Convection is the term used for heat transfer mechanism which takes place in a fluid because of a combination of conduction due to the molecular interactions and energy transport due to the macroscopic motion of the fluid itself. In the above definition, the motion of the fluid is essential otherwise the heat transfer mechanism becomes a static conduction situation as illustrated in Figure 7.2. When the term of convection is used, usually a solid surface is present next to the fluid. There are also cases of convection where only fluids are present, such as a hot jet entering into a cold reservoir. However, most of the industrial applications involve a hot or cold surface transferring heat to the fluid or receiving heat from the fluid.
Figure 7.2 Convective Heat Transfer from Fluid to a Metal Plate
The convective heat transfer can be explained using Newton’s law of cooling which states that the rate of heat transfer per unit area is directly proportional to temperature difference between a surface and the fluid and mathematically can be expressed as follows:
where ts is surface temperature, t∞ in °C, and h is coefficient of proportionality, called the heat transfer coefficient. Its unit is W/m2 °C. Its value depends on properties of fluid as well as its flow conditions.
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