APPLICATION OF THE IRREVERSIBLE PROCESSES THERMODYNAMICS METHOD TO THE STUDY OF THE HEAT AND MASS TRANSFER AT THE BODIES' SURFACE IN THE DRYING PROCESS

Daniela SOVA
Conf.dr. ing. - Universitatea "Transilvania" din Brasov - Facultatea de Inginerie Mecanica
Address: B-dul Eroilor Nr. 29, 500036 Brasov, Romania
Tel: +40 268 412921/int. 193. Fax: +40 268 474761
E-mail: sova.d@unitbv.ro

Adrian POSTELNICU
Prof.dr. ing. - Universitatea "Transilvania" din Brasov - Facultatea de Inginerie Mecanica
Address: B-dul Eroilor Nr. 29, 500036 Brasov, Romania
Tel: +40 268 412921/int. 193. Fax: +40 268 474761
E-mail: adip@unitbv.ro

Abstract: The paper presents the irreversible processes thermodynamics method as applied for the study of the heat and mass transfer in the layers close to the evaporation surface. In the convective drying process of a wet body, for a forced air flow along the free evaporation surface, when heat is absorbed from the hot air, in the air layers close to the evaporation surface occur simultaneously three forms of molecular transfer: momentum, heat and mass transfer. Therefore, the heat and mass transfer can not be studied independently of the other transfer forms, but as a unitary molecular transfer. The use of this method allows the determination of the velocity, temperature and concentration boundary layers characteristics, and also the determination of phenomenological relations that describe the coupled heat and mass fluxes.
      The fundamental equation of the irreversible processes thermodynamics constitutes the equation of the entropy source, which establishes the relationship among thermodynamic forces (the gradients of some intensive properties) and the thermodynamic fluxes conjugated to these forces (heat and mass fluxes), the proportionality factors being the phenomenological coefficients. For the determination of the thermodynamic forces, the mass, momentum, energy and concentration conservation equations were applied to a convective, two-dimensional, heat and mass transfer process. The thermodynamic forces have been determined on the basis of heat and mass transfer criterial equations during moisture evaporation from the water free surface in forced convection. The values of the phenomenological coefficients were obtained from the system of phenomenological equations. Based on the obtained values, the influence of each thermodynamic flux coupled with the mass flux upon the latter one was assessed.
In this way a drying schedule can be conceived by selecting those physical parameters that can accomplish the intensification of the heat and mass transfer process.

Key words: irreversible processes thermodynamics method, heat and mass transfer, drying process.