Mass Transfer by Diffusion and Convection
Transfer of mass, in the sense of chemical or biological species, is the third and last physical mechanism we encounter: with the analysis of diffusion and mass convection a preliminary outlook on transfer phenomena is completed. Strong similarities exist between heat and species transport: we will use our acquired knowledge to describe the species transport in stationary media and a fluid stream that acts as a species carrier between two media in relative motion. After a brief reference to the basic physical mechanism, as we recognize that mass of a given substance is driven by a concentration difference, we start by exploiting first the macroscopic balance for diffusion in the simplest case of binary systems. So far our attention was drawn to the description of temperature and flow fields: now, we derive and integrate governing differential equations as usual in various cases, following the microscopic balance, but as one substance moves relative to another, we arrive to the distribution of a mass concentration scalar. Along the same line that we exploit so far, with few distinctions only, we will find that many concepts have been already laid down, leading to the numerical solution, so we will be ready to move to the last chapter to scrutinize some PCB phenomena of interest.
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