Uday M. answered • 07/30/19
Mass flow is simply the transfer of mass, usually across the boundary of a defined system. So if you have a container of liquid that is connected to inlet and outlet channels, and if you define your system as this container, you would have a positive mass flow coming in via the inlet channel and a negative mass flow exiting via the outlet channel.
There are a few relevant equations for mass flow. One arises from the definition of density where the time derivative is taken. Density = mass / volume, so Density would also = mass flow rate / volume flow rate. Thee explicit equation for mass flow rate is = density * cross-sectional area * velocity.
There's also the principle of continuity. For an incompressible fluid in a tube of varying cross-section, the mass flow rate must be the same everywhere along the tube.