[time-nuts] Thermal time constant

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Mar 11 01:01:46 UTC 2010


Oops forgot a factor of 4
Radiative heat transfer for surface with an emissivity of 1 at 300K is 
about 612uW/square cm/degree C
which is equivalent to about 25mm of styrofoam.

Which raises the question what's the emissivity of the isothermal 
surface used when measuring the thermal resistance of a slab of styrofoam?

Bruce

Bruce Griffiths wrote:
> Radiative heat transfer (for a surface with an emissivity of 1) is 
> around 150uW/square cm /degreeC at 300K.
> That's equivalent to about 10cm of styrofoam.
>
> It seems unlikely that the radiative heat transfer component is 
> included in the thermal resistance rating for Styrofoam.
>
> The radiative component is independent of insulation thickness where 
> the insulation doesnt absorb in the 10-30um infrared region.
>
> Adding carbon black to the foam appears to increase the thermal 
> resistance of 25mm thick foam by about 10%.
>
> Bruce
>
> Bob Camp wrote:
>> Hi
>>
>> That raises the interesting question of weather radiant transfer is 
>> already included in the rated thermal resistance. My guess is that 
>> the 8" of foam is enough to cover any radiation issues and still get 
>> you above 20 C / W.
>>
>> Since you are probably starting with 1 or 2" slabs, including the 
>> aluminum foil would be pretty easy. It can't hurt and it might help.
>>
>> Bob
>>
>> On Mar 10, 2010, at 7:29 PM, Neville Michie wrote:
>>
>>> I have been wondering about achieving the rated thermal resistance 
>>> from plastic foam,
>>> the problem being that radiant transmission may be very strong 
>>> through the foam.
>>> What happens if you interleave concentric sheets of foam plastic 
>>> with aluminium foil? (taking care to keep the foil
>>> on isothermal surfaces)
>>> Will this stop the radiant transfer and leave only the thermal 
>>> conduction of the plastic foam?
>>> cheers, Neville Michie
>>>
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>
>
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