For an isothermal simulation, the temperature field is set to the specified isothermal value.

For other heat transfer models, the temperature field is calculated from an area weighted average of the boundary temperatures (inlets, outlets, openings and walls). If you have used a function or expression to specify the boundary temperature, an area weighted average of the expression over the boundary will be used to calculate an average value for that boundary.

A sensible initial guess for the temperature field is an average of the boundary condition temperatures.