I am trying to approximate mixing of hot gas and a cool air stream in a chamber. Imagine an industrial burner (or jet engine afterburner!) inside a test chamber with air flowing past the flame. For simplicity, let's say the larger air stream is simply a large rectangular cube with an inlet and outlet condition (like a wind chamber) and my burner is simply a cylinder placed within the large volume. I don't want to model the combustion process in detail, I only want to approximate that rapid expansion of gas that occurs at the flame site and see how the hot stream and cold stream mix downstream. I want to put a condition on the cylinder that gives a mass injection (because I know fhe flow rate of combustion air) and total energy flux.
It's almost like I need the option to say "here is another inlet source, but ignore the M/E balance requirements and take my word that hot gas magically appears at this point" since combustion itself cannot be directly modeled.
Anybody tried modeling something like that before?
Is the representation of the burner a cylindrical shell or a fully filled cylinder? If you know how much is going in and out of the cylinder, then why not you model it as a fully filled solid cylinder and apply the boundary conditions on the end surfaces of the cylinder. The cylinder itself can be suppressed if its thermal conduction is not of interest to the analysis. This way the inside of the cylinder will behave as an external system and you will not need to worry whether the mass going through the burner is conserved or not. Of course for this to work, you will have to know how much is going in and out of the burner and how hot it is.
You could also try a resistance region with zero resisrance and a total heat flux applied to it, this way there can be a large temp increase in the air, without anything else occuring.
Thanks to both of you. After some monkeying around after I posted it, I did end up creating a volume region behind the "flame" (on the CAD side) and suppressed it, then applied an inlet to the backside of the flame. That got me a reasonable result if I applied both a mass flow (this I knew) and adiabatic flame temperature. I also tried the second suggestion of making the "flame face" volume as a resistance (value 0) and applied a heat flux to it based on my fuel consumption. In both cases the results were very similar. The downstream temperatures I get in the model are still somewhat higher and don't quite match the trend of our measured reality, so it's time to go back and see what we've excluded from the geometry.
Also remember that you are probably not including any of the radiation effects from you burner. That might account for the higher downstream temperatures in your simulation.