The motion that results from the continual replacement of the heated fluid in the vicinity of the heat source by the cooler fluid nearby is called a natural convection current, and the mass and heat transfer that is enhanced as a result of this natural convection current is called natural convection mass and heat transfer.
Convection currents are generated by density differences in the fluid occurring due to temperature gradients. Convection currents are associated with natural convection, in which any fluid motion occurs by natural means such as buoyancy. In natural convection, matter and heat are moved from one location to another. Convection currents have attracted a great deal of attention from researchers because of their presence both in nature and in engineering applications.
In nature, convection currents formed from air raising above sunlight-warmed land or water are a major feature of all weather systems. Convection currents also seen in the rising plume of hot air from fire, plate tectonics and oceanic circulation.
Creation of Convection Currents
The creation of convection currents is based on three physical assumptions:
- Presence of heat source. The heat source is required because convection currents are generated by density differences in the fluid occurring due to temperature gradients. In natural convection, fluid surrounding a heat source receives heat and by thermal expansion becomes less dense and rises. Thermal expansion of the fluid plays a crucial role. In other words, heavier (more dense) components will fall, while lighter (less dense) components rise, leading to bulk fluid movement.
- Presence of proper acceleration. Natural convection can only occur in a gravitational field or the presence of another proper acceleration, such as acceleration, centrifugal force, and Coriolis force. Natural convection essentially does not operate in the orbit of Earth. For example, in the orbiting International Space Station, other heat transfer mechanisms are required to prevent electronic components from overheating.
- Proper geometry. The presence and magnitude of natural convection also depend on the geometry of the problem. The presence of a fluid density gradient in a gravitational field does not ensure the existence of natural convection currents. This problem is illustrated in the following figure, where a fluid is enclosed by two large, horizontal plates of different temperature (Tupper ≠ Tlower).
- In case A the temperature of the lower plate is higher than the temperature of the upper plate. In this case, the density decreases in the direction of the gravitational force. This geometry induces fluid circulation and heat transfer occurs via natural circulation. The heavier fluid will descend, being warmed in the process, while the lighter fluid will rise, cooling as it moves.
- In case B the temperature of the lower plate is lower than the temperature of the upper plate. In this case, the density increases in the direction of the gravitational force. This geometry leads to stable conditions, stable temperature gradient and does not induce fluid circulation. Heat transfer occurs solely via thermal conduction.
Convection Currents – Atmospheric Circulation
One of the phenomena of the highest importance for Earth climate is the atmospheric circulation, which is the large-scale movement of air and is a means by which thermal energy (together with ocean circulation) is distributed on the surface of the Earth. The Earth’s atmospheric circulation varies from year to year, but the large scale structure of its circulation remains fairly constant.
The atmospheric circulation is a consequence of illumination of Earth by the Sun, and the laws of thermodynamics. The atmospheric circulation can be viewed as a heat engine driven by the Sun’s energy, and whose energy sink, ultimately, is the blackness of space. From this point of view, also wind turbines are powered by the Sun.
Convection Currents – Thermal Column
Another important convection current that is formed in the atmosphere is a thermal column. A thermal column is a column of rising air in the lower altitudes of Earth’s atmosphere, a form of the atmospheric updraft. Thermal columns are created by the uneven heating of the Earth’s surface from solar radiation and are an example of atmospheric natural convection.
The Sun warms the ground, which in turn warms the air directly above it. The warmer air nearer to the surface expands, becoming less dense than the surrounding air. The lighter air rises and cools due to its expansion in the lower pressure at higher altitudes. It stops rising when it has cooled to the same temperature as the surrounding air. Dark Earth, urban areas, and roadways are good sources of thermal columns.