Conjugate heat and mass transfer in drying: A modeling review

Coupled transfer mechanisms are common in a variety of industrial operations, as in food drying, and their virtualization is becoming indispensable. In this paper one such situation, a rectangular food chunk exposed to heat and mass transfer by a turbulent cross flow of air in a channel (Re = 1·104), is investigated by modeling the transfer mechanisms (linked each other by evaporation) in conjunction with the inherent flow transport. This formulation is referred to as a conjugate problem. In the present model, the most limiting parameters that commonly are employed in such studies can be disregarded, i.e. the empirical heat and mass transfer coefficients at the auxiliary air/substrate interface: they refer to unrealistical average conditions and unspecified geometry variations. After proper literature evaluation, the solution of a benchmark drying problem is discussed by assessing the conjugate and coupled features of the analytical development, focusing on some transfer phenomena loci on the sample's exposed surface. It is shown that, in case of a finite protrusion with a unitary form factor (height equals streamwise thickness) the average Nusselt number of about 25 (resumed from associated literature) greatly overestimates the one computed based on the model, everywhere along the exposed surface and during process time: varying up to 6 times along the lateral edges and dropping down to an average value of about 6 after less than 2 h of treatment.