Reactive flows
Fluid Mechanics dynamics of viscous fluid

In some flows, several species are present. This is particularly true of "reactive" flows, i.e. flows in which chemical reactions, such as combustion, take place, but it is also, and more generally, the case when gases of different natures are present in concentrations that vary in space and time. As with other quantities, a balance must be drawn up for each species present in the flow. This balance is totally analogous to the balances for other flow parameters.

If we denote by N _n , the molar concentration (number of moles per unit volume) of species n at a given point in the flow, Fick's first law, analogous to Fourier's law for conductive heat transfer, gives the diffusive transfer of species n as a function of the concentration gradient :