[A_Alrumaidh]

Hello everyone,

I have a problem in setting the boundary conditions for a project I am working on to study the temperature distribution in a pipeline. As you can see in the figure, a fluid (gas) flows to the right through the inner pipe and passes through the nozzle at the end. Due to the expansion of the gas after the nozzle, its temperature will drop. After passing the nozzle, the gas will be directed to the left in the outer pipe.

Now, I know the flow conditions (velocity and pressure) and the initial temperature conditions at the inlet and the surface of the outer pipe, but I dont know the temperature at the outlet.

One more thing, due to the temperature drop across the nozzle, I believe that the colder gas in the outer pipe will reduce the temperature of the inner pipe in the steady state.

I want to know the steady state temp distribution. How can I tackle this problem? Should I use the transient solver?

PS: I am using ANSYS Fluent

[LuckyTran]

You don't actually need temperature at the outlet. You can't apply temperature at the outlet in Fluent anyway. The temperature specification at an outlet is in case there is backflow. The upwind discretization schemes are only valid for high Peclet number flows and the outlet (downstream) temperature should not affect upstream locations in these types of flows.

But to solve this problem you need to know the wall boundary condition of the outer pipe at all time (it's a boundary condition after all). Knowing only the initial temperature in the outer wall is still an initial condition.

If you want only the steady state result, there's no need or benefit to using the transient solver. Use the steady one. But you need to have boundary conditions.

For getting the temperature distribution across the pipe wall, mesh the inner wall also but as a separate cell zone and specify it as a solid. If necessary, create interfaces and build a coupled wall at the wall-fluid interface.

[A_Alrumaidh]

Originally Posted by LuckyTran

If you want only the steady state result, there's no need or benefit to using the transient solver. Use the steady one. But you need to have boundary conditions.

Thank you very much for your reply. I didnt use the transient solver before so I dont know how it works. However from my understanding for the word transient, Fluent should provide the temp distribution as a function of time, from the initial condition (which I know) to the steady state conditions (which I dont know).

My idea of using the transient solver was just to take the final state of the solution as my steady state.

Is that understanding correct?

[FMDenaro]

I suppose the flow is time-dependent, compressible, and subsonic.
That drive us to set one free BC at the inlet and fix one BC at outlet. The outlet you have is in a quiet ambient? Do you have to consider the heat exchange between walls of the inner pipe?

[A_Alrumaidh]

Originally Posted by FMDenaro

I suppose the flow is time-dependent, compressible, and subsonic.
That drive us to set one free BC at the inlet and fix one BC at outlet. The outlet you have is in a quiet ambient? Do you have to consider the heat exchange between walls of the inner pipe?

1- Correct.
2- How can I fix the BC at outlet if I dont know it from the first place?
3- No it is not ambient
4- Yes, I have to consider the heat exchange between walls of inner pipe.