I have a pipe that flows air through a porous media. I calculated the mass flow rate by doing a surface integral across my inlet and outlet. I put fp.rho*v in the integral expression box (where v is the velocity moving perpendicular to the inlet and outlet cross section and fp.rho is the density). The difference between those two results is roughly 0.000802938 kg/s. I then took a volume integral for my air flow domains with fp.rho (density) in the integral expression and got a nearly constant mass of 0.001692 over 600 seconds. Clearly, this is contradictory because the delta between my mass flow rates is large enough to cause a noticeable change in my total mass after 600 seconds. Am I using the integration tools wrong here? If not, what could account for this mathematical impossibility?
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