Technical Fan shroud science. A commonly held thought is that fan blades should recess 50% into the shroud for

The uneducated, wild ass, free-range engineer in me will speak now....... but take it with a grain of salt.
The half in - half out blade position is favored because the front edge of the blade is what "scoops" the air, which then transitions to the back of the blade where no more "scooping" is being done, more like pushing only. An additional advantage to the back half of the blade being unshrouded is the air can get out of the way sideways as well as backways so the blade doesn't have to chew it up twice times. Other setups are not always fails and so can be good enough in many cases.

A couple of down-n-dirty band-aids you could try are.......
1. Slice the supplied shroud in half (front to back) and nest the smaller cone inside the larger (with spacers??) to shorten the overall depth. This has its limits and probably not good for the whole 5".
2. Make a 5" deep cylinder the size of the shroud circle. Attach it inside the back of the shroud extending to the fan blades as sort of an inside-out duct/shroud combo.
Both of these "fixes" (I use the term loosely) will look like dog doo but may get your customer far enough down the road for you to forgetabotit.

 

Much better now

 

found the right shroud for it, eh? looks better

 

That’s actually a early 90s van fan shroud.
I think it looks better too
Thanks

 

Purely from heavy equipment experience..half in,half out,blade picks up air then pushes it through.

 

I've an old magazine article somewhere that explains the actual science behind; design, depth of fan in shroud, airflow, size of fan & number of blades, size of radiator, cavitation etc. I'll have a look when I get home.

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Being part of class 8 truck cooling system testing for many years it was found 1/3 of the fan width inserted into the fan shroud was optimal and became rule of thumb. Now this was dealing with 32” diameter fans but I assume the fluid (air) dynamics would be similar on smaller fans.


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I did quite a bit of fan testing when we designed the revised cooling system for the Light Armoured Vehicle (LAV) /Stryker back in the early 90's when we added A/C for the Saudis. Orig design used a cast magnesium design from Germany with inlet guide vanes. It was expensive, loud, but very efficient. Efficeny was important due to horsepower draw which was reaching 25 Hp under worst conditions. It was decided to go with a "cheap" fibreglass molded design to reduce cost. We had to do a lot of testing to optimize the design of the shroud configuration, but it could never match the efficiency and air volume of the original design. There are so many variables it would incorrect to suggest a one-fit-all solution, but generally speaking a fan will be more efficient if it ducted to some extent.

 

It works quite a bit better too.
Really flings the air, we noticed because a lot more dirt flying lol

 

no more than 50% in the shroud

 

i can't tell you about the depth part of the question, but the point of the duct is to deal with the fairly high losses off the tip of the fan blade.

air is a fluid and just moves around obstacles. at the tip the air displaced by the blade near the tip just curls off at an angle, creates chaotic vortexes and the energy consumed is lost. a duct "eliminates the tip" -- the duct surface prevents the air from curling away.

it's true that for a given fan blade, if you stick it in a duct, it will put a bigger drag on an engine -- because the fan is now more efficiently coupling power to the air. this implies you can use a smaller/thinner/whatever fan in a duct to move the same amoun of air.

hmm i guess this implies that the fan blades must be "far enough" into the duct to work.

not a lot to be found on wikipedia but there's this...
https://en.wikipedia.org/wiki/Ducted_fan