I'm trying to do a computational fluid dynamics test on this rocket engine chamber and nozzle, but for some reason whatever I set my outlet environmental pressure to be, it makes the whole chamber that pressure, which is not true with an inlet mass flow of 4.1lb/s and an initial condition of 450PSI and 6766 DEG Rankine. I don't think it's relevant to helping me, but the working fluids are 70% Liquid Oxygen and 30% Liquid Propane.
Alex, I think you are mixing fluids. FlowSim cannot do combustion or the phase changes from liquid to gas. Just model it as gas [air] only. Apply high pressure to the inlet face - not the fuel mass flow rate. Environment pressure at the outlet. Here is a quick internal example with FloXpress. You can do this as an external flow and just add the inlet higher pressure. You can see that 1013250 Pa ~147 psi generates supersonic flow. Hope this helps!