Hi Rob,
Yes, I would expect it to balance around -23W as well. But as you can see the values deviate a lot.
The specific heat (Cp) of oil is around 1600 J/kg k. At 5000 iteration point, total mass imbalance * Cp = 0.6522. The total heat transfer rate imbalance at this point is 12.5 W (net value considering the heat source).
You are correct about the domain oil fraction. It does not look like it's at equilibrium. But since the temperature change is small it doesn't seem to affect much.
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