Posted 19 January 2013 - 11:14 PM
You know, I appreciate Steve's videos. He did everything right, IMO, by sealing the back of the tube. Doing so allowed some high pressure to build up behind the primary and, instead of leaking out the back, that higher pressure air moved around the front of the primary. It seemed to carry away some smoke (simulated boundary layer.) But, it was not aggressive enough for me, that smoke lingered in the optical path.
The volume of air flow changes with the amount of space it is flowing through. Coming directly off the back of the fan, a given volume of air is fairly compressed into the diameter of the fan. Then it spreads out in all directions around the back of a 10" mirror. This reduces both the velocity and the volume per unit area. The result is a much weaker flow of air from the annular baffle. In fact, it would seem to drive the boundary layer toward the center of the primary, then maybe up the tube as pressure built up on the primary's surface. The result would be a little mixing, but still some eddies of cooler and warmer (density) in the optical path.
Sucking would have the same affect in reverse, but tend to pull the air out of the optical path. However, the low pressure behind the primary could draw air from the huge volume above the primary. So, the effect is still weak, maybe even weaker than blowing air from behind.
A more aggressive approach is to attack the boundary layer directly. Bob showed one such approach earlier with a "spider" mounted fan blowing down directly onto the primary's business end...it's surface. Another is to blow across the primary's surface. But, where does the air go? It still /should be/ vented. So, you can pull it out the back or the side.
Now, there are problems with this design, too. Blowing air across the surface sets up a laminar flow that DOES NOT hug the mirror's surface. The laminar flow departs the mirror's surface and leaves a turbulent area below it. This is exactly what happens when an aircraft wing is near stall (the angle of attack has to be correct to keep the air hugging the surface of the wing or the mirror.)
But, where does this air flow go? Well, there's a good and sensible argument that some of it might be blown out the exhaust holes opposite the side blowing fan. Truth is, some of that air will bounce off the inside of the tube and recirculate into the optical path. But, the nice thing about this set up is the high pressure is applied directly to the mirror's surface and not immediately diminished as it was blowing and expanding along the entire surface of the primary's back. That's pretty aggressive provided you can prevent the laminar flow from departing the mirror's surface.
You could suck the air from the mirror's surface in much the same way - exhausting out the side, except that the fan will, again, be drawing from a large and weak low pressure area inside the tube. It will not be as aggressive as blowing directly across the surface. If you think about it, you cool yourself with a fan by blowing it at you, not by standing behind the fan allowing it to suck air past you. That doesn't work.
Apply some physics and come up with a design that suits you.