The mass flux is the rate at which mass crosses a surface. It has units of mass per unit time per unit area.
The mass of fluid contained in the differential volume, dQ, is obtained by multiplying dQ by the density of the fluid.
The mass flow rate of fluid entering or leaving the differential surface, dS, is obtained by dividing the differential mass, dm, by the increment in time, dt.
The mass flux, J, is defined as the mass flow rate per unit of surface area. The mass flux through the surface, dS, can be found from the above equation to be
Define the mass flux, compute the mass flux, compute the mass flow rate crossing a surface.