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Calculating heat transmission loss in a thick glass

Iknowalittle
Iknowalittle asked
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Last Modified: 2012-05-06
Hi.

Do anybody here knows, if and how much and how quick a thick tempered (60 degrees celcius) glass (85mm) of size 500mm * 500mm looses it temperature?

What I want to know, is the temperature profile "out-of-plane" inside the 50-100mm thick glass and related to time.

The glass is surrounded by air of ambient temperature (20 degrees celcius).

I know there are several programs out there, who could answer this question, such as COMSOL 3.5, ANSYS Multiphysics, NAFEMS etc. But I don't have acces to these fancy software packages.

Did anyone here made some calculations related to the above?

How much heat energy/temperature do a thick window as described above loose within 5, 10 and 15 minutes surrounded by ambient temperature?

Thanks in advance

Frank Nielsen
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Top Expert 2006

Commented:
Do you happen to know the temperature of the glass at a particular time t>0?
If so, then we could work it out using Newton's law of cooling
Top Expert 2006

Commented:
(I've requested that this question is reopened, as I did not actually answer the question. The real experts will hopefully be along soon.)

Authors comment:

"No, sorry. We didn't had any equipment to measure the temperature of the glas.

The glass was originally put into a climate chamber for 24 hours at temperature 60 degrees celcius. It was then removed from the chamber and was supposed to be used for some specific purpose with an expected temperature of the thick glass at 60 degrees. But I could put my fingers and my whole hand on one side of the glass after approx. 10 minutes, and it didn't felt uncomfortable at all to do this; I could keep my hand on the glass without problems, and it didn't felt like 60 degrees at all, not even 50 degrees. Maybe it is just the outer layers of the glass who loosed its temperature (quickly) ? Hope to hear more from you."
CERTIFIED EXPERT

Commented:
did the author not say that the temperature was 60 degrees at t = 0?
-------
"What I want to know, is the temperature profile "out-of-plane" inside the 50-100mm thick glass and related to time."

If I understand your question, the answer to this part is no.
The edges will be at a different temperature than the interior.
------
The glass will begin cooling immediately and will always be below 60 degrees. It will approach 20 degrees. (rapidly at first, then more slowly) The rest of your questions can be answered with more info about the exact type of glass. (Info needed - heat capacity and thermal conductivity.)
Darrell ThomasEngineer
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Author

Commented:
Nothing to write here
Darrell ThomasEngineer

Commented:
1)  No, the model started at 60.  You lose 10 degrees in the first 60 seconds.  There is one unknown value (convective heat transfer coefficient) in which I took a reasonable guess.  I.e. was there wind blowing on it?  was the air still?  etc... the heat transfer between the glass and air is governed by this coeffiient (and the temp difference).  This means the answer is +/- 10% or so.

2) I used a PDE (partial differential equation) solver in Matlab.  There are other open source solvers (scilab, octave, LISA, etc.. as you mentioned above.  I just stumbled across this problem last night and told myself, "I know how to do that! Let me help this guy out!"  Drop me a note if you need anything else.  darrellthomas67 at gmail dot com.

3)  Right now I'm in the Air Force, but I'm getting ready to retire soon.  Now I have to figure out what I want to do when I grow up :).  For study, I have a Master's in Mechanical/Aerospace engineering.

Author

Commented:
No, sorry. We didn't had any equipment to measure the temperature of the glas.

The glass was originally put into a climate chamber for 24 hours at temperature 60 degrees celcius. It was then removed from the chamber and was supposed to be used for some specific purpose with an expected temperature of the thick glass at 60 degrees. But I could put my fingers and my whole hand on one side of the glass after approx. 10 minutes, and it didn't felt uncomfortable at all to do this; I could keep my hand on the glass without problems, and it didn't felt like 60 degrees at all, not even 50 degrees. Maybe it is just the outer layers of the glass who loosed its temperature (quickly) ? Hope to hear more from you.
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