2.1 Mass flow

Volumetric Flow Rate

Volumetric flow rate is the volume of fluid which passes through a given cross-sectional area per second.

        Volumetric flow-rate Q = Av

                               where A = cross-sectional area 

                                v = velocity

SI unit for measuring volumetric flow rate is m³/s (Cubic meters per second)

Mass flow rate

The mass flow rate of a system is a measure of the mass of fluid passing through a given cross sectional area in unit time.

         Mass-flow rate m = ρAV

                 where A = Cross-sectional Area 

                           V = flow velocity

                           ρ = Density

SI unit for measuring mass flow rate is kg/s (kilogram per second)

In a converging / diverging duct, the mass flow rate must be constant (what goes in must come out) and if density is unchanged, volumetric flow rate will also remain constant (This is shown by A1V1 = A2V2). If the cross-sectional area changes then the velocity will change (Area reduces, then velocity increases).