Convection
From Encyclopædia
Convection is the movement of gases or liquids in response to a non-uniform temperature distribution. Heat applied to a fluid generally causes an expansion of the fluid in the region closest to the source of heat. This region now has a lesser
density (is lighter than) its surrounding regions and, under the influence of gravity, travels upward. Similarly, a region of fluid that has been cooled becomes more dense and travels downward. In either
case, a convection current is established that travels through the body of the fluid, transferring heat and causing a temperature redistribution.An important exception to the principle that warmth creates an upward flow is water. Water that is at or near its freezing point of 0 deg C (32 deg F)
contracts when heated until it reaches its maximum
density at 4 deg C (39 deg F). Within this range the warmer regions flow downward. Above 4 deg C, water behaves normally.Convection currents permit buildings to be heated without the use of circulatory devices. The heated air moves solely by gravity. In the
atmosphere, convection causes the wind to blow. Most severe weather conditions, such as tornadoes, result from particularly sharp convection currents. No convection occurs in the
atmosphere of space vehicles in free-fall, which is a consequence of weightlessness.
bibliography: Baker, Martin, Heat Transfer (1986); Burmeister, L. G., Convective Heat Transfer (1980); Jaluria, U., Natural Convection Heat and
Mass Transfer (1980); Rohsenow, Warren, and Hartnett, J. P., Handbook of Heat Transfer Fundamentals, 2d ed. (1985).
