Subsonic Ramjet Model

I am attempting to optimize the design of a ramjet to be created for rocket experimentation. I have been working on the model for the fall semester attempting to refine it. I am now using a 2d axis symmetric turbulent flow and heat transfer models. The heat transfer is being used as a rough model for combustion. I could get the model to converge before I realized the heat transfer model and turbulent flow model were not coupled. But once I coupled them the model will only converge sporadically for very coarse meshes. Could you help me figure out how to get the model to converge? Any tuning that could be done to make the model more accurate would be very helpful as well.

The model beginning with "test coupling" has the two models coupled (at least I think) but will not converge.