Heat transfer and mass transfer are kinetic processes that may occur and be studied separately or jointly. Studying them apart is simpler, but 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. Besides, heat and mass transfer must be jointly considered in some cases like evaporative cooling and ablation.
The usual way to make the best of both approaches is to first consider heat transfer without mass transfer,and present a t a later stage a briefing of similarities and differences between heat transfer and mass transfer, with some specific examples of mass transfer applications. Following that procedure, we forget for the moment about mass transfer (dealt with separately under Mass Transfer ), and concentrate on the simpler problem of heat transfer. There are complex problems where heat and mass transfer processes are combined with chemical reactions, as in combustion; but many times the chemical process is so fast or so slow that it can be decoupled and considered apart, as in the important diffusion
– controlled combustion problems of gas – fuel jets, and condensed fuels (drops and particles), which are covered under Combustion kinetics.