reprinted from Desktop Engineering Magazine, April 1997
With the continuing emphasis on miniaturization and enhanced performance of electronic
devices, the density of electronic components per unit volume and hence the power density
are steadily increasing. This exacerbates heat dissipation problems and may cause elevated
temperatures. Since elevated temperatures deteriorate the performance of electronic
components, there is an urgent need for effective cooling strategies. Older design methods
based on empirical formulas no longer work in these high density environments. Therefore,
it has become imperative for designers to use computer modeling tools that provide a
higher degree of realism and accuracy.
Coolit (R) was specifically designed to address these needs of designers of electronic
equipment. It uses Computational Fluid Dynamics (CFD) methods to provide fully coupled air
flow and heat transfer simulation in electronic equipment. The entire program including
its graphical user interface, solver and graphic postprocessor, were built from scratch
and optimized for electronic cooling applications.
Coolit is capable of modeling all standard objects found in electronic enclosures: the
cabinet, printed circuit boards, fans, vents, etc. which are assembled into a package
using an intuitive graphical user interface (Fig. 1). Coolit
was designed for ease of editing and allows users to see the orientation and size of
components as they are created, assembled, and packaged. It provides features not
previously available in CFD software, such as Undo and Redo functions. Coolit's GUI flows
naturally from creating the enclosure, building and packaging components, solving the
problem, and finally displaying the computed results.
First users create the enclosure and components. An enclosure in Coolit can be the
actual cabinet or simply a volume inside which the solution is computed. Vents or fans can
be placed on the sides of the enclosure and a variety of heat and momentum transfer
boundary conditions can be specified.
A component is created by selecting a component type out of the 7 types provided by
Coolit and assigning it specific geometric dimensions and physical properties. The entire
process of creating a single component takes less then a minute. Previously created
components can be read from a library and newly created components can be saved into a new
library or added to an existing library.
In addition to standard components, Coolit provides the ability to assemble objects
consisting of more than one component. This new complex object can then be manipulated as
a single component. Examples of complex objects include PCB racks, memory chip assemblies,
objects that involve contact resistance and multimaterial objects, such as transformers
consisting of insulation and metal core.
Once components are defined, the user can start packaging them inside an enclosure.
During the packaging, the user can view the process in 3 projections plus a 3D window
using either wireframe or solid model display options selected by clicking on the
appropriate icon. In solid model display, components are color coded in accordance with
the seven Coolit component types. Components can be positioned using interactive dialog or
by selecting and dragging them with the mouse. Several component selection options are
provided on the toolbar to enable users to select components in any projection view, even
if it is blocked by other components. As the mouse cursor is swept over components, the
component's name and position are shown in the status bar.
Solving the Problem
The geometric model of the electronic device created above, now needs to be translated
into discrete volumes (computational grid) for Coolit's finite volume solver. Coolit
generates that grid automatically, which users can edit by interactively moving, adding
and deleting grid lines with the mouse. Finally users specify a few parameters for the
solver: the desired solution precision, the maximum number of iterations, and output
options, which include the display of solution progress (Fig. 2),
listing file, restart/back-up file, and plotting data file. As the solution converges, its
progress can be followed on the solution monitor. The program can be stopped at any time
by pressing the Stop icon. An engineer using Coolit can setup from scratch and solve a
15,000 cell model of a desktop computer shown in Fig. 1 in less than an hour on a Pentium
90 PC. A `what if' iteration of that design may take as little as five minutes to
re-compute and display the results.
The computed solution can be viewed interactively by pressing an appropriate icon on
the toolbar or selecting the Results menu. The graphic display program CoolPlot (R)
automatically loads all the computed results and shows the geometry and temperature and
velocity distribution both in 3D and 2D windows (Fig. 3).
Clicking on any cross section in the 3D window causes that cross section to be displayed
simultaneously in the 2D window, which is used for detailed analysis of the results. As
the cursor is swept over the 2D view, the solution at the cursor location is displayed in
the status bar window. The 3D view displays as many cross sections as desired
simultaneously. A simple mouse movement and click create new cross sections, reorient and
move the existing ones, and select the variables for plotting. The 3D view can be rotated
and zoomed in or panned also simply by moving the mouse. Several options for painting a
component's surface are available: maximum temperature in the component, surface
temperature distribution, and component type.