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Learning
Sauna
Q: What technical background is required to become a Sauna user?
The "target" user is a mechanical engineer who has
completed a heat transfer class in college. Besides mechanical
engineers, electrical engineers can also be successful Sauna users.
Since the average electrical engineer has less of a heat transfer
background than the typical mechanical engineer, the EE user should
be a bit careful about heat transfer assumptions and is urged to
contact technical support when questions arise. The ME to EE ratio
among users is roughly 3:1.
It should be emphasized that these technical requirements are much
less than either FEM or CFD based thermal. The intuitive thermal
resistance network of Sauna is much more accessible.
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Q: How long does it take to learn Sauna?
To get started with Sauna, you should work all seven of the
introductory exercises in the Sauna user manual. For most engineers,
this can be accomplished in one day. This provides the necessary
background to analyze heat sinks and simple box problems. Please
note that most of these introductory exercises are in the evaluation
package booklet, So if you have worked the evaluation exercises, you
have already completed most of the basic preparation.
For more complex simulations, you will need to work some of the
intermediate and supplemental exercises. This could take another day
to complete.
All in all, assume that it takes 1 to 2 days to learn Sauna. This
compares quite favorably with FEM and CFD software.
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Q: I only perform
thermal analysis once in a while. I might be involved with thermal
issues for several weeks, then six months might pass before the next
thermal project. Will I be able to use Sauna on an occasional basis?
Yes! In reality, there are very few Sauna users that do thermal
analysis on a continuous basis. Since Sauna is easy to learn, the
program is well suited for this intermittent type of use.
If the period of non-use if greater than 6 months, it's a good idea
to work one or two exercises in the manual to serve as a refresher
course. This might take an hour or two, but the time savings will be
substantial when you start to build the model.
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Product
Features
Q: What is the
difference between Sauna Standard and Sauna Modeling System?
The differences are described here.
| |
Sauna
Standard |
Sauna
Modeling System |
| Transient
analysis |
No |
Yes,
simple or duty
cycle transient |
Maximum
number
of nodes |
5,000 |
15,000 |
Maximum
number
of assemblies |
25 |
5,000 |
| Free
software updates |
2
months |
1
year |
| Training
session |
Not
included |
One
day class included |
Remember, if you are having trouble deciding between Sauna Standard
and Sauna MS, start with Sauna Standard. This is a logical approach
because you can upgrage with full credit during the first year after
purchase.
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Q: What is the
numerical method used by Sauna?
Sauna is a thermal network program which uses the thermal/electric
analogy. At the lowest level a Sauna thermal model is composed of
nodes, thermal resistors and thermal capacitors. However, users
rarely specify thermal resistances and capacitances. Instead, a user
specifies a material type and plate or board dimensions. Sauna then
creates the appropriate resistance/capacitance network based on
Sauna's library of material properties and the current modeling
setup.
For calculating steady state temperatures, Sauna uses a sparse,
direct matrix solver which is based on Gaussian Elimination.
Transient temperature calculation is based on either an implicit or
explicit solver, depending on the model parameters and user
settings.
If you have further questions, feel free to contact Technical
Support.
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Q: I need to know how
many thermal vias will be required beneath a surface mount
component. Can Sauna model and optimize circuit board vias?
Yes. You would create a "stackup" model. The
layer-to-layer resistance would be based on the via type, size and
density, as calculated with Sauna's Toolbox. For more information,
see the circuit board exercise (basicbrd.pdf) on the evaluation
package CD.
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Q:
Can Sauna be used to find the optimum fin spacing for a heat sink?
Yes. Fin optimization can be accomplished in two different ways.
First, there is a utility for optimizing fin spacing and thickness
within Sauna's Toolbox. Second, it's also possible to optimize fins
by performing what-if modifications of the Sauna model.
Fin spacing can be optimized for both natural and forced air
cooling. Sauna can even optimize fins for unconventional fin
orientations, such as naturally cooled horizontal baseplate heat
sinks.
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Q: Can liquid-cooled
cold plates be modeled?
Yes, although the process is a bit more complicated than for air
cooling networks. For liquid cooling applications, you start by
obtaining the pipe-to-fluid resistance with Sauna's Toolbox. Next,
you create the flow network. As the last step, the pipe-to-fluid
resistance is handled by creating generic resistors between the
plate and the flow ambients.
Sauna now includes a flow network feature.
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Q: Does Sauna have
computational fluid dynamics (CFD) capability?
No. With the flow network capability, the user can define a flow
path (or paths) and Sauna will calculate the appropriate air
temperature increase as air moves along the flow path. However, it
is up to the user to specify all flow volumes. If the box has a
straightforward flow path, the flow volume can be readily calculated
by using the fan characteristics and box dimensions (or simply
entered for what-if studies). The Sauna user manual explains how to
perform flow calculations.
More details on the differences between Sauna and CFD programs are
given here.
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Q: Can Autocad files
be imported? Can Pro-Engineer files be imported?
At the present time, you cannot import data from AUTOCAD, Pro-E,
etc. Although importing CAD data seems like a great idea, there are
many practical limitations. CAD models tend to have too much
dimensional information and not enough physical data. For example, a
solids model of a molded plastic part can be quite complicated due
to draft angles, blends, etc. But in a thermal model it isn't
critical to know every dimension, particularly for materials with
low thermal conductivity. If every detail is included, the model
might take days to calculate but the answer won't be much better
than a more simplified model.
On the other hand, CAD models frequently lack the necessary
properties such as thermal conductivity, specific heat and
emissivity. This is particularly true for circuit boards.
So, while it seems like a great thing, we're not convinced that
importing CAD models is really important. But we might add the
feature anyway, just because so many people ask for it.
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Q: We already own a
finite element (FEM) program (such as Algor, Cosmos, Nastran, Ansys,
etc.) My boss says we should use the FEM package for thermal
analysis instead of purchasing an electronics thermal package like
Sauna. Why should we buy Sauna instead of using our FEM package?
Your company will save money in the long run with Sauna.
There are basically 3 costs associated with a software package: (a)
the purchase price, (b) the training cost and (c) the modeling cost.
If you already own the FEM package, the (a) purchase price will be
zero, of course. The (b) training cost may also be zero if the
thermal modeler is experienced with finite element work. However, if
an engineer is not familiar with the FEM method, there will be a
long startup time for FEM. Sauna, on the other hand, uses the
intuitive thermal resistor network method so it can be easily
learned by any mechanical or electrical engineer (or even
civil/chemical engineers and physical science persons). Finally, and
most importantly, there is (c) the modeling cost. As described here,
the FEM programs really only deal with a part of the overall thermal
problems. So a model which might take an hour or two with Sauna
could take days with the FEM program. This will cost your company
money.
It's important to realize that an engineering man-week now costs
$2,000 or more. So Sauna will pay off quickly when compared with an
FEM program. Nearly all Sauna customers are also owners of an FEM
software package. But these companies use Sauna for thermal problems
because it is so much more efficient.
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Q: How does Sauna
compare with finite element modeling (FEM, FEA) software?
The finite element method (FEM), also known as finite element
analysis (FEA), was developed to analysis deflection and stress in
solid mechanical structures. Once the a finite element mesh has been
created, it's also possible to solve for heat flows and temperatures
within the solid material.
However there are three distinct modes of heat transfer: conduction,
convection and radiation. FEM programs really only deal with the
conduction part of the problem. For the convection part of the
problem, it's usually up to the user to enter a convection
coefficient and, in general, only simple convection to the room is
allowed. For thermal radiation, most FEM packages only include
simple black body radiation to the room environment and there is no
ability for walls of a box to radiate to each other. The result will
be lengthy model creation time and incomplete models.
For the most part, FEM packages are not competitive with Sauna.
Nearly all Sauna users also own an FEM package, but the FEM packages
are reserved for purely mechanical problems.
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Q: How does Sauna
compare with computational fluid dynamics (CFD) software?
Computation fluid dynamic (CFD) software is a numerical simulation
of the Navier-Stokes equation of fluid motion. This method is
computationally intensive and only became practical within the last
5-10 years. The two most popular electronics CFD packages are Icepak
from Fluent Inc. and FLOTHERM from Flomerics Ltd.
CFD's primary advantage is the ability to predict flow. If you want
to model a box with 3 fans, and you don't know where the air is
going, then you may wish to consider the CFD approach. With Sauna,
the user must specify the flow rate (Sauna handles all other aspects
of the problem). If the flow path is simple so that flow volume is
readily calculated (details provided in the Sauna user manual), or
for natural cooling, Sauna can be very effective and accurate. But
there are situations where CFD software is useful.
You should be aware of the drawbacks of CFD:
1. CFD programs focus on air flow and convection. CFD programs are
not always effective at handling the conduction and radiation parts
of the problem. For example, a potential purchaser needs to
investigate the way that multilayer circuit boards are modeled and
whether the CFD program has the ability to model radiation networks
in complex boxes. These are very important features for an
electronics thermal modeling program.
2. Details are important in CFD analysis. For example, it's
important in CFD simulations to correctly specify surface roughness
and vent drag coefficients. Sometimes these values must be measured
experimentally. You can't just create a model and assume that
predicted results are on target.
3. CFD analysis is computationally intensive. It's very difficult to
perform duty cycle analysis because the computing time can be quite
significant. What-if analysis is harder because of the time required
for each iteration.
4. CFD software takes time to learn and requires a skilled user.
Since CFD starts with the Navier Stokes equation, it's important to
have a solid background in both heat transfer and viscous fluid
flow. In addition to the proper background, you can expect to invest
at least a week learning the software.
5. CFD software is expensive. Actually, very expensive.
So CFD is not a "silver bullet" method which excels in all
aspects of thermal modeling. When it comes to ease of use, cost-
effectiveness, radiation modeling and duty cycle capabilities, Sauna
is superior. It really is a question of different programs for
different needs. Many Sauna customers own both Sauna and a CFD
program.
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