Surface-Center Temperature Differences Within Milk Droplets During Convective Drying and Drying-Based Biot Number Analysis

An assumption of uniform temperature is frequently used when evaluating average temperature-time and average moisture content-time profiles for the convective drying of small droplets or thin-layer materials. In most studies, the assumption of uniform temperature was justified by estimating the heat-transfer Biot number at the beginning and end of the drying process. However, the conventional Biot number analysis performed in the literature does not reflect the evaporative effect. In this article, we have examined the temperature uniformity and the heat-transfer Biot number during the drying of skim milk droplets under laboratory drying conditions following the entire drying process. Surface-centre temperature differences and conventional and drying-based Biot numbers are calculated during the drying of skim milk droplets. A simple procedure is outlined to estimate the extent of temperature nonuniformity within the droplet. Results demonstrated that temperature nonuniformity within the skim milk droplets under drying conditions examined is very small, thus the uniform temperature assumption is more likely to be a reliable approach to model heat and mass transfer processes in industrial spray dryers. The analyses provided in this study help in understanding a few assumptions used in literature, and offer a framework that may be used in the future. (C) 2008 American Institute of Chemical Engineers AIChE J, 54: 3273-3290, 2008