Sauna Thermal Modeling - FAQ

When was Sauna first introduced?

Was the Sauna program originally developed at the University Of Michigan?

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.

How long does it take to learn Sauna?

To get started with Sauna, you should work all eight 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.

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.

What is the difference between Sauna Standard and Sauna Modeling System?

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 calculations are based on either an implicit or explicit solver, depending on the model parameters and user settings.

I need to know how many thermal vias will be required beneath a surface mount component. Can Sauna model and optimize circuit board vias?

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.

Can liquid-cooled cold plates be modeled?

Sauna includes a flow network feature. 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 (except for heat sinks, see next paragraph). 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.

For heat sinks, Sauna can perform an integrated fan/flow calculation. You just need to assign a fan curve to the flow network and Sauna will handle all of the flow/pressure calculations. Here again, however, this is handled without CFD.

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 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.

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?

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 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.

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 ANSYS Icepak and Mentor Graphics FloTHERM.

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.

Is Sauna leased or sold as a perpetual license?

Is there a guarantee?

Does the purchase price include technical support?

How do I know that technical support is of good quality?

Do I need a powerful PC to run Sauna?

Can the evaluation package be used for real design problems?

I am looking to install a sauna in my house, how big a heater will I need?