Fluctuating Surface Temperatures in Heat Transfer case: advice on convergence
 

My temperature values on the geometry surface fluctuate within three degrees (74-77°C) even after 6000 iterations. I am running a multi-phase (i.e. oil and air) steady-state heat transfer problem in Fluent.

My problem involves a complex metal geometry with three sides exposed to air, and an oil jet introduced from the top to cool the geometry. The remaining side (i.e. the back wall) is defined as a boundary wall with a constant heat flux of 62,000 W/m2 to simulate heat losses to the environment. All the other boundaries are defined as adiabatic. The plate has an internal power source of 70W. The oil jet inlet temperature is 40 C. Inlet is defined as a velocity inlet with an oil volume fraction of 1. The outlet is defined as a pressure outlet.

The mesh is polyhexcore (32M cells) with 10 boundary layers an orthogonal quality of 0.1 and an aspect ratio of 400. I am using the K omega SST model and my y+ value is below 1. The multiphase model is VOF Implicit with implicit body force, VOF boundary is dispersed. The residuals for continuity and K are not below 1e-3, while the other residuals are. The total heat transfer rate imbalance is 1.5%, and the mass imbalance is 0.7%.

Could there be any issues with the chosen boundary conditions, such as the constant heat flux on the back wall (62,000 W/m2 is a guessed value) or the adiabatic walls on the other sides, causing the temperature fluctuations? I can't think of any other reasons why my temperatures fluctuate like this.

Thank you in advance for your help.