The turbulence effect of the airflow on the calorific losses in foodstuff dryers

摘要

The calorific losses through the sidewalls delay drying, particularly in the higher racks of the foodstuff dryers, which provide us information on total drying time. One notes a temperature gradient of the air during its circulation upwards, between the entry and the exit of the empty dryer. This gradient is more significant with the decrease of the air velocity. The heat transfer coefficient by forced convection between the air and the interior walls is often determined by using the average Nusselt number defined in flow of boundary layer on plane plate (laminar regime). In addition, the presence of the grills, on which slices of product are spread, causes turbulence which is not taken into account by this coefficient of Nusselt number. In this paper, we determine the heat transfer coefficient between the air and the sidewalls of a dryer experimentally. The latter contains grills on which thin layer slices of a dry material (wood) of the same geometry as the initial potato slices are spread. This leads us to determine an expression of the Nusselt number of the form Nu = aRe~bPr~(1/3), without mass transfer. Thereafter, this correlation of the Nusselt number is introduced into the mathematical model, already suggested in our former work, which predicts the performances, in forced convection, of the foodstuff dryers. In the end, the influence of these losses is presented by means of a comparative study between various models, those that take into account and those that ignore these losses between two beds of slices. The results thus obtained are compared with the experiment in an application on potato drying.