|Natural Energy and Vernacular Architecture: Principles and Examples with Reference to Hot Arid Climates (UNU, 1986, 172 pages)|
|Part 1. Man, natural environment, and architecture|
|2. Architectural thermodynamics and human comfort in hot climates|
Natural or free convection is the process whereby a fluid moves because of differences in its density resulting from temperature changes. If the fluid is moved by mechanical means, e.g., by pumps, fans, or wind, the process is called forced convection. Heat may be transferred by convection between a surface and a liquid or a gas.
Discussions of thermal comfort involve the heat transfer between a surface and the neighboring air. When the surface is at a temperature above that of the air, heat is transferred from the surface to the adjacent air by conduction, thereby changing the density of the heated air. Then, even in otherwise still air, air currents result from the gravitational effects due to the differences in density. These natural convection currents cause much greater heat transfer from the surface than would result from conduction in a perfectly still atmosphere. Obviously, the rate of heat transfer by natural convection depends on the temperature difference between the surface and the neighbouring air.
Perfectly still air is rare. Even in a closed compartment, variations in the temperature of the walls and other surfaces set up air currents, so that there is some air movement. If fans are employed or if there are openings to the outside, the air movement may be considerable. These currents increase heat transfer by convection. The speed of the air current and the temperature difference affect the rate of heat transfer by convection.
Air is a gaseous fluid containing by volume (excluding the water vapor content) 21% oxygen, 78% nitrogen, and a remaining 1% consisting of traces of rare gases (argon, neon, and krypton), carbon dioxide (from 0.3 to 0.4 liters per m³), and carbon monoxide (about 0.03 liters per m³ in urban areas and much less in the countryside). Air also contains water vapor from four parts per thousand to two parts per hundred. Dust and soot particles in air are visible as motes in a sunbeam. The oxygen, nitrogen, and other rarer gases are called permanent gases because they only become liquids at temperatures approaching absolute zero, whereas water undergoes continuous change between its gaseous and liquid states within the common range of air temperatures encountered in human climatic zones.