Revue Générale de Thermique最新文献

Mass transfer of weakly soluble gases through gas-liquid turbulent interfaces is analysed by means of the Large-Eddy Simulation technique. Two different flow configurations are considered, namely an open channel flow with a shear-free surface and a liquid layer driven by an interfacial shear. The very good agreement found between the global mass transfer coefficients provided by the simulation and those coming for measurements validates the LES approach which is used for the first time in high-Schmidt-number mass transfer problems. The relationship between the turbulent motions active close to the interface and the instantaneous concentration field is investigated both statistically and dynamically. The validity of available mass transfer models based on the concept of surface renewal is then discussed in the light of this information which is still, to a large degree, unachievable in experiments.

The thermal behaviour of a solid sorption generator of active carbon/alcohol machine, is studied during heating and cooling phases with a preheated air flow. A bicylindrical walls generator that contains 0.9 kg of a granular adsorbent in the presence of residual gas is tested during a cycle of an average duration of 3 hours; the grains of active carbon are rod-shaped of 0.003 m diameter and 0.008 m average length. The thermal contact conductance of adsorbent to the wall has an important influence on the rate of heat transfer between the generator and the external source of heat. In the absence of alcohol, heat transfer occuring without mass transfer in active carbon is essentially due to the conduction. A numerical bidimensional model allows one to justify experimentally the observed evolution and proposes thermal contact conductance between active carbon pellets and the generator wall. A parametric study of the thermal contact conductance gives 6.5 W·m⁻²·K⁻¹ as the best value. A simulation of heating and cooling phases with average conductance values between 5 and 30 W·m⁻²·K⁻¹ gives model estimated heating and cooling phases duration.

In pulsed flow, the surge line of a centrifugal compressor is highly dependent on the upstream and downstream piping systems. In this study, the shift of this surge line in the case of two two-stage turbocharged marine Diesel engines is calculated using a numerical simulation of the compressor/pipe work assembly for each engine. The linkage of two programs using the ACSL language (in a global solution procedure) allows the understanding and quantification of the connection circuit's influence on the operating limits. The tendencies observed on the test bed are reproduced by the simulation, confirming the chosen methodology. This procedure also permits an analysis and understanding of the observed phenomena, especially the identification of the compressor which initiated the surge effects and the explanation of irregularities in the operating limit.

Heat transfer is strongly involved in many scientific and technologic domains and the French heat transfer laboratories and networks cooperating is this field are first located. The analysis of the main industrial activities demanding heat transfer competence helps one first to identify some up-to-date technological challenges. It appears clearly that connecions are to be reinforced between disciplines like heat transfer, fluid mechanics, combustion, material science, optics, biology… Scientific objectives are then scanned through, by splitting the research activities between mature topics (radiation, particularly in semi-transparent media; convection and thermoconvective instabilities; heat transfer in porous media…), emerging (heat transfer with change of phase, convective heat transfer enhancement by active control in the boundary layer, inverse techniques…) and incipient ones. Among some promising new topics, let us mention microscale heat transfer, and also bioheat transfer.

Future 3D electronics packaging systems will require micro cooling systems that can be integrated and permit the continued use of air as a coolant. To achieve this, new types of silicon micro heat exchangers were made using an anisotropic etching process. Various heat exchanger configurations and sizes were made using sandwich and stacking techniques. They can be used either as a heat exchanger for direct cooling with compressed air or as a heat pipe and thermosyphon for indirect cooling with fan-blown air. The performance characteristics of the various cooling systems are stated. The micro-heat-pipe can be used for power loss densities of up to 3 W·cm⁻², the direct air cooling up to 15 W·cm⁻² and the thermosyphon up to 25 W·cm⁻². Cooling performances are achieved that are otherwise only possible with liquid cooling. The practical application of the micro cooling system is demonstrated using the example of the Pentium processor. With a power loss of 15 W, the high-performance micro cooling system is able to limit the increase in operating temperature to 15 K. The volume of the micro heat exchanger is 2.5 cm³ and therefore considerably smaller than that of standard heat sinks.

The aim of the present study is to develop a new experimental methodology that allows one to perform accurate measurements of the local heat transfer distribution before, in, and after a 180° sharp turn in static and rotating channels. Preliminary measurements of convective heat transfer coefficients are performed by means of infrared thermography applied to the steady state ‘heated-thin-foil’ technique. Some preliminary results in terms of Nusselt number Nu distributions and profile, as well as averaged Nu profiles along the channel axis, are presented. Results prove that infrared thermography is capable of measuring heat flux coefficients and detecting particular phenomena linked to the fluid flow configuration such as location of separation bubbles, influence of the channel aspect ratio as well as the influence of the channel rotation.