Experimental two-phase heat transfer study of R245fa in horizontal mini-channels at high saturation temperatures

Heat transfer measurements for R254fa were conducted. The heat transfer coefficient was determined for a smooth stainless steel tube with an inner tube diameter of 3 mm. The experiments were conducted for three heat fluxes (10, 30 and 50 W/m^2), five mass fluxes (100, 300, 500, 700 and 1000 kg/(m^2.s)) and at three saturation temperatures (40°C, 70°C and 125°C). The experimental data was used to determine the influence of the saturation temperature, mass flux, heat flux and vapour quality on the heat transfer coefficient. At a low saturation temperature, the heat transfer coefficient increases with an increasing mass flux. However, at a high saturation temperature the heat transfer coefficient decreases with an increasing mass flux. Furthermore, the heat transfer coefficient increases with increasing vapour quality at a low saturation temperature. On the contrary, the heat transfer coefficient decreases at higher saturation temperatures

Experimental two-phase fluid flow in microchannels

Micro or mini heat spreaders are used in the interest of providing higher cooling capability for microtechnologies. Heat spreaders using micro or mini channels are not yet well studied, for this the fundamentals of two-phase heat transfer in microchannels are being studied. Here, a comprehensive experimental two-phase flow study has been carried out on two single round tubes (D = 0.509 and 0.790 mm) and for two different fluids: R-134a and R-245fa. An optical measurement method for two-phase flow characterization in microtubes has been applied to determine the frequency of bubbles exiting a microevaporator, the coalescence rates of these bubbles and their lengths as well as their mean two-phase vapor velocity. Four principal flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions (bubbly/slug flow and slug/semi-annular flow) were observed. A new type of flow pattern map for evaporating flow in microchannel has been developed. The first zone corresponds to the isolated bubble regime. It includes both bubbly flow or/and slug flow and is present up to the onset of coalescence. The second zone is the coalescing bubble regime. It is present up to the end of coalescence process. The third zone is the annular zone and is limited by the fourth zone of this diabatic map, the onset of critical heat flux. This flow pattern map can be used for heat transfer model and design of micro evaporator. The vapor velocity or cross sectional void fraction have been measured. For R-134a, the flow can be considered to be homogeneous (or near homogeneous). For R-245fa, more tests exhibit instabilities and surprisingly show vapor velocities below those of homogeneous flow. Frictional two-phase pressure drops have been measured over a wide range of conditions for the two microchannels and two fluids. Three regimes are distinguishable when regarding to the variation of the adiabatic frictional pressure drop with the vapor quality or the two-phase friction factor with the two-phase Reynolds number: a laminar regime for ReTP < 2000, a transition regime for 2000 ≤ ReTP ≤ 8000 and a turbulent regime for ReTP ≥ 8000. The turbulent two-phase flows are best predicted by the Müller-Steinhagen correlation. New accurate CHF data have been measured with the test facility. A new microchannel version of the Katto-Ohno correlation has been developed to predict the CHF in circular, uniformly heated microchannels. Moreover, a new transition curve from annular flow to dryout has been proposed

Kinetic Modeling of Vacuum Gas Oil Hydrotreatment using a Molecular Reconstruction Approach

International audienceVacuum Gas Oils (VGO) are heavy petroleum cuts (boiling points ranging from 350 to 550 ˚C) that can be transformed into valuable fuels (gasolines, diesels) by fluid catalytic cracking or hydrocracking. Prior to these conversion processes, hydrotreating is required in order to eliminate the impurities in VGOs. The hydrotreatment process enables to meet the environmental specifications (total sulfur contents) and to prevent nitrogen poisoning of conversion catalysts. In order to develop a kinetic model based on an accurate VGOs molecular description, innovative analytical tools and molecular reconstruction techniques were used in this work. A lumped model using a Langmuir-Hinshelwood representation was developed for hydrodearomatization, hydrodesulfurization and hydrodenitrogenation of the VGO. This lumped model was successfully applied to the experimental feed pretreatment data and was able to predict evolution of concentration of the aromatics, nitrogen and sulfur species

Flow regime based heat transfer correlation for R245fa in a 3 mm tube

241 heat transfer measurements for R254fa were conducted. The heat transfer coefficient was determined for a smooth stainless steel tube with an inner tube diameter of 3 mm. The experiments were conducted for five mass fluxes (100, 300, 500, 700 and 1000 kg/(m2 s)), three heat fluxes (10, 30 and 50 kW/m2) and at three saturation temperatures (40 °C, 70 °C and 125 °C). The experiments were used to determine the influence of the saturation temperature, mass flux, heat flux, vapour quality and flow regime on the heat transfer coefficient. At a low saturation temperature, the heat transfer coefficient increases with an increasing mass flux. However, at a high saturation temperature the heat transfer coefficient decreases with an increasing mass flux. Furthermore, the heat transfer coefficient increases with increasing vapour quality at a low saturation temperature. On the contrary, the heat transfer coefficient decreases at higher saturation temperatures. Due to the fact that most heat transfer models found in literature are developed for low saturation temperatures and one flow regime, the heat transfer coefficients predicted by the existing models do not comply very well with the experimental data. Thus, a new heat transfer correlation for R254fa was proposed. The new correlation has a Mean Absolute Error of 11.7% for the experimental data of a tube with an inner tube diameter of 3 mm. Finally, this new correlation was also verified with R245fa datasets of other authors

Experimental investigation of non-uniform heating effect on flow boiling instabilities in a microchannel-based heat sink

Copyright @ 2011 ElsevierTwo-phase flow boiling in microchannels is one of the most promising cooling technologies for coping with high heat fluxes produced by the next generation of central processor units (CPUs). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as the cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated. It was observed that boiling inside microchannels with axially non-uniform heating leads to high temperature non-uniformity in the transverse direction.This research was supported by the UK Engineering and Physical Sciences Research Council through grant EP/D500109/1

Notion de position du modèle GENELEX et structuration d’une base de données syntaxiques issue des Tables du LADL

Les données syntaxiques du LADL, riches tant par leur large couverture de la langue française que par leur finesse de description, étaient jusqu’à présent restées difficiles d’accès, en raison d’un formalisme de représentation opaque. Notre travail rend directement lisibles les informations syntaxiques contenues dans les tables de verbes et unifie leur représentation par l’usage d’un cadre formel structurant : le modèle GENELEX. La base de données obtenue constitue un fonds de premier intérêt pour toute la communauté linguistique, alliant, à la richesse des données sur les comportements verbaux du français, les capacités de manipulation et de consultation d’une base de données.Syntactical data from LADL detail with an extremely fine granularity French verb syntactic behavior. Up until now, these data have been difficult to process due to a representation framework unsuited to computer processing. Our work renders these syntactic data readable and unifies their format by the use of a highly structuring framework : the GENELEX model. The resulting database is highly interesting for linguistics : it combines the richness of data on French verb behavior with database abilities

Notion de position du modèle GENELEX et structuration d’une base de données syntaxiques issue des Tables du LADL

Les données syntaxiques du LADL, riches tant par leur large couverture de la langue française que par leur finesse de description, étaient jusqu’à présent restées difficiles d’accès, en raison d’un formalisme de représentation opaque. Notre travail rend directement lisibles les informations syntaxiques contenues dans les tables de verbes et unifie leur représentation par l’usage d’un cadre formel structurant : le modèle GENELEX. La base de données obtenue constitue un fonds de premier intérêt pour toute la communauté linguistique, alliant, à la richesse des données sur les comportements verbaux du français, les capacités de manipulation et de consultation d’une base de données.Syntactical data from LADL detail with an extremely fine granularity French verb syntactic behavior. Up until now, these data have been difficult to process due to a representation framework unsuited to computer processing. Our work renders these syntactic data readable and unifies their format by the use of a highly structuring framework : the GENELEX model. The resulting database is highly interesting for linguistics : it combines the richness of data on French verb behavior with database abilities

Two-phase flow characteristics in singularities

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.This paper aims at presenting the latest scientific progress on two-phase flow in singularities through academic research at INSA Lyon as well as proposing some future possible important issues to be investigated. Flow regimes of third- and fourth-generation refrigerants in horizontal and vertical return bends as well as in a horizontal sudden contraction were experimentally investigated. The dynamical behavior of vapor bubbles or slugs in vertical downward flow return bend was reported. A simplified analysis of the forces acting on the bubble was proposed to better understand the vapour trajectory. Furthermore, void fraction was measured along the sudden contraction using an image analysis technique, which gives very original results. Such experimental studies also brought to the fore the upstream and downstream flow disturbances caused by such singularities as contractions and return bends and their impact on the hydrodynamic performance (e.g. pressure drop) of refrigerants. Especially, these disturbances can be analysed in terms of perturbation lengths up- and downstream of the singularities. Lastly, large pressure drop databases for R-410A, R-134a and HFO-1234yf were obtained. Experimental values of pressure drops in singularities were compared against different prediction methods from the literature without any satisfactory results. Finally, these databases were used to develop new twophase pressure drop prediction methods for such singularities as return bends and sudden contractions.dc201

Ébullition

Les différents régimes de l’ébullition en vase et de l’ébullition convective. Modèles pour la prédiction des coefficients d’échange

Extension of Murray's law using a non-Newtonian model of blood flow

<p>Abstract</p> <p>Background</p> <p>So far, none of the existing methods on Murray's law deal with the non-Newtonian behavior of blood flow although the non-Newtonian approach for blood flow modelling looks more accurate.</p> <p>Modeling</p> <p>In the present paper, Murray's law which is applicable to an arterial bifurcation, is generalized to a non-Newtonian blood flow model (power-law model). When the vessel size reaches the capillary limitation, blood can be modeled using a non-Newtonian constitutive equation. It is assumed two different constraints in addition to the pumping power: the volume constraint or the surface constraint (related to the internal surface of the vessel). For a seek of generality, the relationships are given for an arbitrary number of daughter vessels. It is shown that for a cost function including the volume constraint, classical Murray's law remains valid (i.e. Σ<it>R</it>^{<it>c </it>}= <it>cste </it>with <it>c </it>= 3 is verified and is independent of <it>n</it>, the dimensionless index in the viscosity equation; <it>R </it>being the radius of the vessel). On the contrary, for a cost function including the surface constraint, different values of <it>c </it>may be calculated depending on the value of <it>n</it>.</p> <p>Results</p> <p>We find that <it>c </it>varies for blood from 2.42 to 3 depending on the constraint and the fluid properties. For the Newtonian model, the surface constraint leads to <it>c </it>= 2.5. The cost function (based on the surface constraint) can be related to entropy generation, by dividing it by the temperature.</p> <p>Conclusion</p> <p>It is demonstrated that the entropy generated in all the daughter vessels is greater than the entropy generated in the parent vessel. Furthermore, it is shown that the difference of entropy generation between the parent and daughter vessels is smaller for a non-Newtonian fluid than for a Newtonian fluid.</p