Chemical reactions, which act as sources or sinks of species, are interactions between processes that result in the formation or destruction of new molecular species. Quite generally, they may take place in the bulk fluid or be restricted to surfaces or interfaces. The direct consequence of these reactions is a change in the mass of the various species in the system, along with generation or consumption of thermal energy.

Transport phenomena such as diffusion or advection are not unique to chemically reacting flows; they can be found in most models incorporated in Ansys Polyflow. For chemical species, it is quite common for the advection mechanism to dominate the transport, and for diffusion under a gradient of concentration to be small. There is also sometimes a practical difficulty to the measurement of diffusion coefficients for chemical species. Therefore, it is not unusual to neglect the diffusive part of the transport.

Processes such as fusion, evaporation, and precipitation are physical reactions; there is no change in chemical composition associated with these processes. Physical reactions are intra- and inter-molecular interactions that result only in a phase change of the species involved. For instance, the melting of ice into water is a physical reaction. Ice and water share the same chemical composition, though they represent different phases. By treating each phase of a particular chemical species as a separate component, physical reactions can be treated as chemical reactions.

Reactions can be reversible or irreversible. Consider, for instance, a system of three reactions between four species, A, B, C, and D:

The second and third reactions cancel when added together. These reactions are said to be reversible: species A forms species D, which, in turn, re-forms species A. We can express reactions #2 and #3 in a single expression using the following concise notation: A 2D The first reaction, on the other hand, is said to be irreversible, since species C does not react to form species A and B.

The concentration of the various species may be expressed in many ways. In Ansys Polyflow, mass fractions and diffusive mass fluxes relative to the mass averaged velocity of the mixture are used.