It lets designers relatively quickly evaluate concepts and
correct thermal problems long before the proto
type phase. Our company provides enclosures,
backplanes, and subracks, as well as accessories
such as power supplies, front panels, end monitors,
and extender boards. We use Flotherm to calculate
air velocities and flow rates over slots and cards
and for detailed heat-transfer analysis of embedded-chassis designs.
When developing a chassis to be used with high-capacity, high-power-dissipating 6U VME or Compact PCI-based electronics, the first step was to
model the enclosure and circuit card. Flotherm's
extensive materials library came in handy while
modeling the enclosure because it contains a wide
range of common alloys.
We primarily design around plug-in cards specified by customers. So we frequently exploit the
software's capability to generate compact models
or library assemblies. After a detailed analysis of
the card in question, I used volume-resistance elements, computationally efficient representations
of component flow and thermal characteristics.
These elements are basically detailed behavioral
models that react to the environment in which they
are placed. I then save everything as a simplified
model. This substantially reduces computational
time on subsequent enclosure-level analyses containing the same board.
Most chassis we design meet IEEE 1101.1 and
1101.2 standards that specify a 0.8-in. spacing between cards. The software makes it easy to use volume utilization, the percentage of space between
the parallel cards consumed by the components,
and facilitates easy correlation to the pressure-drop characteristics generated by the card at a
specific flow rate.
Power in this case was assumed to be 50 W for cards in the main cage and 20 W
for cards in the rear. We have also
designed chassis with higher power
dissipations using the same Flotherm features.
This chassis application used six tubular fans to generate target flows determined by mass flow
heat-transfer equations. The fans were placed in
the model using SmartParts supplied by the fan
manufacturer and posted to Flomeric's online library at www.SmartParts3D.com . Many companies
that make fans, heat sinks, components, and thermal-interface materials have contributed to the
library. Version 7.1 lets users launch a Web browser
inside the software and simply click on a model to
add it to their thermal simulation.
Once fans were put in the chassis, the software
predicted individual flow rates and power dissipation for each slot. The software's monitor points
efficiently measure junction temperatures of critical components, when located at critical points of
interest.
Chassis flow rates in the example averaged 10.4
CFM. At this rate, the cards dissipate a maximum of
84 W when operating at mean sea level, and 58 W
when operating at 10,000 ft, with an average temperature rise across the cards of 15˚C.
Flotherm also predicts slot velocity-profiles and
temperature distributions. In our case, the profile
proved to be fairly uniform across the cards, but
it was evident that better cooling distribution was
possible by changing the shape of the enclosure.
Analysis assumed unused cards have blockers in
stalled that ensure cooling air goes to the slots.
Simulation showed the chassis exceeded initial design targets. In the example, as in most applications, the full cooling capacity of the system
will not be required. We typically add motor-speed
controls to reduce noise, fan speed, and power
consumption.
Performing a detailed heat-transfer analysis of
an embedded chassis can take a lot of time. So I
generally begin analyzing new designs by creating
simple models that can be run relatively quickly. I
use these models to find design factors with major
influence on thermal-performance variables such
as the temperature of critical components and air
flow velocity over them.
Next I specify a reasonable range for variables.
Flotherm's Command Center selects values of these
variables over a series of runs to explore designs
using the least possible computational resources.
When I leave work at night, I activate the software's
Command Center, and by the next morning, it has
completed all runs and determined optimal values for each variable.
The latest version goes one step further. It adds
a Response Surface Optimization feature that lets
engineers see interactions of design parameters
on design goals and identify best values with more
accuracy. Users can see response surfaces through
either a 2D or 3D chart window, making it easy to
understand the sensitivity of cost to changes in
particular parameters. Optimization provides better design insight and intuition, lets the user rapidly
determine which parameters are crucial, provides
instant assessment of the effects of manufacturing
variations, and so on.
The software comes from Flomerics Inc. , 4 Mount
Royal Ave., Suite 450, Marlborough, MA 01752, (508)
357-2012, flomerics.com.
— Michael Palis
Michael Palis is Senior Simulation Engineer at
Hybricon Inc., Ayer, Mass., hybricon.com
Edited by Leslie Gordon