Are you running this simulation with constant properties? If so, you can save yourself a bundle of time by removing the flow equations from the transient solve. Here would be a possible approach:

  1. Conduct a steady-state flow analysis. (& save!)
  2. Set-up the model as a transient solve.
  3. Patch the temperature of 300° onto the solid body. (Patch is under the Initialization panel in the Tree. Be sure to patch ONLY the temperature as if you do anything else you will destroy the flow field you already solved for and are continuing to use.)
  4. Turn off the flow and turbulence equations and solve only for the energy. This can be set in the Tree under Solution->Controls->Equations
  5. Set up an Monitor points on the body, .cxa animations or other quantities before you run.
  6. Save the case file to a new name (indicate that it is transient)
  7. Run, and enjoy a good cup of coffee while the computer does its thing!

I would also recommend that you do a free-slip wall condition on the upper and lower surfaces of your tunnel. It won't change the results much but be more aesthetically pleasing.