C haptEr 9 design Environments and systems
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mined, as is transient heat conduction through elements such as
walls, windows, floors, and roofs. Some programs include a form
of ground-heat transfer modeling as well. These calculations are
made for each time step selected (e.g., hourly, daily). Thermal
comfort models based on temperature and humidity levels are
included. Outputs include space temperature and humidity predictions, energy consumption in relation to selected steps, and
other information.
Though models of the type we’ve described are in wide use, particularly for energy consumption analyses, they are not as useful
in describing the exact nature of air-flow distributions or other
thermally related phenomena within spaces. Here it is better to use
computational techniques that have been evolved to analyze fluid
or gas flows. Computational fluid dynamics (CFD) models were
developed within the fluid mechanics fields to solve many kinds of
problems that involve the movement of fluids, particularly in relation to the interactions of fluids or gases with surrounding surfaces
of various types. These models can predict airflow, heat transfer,
and the distribution of particles (such as contaminants) in spaces
and around buildings. In these models, a fluid or gas, such as air, is
first characterized as a series of discrete small cells to form a volumetric mesh. For normal viscous fluids, the famous Navier-Stokes
formulations based on conservation laws can then be applied to
solve equations of motions. (These equations describe momentum
changes in infinitesimal volumes as the sum of various forces acting
on them, such as changes in pressure, gravity, or friction. Solving
the equations involves literally millions of calculations for even a
relatively small mesh—a task that’s clearly for computers.) All heattransfer modes, including conduction, convection, and radiation,
can be included.
Other solution types not involving meshes are possible, but the
mesh-based approach is commonly used. As might be expected,
however, there are numerous subtleties in establishing and
working with CFD models that are beyond the scope of this text.
Inputs again involve geometrical and material description of the
spaces, heat sources and sinks, and so forth. Outputs include a
variety of analytical results. Three-dimensional depictions and
associated numerical data can be obtained that show the distribution of air flows throughout a space. Three-dimensional convective flows associated with a heat source, for example, can
be clearly seen and numerically described. Mean radiant temperature and heat-flux distributions can be determined and visually depicted. Surface temperatures can be determined. Particle
316
mined, as is transient heat conduction through elements such as
walls, windows, floors, and roofs. Some programs include a form
of ground-heat transfer modeling as well. These calculations are
made for each time step selected (e.g., hourly, daily). Thermal
comfort models based on temperature and humidity levels are
included. Outputs include space temperature and humidity predictions, energy consumption in relation to selected steps, and
other information.
Though models of the type we’ve described are in wide use, particularly for energy consumption analyses, they are not as useful
in describing the exact nature of air-flow distributions or other
thermally related phenomena within spaces. Here it is better to use
computational techniques that have been evolved to analyze fluid
or gas flows. Computational fluid dynamics (CFD) models were
developed within the fluid mechanics fields to solve many kinds of
problems that involve the movement of fluids, particularly in relation to the interactions of fluids or gases with surrounding surfaces
of various types. These models can predict airflow, heat transfer,
and the distribution of particles (such as contaminants) in spaces
and around buildings. In these models, a fluid or gas, such as air, is
first characterized as a series of discrete small cells to form a volumetric mesh. For normal viscous fluids, the famous Navier-Stokes
formulations based on conservation laws can then be applied to
solve equations of motions. (These equations describe momentum
changes in infinitesimal volumes as the sum of various forces acting
on them, such as changes in pressure, gravity, or friction. Solving
the equations involves literally millions of calculations for even a
relatively small mesh—a task that’s clearly for computers.) All heattransfer modes, including conduction, convection, and radiation,
can be included.
Other solution types not involving meshes are possible, but the
mesh-based approach is commonly used. As might be expected,
however, there are numerous subtleties in establishing and
working with CFD models that are beyond the scope of this text.
Inputs again involve geometrical and material description of the
spaces, heat sources and sinks, and so forth. Outputs include a
variety of analytical results. Three-dimensional depictions and
associated numerical data can be obtained that show the distribution of air flows throughout a space. Three-dimensional convective flows associated with a heat source, for example, can
be clearly seen and numerically described. Mean radiant temperature and heat-flux distributions can be determined and visually depicted. Surface temperatures can be determined. Particle