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Study on convective heat transfer of supercritical carbon dioxyde flow have been performed. An experimental apparatus was build, allowing an experimental investigation of heat transfer and pressure drop during cooling of supercritical CO2 turbulent flows. Horizontal and vertical flows were considered.Experimental results on vertical flows show that heat transfer coefficient reach a peak at a temperature close to the pseudo-critical one and that the peak value is decreasing with increasing pressure. A strong influence of mixed convection has to be taken into account too. Results were presented on a classical form used in mixed convection studies in heating condition, leading to the conclusion that mixed convection behaviors are quite similar in heating and cooling conditions.A numerical model for supercritical fuids flows was developped too, in heating conditions., Une étude des transferts de chaleur convectifs de CO2 supercritique a été menée. Une installation expérimentale a été construite pour la thèse et a permis une investigation expérimentale des transferts de chaleur et des pertes de pressions lors du refroidissement d'écoulements turbulents de CO2 supercritique. Des configurations verticales et horizontales ont été considérées.Les campagnes d'essais en écoulement vertical ont montré que le cœfficient d'échange montrait une forme particulière caractérisée par un pic au voisinage de la température pseudo-critique, la valeur au pic diminue à mesure que la pression augmente. Il apparaît également qu'une forte influence de la convection mixte est à considérée. Les résultats ont été présentés sous une forme caractéristique des études sur la convection mixte en chauffage, confirmant ainsi que les comportements de la convection mixte sont analogues en chauffage et en refroidissement.Parallèlement, un modèle numérique pour la simulation d'écoulement turbulents de fluides supercritiques a été développé.

National audience; Cet article présente les résultats expérimentaux de transfert de chaleur obtenus lors du refroidissement d’écoulements verticaux descendants de CO2 supercritique, en régime turbulent. Le dispositif expérimental développé pour l’étude, ainsi que sa spécificité par rapport à une pompe à chaleur au CO2, sont plus particulièrement abordés. Les résultats expérimentaux mettent plus particulièrement en lumière l’influence de la convection mixte sur les transferts thermiques dans cette configuration.

Due to the increase in energy prices and spiralling consumption, there is a need to greatly reduce the cost of electricity within data centers, where it makes up 50% of the total cost of the IT infrastructure. A technological solution to this is using on-chip cooling with a single-phase or evaporating liquid to replace energy intensive air-cooling. The energy carried away by the liquid or vapour can also potentially be used in district heating, as an example. Thus, the important issue here is “what is the most energy efficient heat removal process?” As an answer, this paper presents a direct comparison of single-phase water, a 50% water ethylene glycol mixture and several two-phase refrigerants, including the new fourth generation refrigerants HFO1234yf and HFO1234ze. Two-phase cooling using HFC134a had an average junction temperature 9 to 15˚C lower than for single-phase cooling, while the required pumping power for the CPU cooling element for single-phase cooling was on the order of 20-130 times higher to achieve the same junction temperature uniformity. Hot-spot simulations also showed that two-phase refrigerant cooling was able to adjust to local hot-spots because of flow boiling's dependency on the local heat flux, with junction temperatures being 20 to 30˚C lower when compared to water and the 50% water-ethylene glycol mixture, respectively. An exergy analysis was developed considering a cooling cycle composed by a pump, a condenser and a multi-microchannel cooler. The focus was to show the exergetic efficiency of each component and of the entire cycle when the subject energy recovery is considered. Water and HFC134a were the working fluids evaluated in such analysis. The overall exergetic efficiency was higher when using HFC134a (about 2%) and the exergy destroyed, i.e. irreversibilities, showed that the cooling cycle proposed still have a huge potential to increase the thermodynamic performance.