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To understand how fan shrouds work, you have to think about what is going on at the very edges of the fan/blade itself when it's spinning. In the middle of the fan is pretty much air flowing straight back, but the main production of the fan is at where the blade is the "fattest" and angular. This is an area of high pressure that really is the "workhorse" area of the blade as long as it's stable and flowing in the desired direction. And remember, the more rpms, the more high pressure and centrifugal force is working to throw off productive airflow!
When a "wing" or blade is flug at speed, the tendancy is for the air to push to the end of the blade is created by this high pressure area at the "fattest"/most angular part and by centrifugal forces at play. The air wants to roll off the end and become enveloped in undefined "eddy" current(s) which are unproductive flows and even could be destructive flows as in the case with vortex formations. You see modern jets with "winglets"(look like vertical stabilizers) that are at the very ends of their wings that prevent this unproductive eddy airflow and vortex formations and keep the flow on the main part of the wing creating more organized lift. This improves the efficiency of the wing. Same thing in fans.....if you can prevent the air from rolling off the end, this really increases the efficiency of the fan. The question is how to do it with a fan. It's not practical to put "winglets" on the fan.
So, the exact perfect position of the fan in the shroud is really depening on several other anomolies. The fan shroud collar length (the collar is the "tube" looking part that the fan fits into and some shrouds have no collar at all (due to space limitations) which is less efficient but gets the job done...however if you remember some of the 70s Montes and Grand Prix...they had very long odd looking collars making the shroud very efficient, but ugly), fan pitch, rpms, and especially the space between the end of the blade and the shroud opening! You can actually "tune" a fan and shroud by moving the fan in and out and/or size of the fan or the opening and measuring the pressure gradient (negative pressures) inside the shroud. I actually did this on my wife's Camaro with differing spacers for the fan and made a custom spacer to exact the fit. The fan ended up about 75% inside the shroud. The fan was a 17" inside a 19" opening and I could have put a 18" fan on, but the 17" produced less drag and almost the same airflow.
In general, if you put the fan just outside the shroud, the end of the blades are left in the open allowing for the main part of the air to be pushed and flug off the ends of the blades and creating these eddy currents reducing it's effectiveness obviously. If far enough, you may be recirculating air and not pulling it thru the radiator. If you put the fan too far inside the shroud, and depending on the design, the blades can actually do the very same thing and create eddys inside the shroud and inside a "collarless" shroud without some sort of pressure set up at the ends to contain the high pressure area on the main part of the blades. You also can put it so far in that it works to "push" the air into the shroud which creates unwanted pressure behind the fan. When you have a collared shroud of sorts, this is the best design whereas the ends of the blades let some of the air roll off, but an organized high pressure area is set up at the ends of the blades which prevent the airflow from just rolling off and/or creating eddys or vortexes, all destructive to airflow. The reason that it is desirable to have SOME of the blade outside the shroud is that some of the blade is pulling and some of the blade is pushing the air. Thus a low pressure in front and high pressure behind. The efficiency trick is to lower the "pushing" effort of the fan airflow which then means that at some point in the exit process of the air can be compressed and/or working against static air pressure at the rear to the extent of lowering flow. By leaving a part of the blade on the outside of the shroud this reduces the "pushing" pressure and raises the overall efficiency.
So, the general and common concensus is that the fan should be located from 50% to 75% INSIDE the shroud for best results and depends on system design moreover. For shrouds that have no collars such as the case for most rods, I would put it at 50-66% inside for a good safe bet. If it has a collar, then 75% seems to be better. Some of this has to do with what type of fan you are employing too. Straight bladed fans seem to like smaller edge to shroud distances and big curved fans are not as picky about the distance for effectiveness.
If you notice, most quality and productive electric fans have a built in shroud. By controlling the location, speed, diameter and spacing, the constant speed electric fan can be made very efficient. This is one reason why they are so effective at moving air!
I hope this helps even tho it's toooooooo long and probably boring!
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Steve "Jack'Stands" Jack
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