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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Contribution à la caractérisation expérimentale des transferts couplés en écoulement turbulent en conduite horizontale avec ou sans condensation / Experimental analyses of oupled transfer phenomena with or without condensation in a turbulent channel flow fied

Sakay, Danilo 09 December 2014 (has links)
Ce travail expérimental concerne l'analyse du transfert de chaleur au sem d'un écoulementturbulent d'air humide en présence ou non d'un phénomène de condensation. Pour cela, une souffleriespécifique a été conçue au sein du laboratoire afi n de générer un écoulement d'air contrôlé en vitesse,en température et en teneur en humidité. Cet écoulement traverse un canal d'étude rectangulairehorizontal dont les parois inférieures sont maintenues à température constante. Une première partie destravaux permet de décrire l'écoulement au se in du canal par l'analyse des champs de vitesse PlVmoyenne et fluctuante pour différents nombres de Reynolds (Re = 10056 à Re = 55333) et dedétailler les régimes dynamiques obtenus. Dans la zone pleinement turbulente, une attentionparticulière a été portée sur la caractérisation de la sous-couche visqueuse par PlV à haute résolutionspatiale. L'analyse fine du profil de température d'air en proche paroi permet d'estimer avec précisionle transfert convectif local. Les résultats obtenus sans changement de phase ne montrent pasd'influence de la teneur en humidité sur le flux convectif. Au-delà des résu ltats en milieu non-saturé,le phénomène de condensation est abordé. La visualisation du dépôt de vapeur d'eau condenséepermet d' identifier macroscopiquement les différents régimes de condensation (film, gouttes oumixte). L' analyse fine à échelle locale permet de définir différentes étapes de la condensation:développement par grossissement et par fusion entre plusieurs gouttes, mouvements de frontièresaprès fusion. Cette étude a été menée pour des substrats de nature et de conditions de mouillabilitédifférentes, ainsi que pour des taux d'humidité absolue variables. Enfin, l' identification de ces étapesest complétée par la quantification de la condensation à partir des mesures de masse condensée.Celles-ci sont exploitées pour estimer les flux de chaleur latente et pour comparer le comportement surdifférents substrats. / Heat transfer in a turbulent moist airflow field is under investigation with or without condensation phenomena. A dedicated experimental set-up was developed and with special attention was given to the control of mass flow rate, temperature and moisture content at inlet. Flow field then develops within a rectangular channel with thermal regulation at walls. Characteristic Reynolds number range studied is between 10 056 and 53 333 and average and fluctuating dynamic and thermal fields were depicted by PIV and thermocouple investigation respectively. In the turbulent region, a refined analysis was carried to characterize the sub-layer viscous region (from high-resolution PIV measurements) and temperature profiles at the very close vicinity of the wall allows one to locally estimate convective heat flux. Results underline that the degree of moisture of unsaturated humid air does not play any major role on the thermal flux exchanged. Beyond the saturation regime, condensation occurs and water vapor deposited along the flat horizontal wall is analyzed. Condensation is driven by several regimes (film-wise, dropwise and mixed) and within drop-wise condensation, size distribution as well as drop coalescence is detailed. Our analyses was carried on three substrates presenting different wettability conditions while moisture degree was considered as an additional parameter. Finally, the mass of condensed water was measured in time and latent heattransfer was estimated and compared between the different substrates.
2

Experimental Investigations on Non-Wetting Surfaces

Stoddard, Ryan Manse 24 May 2021 (has links)
Superhydrophobic (SHS) and lubricant-infused surfaces (LIS) exhibit exceptional non-wetting characteristics that make them attractive for energy production applications including steam condensation and fouling mitigation. The dissertation work focuses on application of non-wetting surfaces to energy production using a systematic approach examining each component of surface fabrication in three functional areas. First, SHS and LIS are fabricated using robust, scalable methods and tested for durability in heated, wet conditions and under high-energy water jet impingement. Clear performance differences are shown based on surface texturing, functionalizing agent, and infused lubricant. Second, SHS and LIS are applied to tube exteriors and evaluated for their ability to produce sustained dropwise condensation in a typical power plant condenser environment. The surfaces are shown to produce heat transfer coefficients up to 7-10 times that of film-wise condensation, with condenser effectiveness of 0.92 or better compared to effectiveness of about 0.6 in conventional condensers. Third, LIS on the interior of tubes are assessed in accelerated mineral fouling conditions. LIS are shown to mitigate calcium sulfate and calcium carbonate fouling under laminar conditions. The results of the study bear profound benefits to reducing the levelized cost of condensers and water uptake in thermoelectric power plants, that currently consume about 50% of the total water use in the U.S. / Doctor of Philosophy / Creating durable, hybrid surfaces for improved steam condensation and fouling mitigation would provide substantial impact to power generation worldwide. Bioinspired, non-wetting surfaces, such as superhydrophobic (SHS) and lubricant-infused surfaces (LIS) exhibit exceptional non-wetting characteristics that make them attractive for energy applications. Each of these non-wetting technologies, however, faces durability and scalability challenges that make them unfeasible for widespread, practical adoption. As a result, decades of materials science research have stagnated in the research laboratories with limited demonstrations of dropwise condensation and fouling mitigation in static situations. The dissertation work focuses on application of SHS and LIS to energy production using a systematic approach examining each component of surface fabrication in three functional areas. First, SHS and LIS are fabricated using robust, scalable methods and tested for durability using ASTM standard static and dynamic evaluation methods. Clear performance differences are shown based on surface texturing, functionalizing agent, and infused lubricant. Second, dropwise steam condensation on the surfaces are shown to exhibit heat transfer performance an order of magnitude greater than film-wise condensation in a typical power plant condenser environment. The surfaces are shown to produce heat transfer coefficients up to 7-10 times that of film-wise condensation, with condenser effectiveness of 0.92 or better compared to effectiveness of about 0.6 in conventional condensers. This work presents for the first time, a non-dimensional correlation for a priori prediction of LIS heat transfer performance given known qualities of the LIS. Third, challenges of fouling mitigation in power plants have been studied for over a decade. This work demonstrates for the first time that LIS applied to the interior of tubes mitigate calcium sulfate and calcium carbonate fouling in both static and laminar flow conditions.

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