Learning Forum

[vsjay3]

Hi all,

So I am modelling a simple heat exchange design using Ansys Fluent but have a small doubt that I believe you could clarify for me. The design is provided below. A U-shaped pipe is embedded inside a sand layer and water is flowing inside the pipe. The pipe flow is horizontal and the sand is being heated from the top. So the heat is flowing vertically through the sand layer and heating the sand layer. Meanwhile, cold water is pumped into the pipe (from an external pump) and is flowing horizontally inside the pipe (turbulent flow conditions) while the heat from the warm sand layer will transfer to the flowing water through the pipe wall.

[Karthik Remella]

Hello You will need to first determine if the heat transfer is natural or forced. Estimating the Rayleigh number might help. Once you confirm that natural convection in important to understand the heat transfer, you will need to include the gravitational terms in your equations. Otherwise, it may not be necessary.
Karthik

[vsjay3]

Thank you so much for replying. Appreciate it. I can calculate the Richardson number and see if natural convection is dominant or not. I understand that gravity option needs to be enabled when natural convection (or buoyancy effects) are dominant in the flow.
But keeping the natural convection aside for now - does activating the gravity option have anything to do with the flow direction as well? For example, assuming hypothetically that I have a vertical pipe flow and it is under purely forced convection conditions (zero natural convection) - do I still need to include the gravity option just because the flow is in the vertical direction? Or does the direction of flow not matter (and does not need to be considered) when enabling the gravity option? Until now I thought even if natural convection is not present in the flow, we should still enable the gravity option if the flow direction is non-horizontal (i.e., either vertical or at an angle) (I thought this because in normal physics, we take into account the gravity force when we analyze forces in vertical direction or at angles)
Your kind response would be highly appreciated!
Thank you!

[vsjay3]

[Karthik Remella]

Hello The gravitational term might be important in your question from a momentum balance standpoint. You might want to compare the inertia and viscous forces with the gravitational terms. If you wish to accurately capture all the forces acting on the fluid, I would include it in your analysis. If it turns out that including this term is not impacting your heat transfer, then the influence of the gravitational term is indeed weak.
Karthik

[vsjay3]

Thank you very much for your answer. Appreciate it.
I saw that the option to enable the gravity is present in two places in FLUENT (image-A and image-B). Does enabling the gravity option in both image-A and B have the same effect on the modelling result?
Image-A

Image-B

[Rob]

It's tagging the same value in the solver, we have 2-3 ways of setting some values due to the way the code has evolved over the years. Do not turn gravity on and set as zero, it's a waste of solver resource and may not improve stability.

[Arun Kumar]

Hi,

My problem is almost similar to above can you please share boundary condition to be used. I want to know temperature distribution in pipe to check thermal expension of pipe.