MASS DIFFUSION 161;Bienvenidos!（大规模扩散 161;Bienvenidos !）.pdf

MASS DIFFUSION Mass Transfer 1 What it is and what it isnt 1 What it is for. Applications 2 How to study it. Similarities and differences between Mass Transfer and Heat Transfer 2 Forces and fluxes 4 Specifying composition. Nomenclature 4 Specifying boundary conditions for composition 6 Species balance equation 9 Diffusion rate: Ficks law 10 The diffusion equation for mass transfer 14 Some analytical solutions to mass diffusion 15 Instantaneous point-source 15 Semi-infinite planar diffusion 16 Diffusion through a wall 17 Summary table of analytical solutions to diffusion problems 19 Evaporation rate 22 References 23 MASS TRANSFER WHAT IT IS AND WHAT IT ISNT The subject of Mass Transfer studies the relative motion of some chemical species with respect to others (i.e. separation and mixing processes), driven by concentration gradients (really, an imbalance in chemical potential, as explained in Entropy). Fluid flow without mass transfer is not part of the Mass Transfer field but of Fluid Mechanics. Heat transfer and mass transfer are kinetic processes that may occur and be studied separately or jointly. Studying them apart is simpler, but it is most convenient (to optimise the effort) to realise that both processes are modelled by similar mathematical equations in the case of diffusion and convection (there is no mass-transfer similarity to heat radiation), and it is thus more efficient to consider them jointly. On the other hand, the subject of Mass Transfer is directly linked to Fluid Mechanics, where the single- component fluid-flow is studied, but the approach usually followed is more similar to that used in Heat Transfer, where