Conjugate heat and mass transfer by jet impingement over a moist protrusion

Abstract

Biosubstrates drying can be intensified, controlled and optimized, even in blunt shapes, by providing exposure to air jet impingement. In this paper round air jet impingement on cylinder protrusions of a model substrate is investigated, for moderate Reynolds numbers and various geometry arrangements.A comprehensive numerical model, featuring conjugate interface transport (local fluid dynamic effects), multiphase coupling (local surface evaporation) and moisture diffusion notations, is first validated with the corresponding experimental results. Then quantitative distributions of temperature and moisture within the protrusion and along its exposed surface are presented, focussing on the dependence of surface heat and mass transfer on geometry arrangement and fluid dynamic regime. Two values of Reynolds number, two jet heights and two protrusion/jet diameter ratio combinations are investigated.It is pointed out that, within the investigated range of variables, a protrusion/jet diameter ratio equal to 1 allows for flow patterns that foster process enhancement, but at the expenses of treatment uniformity: after 15min of treatment the 10% of protrusion only is still relatively moist, but with a strong internal non-uniformity, whereas with a protrusion/jet diameter ratio equal to 3 the untreated part accounts to the 85%, with a smoother internal distribution.