Help needed: Simple pipeline flow heat exchange, yet the total heat transfer rate re
 

Simple pipeline flow heat exchange, yet the total heat transfer rate report is not conserving.
(Apologize for the incomplete title， it's a little bit long)

Hello

I am currently using Fluent to simulate the heat transfer process in a concentric tube heat exchanger. The three consecutive domains from inner to outer are HTF, TUBE, and a solid domain.

Background：The HTF inlet flow rate is 0.004545 kg/s, with a constant temperature of 350 K. In the simulation, I have set up a monitor to record the total surface heat flux of the HTF-tube heat transfer surface in an integral manner. I am also monitoring the facet average temperatures at the inlet and outlet. The convergence residual for the energy equation is set to 10^-9, with a maximum of 50 iterations per time step, and the energy equation can converge to a level of 10^-8 during this process.

Problem： I found that the net heat transfer rate at the inlet and outlet calculated using Fluent's report module （REPORTS> fluxes>total heat transfer rate @inlet & outlet） does not match the heat transfer rate on the HTF-TUBE interface（REPORTS> fluxes>total heat transfer rate @couple interface）. When I calculate the heat transfer rate using the temperature difference monitored ((T_in-T-out)*mass_flowrate*Cp), the resulting value does not match either of the previously mentioned values.:confused:
Meanwhile, I also found that the mass flow rate at the inlet and outlet are consistent though, the velocity at inlet and outlet are inconsistent (the area are identical for both inlet and outlet).

I don't know if this is related to the setting of the reference temperature. My understanding is that the setting of the reference temperature will affect the calculation of enthalpy, and the calculation process of heat flow should offset the reference temperature part (probably?).

I would like to ask for help

thanks in advance