Isobaric dynamic mass balance

In the volumes (and derived components) dynamic_mass_bamancecan be set to:

• false → BQ=0: inflows equal outflows.
• true → BQ=V*der(rho) : the inflow/outflow inbalance compensate compression/dilation.

In ThermoSysPro the der(rho) term is generally decomposed in the ddph*der(P) (isenthalpic) and ddhp*der(h) (isobaric) terms.

I'am wondering whether an intermediate choice to take into account only the isobaric term, aka the thermal dilatation, would be a good enhancement for the library. I think this would make sense at least for liquid water for a couple of reasons:

• Liquid water is generally considered as incompressible (ddph~0).
• Variations of h travel slowly (transported by the mass flow) compared to Pvariations: it should be easier to solve the isobaric term only than the whole equation.

This could be useful for example to model the thermal dilatation in the primary circuit of a PWR ( @viola.ferrara).