Introduction to Natural Convection Cooling in Electronic Systems

When it comes to the thermal management of electronic systems there are four types of cooling techniques. Those are Conduction, Natural Convection, Forced Air Convection, and Liquid Cooling. In our previous post we discussed the basics of conduction cooling. Here we will introduce Natural Convection cooling and the equations that you need to know to get started.

So what is Natural Convection Cooling?

As it relates to electronic systems, Natural Convection cooling involves the transfer of heat generated by the electronic systems to the atmosphere. Typically the heat is transferred from the high powered electrical components on the PCB via conduction cooling through a heat frame. The heat frame is then connected to a heat sink, which is the means of transferring the heat to the atmosphere. When there exists a temperature difference between the surface of the heat sink and the atmosphere there will also exist heat transfer via convection. The air surrounding the heat sink will begin to warm up due to the hot surface temperature. This increase in air temperature is accompanied by a decrease in air density, which will cause the air to rise. This is also known as natural convection currents. The heat transfer is most optimized when the flow path has the least amount of flow resistance, in other words the least amount of obstacles in the way.

Convection Heat Transfer Equation

The equation for convection heat transfer is shown below.

This equation effectively states that the heat transfer due to convection is equal to the convection coefficient multiplied by the surface area multiplied by the temperature difference between the surface and the fluid (in most cases the fluid is air, however if you have a liquid cooled application your fluid may be a refrigerant or water). The convection coefficient is a function of a few variables such as flow velocity, type of geometry, type of flow, among others.

Convection Heat Transfer Coefficient

Typically, the natural convection currents start out as laminar flow, but turn to turbulent flow rather quickly as the temperature and characteristic length of the thermal surface increases. In most applications regarding natural convection for electronic systems the flow is laminar, so for the purposes of simplicity we will assume this here. Therefore, with air as the media, the convection heat transfer coefficient can be determined with the equation below.

The equation states that the temperature difference between the thermal surface and air, divided by the characteristic length of the thermal surface, raised to the power of 0.25, multiplied by a constant is equal to the convection coefficient. This applies to laminar flow and a media of air. The constant is a function of the geometry and orientation of the thermal surface, see the chart below for a list of common geometries encountered in electronic systems.

Convection Coefficient Constant Chart

Radiation Heat Transfer

Radiation heat transfer is a result of the energy emitted by the thermal surface in the form of electromagnetic waves. This form of heat transfer is the fastest and does not require any media to intervene. The magnitude of radiation heat transfer is comparable with natural convection so it is prudent to include it in the analysis. When looking at systems involved forced air convection or liquid cooling, radiation heat transfer is not typically included as it is off negligible benefit. Surfaces with very low emissivity typically have negligible benefit from radiation heat transfer as well, such as surface that are polished. Thermal surfaces that are close to unity, such as plastics and painted surfaces are much more effective. The equation for radiation heat transfer is show below.

Here the equation reads the radiation heat transfer is equal to the surface emissivity, multiplied by the surface area, multiplied by the Stefan-Boltzmann constant,

Applications of Natural Convection

Natural Convection Cooling is reserved for low powered electronics, typically 100W or less. It is possible to cool higher-powered electronics by natural convection, however the surface area and ambient temperatures will need to be adjusted to accommodate the increased power density. In many commercial applications, the PCB’s are mounted in computing racks, evenly spaced apart. It is important to mount the PCB’s vertically as to maximize the natural convection currents flow path. Furthermore, the spacing of the PCB’s is important as well, considering the flow path can result in “choked” flow if the PCB’s are to close and an un-necessary amount of space can be used if they are too far apart.

When the PCB’s are mounted closely together, the resulting radiation heat transfer will be negligible, since the surrounding temperature is hot. Therefore, it is prudent for the design engineer to place the high-powered components facing the edge of the mounting rack, which is a much cooler surface. Another option would be to conduct the generated heat on each circuit card assembly to a universal heatsink. This can be done using a rugged heat frame, which is typical in Aerospace and Defense applications. A universal heatsink can utilize a finned surface, which will effectively maximize both Natural Convection and Radiation heat transfer. The surface area and fin spacing can be further optimized with respect to weight and performance. Typical materials used for universal heat sinks and circuit card heat frames are aluminum and copper. Copper has a larger thermal conductivity then aluminum, however it cost and weighs significantly more. If you can live with the performance impact, aluminum is an industry standard material as it relates to thermal surfaces.

Conclusion

This concludes an introduction to natural convection cooling on electronic systems. In our next blog post, we will look into the basics of forced convection systems.

If you have an application involving thermal management of electronic or mechanical systems, Dreamspace Engineering Consulting has the tools and expertise to get the job done. Visit our website at www.designwithdreamspace.com to learn about the exciting services we offer. We utilize industry standard software such as Solidworks Simulation and Flow Simulation to optimize any design.

Warmest Regards,

Thomas A. Smith

References

1. Yunus Cengel and Afshin Ghajar. Heat and Mass Transfer: Fundamentals and Applications, Fifth Edition. New York: McGraw-Hill Education, 2011. Print.