ADVANCES IN TRANSPORT PHENOMENA DURING CONVECTIVE DRYING WITH SUPERHEATED STEAM AND MOIST AIR

The latest developments in high temperature convective drying are presented. The process has been investigated experimentally and theoretically for two drying fluids: moist air and superheated steam, and for two materials: light concrete and softwood. The experimental investigation was made in an aerodynamic return flow wind-tunnel, using a new type of sensor which enables measurement of both temperature and pressure at the same point. The numerical model of heat and mass transfer developed over several years has been used to simulate the experimental test, in 1-D for concrete and in 2-D for softwood. Good agreement is obtained between the calculations and experiments. An analytical model taking into account the transfer mechanisms between the exchange surface and an assumed drying-front has also been developed, and enables the explanation of most of the phenomena observed, both in the experiments and the simulation. The main purpose of the present work was not only to determine the differences on the pressure and temperature fields, but also to give a physical explanation for the moisture migration at temperatures when the wet bulb is other than the boiling point. Finally, it is shown that the process can be divided into three domains: (a) an initial drying period, where the medium is at the wet bulb temperature; (b) a second phase with a stage more or less pronounced, at a temperature which depends on the air flow and on the material: and (c) a final stage, during which all the temperatures approach the dry bulb temperature.