimentaux nous ont permis de valider dans une première approche cette nouvelle méthode d'estimation pour la caractérisation de diffusivités thermiques longitudinales. Des applications dans le domaine du contrôle non destructif des matériaux sont également proposées. Une ancienne problématique qui consiste à retrouver les champs de température initiaux à partir de données bruitées a été abordée sous un nouveau jour. La nécessité de connaitre les diffusivités thermiques du matériau orthotrope et la prise en compte des transferts souvent tridimensionnels sont complexes à gérer. L'application de la double décomposition en valeurs singulières a permis d'obtenir des résultats intéressants compte tenu de la simplicité de la méthode. En effet, les méthodes modales sont basées sur des approches statistiques de traitement d'une grande quantité de données, censément plus robustes quant au bruit de mesure, comme cela a pu être observé
Infrared thermography is a widely used method for characterization of thermophysical properties of materials. The advent of the laser diodes, which are handy, inexpensive, with a broad spectrum of characteristics, extend metrological possibilities of infrared cameras and provide a combination of new powerful tools for thermal characterization and non destructive evaluation. However, this new dynamic has also brought numerous difficulties that must be overcome, such as high volume noisy data processing and low sensitivity to estimated parameters of such data. This requires revisiting the existing methods of signal processing, adopting new sophisticated mathematical tools for data compression and processing of relevant information.New strategies consist in using orthogonal transforms of the signal as a prior data compression tools, which allow noise reduction and control over it. Correlation analysis, based on the local cerrelation study between partial derivatives of the experimental signal, completes these new strategies. A theoretical analogy in Fourier space has been performed in order to better understand the «physical» meaning of modal approaches.The response to the instantaneous point source of heat, has been revisited both numerically and experimentally. By using separable temperature fields, a new inversion technique based on a double singular value decomposition of experimental signal has been introduced. In comparison with previous methods, it takes into account two or three-dimensional heat diffusion and therefore offers a better exploitation of the spatial content of infrared images. Numerical and experimental examples have allowed us to validate in the first approach our new estimation method of longitudinal thermal diffusivities. Non destructive testing applications based on the new technique have also been introduced.An old issue, which consists in determining the initial temperature field from noisy data, has been approached in a new light. The necessity to know the thermal diffusivities of an orthotropic medium and the need to take into account often three-dimensional heat transfer, are complicated issues. The implementation of the double singular value decomposition allowed us to achieve interesting results according to its ease of use. Indeed, modal approaches are statistical methods based on high volume data processing, supposedly robust as to the measurement noise

Yes
Fouling on heat transfer surfaces due to scale formation is the most concerned item in thermal desalination industry. Here, a dynamic fouling model is developed and incorporated into the MSF dynamic process model to predict fouling at high temperature and high velocity. The proposed dynamic model considers the attachment and removal mechanisms in the fouling phenomena with more relaxation of the assumptions such as the density of the fouling layer and salinity of the recycle brine. While calcium sulphate might precipitate at very high temperature, only the crystallization of calcium carbonate and magnesium hydroxide are considered in this work. Though the model is applied in a 24 stages brine recycle MSF plant, only the heat recovery section (21 stages) is considered under this study. The effect of flow velocity and surface temperature are investigated. By including both diffusion and reaction mechanism in the fouling model, the results of the fouling prediction model are in good agreement with most recent studies in the literature. The deposition of magnesium hydroxide increases with the increase in surface temperature and flow velocity while calcium carbonate deposition increases with the increase in the surface temperature and decreases with the increase in the flow velocity.

Eulerian-Eulerian Computational Fluid Dynamics (CFD) techniques continue to show promise for characterizing the internal flow and near-field spray for various fuel injection systems. These regions are difficult to observe experimentally, and simulations of such regions are limited by computational expense or reliance on empiricism using other methods. The physics governing spray atomization are first introduced. Impinging jet sprays and Gasoline Direct Injection (GDI) are selected as applications, and modern computational/experimental approaches to their study are reviewed. Two in-house CFD solvers are described and subsequently applied in several case studies. Accurate prediction of the liquid distribution in a like-doublet impinging jet spray is demonstrated via validation against X-Ray data. Turbulence modeling approaches are compared for GDI simulations with dynamic mesh motion, with results validated against previously available experimental data. A new model for turbulent mixing is discussed. Code performance is thoroughly tested, with new mesh motion techniques suggested to improve scaling. Finally, a new workflow is developed for incorporating X-Ray scanned geometries into moving-needle GDI simulations, with full-duration injection events successfully simulated for both sub-cooled and flash-boiling conditions.

Yes
In this paper, a dynamic model of fouling is presented to predict the crystallization of calcium carbonate and magnesium hydroxide inside the condenser tubes of Once-Through Multistage Flash (MSF-OT) desalination process. The model considers the combination of kinetic and mass diffusion rates taking into account the effect of temperature, velocity and salinity of the seawater. The equations for seawater carbonate system are used to calculate the concentration of the seawater species. The effects of salinity and temperature on the solubility of calcium carbonate and magnesium hydroxide are also considered. The results reveal an increase in the fouling inside the tubes caused by crystallization of CaCO3 and Mg(OH)2 with increase in the stage temperature. The intake seawater temperature and the Top Brine Temperature (TBT) are varied to investigate their impact on the fouling process. The results show that the (TBT) has greater impact than the seawater temperature on increasing the fouling.

Vertical transmission of HIV is still a growing concern in South Africa. Breastfed infants are still at risk as HIV is present in breast milk, leaving HIV-positive mothers unsure of the best feeding option for their infants. However, there are various infant feeding techniques that HIV-positive mothers can use to supplement breastfeeding and flash-heat is one of them. Flash-heat is heat treating expressed breast milk to deactivate HIV for infant feeding. This study explored the possibility of HIV-positive mothers to practice flash-heating method for their infants exclusively for four months as a strategy to prevent vertical transmission of HIV. A descriptive, explorative and contextual design using a mixed method was used to obtain data from mothers in a post natal ward at Tembisa hospital. The mixed method used was useful in identifying the number of HIV-positive mothers who would adopt the flash-heat technique, the characteristics of mothers whom the technique could be promoted to, the factors that influence/affect the choice of infant feeding for these mothers, as well as their feelings associated with the feeding technique. Most (74%) mothers had a positive response to the flash-heat technique compared to 10% who were uncertain. They believed that heat treating their breast milk would result in their infants being HIV-free. In addition they believed that this method was cheaper than formula feeding and expressed positive feelings about touching their breast milk while expressing with no adverse feelings of expressing into a glass jar. Furthermore, findings of this study indicated that HIV-positive mothers in a public health facility would adopt flash-heat as an alternative infant feeding method. Thus practical guidelines to promote this feeding method were proposed. The proposed draft guidelines which promote the use of the flash-heat infant feeding method for HIV-positive mothers in public sector facilities will be communicated to relevant authorities such as the National Department of Health. These guidelines support the new policy shift to exclusive breastfeeding as a child survival strategy in South Africa.
Health Studies
D.Litt. et Phil. (Health Studies)