Cooling fan question

I recently got a used 13.1" custom dob (sonotube) and am planning on adding a fan to decrease the cool-down time. I don't want to cut into the tube to pull air across the mirror face and so am going to mount the fan on the back of mirror cell assembly. (The fan I have is a 4x4x1" 12v fan from a computer).

Does anyone have input re: whether it better to have the fan blowing air towards the mirror through the existing holes in the cell assembly or have the fan pull air away from the mirror?

Thanks.

Does anyone have input re: whether it better to have the fan blowing air towards the mirror through the existing holes in the cell assembly or have the fan pull air away from the mirror?

Thanks.
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A much debated question...

I believe blowing the air towards the mirror will allow for more heat transfer between the mirror and the air. My simple fluid dynamics model in the old brain says there are serious stagnation points on both sides of the mirror when pulling the air, probably only one on the mirror side when blowing the air.

Jon

Hi Mitch,
I make fans for people and I can tell you that it works best if you blow air onto the rear of the mirror with a closed rear cover. This forces air past the sides of the mirror due to pressure differential and causes great a vortex on the front side which scrubs the boundary layer effectively.
Here is a picture to show what I mean by this. The cover should be just inside the ring, but not touch it to adoid vibration. The cover can be made from anythin that you wish, I use ABS sheet 1/16" thick.
The holes are for the collimation screws of course.

Attached Image:

An arrangement I personally prefer is to have the
fan pulling air out the rear of the scope with no
other openings in the scope except the front.
In other words, you pull cold air in the front
of the scope, down the tube in a laminar fashion,
around the mirror, and out the rear of the scope.
This provides a relatively "clean" optical path
through the scope. Whether or not the mirror
cools quickly is less important, since thre are no
heat plumes, etc. -- all the heated air is going
out the back of the scope. There are no heat
plumes rolling up the tube; it is all getting sucked
out the back of the scope. I use about 120 CFM
(11" Dob) with some speed adjustment on the fans.

Adding a small fan right in front of the mirror
blowing at the surface can help with the boundary
layer issues right at the mirror. This can be a
small 1.5" - 2" fan mounted just off the tube surface
(no holes in the tube).

I guess the debate continues....

Mitch, as you can see there is a lot of support for either orientation. It's easy enough to flip the fan around, so I plan to try both ways in my next scope (6" f/4.5 RFT Newt) to see what works best. I'll probably mount it using Velcro to make flipping it easy.

I'll post pics and results when I get it done.

Doug