boundary condition at the outlet of a gas turbine combustion chamber
I'm currently trying to simulate a compressible flow without combustion through a combustion chamber for a gas turbine using Fluent. Therefore I need to determine the loss of pressure over the flame tube.
My question is: Which boundary condition at the outlet of my combustion chamber should I use?
The given parameters are: the mass flow, pressure and temperature at the end of my compressor/the inlet of my combustion chamber.
I know that Fluent would accept the "Outflow"-boundary (in an incompressible case) to solve this problem without the use of any further information (e.g. velocity or pressure). If I try to use this kind of boundary in my compressible case, Fluent tells me to use the "Pressure Outlet"-boundary instead. The problem is, I don't know which pressure to set at the outlet, since the pressure loss over the chamber is the variable I'm trying to calculate.
Is it somehow possible to use a Neumann-boundary condition (gradient of pressure equals zero) instead of using a constant value for the pressure?
Does someone have an idea how to solve this? I would be very grateful!
For the pressure outlet (which I agree is the correct boundary condition) you can enable the target mass flow rate option. This will keep the pressure as variable, the solver will iterate this to be the pressure needed to achieve a given mass flow rate across the boundary (which you do know). For compressible flows you need to know mass flow rate otherwise you do not have enough boundary conditions so I assume you already know it.
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