I have a tiny amount of experience with axial flow within a jacketed tube, i.e., one large tube within a slightly larger shell, the whole assembly used as an exhaust pipe for a gas turbine powering a boat, with water pumped through the annulus so that the exterior wouldn't burn persons who fell or leaned against it.
There was some concern about temperature gradients around the periphery of the outer tube, e.g., if the flow were not forced into some semblance of uniform velocity all around, that areas with lower flow would get too hot and burn people. So we laser-cut some spacer rings with a circular array of orifices to be installed near the inlets and outlets of the jacket/shell. The pipes performed as intended, and made the customer happy, and we got paid. There was no CFD performed. We sized the orifices to produce a 1/2 psi drop at each stage at the design coolant flow.
... none of which is of great interest to you;
I just wanted to point out that even with a tube bundle within a long shell, the shell connections can't be axial, so the flow can't be assumed uniform over the transverse area, unless you force it with some sort of pierced baffle/ screen as generally described above.
Even if you do that, you will probably find density gradients and resulting velocity gradients and temperature gradients across the bundle when it exceeds some unknown minimum length, so you may need intermediate baffles to uniformly redistribute the flow, and don't forget that you need baffles or something to support the tubes to keep them from abrading each other.
Given that you need baffles to support the tubes anyway, it makes sense to turn them into partial baffles to induce crossflow, but maybe you can find a way around that.
Also don't forget the difference in thermal expansion between the tubes and the shell, which is likely to cause buckling of one or the other as the exchanger goes from dead cold to operating temperature and back. ... which is one very good argument for using u-shaped tubes with both ends anchored in the same tube sheet. If you use long straight tubes and two tube sheets, how do get the bundle into the shell? Not impossible, but it adds complexity.
There are valid reasons why heat exchangers have evolved to use a limited number of possible topologies. Exploration of some of the possible evolutionary branches happened a long, long time ago. If you look back far enough, you might find that someone has already been where you plan to go. ... but if their branch turned out to be a dead end, they had little incentive to record their adventure. It might be great fun to explore someone's library of engineering logbooks from a hundred years ago. ... if said library still exists, and you could gain access to it, and had the patience to wade through the chaff and misunderstanding.
Have fun with your adventure.
Mike Halloran
Pembroke Pines, FL, USA
