DOE-2 Engineers Manual Version 2.1A - DOE-2.com

In the sketch, Rc is the convective film resistance, RR is the radiative
resistance, and RRc is the combined radiative and convective resistance,
usually chosen such that
(I !.lOn
In the DOE-2 model, the only path for energy from an interior wall surface is
to the zone air. Because the zone air represents a heat sink at a fixed temperature
(the zone reference temperature) during the load calculation, there
is no path for heat flow between walls in the DOE-2 model. The realistic model
indicates that energy can flow between walls 1 and 2 if there is a temperature
difference between the inside surfaces of the walls. Even though both walls
are exposed to the same exterior and room air temperatures, they could exhibit
different interior surface temperatures if they have different thermal properties
or construction.
Even though the need for conduction weighting factors can be seen, a
calculation technique is not obvious. Conduction weighting factors should
represent the coo 1 i ng-l oad sequence that resu lts from a pu 1 se of conduct ion
energy at the interior wall surface. However, generating a pulse of conduction
flux at interior wall surfaces for weighting-factor calculations would
require temperature gradients in the walls. These temperature changes in the
walls would effectively store some energy in the walls, as well as creating
the conducting flux. During the weighting-factor calculation, it would not be
possible to distinguish the energy initially stored in the walls from that in
the conduction pulse. However, it is possible to estimate values of the conduction
weighting factors by analyzing only the energy inside the interior
wall surfaces in the same manner that was done for lighting, people, and equipment
weighting factors. The analysis is based on the following assumptions.
1. Conduction-energy transfer from interior wall surfaces can occur by two
modes, convect ion and radi at ion. The re 1 at i ve amounts of convect i ve and
radiative energy are constant.
2. Energy transfer by convection represents an immediate cooling or heating
load.
3. Energy transfer by radiation can be analyzed by assuming that radiant
energy from a wall has uniform intensity on all other walls in the zone.
The cooling or heating loads from a pulse of this radiation, multiplied
by the fraction of energy assumed to be radiant energy, represent conduction
loads.
4. The unweighted conduction energy transfer rate was calculated using an
appropriate value of the combined film resistance.
The process for cal cul at i ng conduct ion wei ght i ng factors i nvo 1 ves the
specification of the fractions of energy assumed to be radiant (fR) and
convective (fc)' By the first assumption above, fR + fc = 1. The sequence of
cooling loads obtained from a uniform radiant intensity on all of the room
walls [Qu(i8)] is the same sequence required for calculating weighting factors
II .86