I am solving a transient thermal driven cavity problem. However, I see that mass is not conserved even though there are no inlets and outlets. Why is this?

 We have an initial mass corresponding to the pressure and temperature. The temperature boundary conditions then throw heat into and out of the system at the walls as the transient develops. FLOTRAN looks up density as a function of temperature and pressure. The average density of the fluid in the cavity is not constant, and mass is not conserved.That problem should be changing the internal energy with the heat flow that occurs because of the temperature boundary conditions, and then calculating a new pressure based on the constant average density and internal energy.For this case, FLOTRAN says that "rho=f(T,P)", but the reality is that "P=f(rho,T)".The ideal gas law is not the problem. The problem is that for many thermodynamic processes, one cannot assume constant reference pressure.

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