The growing demand for the development of efficient miniature cooling systems has led to stimulating numerous investigations on two-phase flow boiling in small to microscale tubes. Because of the variation in properties of synthetic cooling fluids, this causes an inaccuracy of existing flow boiling prediction models or correlations in the literature to interpolate or extrapolate the two-phase flow boiling heat transfer and pressure drop. The purpose of this investigation was to study experimentally the parametric aspects of flow boiling characteristics inside vertical stainless-steel tubes with four different internal diameters (1.1, 2.01, 2.88 and 4.26 mm). The R245fa (1,1,1,3,3-pentafluoropropane, HFC-245fa) was used as working fluid. The experiments were carried out under a system pressure range of 185 - 310 kPa (which correspond to a saturation temperature range of 31 - 46 °C), mass flux range of 200 - 500 kg/m²s, heat flux range of 3 - 188.5 kW/m², vapour quality up to the onset of dryout and 5 K inlet subcooling. Flow pattern visualisations, two-phase pressure drops and saturated flow boiling heat transfer coefficients were presented. The experimental data of R134a employed for comparison is acquired from the previous studies of Huo et al. (2007), Shiferaw et al. (2011) and Mahmoud et al. (2014a). These studies were carried out in the same experimental facility and under the similar operating conditions. The Two-phase flow regimes inside four tubes were visualised in a borosilicate glass tube located at the heated section outlet to capture the dominant flow patterns which assist to elucidate the heat transfer results. The flow boiling visualisation was recorded by a high-speed camera with experiments of increasing and decreasing heat flux. The four observed flow regimes are identified as bubbly flow, slug flow, churn flow and annular flow. In increasing heat flux experiments, the churn and annular flows were only the dominant patterns in all four tubes. The slug flow was often discerned at lower mass flux except for the tube of 1.1 mm where it was not observed at all. This is contrary to decreasing heat flux experiments where all flow patterns including the bubble flow were observed in all the tubes. This shows a strong impact of hysteresis, which is a result of nucleation sites remained active as the heat flux is reduced. The flow patterns and transition boundaries for R245fa are affected by mass flux, system pressure, and tube diameter. The vapour quality corresponding to flow pattern transition boundary tends to decrease with increasing mass flux and tends to increase with increasing system pressure and decreasing tube diameter. Except for the bubbly-slug boundary, its vapour quality decreases with decreasing tube diameter. The experimental flow pattern maps of R245fa were fairly predicted with the predictive models developed for mini- and micro-channels by Tibiriçá et al. (2017). The two-phase pressure drop of R245fa is affected by mass flux, heat flux, system pressure, tube diameter and surface topography. The two-phase pressure drop increases with increasing mass flux and heat flux (vapour quality) and decreases with increasing system pressure and tube diameter. The two-phase pressure drop of the coated tube is higher than that of the uncoated tube. This is attributed to the coated tube having a higher surface roughness compared to the uncoated tube. The comparison between R245fa and R134a shows that the measured two-phase pressure drop of R245fa is dramatically higher than that of R134a. This arises from the difference in physical properties of the two fluids. The experimental data of two-phase pressure drop for 4.26 mm tube were reasonably predicted by Müller-Steinhagen and Heck (1986) correlation. Further, the experimental data of 2.88 mm and 2.01 mm tubes were well predicted by Chisholm (1973a), and Kim and Mudawar (2013), respectively. The experimental data of 1.1 mm tube were not well predicted by any of the selected predictive methods. The local heat transfer coefficient of all tubes increases with increasing heat flux for low and intermediate vapour qualities. After this vapour qualities, the heat flux effect diminishes. Then, the local heat transfer coefficient increased slightly with vapour quality, especially for higher heat flux near the outlet of the tube. However, the dryout inception in the 1.1 mm tube occurs after the intermediate vapour quality value and expands along the high vapour quality region. The behaviour of the local heat transfer coefficients of 1.1 and 2.88 mm tube is slightly dependent on the mass flux and vapour quality. Contrarily, there is insignificant effect of mass flux along 2.01 and 4.26 mm tube. This gives an indication of the contribution of nucleate boiling in the heat transfer process at lower and medium heat fluxes and nucleate boiling plus convective evaporation at higher heat fluxes near the tube outlet. Further, the local heat transfer coefficient increases as the system pressure increases. The tube diameter has a strong influence on the enhancement of local heat transfer coefficient. The enhancement in average heat transfer coefficient approaches 83% when the tube diameter is reduced from 4.26 to 1.1 mm. The trend of the local heat transfer coefficient of R134a was almost similar to that of R245fa with the exception of local dryout. The average heat transfer coefficient of R134a is about 106-151% larger than that of R245fa for the operational range studied. The dominant heat transfer mechanism is also represented by nucleate boiling for both fluids, particularly for 4.26 mm tube tested in this study. Also, the average heat transfer coefficient was enhanced by 33% when the inner tube surface coated with a copper coating. Finally, the correlation of Fang et al. (2017) predicted all experimental data for the four tubes with fair and similar accuracy.

Cette étude est dédiée à l'amélioration du design des microréacteurs gaz-liquide. Le terme de microréacteur correspond à des appareils composés de canaux dont les dimensions sont de l’ordre de quelques dizaines à quelques centaines de microns. Grâce à la valeur importante du ratio surface/volume, ces appareils constituent une issue prometteuse pour contrôler les réactions rapides fortement exothermiques, souvent rencontrées en chimie fine et pharmaceutique. Dans le cas des systèmes gaz-liquide, on peut citer par exemple les réactions de fluoration, d’hydrogénation ou d’oxydation. Comparés à des appareils conventionnels, les microréacteurs permettent de supprimer le risque d’apparition de points chauds, et d’envisager le fonctionnement dans des conditions plus critiques, par exemple avec des concentrations de réactifs plus élevées. En même temps, la sélectivité peut être augmentée et les coûts opératoires diminués. Ainsi, les technologies de microréacteurs s’inscrivent bien dans les nouveaux challenges auxquels l'industrie chimique est confrontée ; on peut citer en particulier la réduction de la consommation énergétique et la gestion des stocks de produits intermédiaires. Les principaux phénomènes qui doivent être étudiés lors de la conception d’un microréacteur sont le transfert de matière et le transfert thermique. Dans les systèmes diphasiques, ces transferts sont fortement influencés par la nature des écoulements, et l'hydrodynamique joue donc un rôle central. Par conséquent, nous avons focalisé notre travail sur l’hydrodynamique de l’écoulement diphasique dans les microcanaux et sur les couplages constatés avec le transfert de masse. Dans ce contexte, nous nous sommes dans un premier temps intéressés aux régimes d’écoulement et aux paramètres contrôlant la transition entre les différents régimes. Au vu des capacités de transfert de matière et à la flexibilité offerte en terme de conditions opératoires, le régime de Taylor semble le plus prometteur pour mettre en œuvre des réactions rapides fortement exothermiques et limitées par le transfert de matière. Ce régime d'écoulement est caractérisé par des bulles allongées entourées par un film liquide et séparées les unes des autres par une poche liquide. En plus du fait que ce régime est accessible à partir d’une large gamme de débits gazeux et liquide, l'aire interfaciale développée est assez élevée, et les mouvements de recirculation du liquide induits au sein de chaque poche sont supposés améliorer le transport des molécules entre la zone interfaciale et le liquide. A partir d'une étude de l’hydrodynamique locale d’un écoulement de Taylor, il s’est avéré que la perte de charge et le transfert de matière sont contrôlés par la vitesse des bulles, et la longueur des bulles et des poches. Dans l’étape suivante, nous avons étudié l'influence des paramètres de fonctionnement sur ces caractéristiques de l’écoulement. Une première phase de notre travail expérimental a porté sur la formation des bulles et des poches et la mesure des champs de vitesse de la phase liquide dans des microcanaux de section rectangulaire. Nous avons également pris en compte le phénomène de démouillage, qui joue un rôle important au niveau de la perte de charge et du transfert de matière. Des mesures du coefficient de transfert de matière (kLa) ont été réalisées tandis que l'écoulement associé était enregistré. Les vitesses de bulles, longueurs de bulles et de poches, ainsi que les caractéristiques issues de l’exploitation des champs de vitesse précédemment obtenus, ont été utilisées afin de proposer un modèle modifié pour la prédiction du kLa dans des microcanaux de section rectangulaire. En mettant en évidence l'influence du design du microcanal sur l’hydrodynamique et le transfert de matière, notre travail apporte une contribution importante dans le contrôle en microréacteur des réactions rapides fortement exothermiques et limitées par le transfert de matière. De plus, ce travail a permis d'identifier certaines lacunes en termes de connaissance, ce qui devrait pouvoir constituer l'objet de futures recherches. / The present project deals with the improvement of the design of gas-liquid microreactors. The term microreactor characterizes devices composed of channels that have dimensions in the several tens to several hundreds of microns. Due to their increased surface to volume ratios these devices are a promising way to control fast and highly exothermic reactions, often employed in the production of fine chemicals and pharmaceutical compounds. In the case of gas-liquid systems, these are for example direct fluorination, hydrogenation or oxidation reactions. Compared to conventional equipment microreactors offer the possibility to suppress hot spots and to operate hazardous reaction systems at increased reactant concentrations. Thereby selectivity may be increased and operating costs decreased. In this manner microreaction technology well fits in the challenges the chemical industry is continuously confronted to, which are amongst others the reduction of energy consumption and better feedstock utilization. The main topics which have to be considered with respect to the design of gasliquid μ-reactors are heat and mass transfer. In two phase systems both are strongly influenced by the nature of the flow and thus hydrodynamics play a central role. Consequently we focused our work on the hydrodynamics of the two-phase flow in microchannels and the description of the inter-linkage to gas-liquid mass transfer. In this context we were initially concerned with the topic of gas-liquid flow regimes and the main parameters prescribing flow pattern transitions. From a comparison of flow patterns with respect to their mass transfer capacity, as well as the flexibility offered with respect to operating conditions, the Taylor flow pattern appears to be the most promising flow characteristic for performing fast, highly exothermic and mass transfer limited reactions. This flow pattern is characterized by elongated bubbles surrounded by a liquid film and separated from each other by liquid slugs. In addition to the fact that this flow regime is accessible within a large range of gas and liquid flow rates, and has a relatively high specific interfacial area, Taylor flow features a recirculation motion within the liquid slugs, which is generally assumed to increase molecular transport between the gas-liquid interface and the bulk of the liquid phase. From a closer look on the local hydrodynamics of Taylor flow, including the fundamentals of bubble transport and the description of the recirculation flow within the liquid phase, it turned out that two-phase pressure drop and gas-liquid mass transfer are governed by the bubble velocity, bubble lengths and slug lengths. In the following step we have dealt with the prediction of these key hydrodynamic parameters. In this connection the first part of our experimental study was concerned with the investigation of the formation of bubbles and slugs and the characterization of the liquid phase velocity field in microchannels of rectangular cross-section. In addition we also addressed the phenomenon of film dewetting, which plays an important rôle concerning pressure drop and mass-transfer in Taylor flow. In the second part we focused on the prediction of gas-liquid mass transfer in Taylor flow. Measurements of the volumetric liquid side mass transfer coefficient (kLa-value) were conducted and the related two-phase flow was recorded. The measured bubble velocities, bubble lengths and slug lengths, as well as the findings previously obtained from the characterization of the velocity field were used to set-up a modified model for the prediction of kLa-values in μ-channels of rectangular cross-section. Describing the interaction of channel design hydrodynamics and mass transfer our work thus provides an important contribution towards the control of the operation of fast, highly exothermic and mass transfer limited gas-liquid reactions in microchannels. In addition it enabled us to identify gaps of knowledge, whose investigation should be items of further research.