How could I increase the domain length without affecting the calculations in the Flue
 

Hello everybody,
I am trying to solve a heat transfer problem(conjugate heat transfer )by the fluent, which is a heat transfer enhancement in a rectangular channel with baffles. The problem is a rectangular channel with a 1 m length and there is heating by constant heat flux from the above and lower plates(The aluminium heating plates are 3 mm in thickness) in the channel and there are solid baffles in a staggered arrangement on the lower and upper plates. During my calculations, I noticed that there is a reverse flow in many faces(about 1750 faces) at the pressure outlet, this number started small at the 25th iteration and increased to 1750 faces at the 450th iteration and remained constant after that. Before the exit section, there is a baffle on the upper surface and it is about 5 cm away from the exit section. I think this small distance between the baffle and the exit section is the main cause of reverse flow at the outlet, so I need to shift the exit section with enough distance from the baffle to prevent this phenomenon, What should I do in order to do this without any effects on the boundary conditions and the calculations?
Thanks in advance.

Domain Increase
 

If there is a domain and it is expected that nothing changes in the domain, then the domain is not useful. There is a reason that you want to add an extra domain; to reduce the flow reversal. Therefore, you will have to accept the changes caused by this domain since you want the domain to affect the flow. The only thing you need to keep in mind is that there is an extra domain that does not take away or add any heat, however, if thermal field is not fully developed, which it won't be, then the thermal field will develop further. But you can always report the results at the end of your actual section. You don't need to consider the results at the outlet of the total domain, such as, you can report the thermal field or average temperature at the outlet of your actual domain, the domain without the extension.