CHT Simulation: Poor Convection to One Phase using VOF-Modell

Hello everyone,

In my simulation using the VOF model combined with a CHT method, I have defined oil and air as fluid media, with a constant surface tension coefficient of 0.027 N/m. Both air and oil share an interface with the housing. The boundary condition on the exterior of the housing is set for convection to the air at 5 W/m²·K and 22°C. Additionally, I have defined certain internal surfaces as heat sources by specifying a wall thickness and a volumetric heat flux (coupled).

While the pressure, temperature, and velocity results yield a converged solution, I am encountering an issue with heat transfer to the oil. The oil temperature remains close to its initial value throughout the simulation. In contrast, the convection to the air appears to be effective (as shown in the attached image). Interaction between the oil and air is evident, with air bubbles within the oil displaying a higher temperature, but the overall temperature of the oil stays nearly constant.

What could be the reason for this behavior? Any insights or suggestions would be greatly appreciated. Thank you for your help.