[esp-r] Re: R: Mean Radiant Temperature

[Geissler Achim]

Dear Francesco,
the definitions I know of for mean radiant temperature do not take emissivity into account. The interesting question probably is, is the measure "MRT" useful for environments with large differences in emissivity? Of course, for radiation heat transfer, the emissivity is important (that is what I think you want to describe?).
I would assume that the definition of MRT is based on the "normal" case, that all surfaces of an indoor space have an emissivity of approx. 0.9. Thus, the description of the radiative surroundings by temperature alone is sufficient.
For your case, you may need to define a "new" MRT - maybe more a "mean radiant heat exchange" type of coefficient (or try PPD / PMV calculations, however, these also use standard mean radiant temperature for their calculation).
Regards,
Achim


> -----Original Message-----
> From: esp-r-bounces at lists.strath.ac.uk [mailto:esp-r-
> bounces at lists.strath.ac.uk] On Behalf Of Francesco Frontini
> Sent: Tuesday, May 20, 2008 10:35 AM
> To: Geissler Achim; esp-r at lists.strath.ac.uk
> Subject: [esp-r] R: Mean Radiant Temperature
> 
> Dear Achim
> Yes I have obtained a MRT, close to the blind, higher for the solution
> with
> low-e blind (that has a higer surface temperature). But for my opinion the
> MRT has to take into account not only the view factor and the temperature
> of
> the surface but also the emissivity. For that reason I'm also supposing
> that
> with a low emmissivity also the MRT should be low (for exemple if the view
> factor is the same: 43,88°x0,9>48.21°x0,5! Where the formula is
> (T1(°C)blind)*(emissivity(1))>(T2(°C)blind)*(emissivity(2))!!!). It's not
> correct?
> 
> Best regards,
> Francesco
> 
> -----Messaggio originale-----
> Da: Geissler Achim [mailto:Achim.Geissler at josef-gartner.ch]
> Inviato: martedì 20 maggio 2008 9.23
> A: francesco.frontini at polimi.it; esp-r at lists.strath.ac.uk
> Oggetto: RE: [esp-r] Mean Radiant Temperature
> 
> Dear Francesco,
> 
> > We want to assess the impact on the operative temperature in an office
> > if we use an internal blind with a low-emissivity coating (0,5
> > emissivity for indoor-facing surface) instead of 0.9 emissivity.
> > We made two different simulation: static (with WIS program) and
> > Dynamic (ESP-r).
> 
> > The results from WIS and from ESP-R for the blind surface temperature
> > are consistent (blind temperature about 5K higher for the
> > low-emissivity blind).
> That is good news.
> 
> > However, the mean radiant temperature at a position close to the blind
> > (0.5 m in ESP-R) is lower for the high-emissivity blind than the
> > low-emissivity blind with the ESP-R calculation.
> 
> As the mean radiant temperature is defined by view factors and surface
> temperature, only, all is well, no? The low-e blind has a higher
> temperature, thus I would expect the mean radiant temperature close to the
> blind to be higher. That is what you seem to have obtained as result?
> 
> Best regards,
> Achim
> 
> 
> 
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