Advanced Building Simulation

154 Addington
emerge, particularly in relationship to micro- versus macro-scale modeling. High
momentum air systems (HVAC as well as wind driven) tend to supplant local air
movement such that the resulting scale at which the behavior is manifest depends on
the length, for example, of the diffuser throw and is therefore relative to room- or
macro-scale. As a result, the CFD model must also be macro-scale and thus the thermal
boundary layers and the specifics of buoyant flow are not relevant. When the
high-momentum system is eliminated from the analysis, individual buoyancy behaviors
will predominate, and the discrete boundary becomes significant. The scale of
interest for investigating the thermal behavior relates to the thickness of the boundary
layer and thus micro-scale. Room-scale is no longer relevant, and the large grid
finite volume models typically used to model room air behavior have no application.
Buoyancy behavior in buildings has more in common with microelectronic heat transfer
than it does with high-momentum air distribution.
One issue that remains regardless as to whether a macro- or micro-model is used is
determining the nature and purpose of the ambient surround—the fluid medium. In
microelectronics, the operative assumption is that the fluid medium acts as an infinite
sink. There is only one objective: the rapid dissipation of heat away from the object.
In buildings, however, we have typically assumed that our objective is the maintenance
of the fluid medium. The heat transfer from the object is only important insofar
as it affects the ambient surround. The thermal objects in a building, however,
may include all the physical surfaces—structure, equipment and people—all of which
have different thermal production rates and dissipation needs. The inertial mass of
the homogeneous fluid medium has been sufficient to absorb these diverse thermal
inputs, but it demands that the room or building volume be controlled. While reasonably
effective, after all this approach has been used for over a century, it is not
only an extremely inefficient means of controlling local heat dissipation from objects,
but it also provides at best a compromise—no single object’s heat transfer can be optimized.
If instead of trying to macro-model the fluid medium, we began to micromodel
the boundary layer of the object, we may begin to be able to mitigate or even
control the heat transfer from the object without depending on the inertia of the
ambient.
Heat transfer is dependent upon characteristic length. Characteristic length is traditionally
considered to be a property of an object or a condition of the prevailing
flow description. Room walls have specific dimensions and locations; luminaires and
computers are built to consistent specifications. Both sources and sinks are relatively
fixed and unchanging in the typical building, this then “fixes” the dimensions and the
flow patterns, thus predetermining the characteristic length and the resulting heat
transfer. Micro-scale modeling, however, allows us to treat the characteristic length
as a variable, affording the opportunity to control the heat transfer at the source.
Characteristic length can be readily changed by adjusting the height of an object,
even if the total area of the object is maintained. For example, if a window with
dimensions of 2ft high by 1ft wide were to be rotated 90� such that it became 1ft
high and 2ft wide, the total heat transfer would be reduced approximately in half.
Fourier’s Law of heat conduction cannot account for this, nor can the wall functions
currently used in macro-scale modeling of no-slip surfaces. It can only be determined
from a micro-scale analysis of the window’s boundary layer. The heat transfer could
further be reduced by an order of magnitude if the height reduction also brought