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Unhappy about active cooling attempts...

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#1 azure1961p

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Posted 19 November 2012 - 07:35 AM

I am happy I get some reduction in thermals via my fans but its far from effective as it could be.

1. First I blew air at the back of the mirror up the tube. OK it helped.
2. Then I put in an 80mm fan for boundary layer reduction followed by exhaust ports in the othetside of the tube. No difference whatsoever.
3. Last night I reversed the rear fan so its sucking the best out behind the mirror. No difference infact its worse as it ducks my breath down the tube even yh3 heat from my hand guiding the end of the tube.

Yes it's improved but right now its awful. Seems the boundary fan is too weak and frankly it was best only when the air blew over the hot mirror.

Help.


Pete

#2 Asbytec

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Posted 19 November 2012 - 08:23 AM

Okay, so you put a side blowing fan with exhaust and there was no difference...from the fan blowing on the back. Visually, you mean. Interesting.

Surely the fan blowing on the back had little effect across the face of the primary - as the side blowing fan would have. The back blowing fan, I believe, is primarily just for cooling and eventual elimination of any boundary layer as the mirror cools. The side blowing fan is for immediate elimination of the boundary layer and eventual cooling. So, you scrubbed a boundary layer and noticed no difference? Was the mirror already cooled to ambient?

Now, sucking cool, clean air down the tube draws in your breath and body heat. Then it deposits it on the primary 4 feet away? Maybe not. Maybe you can certainly see some body heat in a de-focused star, but that heat can be anywhere. And it is, across the light path at the tubes open end. It does not necessarily have to be drawn onto the primary. I dunno, can you feel a breeze entering the top of the tube?

Same with breath. That would need quite a bit of suction from a pretty small fan moving a little air. (How many cubic feet per minute?) But, the kicker is, if the bottom of the tube is open, the fan can more easily draw air from there. Not all of it has to come down from the top of the tube - unless the primary end is sealed air tight. The air being drawn down the tube would mix readily, I would think.

I am a big fan (pun intended) of the side blowing set up. Go for immediate boundary layer removal (if any) and let cooling happen naturally. Do it in a way that helps warm air do what it wants to do - rise. The fan, unless it's an air boat engine, just aids nature helping the warm air along it's natural path.

In this sense, you can pull it from the primary toward the upward facing tube, and maybe have some intake port opposite. Or your could blow it "upward" toward the exhaust port mounted on the upward facing side of the tube. And if you can exhaust it, then temp (density) differences should be minimal.

In any case, if it made no difference than blowing on the mirror back, and it did not take your breath away - so to speak - then I'd be inclined to give sideways a second chance.

#3 Project Galileo

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Posted 19 November 2012 - 12:25 PM

Have you tried a Mauro da Lio baffle? Much has been written on them here. It may help.

#4 Mike B

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Posted 19 November 2012 - 01:09 PM

Do it in a way that helps warm air do what it wants to do - rise. The fan, unless it's an air boat engine, just aids nature helping the warm air along it's natural path.


Precisely!

And let me add a thot to this:
You've gotta look thru the air in your tube. Period. How do you make that air behave as nicely as possible? You want it to be calm, not turbulent. If it's gonna be moving, then you'll want it marching up the tube in uniformity, not re-enacting scenes from a Keystone Cops movie as it goes! Ideally, you'd want any warmer (& thereby turbulent) air streamlined into its narrowest profile, and hugging the tube's walls as it chimney'd its way up & out.... not billowing & bubbling its turbulence into the light's path- and remember, it's a DOUBLE pathway, first down the tube, then reflected back up!

A rear-blower (not sucking) fan on a tubed Newtonian should go a lonnng way toward accomplishing the above objectives!
1) speeds the mirror's cooling to (nearer) ambient,
2) helps maintain that temp as the night's air continues to cool faster than glass can normally keep up with,
3) pushes the air inside the tube into more of a streamlined shape as it moves up the "chimney",
4) the lightpath should then be filled with uniformly moving air,
5) any "boundary layer" hugging the face of the mirror, caused by the temp difference b/t the glass & the air, WILL get siphoned off- at least to some degree- by the air moving upward around its edges... it cannot & WILL not remain static.

Even in my 15" Dob, open truss for sides, and a single PC fan blowing at the back of the mirror (note: my mirror box has a *solid* back, not "open"... there's a small circular hole for the fan to blow thru- so it's essentially a "baffled" fan set-up ;)), when i defocus a bright star into a large, bright disk, i can easily *SEE* the air in front of the mirror moving! It's slowly & gently swirling in a circle- a motion clearly induced by the rear-fan!

So after a while of this fan running , and the mirror nearing ambient, i'm pretty confident there is *NO* appreciable boundary layer remaining!

If you can envision the scope + tube bein' a typical kitchen "blender", with the spinning blade at the bottom, look at the motion that's induced into the fluids in the blender.

Now consider this: In this nicely behaved, gently swirling & ordered march of air moving up one's tube... what would a side-blowing fan cause to happen? Even if there were "exit holes" in the tube's wall opposite this side-blower, do you really believe THAT air will pass thru the upward marching air without affecting it, and then exit those holes like well-behaved molecules? :scratchhead:

Needless to say, i have my own suspicions about the efficacy of dual-fan systems. ;) But i'm a HUGE fan of the rear-blower configuration!
:lol:

#5 Starman1

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Posted 19 November 2012 - 02:54 PM

I run 3 fans between sunset and darkness 90 minutes later: 1 blows on the back, and 2 blow across the face of the mirror. The goal is to cool the mirror as fast as possible by having the entire mirror being blown upon by air colder than the mirror.
Once the mirror has cooled (and a laser thermometer shows the mirror is at ambient when it gets dark), the single rear fan is sufficient to keep the mirror at ambient temperature. I only turn it off if the night ceases to cool and the temperature is the same a few hours in a row.
I have tried turning the fan off to see if the out of focus star image begins to exhibit heat related movement, and IT ALWAYS DOES WITHIN A MINUTE OF TURNING THE FAN OFF. So I keep the rear fan going. It's damped, so I see no vibration at 304X-456X in the star images. But I DO see "heat signature" star movement when the fan is turned off, even after 6 hours outside.
I observed for over ten hours on the 11th, and even after that long a period, the fan still helped.

I wish it could turn 2 arc-second seeing into 0.2 arc-second seeing. But the fan that could blow that kind of hole in the atmosphere would be a powerful fan indeed. :lol:

#6 azure1961p

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Posted 19 November 2012 - 05:53 PM

Hi guys,

Thanks for the replies. Just to clarify a few points....

The side mounted fan is where Alan Adler has his in the illustrations to his sky tell article. My trouble here is two fold: 1. The fan even with its body 6v

#7 azure1961p

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Posted 19 November 2012 - 06:11 PM

Hi guys,

Thanks for the replies. Just to clarify a few points....

The side mounted fan is where Alan Adler has his in the illustrations to his sky tell article. My trouble here is two fold: 1. The fan even with its boxy 6v battery is still to weak. I can barely feel a draft out the exhaust ports. 2. The fan is blowing OVER. The mirror not onto it. I would need to tilt it 45 degrees down to put the airflow ON the mirror. That's the major problem here.

So here's what I'm doing: 1. Making a fan box with a big 5" fan like the rear fan size. THIS is the airflow needed. From the airflow out of the box that'll mount to my dob I will connect the airflow to the tube via a flexible hose much like the hose attached to the back of a laundry dryer. Hose can't be too long cause of its own movement but it'll dampen the other 5" . 2. Where the dryer hose attaches to my tube I will have a louver of some kind. Maybe a section from an old AC or somesuch just so it vectors the air down onto the mirror face.

I'm sorry I followed the sky and tel illustration. Adler's explanation is correct but the illustration is flat out wrong.

Don, Norme, Mike, guys thanks for the input. On balance if it never got better than a rear fan cool effect I'd still love my scope. Reading about the fact that it CAN be grestly reduced to eliminated has made me intolerant of it.

At the very least that fan is blowing the hell forward. Norme the big 5" fan was sucking my breath down the tube. Puttting my hand at the tube - LOL intake I could feel the suction draft. Badly no but it definately was felt.

I'm bushed hit the sack at two last night and got up at 6 this morning. No third observing night in a row lime I planned and its clear out. :(

Pets

#8 demiles

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Posted 19 November 2012 - 07:22 PM

One thing to consider is the air/mirror temperature differential when you start. If your mirror has been in a warm house at 70 degrees and the air temps are say 50 at sunset, that's a large differential to over come. That's going to be ugly regardless of fan configuration. Add the fact that temps may drop 5 degrees per hour for the first 3-5 hours of observing. Try to store your scope outside if you know you will be observing to get as close to ambient as possible if you can.

#9 azure1961p

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Posted 19 November 2012 - 09:40 PM

Thanks good points.

Pete

#10 Asbytec

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Posted 19 November 2012 - 10:40 PM

Mike's comment above spurred some thought. It's all about pressure. Pressure moves air, even huge atmospheric weather patterns. If you can create high pressure at the primary, all that air will move toward low pressure.

Trick is, to isolate that low pressure to the top of the tube to better assist the chimney effect. Sealing the bottom of the tube, with neoprene or something, would force high pressure area created by the fan to move in the only direction it can. Up and out the tube. (With the fan you're using, looking down the tube might cause your cheeks to flutter! :lol:) With an open bottom, some of that high pressure blows out the back of the tube and the effect is wasted.

If it siphons off some of the boundary layer, all the better. And it should, much like cigarette smoke making its way toward a cracked window in a car door. The air moving up the tube wall will create lower pressure relative to the calm air over the primary face. This would draw the boundary layer toward the tube edge in exactly the same way cigarette smoke makes its way out of a car moving down the highway. How much is siphoned depends on the amount of pressure differential (wind speed), the stuff that makes airplane wings fly.

Alternatively, you could seal off the bottom of the tube and have the fan exhaust air out the bottom. This would create low pressure at the primary and pull high pressure air down the tube. Along with your humid breath, of course. :)

I like the fan blowing "up" configuration, at least to try it.

#11 Asbytec

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Posted 19 November 2012 - 10:53 PM

I wish it could turn 2 arc-second seeing into 0.2 arc-second seeing. But the fan that could blow that kind of hole in the atmosphere would be a powerful fan indeed. :lol:


With such a fan, cloud cover would no longer be a problem. Imagine the weather guy trying to explain suddenly clear skies just over your house. Airline passengers would definitely feel the bump and they passed over. Laminar flow fired directly into space. Creating your own observing hole, I love it!

:rofl2: :rofl2: :rofl2: :rofl2:

Hmmm...or maybe it would create one huge cumulonimbus cloud right above your scope on an otherwise clear night. I mean, if you had Pete's moisture laden breath shooting skyward and rapidly cooling. Friggin lighting bolts! A vortex! The updraft flapping roofing tiles for blocks!

:rofl2: :rofl2: :rofl2: :rofl2:

Ah, *BLEEP*, got tears in my eyes...:rofl2:

#12 Mike B

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Posted 19 November 2012 - 10:57 PM

Certainly would help solve the conundrum of designing a "cloud-filter"...
;)






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