The equation for conservation of mass, or continuity equation, can be written as follows:

(1–1)

Equation 1–1 is the general form of the mass conservation equation and is valid for incompressible as well as compressible flows. The source is the mass added to the continuous phase from the dispersed second phase (for example, due to vaporization of liquid droplets) and any user-defined sources.

For 2D axisymmetric geometries, the continuity equation is given by

(1–2)

where is the axial coordinate, is the radial coordinate, is the axial velocity, and is the radial velocity.