The following convection diffusion equation is considered here
In the above equation, four terms represents transience, convection, diffusion and a source term respectively, where
*  is the temperature in particular case of heat transfer otherwise it is the variable of interest
*  is time
*  is the specific heat
*  is velocity
*  is porosity that is the ratio of liquid volume to the total volume
*  is mass density
*  is thermal conductivity
*  is source term representing the capacity of internal sources
The equation above can be written in the form
where  is the diffusion coefficient.