Energy balance in MFix

I am performing a simple heat energy balance in MFix. I have attached the relevant mfix .mfx file and the obtained plot for energy balance.

In the plot, LHS and RHS represent two sides of the energy balance equation for heat. Ein_rate-Eout_rate=Estored_rate.

Here, air is entering the domain with some heat and exiting with some heat. Simultaneously the right wall of the 2D domain is providing a heat flux driven by a temperature difference.

Why am I seeing such an erratic energy balance response from the storage terms? I am using mcpT for heat stored term applied to air and solid separately. The energy residual stayed under 1e-04 throughout the simulation.

tfm2d.mfx (11.9 KB)

Would appreciate some guidance here.