Role of Buoyancy in heat transfer
Buoyancy enables heat transfer without cost. Because it maintains a sharp temperature gradient between the body and the surrounding air or water, convection is a very efficient method of heat transfer. In convection, the heat is transferred through both diffusion and advection.
Buoyancy is an upwards force
A fluid exerts an upward force that opposes the weight of a partially or fully immersed item. The weight of the overlying fluid causes pressure to rise with depth in a fluid column. As a result, the pressure at the bottom of a fluid column is greater than the pressure at the top. The pressure at the bottom of an object submerged in a fluid is also larger than at the top. The object is subjected to a net upward force due to the pressure differential. The magnitude of the force is proportional to the pressure differential and is equal to the weight of the fluid that would otherwise occupy the space (as explained by Archimedes' principle).
Buoyancy is the pressure generated by kinetic energy. The kinetic energy of all the fluid molecules moving about and interacting with each other, as well as any other object in the vicinity, generates buoyancy. While kinetic energy pushes in every direction, gravity is the most powerful opposing force. Buoyancy is thus produced by gravity.
The science behind buoyancy force
The quantity of buoyancy is determined by the fluid's density. Fluids that are denser or cooler have less kinetic energy than fluids that are more excited, therefore they exert less pressure on the molecules around them. The force of gravity has a greater effect on this fluid than on warmer fluids because it generates less pressure. Cool air descends and warm air rises because of this.
Role of buoyancy in heat transfer
Natural convection
Natural convection is driven by buoyancy, which occurs when the pressure differential between heated air and surrounding ambient air pushes the hot air higher and away from the heat source. The energy received by the air causes it to rise, removing it from the area around the heat sources. The warm air that has risen up is then replaced by cooler ambient air. This rising and replacing of hot and cold air produce a steady flow without the use of any active devices.
Stack effect or chimney effect
There is a pressure difference between the outside air and the air inside the building caused by the difference in temperature between the outside air and the inside air. That pressure difference (ΔP) is the driving force for the stack effect.
The stack effect, also known as the chimney effect, is the movement of air into and out of structures due to air buoyancy through unsealed holes, chimneys, flue-gas stacks, or other containers. The difference in indoor-to-outdoor air density caused by temperature and moisture changes causes buoyancy. A positive or negative buoyancy force is the outcome. The buoyancy force, and hence the stack effect, increases as the heat differential and structure height increase. Natural ventilation is aided by the stack effect.
Buoyancy effect in heat transfer: Convection
The transfer of heat between two bodies via currents of moving gas or fluid is known as convective heat transfer. In free convection, warm air or water rises and is replaced by a cooler parcel of air or water as it moves away from the heated body. Forced convection removes heat from the body by forcing air or water to move over the body surface (as in wind or wind-generated water currents). Because it maintains a sharp temperature gradient between the body and the surrounding air or water, convection is a very efficient method of heat transmission.
Convection is a type of heat transmission that occurs when a large number of heated atoms and molecules move together. Convection necessitates the movement of material particles, whereas conduction transfers heat by vibration without requiring the atoms or molecules to leave their original positions. Heat is transferred through both diffusion and advection in convection.
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