Convection

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Convection is energy transfer by bulk motion. Unlike conduction, which is a microscopic phenomenon, convection involves the macroscopic interchange of energy between two mediums. Convection is most prominent in gases and liquids, where the distances between molecules are too large for conduction to be effective. Convection can be classified as natural or forced. In natural convection, fluid motion is a result of a density gradient, whereas forced convection is mainly due to a pressure gradient. Hot air balloons rise as a result of natural convection because they contain hotter, lower-density air. Helicopters are lifted by forced convection resulting from the pressure difference across their propeller blades. Dogs often pant to get rid of excess heat by forced convection.

It should be noted that heat transfer by convection and conduction are closely linked. Consider for example, the cooling of a hot plate by blowing air over it. As the cold fluid replaces the warmer fluid (convection), more heat is conducted away from the plate; the process continues until the surface temperature approaches that of the fluid, in this case the surrounding air. A fan placed in front of a car radiator functions in a similar manner.

In buildings, convection losses are due to air infiltration through the cracks, windows, and other openings in walls. Additional losses occur due to air movement inside and wind motion outside the exterior glasses and windows. In a typical building infiltration losses are the most significant and are comparable to losses by conduction. Common insulation materials such as fiberglass, rigid boards, cotton, and feathers work by creating tiny air pockets that slow down the convection flow of heat.

Convection is not only important at the local level, but also plays a role in large-scale movements of the atmosphere. The major winds are convection currents driven by temperature differences caused by non-uniform heating of the earth by solar radiation. The winds, in turn, drive the ocean currents.

Question: To keep a body warm in a cold winter, would it be more beneficial to wear two layers of light clothing or one layer of clothing twice as thick?

Answer: Two layers will work best because there is always some air trapped between the layers, providing additional insulation.

Question: Is it best to add creamer to coffee immediately after the coffee is poured or to add it right before drinking the coffee? Assume that coffee is most desirable when it is hot.

Answer: Creamer should be added as soon as the coffee is poured. Two effects are of importance: First, creamer makes coffee lighter in color, reducing its emissivity and heat loss from radiation. Secondly, adding creamer sooner decreases the temperature difference between coffee and the environment, reducing the rate of conductive and convective losses.

References

(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

El-Sayed, Y., The Thermodynamics of Energy Conversions, Elsevier Direct Science, 2003.

Cengel, Y. A., Heat Transfer: A Practical Approach, McGraw-Hill, Inc., 1998.

Rifkin, J., Entropy, The Viking Press, 1980.

El-Wakil, M/ M., Power Plant Technology, McGraw-Hill, Inc., 1984.

Energy and Buildings, Science Direct Elsevier Publishing Company. An international journal publishing articles about energy use in buildings and indoor environment quality.

Energy Conversion and Management, Science Direct Elsevier Publishing Company. This journal focuses on energy efficiency and management; heat pipes; space and terrestrial power systems; hydrogen production and storage; renewable energy; nuclear power; fuel cells and advanced batteries.

Energy and Buildings, Science Direct Elsevier Publishing Company, An international journal dedicated to investigations of energy use and efficiency in buildings.

External Links

How Things Work (http://howthingswork.virginia.edu).

How Stuff Works (http://www.howstuffworks.com).

California Energy Commission Consumer Energy Center (http://www.consumerenergycenter.org).