Transferts couples de chaleur et de masse dans des matériaux consolides utilises en génie civil

The method of measuring the humidity in the materials based on the determination of the thermal conductivity is presented and the principal characteristics are analysed. The water diffusion coefficients due to the water content and temperature gradients are measured and utilized in the numerical calculation of a drying process. These results are compared to an experiment and discussed. We note the difficulty of describing the superficial mass transfer with a single coefficient function of the heat transfer coefficient. We present some enthalpic balance which proves of interest in taking into account the presence of humidity in walls of buildings.     

Séchage superficiel d'un matériau poreux humide par convection forcée d'air chaud: couplage entre les équations de transfert dans le matériau et celles de la couche limite

The authors considered drying of a thick slab of humid porous material immersed in a laminar steady flow of hot air parallel to its surface. They wrote the boundary layer equations in air (continuity, momentum, energy and mass), those describing humidity and heat transfer in the porous medium deduced from Luikov's theory. Then, they coupled them at the air-product interface by expressing the continuity of the thermal and mass fluxes taking into account the evaporation. They solved numerically the resulting system of differential equations using an implicit finite-difference method. They determined the instantaneous evaluation of the spatial distributions of heat and humidity, the local values of the Nusselt and Sherwood numbers. They also studied the influences of principal parameters of the system.     

Evolution thermique d'une sonde électrostatique dans un gaz raréfié et partiellement ionisé

Based on the classical Langmuir theory, an electrostatic probe can be used in an ionized gas to measure electron temperature, electron density and electron distribution function. In many experimental cases the characteristic curve (applied potential vs total current collected) presents a complex and reproducible hysteresis effect. Consequently, electron measurements are impossible. The hysteresis effect decreases when the probe potential frequency increases and is related to the thermal probe evolution. The thermal probe evolution is determined by a numerical method taking into account plasma energy flux, radiation and probe conduction. The numerical results show that the hysteresis effect appears simultaneously with a thermal probe hysteresis.     

Convection mixte dans un écoulement de poiseuille vertical: Étude comparative de chauffage volumique et pariétal à flux constant

Numerical study of heat transfer in a laminar flow between two vertical parallel plates is reported. Two configurations are considered: the first consists of an imposed constant wall-heat-flux boundary conditions; the second corresponds to a bulk energy dissipation by direct electric-conduction (Joule effect) in the fluidIn the two cases, we have kept the same average temperature-difference between the fluid at the entrance and at the exit of the channel. The flow rate is also the same in both cases. Comparison is made between the axial evolution of Nusselt number obtained from this computation and the Nusselt number given by a classical correlation, in forced convection regime. It has been found that they are in good agreement.Heat transfer enhancement in the mixed convection regime is analysed We show particularly that the temperature distribution at the exit section of the channel is more uniform in the bulk heating case than in the constant wall-heat-flux condition.The secondary-flow induced by free convection and its contribution to the temperature uniformity is also studied We show the similarity between the two configurations from the point of view of the secondary-flow structure. In both cases we cam observe two convective cells whose centres are located closer to the exit from the channel. It is explained that this phenomenon is due to the presence, in the two cases, of the wall temperature increase along the channel in the downstream direction.     

Etude des structures convectives et du transfert thermique autour d'un cylindre en deuxième nappe dans une configuration en quinconce,en écoulement d'air chargé de gouttelettes d'eau

We describe an experimental study of the influence of longitudinal and transversal pitch on the convective transfer around a cylinder on the second row of a three cylinder staggered bank, operating in an airstream charged with sprayed water at moderate Reynolds number. The analysis of the wall velocity gradient and its fluctuation shows the effects of geometrical parameters on various characteristic zones of the flow: laminar boundary layer, separation and vortices. The modifications of local Nusselt number evolution compared with the single cylinder are explained. For the disposition which gives the highest heat transfer, a correlation of local Nusselt number is proposed.