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Re: [EnergyPlus_Support] View Factors
I assume that the surfaces do not have identical outside boundary conditions
and constructions, otherwise they would all be at the same inside temperature
and radiant transfer would go to zero regardless of absorptance. If you view
the problem as a steady-state circuit diagram, there is a resistance between
each surface to the zone air node through convection, and there is a resistance
from each surface to each other surface through radiation. Because the space
is uncontrolled and the internal gain is 100% convective, the net convective
flow into the surfaces is equal to the internal gain. The radiant exchange
just redistributes that flow between the surfaces. So, it is very plausible
that changing the radiant resistances (by changing the absorptance) will
redistribute the heat flows such that a different air node temperature is
required to balance the flows. Remember if you are using automatic convection
coefficients, the convective resistance varies from surface to surface.
Mike
On 24 Oct 2004, at 9:04, Peter G. Loutzenhiser wrote:
>
> The internal gain is electric equipment with no radiative component. This may
> help though. I am letting the temperature float and controlling the boundary
> conditions with outside surface temperatures. Would that have any impact? I
> guess what I am seeing is a change in the zone temperature when adjusting the
> thermal absorbtance. Does that provide enough information?
>
> Peter
>
> >
> > <html><body>
> >
> >
> > <tt>
> > Are there any internal gains (people, lights, etc.) that have a radiant <BR>
> > component? How are you reporting or inferring the amount of radiant
> exchange. <BR>
> > While it is true that the total radiant exchange should sum to zero, the <BR>
> > magnitude should change. Lower absorptances become a resistance to radiant
> <BR>
> > heat exachange. So, I would expect to see more heat flowing from warm to
> cool <BR>
> > surfaces when the absorptance is high, and less flow when the absorptance is
> <BR>
> > low. Is that consistent with the results you are getting?<BR>
> > <BR>
> > Mike<BR>
> > <BR>
> > <BR>
> > On 21 Oct 2004, at 6:46, Peter G. Loutzenhiser wrote:<BR>
> > <BR>
> > > <BR>
> > > I am modeling a small interior room with no windows--so no additional <BR>
> > > irradiation enters the space, and I have specified no thermal mass in the
> <BR>
> > > space. I look at the calculation method for radiation exchange and
> appears to<BR>
> > > be an energy balance between all the surface. I would expect from an
> energy<BR>
> > > balance that the heat transfer from some wall would be negative and some to
> be<BR>
> > > positive and, therefore, from a first law balance assuming steady-state<BR>
>
> > > conditions, the total heat exchange due to radiation would sum to zero.
> <BR>
> > > However, I have found that radiation exchange varies significantly (for
> this<BR>
> > > size of space) when I alter the thermal absorbtance. Perhaps I am
> misinformed<BR>
> > > and am completely missing an assumption or not factoring in additional
> elements<BR>
> > > used in the algorithm, but how can this be?<BR>
> > > <BR>
> > > Thanks in advance,<BR>
> > > Regards,<BR>
> > > Peter Loutzenhiser<BR>
> > > ISU <BR>
> > > <BR>
> > > <BR>
> > > <BR>
> > > <BR>
> > > <BR>
> > > <BR>
> > > The primary EnergyPlus web site is found at:<BR>
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> > > <BR>
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> > ========================================================<BR>
> > Michael J. Witte, GARD Analytics, Inc.<BR>
> > EnergyPlus Testing and Support <BR>
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