Conductances for Heat and Mass Transfer

This chapter continues the discussion of transport, and focuses on methods for computing the conductances and resistances needed for the calculations in Ch. 6. We first discuss conductances on the smallest spatial scale; molecular diffusion. It is by this process that heat and mass are transported in still air or water, such as in parts of the lungs of animals, in soils, in the substomatal cavities of leaves, and in animal coats. The equations for turbulent transport of heat and mass on larger scales in the atmosphere are similar to those for molecular diffusion, so those equations are discussed following the molecular diffusion equations. After diffusion processes are discussed, we consider an intermediate scale; namely, convective heat and mass transfer theory as it applies to fluids moving over plates, cylinders, and spheres (simulating leaves, stems, fruits, and animals).