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Thermal Issues and Fans Successes

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#326 Starman1

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Posted 17 January 2013 - 02:12 PM

Thanks for the comments everyone. I'm interested in improving both cool-down and the boundry layer, so I'm open to new ideas. I need to run more tests and plan on trying the following:

Measure and time the cool-down with the current configuration.
Test for fan vibration with the current configuration (haven't noticed anything yet).

Reverse the fan so that it's "sucking" air from below, and I'll try the smoke test again and in addition, will measure & time the cool-down via an infrared thermometer.

Lastly, after reading through this thread, I'm interested in adding a 2nd fan, high-quality low vibration type, to either blow across the face of the mirror, or more likely just to exhaust the air from just above the mirror (exhausting outside the tube).

At this point I can't make many claims on these improvements, other than adding the rear baffle dramatically improves air movement around the mirror. Cool-down, here in the Pac NW, is still surprisingly long - at least one hour in winter!

If you get close to ambient in one hour with a single fan and a 10" mirror, you're doing VERY well.
Try different scenarios with this mirror cooling calculator and you'll see that 1 hour cooldown is quick! If you didn't have a fan, it might never make it to ambient.
http://www.cruxis.co...rrorcooling.htm

#327 SteveG

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Posted 18 January 2013 - 01:07 PM

Yes - Don - I actually think it's more like 1.5 hrs! Still too long in winter.

Mike - Its 1/4" thk black Plexiglas. Shiny on the outside, flat on the inside. I'm very lucky to have a good plastics shop doing custom work for my business, so I can just draw up an astronomy part and get it made. I just received a really nice eyepiece tray that I designed for my Vixen Porta II.

#328 Mark Peterman

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Posted 18 January 2013 - 07:33 PM

Thanks to the work of all those who have gone before us, I think that we can all agree on the following for a solid tube telescope design with a single, rear fan:

A sealed fan baffle is required for optimum effect.

An annulus in front of the mirror is necessary for a rear fan to effectively scrub the boundry layer on the front of the mirror.

The questions that still I have are:

For a fan located behind the mirror, which is better, blowing or sucking? It appears the annulus will direct air over the mirror with either method but there must be advantages/disadvantages to each method.

What size fan (CFM) is adequate for a given mirror size? Is the bigger the better or do you get to a point of diminishing returns?

Is there an optimum distance above the mirror to mount the annulus?

Mark

#329 azure1961p

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Posted 18 January 2013 - 10:06 PM

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#330 Bob S.

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Posted 18 January 2013 - 10:25 PM

Pete, It would be nice if you could tone down the heat a little bit and allow others to have opinions or better yet share their empirical knowledge. The work of Mara Da Lio, myself and many others should not be strongly discounted. If you have a lot of experience and maybe 10-20 scopes that you have experimented with, then you join the club of many of us who do have this amount of experience with closed and open tubes. It appears that a sucking fan with an annulus might be one of the most efficacious strategies for small closed tube telescopes? I am not saying it as an absolute or with ultra strong push but based on the experiments to date, it seems like one of the better strategies. Bob

#331 Chucky

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Posted 19 January 2013 - 07:24 AM

<< I think that we can all agree >>

Will never happen <g>

#332 Mark Peterman

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Posted 19 January 2013 - 10:39 AM

Thanks to the work of all those who have gone before us, I think that we can all agree on the following for a solid tube telescope design with a single, rear fan:

A sealed fan baffle is required for optimum effect.

An annulus in front of the mirror is necessary for a rear fan to effectively scrub the boundry layer on the front of the mirror.


No it's NOT necessary and a side mounted fan has stronger directional airflow. The annulus works but its hardly the last word. Frankly your finds are a little puzzling, particularly on a thread this large.

Pete


Pete, first thanks for starting this thread, it has been a very interesting topic and I have learned a lot from everyone's experiences.

Regarding my post, have you viewed SteveG's videos? Did you see the difference in a sealed baffle versus unsealed?

My second point, about the annulus, relates to my scenario of a SINGLE fan, mounted on the REAR of the scope. I have no desire to cut a hole in the side of my scope (at this time). While I don't doubt that without the annulus, there is some 'pulling' of the boundry layer as the air flows linear up (or sucked down) the tube, I doubt it is as effective as when using the annulus.

Do you have any knowledge to share regarding the questions I asked in the same post?

#333 Mark Peterman

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Posted 19 January 2013 - 10:47 AM

<< I think that we can all agree >>

Will never happen <g>


LOL, yeah, what was I thinking! Take a two minute video, have two people watch it, and they COULD tell you two different stories about what they saw.

#334 Sarkikos

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Posted 19 January 2013 - 11:13 AM

Mark,

No it's NOT necessary and a side mounted fan has stronger directional airflow. The annulus works but its hardly the last word. Frankly your finds are a little puzzling, particularly on a thread this large.

Pete


Pete, have you viewed SteveG's videos? Did you see the difference in a sealed baffle versus unsealed?

My second point, about the annulus, relates to my scenario of a SINGLE fan, mounted on the REAR of the scope. I have no desire to cut a hole in the side of my scope (at this time). While I don't doubt that without the annulus, there is some 'pulling' of the boundry layer as the air flows linear up the tube, I doubt it is as effective as when using the annulus.

Mark


Keep in mind that Steve has a 10" truss scope, while Pete is working with an 8" solid-tube f/8, I believe. Steve's scope has a relatively short mirror box, while Pete has a long solid-tube to deal with. If Steve does not wrap his scope in a shroud, he already has a very large hole in the middle of his "tube" (though admittedly not at the level of the mirror face). Pete has an enclosed solid-tube OTA about 5' long. These differences might be important. :thinking:

Steve's video agrees with my own experience so far. My primary fan is blowing onto back of the mirror, and for my Z8 I have an annulus above the mirror. I'd probably be doing better if I sealed the baffle around the fan. And I'm anxious to see the results in Steve's next video when he reverses the direction of airflow from the fan.

But things might be somewhat different for a solid-tube scope beyond the length of a mirror box. A boundary layer fan and / or venting holes do make sense for a solid-tube scope.

Now, if we don't want to drill or saw-cut holes in our OTA's, that's another matter altogether. I don't like doing that, either. But not wanting to do something is not a valid argument per se for not doing it ... though I do try to get away with that rationale here at home. :grin:

Mike

#335 cjc

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Posted 19 January 2013 - 11:43 AM

...
What size fan (CFM) is adequate for a given mirror size? Is the bigger the better or do you get to a point of diminishing returns?


Bryan Greer suggests some sizes here: Fan_Select. It is also worth looking at the articles by Gary Seronik: Beat_the_heat and Alan Adler: Cross Flow Fan.

I currently have a small, quiet 40mm fan on the back of my solid tube 8", blowing. It has lowered cooling time to under 1 hour from 2 hours to never, when it had an almost completely closed back. Further improvement is needed and I have found this thread both interesting and useful.

#336 Mark Peterman

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Posted 19 January 2013 - 11:53 AM

Mike,

Yes, I am aware of the scope SteveG was using. The lower end of that scope is a 'solid tube' construction (maybe 24-26 inches tall). I believe the video was good eveidence of what is happening around the mirror in a round, solid tube, configuration.

#337 Mark Peterman

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Posted 19 January 2013 - 12:01 PM

Bryan Greer suggests some sizes here: Fan_Select. It is also worth looking at the articles by Gary Seronik: Beat_the_heat and Alan Adler: Cross Flow Fan.

I currently have a small, quiet 40mm fan on the back of my solid tube 8", blowing. It has lowered cooling time to under 1 hour from 2 hours to never, when it had an almost completely closed back. Further improvement is needed and I have found this thread both interesting and useful.


Thanks cjc.

It has been a while since I read Mr. Greer's article so I will revisit it but I believe it was more or less centered around cooling the mirror and not so much the 'scrubbing' of the boundry layer so I am wondering if the stated CFM's per mirror size still applies.


Mark

#338 Sarkikos

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Posted 19 January 2013 - 01:01 PM

Mark,

Yes, I am aware of the scope SteveG was using. The lower end of that scope is a 'solid tube' construction (maybe 24-26 inches tall). I believe the video was good eveidence of what is happening around the mirror in a round, solid tube, configuration.


I agree. I can't - well, shouldn't - argue against the evidence presented to my eyes by Steve's videos. And I'm not. But I'm thinking about what additional measures may be needed to solve thermal problems in a completely solid-tube Dob. I'm aware that we may be comparing apples and oranges. They are both fruit, but they are still different in some ways.

Mike

#339 demiles

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Posted 19 January 2013 - 02:20 PM

Bottom line is this, a simple rear mounted fan blowing against the back is effective in cooling your mirror. Boundary layer solutions are more complicated to execute and may be too difficult for some in that means modifying your tube or mirror box. There are so many variables involved , what works for one person may not for another with different climate conditions. Do your own testing and find out what works for you, there are alot of good ideas put forth in this thread. I'll be interested in your findings

#340 Sarkikos

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Posted 19 January 2013 - 03:03 PM

The ancient Romans had a saying that sums up how we should go about solving our own thermal issues: Festina lente. Make haste slowly! I've been trying to follow that advice, but it's not easy.

Festina lente

And I imagine it is a bit easier to mount a boundary-layer fan in a mirror box than to saw-cut a hole in a solid-tube OTA and attach a fan with a rubber baffle.

:grin:
Mike

#341 Mark Peterman

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Posted 19 January 2013 - 03:44 PM

Bottom line is this, a simple rear mounted fan blowing against the back is effective in cooling your mirror. Boundary layer solutions are more complicated to execute and may be too difficult for some in that means modifying your tube or mirror box. There are so many variables involved , what works for one person may not for another with different climate conditions. Do your own testing and find out what works for you, there are alot of good ideas put forth in this thread. I'll be interested in your findings


Well said. No argument here.

#342 azure1961p

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Posted 19 January 2013 - 08:05 PM

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#343 johnnyha

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Posted 19 January 2013 - 08:11 PM

:gotpopcorn:

#344 Asbytec

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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.

#345 Sarkikos

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Posted 20 January 2013 - 02:08 AM

Norme,

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.)


Then what angle would the side fan need to have in relation to the surface of the primary in order to generate a consistent laminar flow across the entire surface?

There are differences between an aircraft wing and a primary mirror. An aircraft wing is an airfoil. The primary's surface is a parabola, but might be considered planar in the macroworld of air flow. What impact would these differences have in terms of breaking up the boundary layer?

Also, would more than one fan or maybe a larger fan to cover more of the mirror's surface be more effective than a single smaller fan in transforming the boundary layer into a laminar flow?

Mike

#346 Gray

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Posted 20 January 2013 - 02:18 AM

Lol, here we go. :gotpopcorn:

#347 Bob S.

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Posted 20 January 2013 - 02:49 AM

Pete, It would be nice if you could tone down the heat a little bit and allow others to have opinions or better yet share their empirical knowledge. The work of Mara Da Lio, myself and many others should not be strongly discounted. If you have a lot of experience and maybe 10-20 scopes that you have experimented with, then you join the club of many of us who do have this amount of experience with closed and open tubes. It appears


You got to be kidding me Bob. Turn down the heat so long as Im a fan of yours? At this point Im trying to figure out where to vent off the hot air in your post. Your stance on sucking in this case is entirely appropriate.

Pete


Pete, Thanks for starting this thread. It is an important issue that has been frequently overlooked. I would prefer to not engage in personal attacks and will opt out of your thread to reduce any perceived noise from me that you are experiencing. Much good luck on your quest to optomize boundary layer mitigation issues. Bob

#348 Asbytec

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Posted 20 January 2013 - 04:20 AM

Mike, I wish I knew what a good angle would be. Maybe one that just skimmed or slightly above the surface. There was a link a while back in this thread that had a pretty extensive link to a video on fluid dynamics. I watched some of it. This laminar break is one thing they discussed.

I believe it had to do with the hard edge of the primary as opposed to an air foil. Tilt an air foil enough and it puts a hard edge to the oncoming wind and the laminar flow breaks from the surface. But, I am no expert in the field, just regurgitating what I took from that link and applying it to what I think I know.

One would think a broader air flow either by a larger fan or several small fans would more efficiently cover the entire surface.

#349 nevy

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Posted 20 January 2013 - 08:22 AM

I think all these ideas on this subject are good and I enjoyed reading them but its getting a bit complicated and it's started to turn into a bit of an argument , I haven't got the time to experiment with all these options , I have a largish truss scope and a medium solid tube . All I know is from experience with my lightbridge is that planetary views are much nicer with the standard fan blowing the back of the mirror , it also cools the mirror quicker than not having a fan , that's good enough for me so I will be putting a fan on my 12" in the same way as the lightbridge ( blowing on the back of the mirror to cool & to sharpen up the image when cooled) but good luck with whatever way you all find best on your scopes.

#350 azure1961p

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Posted 20 January 2013 - 09:43 AM

I think all these ideas on this subject are good and I enjoyed reading them but its getting a bit complicated and it's started to turn into a bit of an argument , I haven't got the time to experiment with all these options , I have a largish truss scope and a medium solid tube . All I know is from experience with my lightbridge is that planetary views are much nicer with the standard fan blowing the back of the mirror , it also cools the mirror quicker than not having a fan , that's good enough for me so I will be putting a fan on my 12" in the same way as the lightbridge ( blowing on the back of the mirror to cool & to sharpen up the image when cooled) but good luck with whatever way you all find best on your scopes.


The end game of it all which you've pointed out well is, it's simply a matter of what works best for you. There's theory a plenty and variables higher and yon but ultimately, you look through the eyepiece, slight defocus a star and observe . Like the star test itself its ruthlessly obvious wether or not your thermal management is successful or not and often the solutions to opt for are incrediy simple - like reversing fan flow. In the end what finally works for you is independent of countering theory, hypothetical models and so much testimony. The negative airflow or reverse sucki g flow for my closed system essentially made it a vacuum cleaner intake for dew and warm breath. In the face of that it would seem some folks with THEIR design have found success with it still. One being a lifetime ALPO mars observer with a 16" reflector. My disaster is his solution.

The one thing that can be stated with blanket authority is that optical tube assemblies vary enough in thermal properties for any number of reason s that one shoe does not fit all - no matter how well it fits one. I'm still mystified negative airflow in the tube can be beneficial for anybody but such are the details in the variables .

Pete






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