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Thermal fundamentals: How heat transfer shapes system performance

Buoyancy: the driving force of natural convection

What is buoyancy?

Buoyancy is what allows boats to float and hot air balloons to fly. It is the reason that the water in the ocean is separated from the air in the sky. This is due to the variation in the densities of these different fluids.

Buoyancy is pressure generated by the kinetic energy of all the fluid molecules moving around and colliding with each other and any other object within the vicinity. While this kinetic energy pushes in all directions, its largest opposing force is gravity

The science behind the force of buoyancy

The amount of buoyancy is dependent on the density of the fluid. Denser or cooler fluids have less kinetic energy than more excited fluids so they exert less pressure against other surrounding molecules. Since there is less pressure generated by this fluid, the force of gravity has more effect on this fluid than warmer fluids. This is why cool air falls and warm air rises.

Why does buoyancy matter in thermal management?

Natural convection

Buoyancy is what drives natural convection, where the pressure differential between air heated by heat sources and surrounding ambient air drives the hot air upwards and away from the heat source. The air rises up with the energy it absorbed and thus removes that energy from the area around the heat sources. Cooler ambient air then moves in and replaces the heated air that moved up. This rising and replacement of hot and cool air generates a consistent flow without the need of any active mechanisms to drive it.

Benefits of natural convection

One of the key points of natural convection is enabling heat transfer without the extra cost and assembly time associated with adding a fan into the product. Fans or blowers can decrease the overall reliability of the device as they are driven by motors that wear down and can break over extended use of the product.

How to make buoyancy and natural convection work for you

Since buoyancy enables fluids to rise against gravity, surfaces that dissipate heat should be oriented in the same direction as the gravity vector. Typically this surface is your heat sink. The longest dimension of the heat sink surfaces should be vertical, so as the cool air contacts the bottom of the heat sink it travels upwards and continues to be heated up and increases the pressure differential.

Natural convection and the chimney effect

By optimizing the amount of air that can be pulled into the fin gaps and the length of the heat sink, you can easily use natural convection to cool devices connected to your heat sink surface. You’ll typically see good natural convection heat sinks have a fin gap of around 0.25″ or 6.35mm. This allows enough space between the fins to accommodate boundary layers that form on each of the fin surfaces and still allow upward flow of heated air.

Heat sink orientation for natural convection

Since natural convection relies on gravity to drive buoyant air flow, designers must consider heat sink orientation. Ideally, the flow length of heat sink fins should be in the same axis as the gravity vector. This creates the channel for heated air to naturally rise upwards as it decreases in density.

If the application prevents the the vertical orientation of the fin flow length, the fin tips should point upwards and the base of the heat sink facing down.

The two heat sink orientations to avoid include the base on the sides with the flow length going sideways or the base facing upwards. Both of these orientations block heated air from easily rising away from the heat sink, limiting the effectiveness of natural convection.

Eaton is an intelligent power management company dedicated to improving the quality of life and protecting the environment for people everywhere. We are guided by our commitment to do business right, to operate sustainably and to help our customers manage power ─ today and well into the future. By capitalizing on the global growth trends of electrification and digitalization, we’re accelerating the planet’s transition to renewable energy and helping to solve the world’s most urgent power management challenges.

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Buoyancy: the driving force of natural convection