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Equipment Discussions >> Reflectors

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Scanning4Comets
Markus
*****

Reged: 12/26/04

Loc: Ontario, Canada
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5637740 - 01/22/13 08:56 PM

Quote:

I can also feel a breeze on my face or hands as I look into the sky end of my solid-tube 8" or 10" Dob when the fan is blowing onto the bottom of the primary. It is not a strong wind, but a noticeable breeze.

Mike




...But, does it blow dry your hair?


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Scanning4Comets]
      #5637747 - 01/22/13 08:59 PM

No, it is a cooling breeze, not a dry heat.


Mike


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azure1961p
Postmaster
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Reged: 01/17/09

Loc: USA
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5637777 - 01/22/13 09:08 PM

Lol depends how thick that mirror is!

Pete


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: azure1961p]
      #5637917 - 01/22/13 10:40 PM

Pete,

Thanks for taking the time to write all those details. I will be sure to put them into practice the next time I have the scope out. Good info for sure!

Btw, I purchased a 120mm fan as well but I wanted to start with the 80mm first to see how it did. I can then just cut a bigger hole in the existing test baffle to try the bigger fan. I am sort of concerned that the 74 CFM of the larger fan would be too much air, vibration, turbulence versus the 32 CFM of the 80mm fan.

It's possible the reason I did not feel air out the top of the tube with my original setup is because of the way the mirror cell is built. The mirror sits really close to the back of the cell and the room around the back half of the mirror is really cramped. The air has to weave back and forth before making it to the side of the mirror and then into the open tube. I think all those deflections kept the pressure high so most of the air just bounced off the back of the mirror and back outside the tube. That's just a guess.

I enjoy observing for sure, but this tinkering is fun stuff too!


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: Thermal Issues and Fans Successes new [Re: Mike B]
      #5639097 - 01/23/13 03:24 PM

I have just come back from talking with an expert in fluid dynamics. We discussed the problem of clearing the air film from the front of the mirror with a fan behind. It is similar to a problem he tackled in the past of keeping the window in front of an infra-red sensor clear while monitoring the temperature of turbo fan blades. In that case radial jets were positioned round the window. In this case he made the following comments, some of which were quite counter intuitive to me.

This is flow round a bluff object and flow in front of such an object at low velocity (Reynold's number) is lamina, so will efficiently move air at the (mirror) surface across that surface. Round the back the air is turbulent with recirculation which would be less efficient. Added to this, air from the fan is already turbulent, so he felt that a fan at the back sucking is much more likely to be effective than blowing (I have always had my fans blowing, so far). To reduce turbulence further he suggests an exponential horn (like that of a trumpet) on the end of the telescope, which would also serve to insulate the intake from air warmed by the observer's body. He felt convection would not be a problem. Air flowing down the telescope would be at ambient, not warmed by the mirror and I remain concerned about the secondary dewing.

Air should encouraged to move across the mirror surface so a baffle in front of the mirror would be a good idea, creating a circumferential orifice. Shaping this and perhaps the mirror edge to pull the air sideways would be a good idea. There will always be a null point where there is no flow, but the centre of the mirror, which is obstructed by the secondary is a good place for this so a symmetrical arrangement is indicated. The core of a fluid flow into (in this case, but more commonly from) an orifice extends for approximately five times the size of the orifice. So in this case the baffle should be about 1/5th the radius of the mirror in front of the mirror (though perhaps slightly less for a circumferential orifice like this one with area reducing away from the edge). It was counter intuitive that the bigger this orifice, the further the jet would extend.

He suggests that I work with my current small fan (4.7cu ft/min) rather than put a larger one in, because the Reynolds number needs to be kept low. He was surprised at how much air such a small fan can apparently move.

Getting a radial flow past the mirror may be a challenge in my 200mm because the tube is only about 220mm in diameter giving a gap at the side of only 10mm (and I want to suck past a baffle that will be about 20mm up the tube. I shall do an experiment however.

I hope to be able to model this with a CFD code in the near future, but in the mean time I have been loaned some heavy reading matter and have a lot to think about...

Edited by cjc (01/23/13 03:27 PM)


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Project Galileo
scholastic sledgehammer
*****

Reged: 11/14/07

Loc: Jefferson County, Colorado
Re: Thermal Issues and Fans Successes new [Re: cjc]
      #5639324 - 01/23/13 05:57 PM

That is a very interesting post Chris. Thank you for sharing. I can attest to the benefit of this sucking system in application. It is a definite thermal issue fan success. When I first closed the back of my 16" telescope intending to install a sucking system I tried the fan blowing and sucking just to be sure. The research I had done pointed to what you talk about in your post but I wanted to be sure. While testing each way, sucking won out over a period of six different nights. Recently, because of this thread actually, and because turning over a fan is easy, I retested sucking vs. blowing over three nights. Sucking still wins. The air in the tube is just more laminar, cleaner moving, and smoother heading down the tube with a sucking fan. At the eyepiece there was a clear difference. With the fan blowing I could see out of focus more turbulence in the tube. I say tube because with the light shroud my trusser is functionally turned into a tube. I implore people to test it themselves on their own systems. This is a performance tweak that can be easily be undone if you don't like the results.

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johnnyha
Postmaster
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: Project Galileo]
      #5639672 - 01/23/13 10:28 PM Attachment (18 downloads)

Aha! I was digging around in my stuff tonight and found this fan, I intend to mount it somewhere on the top of my Obsession mirror box. It has a flexible stalk, and it can be spun around to blow in any direction in the circular housing. Brushless motor and 3 speeds.

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demiles
professor emeritus
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Reged: 11/07/06

Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5640039 - 01/24/13 05:40 AM

Johnny, my advice is too try to temporarily mount it and test before doing any thing permanent. I posted a pic earlier in this thread of my top mounted fan on the corner of the mirror box on my 15. It's isolated from the box with rubber expansion nuts and I have added a safety tether so it doesn't accidentally fall into the mirror.

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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: Thermal Issues and Fans Successes new [Re: Project Galileo]
      #5640298 - 01/24/13 09:55 AM

Chris and Project,

Thanks for the information. I think I am going to have to give the exhausting configuration a try. Chris alluded to a low flow rate as being optimum for this but I don't understand the theory behind it and Project G's fan is strong enough to suck the shroud tight.

Someone ealier had commented that you don't stand behind a fan to get cool and logically, that makes sense. However, if you are standing in a long tunnel and have a fan pulling air through it, I guarantee you that you will feel a breeze. I've even felt this in parking garages where they have large fans running to exhaust the fumes.


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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Mark Peterman]
      #5640318 - 01/24/13 10:06 AM

FWIW, my intuition is that a stronger air flow is best for cooling the primary, but a low flow rate is better for smoothing out the boundary layer. Probably we should try not to confuse these two problems and their solutions.

Mike


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peleuba
scholastic sledgehammer


Reged: 12/01/04

Loc: Southern PA
Re: Thermal Issues and Fans Successes new [Re: johnnyha]
      #5640891 - 01/24/13 03:07 PM

Quote:

Aha! I was digging around in my stuff tonight and found this fan, I intend to mount it somewhere on the top of my Obsession mirror box. It has a flexible stalk, and it can be spun around to blow in any direction in the circular housing. Brushless motor and 3 speeds.




This is an excellent idea. I have thought about installing one of these inside the sphere of my Portaball. But, this fan emits an obtrusive blue glow from an LED. How did you disable the LED, I've not determined a way to do this yet.

Thanks.


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nevy
professor emeritus
*****

Reged: 02/07/12

Loc: UK
Re: Thermal Issues and Fans Successes new [Re: peleuba]
      #5640975 - 01/24/13 04:05 PM

Quote:

Quote:

Aha! I was digging around in my stuff tonight and found this fan, I intend to mount it somewhere on the top of my Obsession mirror box. It has a flexible stalk, and it can be spun around to blow in any direction in the circular housing. Brushless motor and 3 speeds.




This is an excellent idea. I have thought about installing one of these inside the sphere of my Portaball. But, this fan emits an obtrusive blue glow from an LED. How did you disable the LED, I've not determined a way to do this yet.

Thanks.



Nip it & bust it with pliyers


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MDavid
sage


Reged: 11/15/11

Loc: 29N (Texas)
Re: Thermal Issues and Fans Successes new [Re: Sarkikos]
      #5641001 - 01/24/13 04:28 PM

Quote:

FWIW, my intuition is that a stronger air flow is best for cooling the primary, but a low flow rate is better for smoothing out the boundary layer. Probably we should try not to confuse these two problems and their solutions.

Mike




So, I'm wondering if they make or if there is a way to make a fan bi-directional?? "Blow" to cool the mirror then switch to "suck" in order to keep it clear.


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Thermal Issues and Fans Successes new [Re: MDavid]
      #5641053 - 01/24/13 04:55 PM

Quote:

Quote:

FWIW, my intuition is that a stronger air flow is best for cooling the primary, but a low flow rate is better for smoothing out the boundary layer. Probably we should try not to confuse these two problems and their solutions.

Mike




So, I'm wondering if they make or if there is a way to make a fan bi-directional?? "Blow" to cool the mirror then switch to "suck" in order to keep it clear.



These small fans have "cupped" blades, so are not reversible. The only way to reverse their actions is to reverse the directions in which the fans point.
It might be possible to have sets of fans and a switch that controls them: say, 3 fans to push during cooling and one fan to pull when cooling is done.


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rlmxracer
sage
*****

Reged: 11/09/11

Loc: Motocross Mecca (Riverside) , ...
Re: Thermal Issues and Fans Successes new [Re: MDavid]
      #5641058 - 01/24/13 04:58 PM

Quote:



So, I'm wondering if they make or if there is a way to make a fan bi-directional?? "Blow" to cool the mirror then switch to "suck" in order to keep it clear.



Most of the modern low vibration fans have blades that are designed to go in one direction only. On most of the set ups I've seen in this post it would only take a minute or two to just flip the fan over. When these clouds clear out I am going to flip my fan over so it is blowing up the tube just to say I've tried it.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Thermal Issues and Fans Successes new [Re: rlmxracer]
      #5641197 - 01/24/13 06:21 PM

1) Attach fan to scope with velcro.
2) put velcro pieces on both sides of fan.
3) it will take 5 seconds to reverse fan direction by flipping fan over


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johnnyha
Postmaster
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Reged: 11/12/06

Loc: Sherman Oaks, CA
Re: Thermal Issues and Fans Successes new [Re: nevy]
      #5641270 - 01/24/13 07:07 PM

Quote:

Quote:

Quote:

Aha! I was digging around in my stuff tonight and found this fan, I intend to mount it somewhere on the top of my Obsession mirror box. It has a flexible stalk, and it can be spun around to blow in any direction in the circular housing. Brushless motor and 3 speeds.




This is an excellent idea. I have thought about installing one of these inside the sphere of my Portaball. But, this fan emits an obtrusive blue glow from an LED. How did you disable the LED, I've not determined a way to do this yet.

Thanks.



Nip it & bust it with pliyers



That's exactly what I did. I forgot about those blue lights!


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Nyctophobia, Maryland, USA
Re: Thermal Issues and Fans Successes new [Re: Starman1]
      #5641308 - 01/24/13 07:30 PM

Don,

Quote:

1) Attach fan to scope with velcro.
2) put velcro pieces on both sides of fan.
3) it will take 5 seconds to reverse fan direction by flipping fan over




Velcro is a marvelous and underutilized material. It puts duct tape to shame. The only DIY building stuff that is more neglected than Velcro is black foam core. Velcro and black foam core is a match made in heaven ... or somewhere in New Hampshire, maybe.

Mike


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StevenYood
journeyman


Reged: 01/29/10

Re: Thermal Issues and Fans Successes [Re: Sarkikos]
      #5641952 - 01/25/13 05:12 AM

I have done the back fan with velcro experiment over the past several months, myself with a 18 inch Starmaster. My findings were that the exhaust fan configuration was vastly preferable for my situation. It definitely yeilded better results for viewing with the fan running at relatively low speeds.
The cooling also was better accomplished with exhaust in my area. In central GA, we can have very extreme dewing problems. When I was the fan blowing directly on the mirror, I sometimes, inadvertantly, overcooled the mirror inducing dew! After a couple of occurances of this, I went to the exhaust option and never looked back.
I am, now, gathering materials to add a front fan in the shadow of the secondary. I can post details of the project and result in a few weeks.


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MDavid
sage


Reged: 11/15/11

Loc: 29N (Texas)
Re: Thermal Issues and Fans Successes [Re: Starman1]
      #5643003 - 01/25/13 05:20 PM

Quote:

1) Attach fan to scope with velcro.
2) put velcro pieces on both sides of fan.
3) it will take 5 seconds to reverse fan direction by flipping fan over




Simple, easy to implement, and good common sense...I like it!


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