ANALYSIS OF HEAT AND MASS FLUXES DURING MICROWAVE DRYING

The kinetics of combined convective and micro-wave drying presents uni versally more than the two habitual drying phases. Superimposed to a g eneral heating of the product, a brutal and short acceleration of the drying rate raises all flux densities at a very important level. This is particularly evident with the class of product we used in this stud y : non porous polymeric gels. Moreover, an unusual correlation betwee n the kinetics and the surface temperature around the micro-wave speci fic phase might appear : unexpectedly, the evaporation flux density de creases when the surface temperature increases noticeably. In order to elucidate this apparent complexity and understand the underlying phys ical phenomena, we analysed term by term all fluxes according to their origin. The splitting was achieved through mathematical modelling and partially validated by the recording of temperatures and mass fluxes. Usually with convective drying, all rate changes are governed by the state of the product : hygroscopicity and porosity development. Here i n the case of combined micro-wave drying, all accidents on the drying curves are determined by the heat generation and the heat transfer sit uation. The constant rate period is also governed by the balance of he at fluxes. The accelerated period corresponds to a heat wave arriving to the surface. Most of the time, the product gives out heat to the su rrounding air : this heat is lost for the drying but this cooling effe ct may be used for the protection of the sample.