| Topic: |
Science > Physics |
| User: |
"Quentin Grady" |
| Date: |
23 Apr 2007 02:41:03 AM |
| Object: |
How many surfaces? |
G'day G'day Folks,
I was doing some elementary calculations on thermal conduction when
I came across a situation that gave me cause to doubt my
understanding. For the following discussion let(D) represent delta.
The following is a common formula for conduction
Q = kA(D)T/x
though different texts use different symbols.
When dealing with three layers of insulation the formula becomes
Q = A(D)T /(1/hi + x1/k1 + x2/k2 + x3/k3 + 1/ho)
where hi and ho represent surface coefficients.
My doubts arose when dealing with a double glazed window.
Does it have two surfaces or four?
To put it another way does one count the surfaces where the air is
trapped in the glass sandwich?
If one does, which surface coefficient should one use?
Are they "inside" surfaces for which one should use hi?
Thank you.
Best wishes,
--
Quentin Grady ^ ^ /
New Zealand, >#,#< [
/ \ /\
"... and the blind dog was leading."
http://homepages.paradise.net.nz/quentin
.
|
|
| User: "" |
|
| Title: Re: How many surfaces? |
23 Apr 2007 03:03:03 AM |
|
|
In article <ktno23d6kbf16u7h07gnmimqkuhk0vi9sq@4ax.com>, Quentin Grady <quentin@paradise.net.nz> writes:
G'day G'day Folks,
I was doing some elementary calculations on thermal conduction when
I came across a situation that gave me cause to doubt my
understanding. For the following discussion let(D) represent delta.
The following is a common formula for conduction
Q = kA(D)T/x
though different texts use different symbols.
When dealing with three layers of insulation the formula becomes
Q = A(D)T /(1/hi + x1/k1 + x2/k2 + x3/k3 + 1/ho)
where hi and ho represent surface coefficients.
My doubts arose when dealing with a double glazed window.
Does it have two surfaces or four?
If you say "two" then I'm not sure what is your k2, meaning what
conductivity value you use for the trapped air (in principle, k
applies to solids. And if you say "four" then ...
To put it another way does one count the surfaces where the air is
trapped in the glass sandwich?
If one does, which surface coefficient should one use?
That's always the biggest question in heat conduction problems.
Ususally, the answer is empirical or semi-empirical. You should
contact some window manufacturers for info.
Are they "inside" surfaces for which one should use hi?
In principle, yes. However, as there is little room for convection
between the two glass plates, perhaps some effective conductivity can
be assigned to the trapped air.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
.
|
|
|
| User: "Quentin Grady" |
|
| Title: Re: How many surfaces? |
23 Apr 2007 03:20:49 AM |
|
|
This post not CC'd by email
On Mon, 23 Apr 2007 08:03:03 GMT, wrote:
Are they "inside" surfaces for which one should use hi?
In principle, yes. However, as there is little room for convection
between the two glass plates, perhaps some effective conductivity can
be assigned to the trapped air.
G'day G'day Mati,
Thank you for your prompt reply.
One point I'd like to deal with immediately is the matter of whether
it is possible to assign a value of k for trapped air.
The text books I've consulted assign a value of 0.023 W/m.K to the
value of k(air). (Interesting they also give values for k for various
foams. These have slightly higher values.) What is means is most of
the insulation comes from the trapped air with the surface
coefficients being the next in importance.
Best wishes,
--
Quentin Grady ^ ^ /
New Zealand, >#,#< [
/ \ /\
"... and the blind dog was leading."
http://homepages.paradise.net.nz/quentin
.
|
|
|
| User: "" |
|
| Title: Re: How many surfaces? |
23 Apr 2007 03:53:46 AM |
|
|
In article <kjqo23l0u8nist0g659uuugdbkspo0h6oh@4ax.com>, Quentin Grady <quentin@paradise.net.nz> writes:
This post not CC'd by email
On Mon, 23 Apr 2007 08:03:03 GMT, wrote:
Are they "inside" surfaces for which one should use hi?
In principle, yes. However, as there is little room for convection
between the two glass plates, perhaps some effective conductivity can
be assigned to the trapped air.
G'day G'day Mati,
Thank you for your prompt reply.
You're very welcome.
One point I'd like to deal with immediately is the matter of whether
it is possible to assign a value of k for trapped air.
I would say that yes, it is possible provided there is not enough room
for significant convection. I would guess that the space between the
glass panels fits the bill for all reasonable thermal gradients.
The text books I've consulted assign a value of 0.023 W/m.K to the
value of k(air). (Interesting they also give values for k for various
foams. These have slightly higher values.) What is means is most of
the insulation comes from the trapped air with the surface
coefficients being the next in importance.
This may very well be so. Air is not a great conductor (meaning, it
is a good insulator).
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
.
|
|
|
|
|
|
|
Related Articles |
|
|
