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.

An experimental study of condensate was performed by exposing a sheet of glass, cooled at its bottom edge, to an enclosure with a controlled environment. The air in the enclosure was maintained at a constant relative humidity (RH) and a constant dry bulb temperature (Tdb). Experiments were conducted at Tdb = 22.1°C and RH of 30%, 35%, 40%, 45%, and 50%. It was found that the time until initial condensation run-off was sensitive to low RH (RH = 30%, 35%, 40%) and insensitive to high RH (RH = 45%, 50%). Time until run-off decreased with increasing RH. It was found that, at first, condensation run-off occurred near the bottom of the glass and left one to believe that the remaining condensate was in steady state with the enclosure. Over a 16 hour period however, it was found that the condensation run-off front, in every case, progressed upward to include the entire condensate area. Similar to time of initial run-off, speed of condensation front movement increased with RH and was also insensitive at high RH. A summary plot showing run-off front position is presented. This chart can be used to predict initial run-off and front progression at the bottom edge of any window if the surface temperature profile is known.

An experimental study of condensate was performed by exposing a sheet of glass, cooled at its bottom edge, to an enclosure with a controlled environment. The air in the enclosure was maintained at a constant relative humidity (RH) and a constant dry bulb temperature (Tdb). Experiments were conducted at Tdb = 22.1°C and RH of 30%, 35%, 40%, 45%, and 50%. It was found that the time until initial condensation run-off was sensitive to low RH (RH = 30%, 35%, 40%) and insensitive to high RH (RH = 45%, 50%). Time until run-off decreased with increasing RH. It was found that, at first, condensation run-off occurred near the bottom of the glass and left one to believe that the remaining condensate was in steady state with the enclosure. Over a 16 hour period however, it was found that the condensation run-off front, in every case, progressed upward to include the entire condensate area. Similar to time of initial run-off, speed of condensation front movement increased with RH and was also insensitive at high RH. A summary plot showing run-off front position is presented. This chart can be used to predict initial run-off and front progression at the bottom edge of any window if the surface temperature profile is known.

Call number: LD2668 .T4 1952 Y4 / Master of Science

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A new test facility, with a vertical reflux condenser of 1500mm overall length and 45mm internal diameter, has been commissioned and tested and methods developed for measuring key process parameters. An experimental study of reflux condensation in a single tube using n-pentane and iso-octane and binary mixtures of these single component hydrocarbons has been undertaken. Using water as the cooling medium, a correlation was developed for determining the coolant-side heat transfer coefficient in the reflux condenser based on the Wilson plot method. The composition of binary liquid mixture samples from the test facility was determined using an empirical correlation developed using density measurements from a vibrating u-tube densitometer. The single components were condensed in the range 32.0-48.4°C and 0.106-1.515bara by adjusting the test condenser heat load for fixed conditions on the coolant side to investigate how the condensate-film heat transfer coefficient varied with the condensate film Reynolds number. The results show good agreement with the method recommended by HTFS for correcting the Nusselt theory for the effects of waves. A further small correction was made to improve the fit to the data. The binary hydrocarbon mixtures were condensed across the range 65.9-90.1°C and 0.729-1.531bara by conducting similar experiments where the feed vapour contained 50% and 70% n-pentane. Composition measurements of the condensate and vapour leaving the test condenser were made to examine the separation of components during partial reflux condensation. The results suggest that this separation is influenced by heat flux and that it would be improved if the test condenser were operated at a lower heat flux. Further experimental work is needed to verify this, and to investigate how this influences the number of thermodynamic stages, which was found to be less than one with the conditions reported here. Analysis of the heat transfer resistances on the vapour side showed that the standard procedure of using a dry-gas heat transfer coefficient, with or without a mass transfer correction term based on the film theory, poorly predicted the experimental values. These predictions were improved by the use of an enhancement factor, which may be more relevant in counter-current than co-current condensing situations. The results indicate that use of a dry-gas heat transfer coefficient with the film theory correction factor, over-predicts the mass transfer resistance. Comparison was made between the data and predictions based on the integral condensation curve, as might be used in Silver's method for condenser thermal design. It was shown that this method poorly predicted the surface area and the separation achieved in the test condenser. The results indicate that the heat and mass transfer coefficients obtained in a plain tube are significantly higher than those based on using a dry-gas heat transfer coefficient corrected by film theory. Implications for the design of reflux condensers have been presented.

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