R is the net rate of production of species i by chemical reaction and S is the rate of creation by addition from the dispersed phase and the user defined source. J is the diffusion flux of species i, which arises due to concentration gradients and differs in both laminar and turbulent flows. In turbulent flows, computational fluid dynamics also considers the effects of turbulent diffusivity. The net source of chemical species i due to reaction, R which appeared as the source term in the species transport equation is computed as the sum of the reaction sources over the N reactions among the species.