Advanced Building Simulation

New perspectives on CFD simulation 155
about a regime change from turbulent to laminar flow. In addition, the characteristic
length could be altered in the reverse direction to increase the heat transfer from
radiant or chilled panels.
More significant, however, and with potentially greater applicability, is the direct
manipulation of the characteristic length through an intervention to the flow behavior.
Objects and surfaces would not have to be modified or repositioned, rather a
careful positioning of an intervention or a response behavior could shift the flow
behavior such that a different characteristic length would drive the heat transfer. For
example, for a given heated surface, such as a west-facing wall on a summer afternoon,
simply shifting the location of the response behavior, whether a cooled surface
or a low-momentum air supply, can substantially impact the resulting heat transfer.
A chilled ceiling or a cool air supply near the ceiling will result in nearly a 70%
greater heat transfer from the heated wall than if the cool sink were to be moved to
the floor (Addington 1997). Although a common belief is that cool air when supplied
low is least efficient at dissipating heat, it is most efficacious at reducing the amount
of heat that must be dissipated to begin with.
The relative location of the cold sink is one of the most straightforward means to
manipulate characteristic length and thus heat transfer (Figure 6.3). Although conventional
HVAC systems could be modified to allow this shifting, recent developments
in micro- and meso-thermal devices may provide the ideal response.
Researchers have toyed with these devices for many years, hoping that they would
eventually help to replace HVAC systems. In 1994, the Department of Energy stated
that its primary research objective was the development of micro- and mesotechnologies
for heating and cooling (Wegeng and Drost 1994). They had imagined
that a micro-heat pump could be assembled in series into a large sheet, much like
wallpaper, such that a relatively small surface area of this thin sheet could easily
provide enough capacity to heat and cool a building. The initial projections were that
1m 2 of the sheet would be all that was necessary for heating and cooling the typical
home. Their expectations for the micro-heat pump capability have been far exceeded,
with today’s micro-heat pump capable of transferring two orders of magnitude
more heat than the basis for their original calculation, yet the project has been stalled.
Figure 6.3 Temperature profile comparisons as the relationship between source and sink is shifted.
(See Plate V.)