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Erschienen in:
2015 | OriginalPaper | Buchkapitel
13. Evaporation and Mass Transfer Fundamentals
verfasst von : George Sidebotham
Erschienen in: Heat Transfer Modeling
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Abstract
This chapter introduces fundamental mass transfer concepts through the modeling of the evaporation process that takes place at the free surface of a mug of coffee. An evaporative heat transfer coefficient, hevap, is defined that can be included in any of the transient numerical simulations from earlier chapters (i.e., lumped 1-node, few node, multi-node). Evaporation is driven by concentration gradients, not a temperature difference, and defining an evaporation coefficient this way allows direct comparison of the relative effects of evaporation, convection, and radiation on the cooling of the free surface. As will become apparent, this approach makes sense only for cases when the liquid surface temperature differs significantly from ambient. The case where the coffee reaches thermal equilibrium with the environment and the slower evaporation continues is treated differently, and it will be seen that the coffee attains a temperature below ambient, a form of evaporative cooling.