Tag Archives: thermal design

Some Basic Principles of Wind Tunnel Design

Wind tunnels generate uniform air flows, with low turbulence intensity, for thermal and hydraulic testing. These devices have been around for more than a century, and are used in many industries, including aerospace, automotive, and defense. They also play a key role in electronics thermal management. Wind tunnels are made in different shapes and sizes, from just 30 cm long to large enough to contain a passenger airplane. But the basic idea behind all wind tunnels is universal.

There are two basic kinds of wind tunnels. One is the open type, which draws its air from the ambient and exits it back to the ambient. This kind of wind tunnel provides no temperature controls. The air follows the ambient temperature. The second type of wind tunnel is the closed loop wind tunnel, whose internal air circulates in a loop, separating it from outside ambient air. The temperature in a closed loop wind tunnel can be controlled using a combination of heaters and heat exchangers. Air temperatures can be varied from sub-ambient to over 100oC. Figure 1 shows a schematic of a closed loop wind tunnel.

In general, closed loop wind tunnels are made with the following sections:

1-Test section

2-Settling chamber

3-Contraction area


5-ÂBlower assembly

6-Heater/heat exchanger assembly

Figure 1. Schematic of an ATS Closed Loop Wind Tunnel.

A good quality wind tunnel will have a flow uniformity of 0.5-2% and turbulence intensity of 0.5-2%. It should provide temperature uniformity within 0.1-0.5oC at the inlet of the test section [1].

108K different push pin heat sink assembly configurations featuring 3 different pitch heat sink types, 3 different fin geometries, brass and plastic push pins


To achieve uniform, high quality flow in the test section, the settling chamber and the contraction area are used to smooth the flow. The role of the settling chamber, which is upstream of the contraction area, is to eliminate swirl and unsteadiness from the flow. The settling chamber includes a special honeycomb and a series of screens. As long as a flows yaw angles are not greater than about 10o, a honeycomb is the most efficient device for removing swirl and lateral velocity variations and to make the flow more parallel to the axial axis [2]. Large yaw angles will cause honeycomb cells to stall, which increases the pressure drop and causes non-uniformity in the flow. For large swirl angles, screen meshes should be placed before the honeycomb. For swirl angles of 40o, a screen with a loss factor of 1.45 will reduce yaw and swirl angles by a factor of 0.7. Several screens are needed upstream of the honeycomb to bring the swirl down to 10o.

Using a honeycomb will also suppress the lateral components of turbulence. Complete turbulence annihilation can be achieved in a length of 5-10 cell diameters [2]. Honeycombs are also known to remove the small scale turbulence caused by the instability of the shear layer in front of them. This instability is proportional to the shear layer thickness, which implies a short honeycomb has a better ratio of suppressed turbulence to that generated.

Screens break up large eddies into smaller ones which decay faster. They lower turbulence drastically when several screens are placed in a row. Screens also make flow more uniform by imposing a static pressure drop which is proportional to velocity squared. A screen with a pressure drop coefficient of 2 removes nearly all variations of longitudinal mean velocity. Low open area screens usually create instabilities. In general, screens should have openings larger than 57%, with wire diameters about 0.14 to 0.19 mm. Sufficient distance is needed between multiple screens to stabilize static pressure from perturbation. This distance is typically a percentage of the settling chamber diameter.

The contraction area is perhaps the most important part of a wind tunnel’s design. Its main purpose is to make the flow more uniform. It also increases the flow at the test section, which allows flow conditioning devices to be at lower flow section with less pressure drop. Batchelor used the rapid distortion theory and estimated the variation in mean velocity and turbulence intensity [3]

A considerable number of shapes have been investigated for contraction, including 2-D, 3-D and axisymmetric shapes with various side profiles.

The shape of the contraction can be found using potential flow analysis. Consider the axisymmetric contraction shown in Figure 2 [4]

Figure 2. Schematic of an Axisymmetric Contraction [4].

The design of a wind tunnel is a lengthy process and, as shown above, it requires extensive knowledge and experience in both theory and construction. A novice might attempt to construct a tunnel, but considering the time spent, it might not be justified economically. Wind tunnel design also depends on economic and space constraints. Larger wind tunnels allow more space to have all the conditioning elements in place. A space-constrained wind tunnel must compromise some features at the cost of reduced flow quality, but can still be acceptable for practical engineering purposes.


  1. Azar, K., Thermal Measurements in Electronics Cooling, Electronics Cooling Magazine, May 2003.
  2. Bell, J. and Mehta, R., Design and Calibration of the Mixing Layer and Wind Tunnel, Stanford University, Department of Aeronautics and Astronautics, May 1989.
  3. Batchelor, G., The Theory of Homogeneous Turbulence, Cambridge University Press, 1953.
  4. Edson, D. and Joao, B., Design and Construction of Small Axisymmetric Contractions, Faculdade de Engenharia de Ilha Solteira, Brazil, 1999.

How To Make a Thermocouple Video from ATS

ATS’s Latest Video, “How To Make a Thermocouple” has just been published.  If you’ve ever needed to make a thermocouple in your lab or shop, this is the video for you. Let Greg from our engineering team show you how it’s done.

And for a more robust alternative to a thermocouple, consider ATS Spot Sensor,  you can learn more about our spot sensor at this link or get a quote at this link.

How to Design Out Your Heat Sinks with Smart PCB Thermal Design, December 15th Webinar

We’ve got another webinar on tap, this one for next week, December 15th, 2011 at 2PM EST. Our topic this month is, “How to Design Out Your Heat Sinks with Smart PCB Thermal Design”.

It seems like a contradiction in terms doesn’t it? Why would a thermal management company like ATS teach you to design out your heat sinks? Wouldn’t we be better business people if we helped you design in more heat sinks?

Well, no. And here’s why.

At ATS, we are the leader in Innovations in Thermal Management. And for us, those innovations have to be useful.  There are many innovations in thermal management in the market but most simply aren’t truly practical to deploy except in a lab.

Here’s the link to join our free webinar, Thursday, December 15th, 2PM EST, “How to Design Out Your Heat Sinks with Smart PCB Thermal Design”.

And, many innovations are not products. They are know how, experience and understanding of the vectors involved in todays complex thermal management problems. And that’s why ATS’ team would teach our customer’s how to design out their heat sinks. Because we believe in innovations in thermal management that work. Understanding how to optimize air flow with your PCB works. Once  your optimization is set, then buy your heat sinks (hopefully from us!).

Can’t make our webinar? Then visit the following links to reach important resources on this topic:

Selecting a Fan for Your Thermal Management System (part 2 of 2)

In part 1 of our 2 part series this week, “Selecting a Fan for Your Thermal Management System”, we talked about how the type of fan needs to be chosen based on chassis design and allowable space. We discussed a bit about fan types and how to estimate the amount of airflow a system needs. Today in part 2, we’ll talk about fan impendance curves, pressure drop, and the effect of multiple fans.

For high heat loads, with concentrated heat sources you must design to the worst case component. Spot cooling may be accomplished with internal fans, heatsinks, ducting etc.

Next the total system impedance curve is needed, or the very least, the system pressure drop at the desired flow rate. System resistance is defined as:

System Pressure Drop

System Resistance Calculations

For practical purposes the value of can only be found experimentally or using computational fluid dynamics. Due to the complex nature of modern electronics enclosures an accurate value of cannot be derived analytically. In a modern electronics enclosure the airflow is turbulent and the value of can be conservatively chosen as 2.

Typical Fan ImpedanceTypical Fan and Impedance Curves (3)

The calculated flowrate at a specific static pressure can then be compared to a specific fan curve to determine if the fan will be adequate. An example fan curve is shown in figure 5. Point A is known as the “no flow” point of the fan curve, where the fan is producing the highest pressure possible. Next is the stall region of the fan, B, which is an instable operating region and should be avoided. The area from point C to D is the low pressure region of the fan curve; this is a stable region of fan operation and should be the design goal. It is best to select a fan that operates to the higher flow area of this region to improve fan efficiency and compensate for filter clogging.

In many systems a single fan cannot deliver the entire volumetric flow rate needed. In these situations multiple fans can be used, either in parallel or series configurations. In order to determine which configuration is more appropriate the system impedance curve is once again needed (figure 6). For a high impedance system two fans in series will produce a higher flow rate than in parallel. The opposite behavior can be expected in a low impedance system, where parallel fans are preferred.

Effect of Multiple Fans on Air Flow in a System

Figure 6. Effect of multiple fans on system pressure and flow rate [Comair Rotron]

An important consideration that needs to be addressed once the fan has been selected is to configure them in a blow through or pull through configuration. The airflow into a fan can roughly be modeled as laminar, whereas the exit airflow is highly turbulent. This phenomenon can be useful in thermal management, for instance in a typical telecom sub rack. Due to the varied resistance that the PCBs impose and the close proximity of the fans (fan tray) to the cards, the laminar flow will assist in better velocity distribution in the sub-rack, which also functions as a plenum.

In the blow through configuration the turbulent air has a positive effect on the heat transfer coefficient which can be useful when dealing with concentrated heat sources. The blow through design allows the fan to push cooler air, which improves its pressure capability, and extends the life of the fan. In this configuration the enclosure is slightly pressurized which prevents unfiltered air from being drawn through the joints and gaps in the chassis.


Common Fan Laws

1. Fan Cooled Enclosure Analysis Using a First Order Method, Ellison, Gordon, N., Electronics Cooling, Vol. 1, No. 2, October 1995, pp. 16-19.

2. Practical Guide to Fan Engineering, Daly, Woods, Woods of Colchester, Ltd, 1992

3. All you need to know about fans, Mike Turner, Electronics Cooling, Vol.1 May 1996

4. Comair Rotron, Establishing Cooling Requirements: Air Flow vs. Pressure, www.comairrotron.com/airflow_noe.shtml, March 12, 2007

How to Select a Heat Sink: Why Thermal Management is a Challenge

Today we are kicking off a series of articles on how to select a heat sink for an OEM project. The principles are the same for overclockers building their own systems with one difference. In OEM projects, mechanical engineers usually (but not always) have a chance to simulate the design before hand and suggest changes to chassis and layout to help with airflow.

So, let’s get started with some basics. First, why is thermal management a challenge? There’s a few reasons and many of these are only getting worse if you consider them from the world of thermal engineering.

First on our list is higher frequency circuits. The International Technology Roadmap for Semiconductors notes that, “projected power density and junction-to-ambient thermal resistance for high-performance chips at the 14 nm generation are >100 W/cm2 and <0.2°C/W, respectively.” In other words, semiconductors are simply getting hotter as their clock speeds are increased.

Second, generally, semiconductors are being assembled into smaller packages. The packages are smaller, the circuits are denser and this combination means that they are warmer.

Third on our list of why thermal management is a (growing) challenge is low acoustic noise requirements. End users don’t want to be deaf just for using electronics. The result is many specifications that set a reasonable acoustic range for their equipment, often in the 100LFM to 400LFM range. This relatively low airflow is great for end users but creates a real challenge for mechanical engineers and systems integrators trying to create a solid system that meets end users needs and still operates at its optimal levels.

Fourth, circuit designers determine component placement. On the surface of this, this is how it should be. Electrical engineers have alot of pressure on them to reduce board latency and design for performance. While they often consider the thermal needs of the systems and circuits, it’s not their primary design point. For mechanical engineers that is what we do and so our challenge is the balancing act of working with EE’s to insure great placement, but also great airflow.

Fifth and finally, thermal management is a challenge because EMI shielding. Higher frequency components require better shielding and that shielding can restrict airflow.

When we pick this topic up next, we’ll cover why temperature is so important to manage. If you have any questions in the mean time about heat sinks or thermal management, contact us and lets see how can make your next project a success! Email us at ATS thermal engineersats-hq@qats.com , call us at 781-769-2800 or visit our heat sink catalog at qats.com