Is this an open bundle? Just with flow down across it?

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Flow comes in crossflow (from above) on cylinders (horizontally oriented). Can be considered as open, yes.

Tube spacing trumps fluid velocity uniformity, I think. What is the depth of the bundle in the velocity direction?

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Needs a drawing to understand what you're trying to do.

I can't see how you're going to get a constant velocity when flowing at 90 degrees to a set of tubes.

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I have seen the call waterfall or cascade coolers.
The use a long rectangular box at the top that the size of the bank of tubes.
There are a series of vertical tubes in the box. The top (inlet) ends are up off of the bottom of the box and they are all exactly the same height and are level. The bottom end of these tubes serves to distribute the flow and is often flattened to spread the flow along the HX tubes. These are usually long enough that they can be bent slightly to improve the uniformity of coverage.
There is often also a small flow restrictor inside of each downtube, this serves to keep the flow more uniform between them.
The bank of HX tubes can be a single panel of tubes or they may be 2 or 3 tubes deep (front to back) and 10-20 tubes tall.

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P.E. Metallurgy, consulting work welcomed

Thank you all for your answers.

Ed: I would appreciate a refferal to such application, seems interesting.