The total accumulated heat rate variable, dEiInt, is the variation of internal energy per unit time in the domain:
The total net heat rate, ntfluxInt, is the integral of
Total Heat Flux (Heat Transfer interface) over all external boundaries. In the case of a fluid domain, it reads:
The total heat source, QInt, accounts for all domain sources, interior boundary, edge and point sources, and radiative sources at interior boundaries:
The total fluid losses, WnsInt, correspond to the work lost by a fluid by degradation of energy. These works are transmitted to the system through pressure work and viscous dissipation:
The total accumulated energy rate, dEi0Int, is the variation of total internal energy per unit time in the domain:
The total net energy rate, ntefluxInt, is the integral of
Total Energy Flux (Heat Transfer interface) over all external boundaries. In the case of a fluid domain, it reads:
According to Equation 4-145, the following equality between COMSOL Multiphysics variables holds:
The sign convention used in COMSOL Multiphysics for QInt is positive when energy is produced (as for a heater) and negative when energy is consumed (as for a cooler). For
WnsInt, the losses that heat up the system are positive and the gains that cool down the system are negative.
According to Equation 4-146, the following equality between COMSOL Multiphysics predefined variables holds:
In stationary models, dEi0Int is zero so the energy balance simplifies into:
At steady state, and without any additional heat source (QInt equal to zero), the integral of the net energy flux on all boundaries of the flow domain,
ntefluxInt, vanishes. On the other hand, the corresponding integral of the net heat flux does not, in general, vanish. It corresponds instead to the losses from mass and momentum equations, such as
WnsInt for pressure work and viscous dissipation in fluids. Hence, energy is the conserved quantity, not heat.
