Here's my address (and full thought process!) to #7:
Option 1: extend the fins in the center area into the nozzle, in the shape of a blade.
Pros: will split the flow 4 ways, may add a bit more flow in the center
Cons: it was already split 4 ways. Effect would be negligeable
Option 2: extend the fins (same as #1) but wrap the inner hose tightly around them
Pros: might serve to put more flow in the middle
Cons: no provision for different nozzles, nozzle is tube size
Option #3: add a 1.5 mm needle in the middle
pros: none, really
cons: unnecessarily complex
Option 4: lower the fin height (cone) in the middle
pros: Excellent distribution
cons: no cooling from the fins (no fins!)
Option 5: same as #4, but from the bottom of the fins
pros: ?
cons: effect negligeable, impossible to do?
So the best solution being #4, a cone cut into the middle, there's still a number of issues (or sub-issues):
1-weaker structure
2-no cooling effect from fins
Answer:
The cone does not have to extend down to the baseplate. Is a compromise possible... not really.
How about a ball end mill, 6mm in diameter? Not much different than a cone. A cone cut would leave a triangle that ends in the center. This would be the reinforcement, as an extension from the top.
The structure at that point is not reinforced straight down by the top, regardless of any option, so the structure issue is minor, except for the rigidity provided by the solid main fins, as pointed out by Nicozeg. With a 9/32 nozzle, the radius is 3.57 mm, so with a fin height of 5 mm, the angle would be about 35 degrees from vertical. (relevance?)
Conclusion: cone cut in the center, about 35 degrees from vertical axis.
I still have a couple of options that I'm pondering. More when they're finalized.
Headache time.
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