flow in annulus

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Hi, I am Munzir Mechanical Eng. I have practical case of flow in annulus, I'd like to simulate and study it in details with fluent,

my problem is the annulus i have is 3 meter in length and the flow enter and exit in vertical direction i am most interested in the effect of the entrance and exit as well as the horizontal flow along the annulus.

My current inquiry about the geometry, how can I make it shall I involve all the 3 meter in my simulation or what exactly.

My case will involve flow and heat transfer for laminar and turbulent regime for both Newtonian and non-Newtonian fluid.