This page provides notes on heat transfer that may be useful to mechanical engineers. The subject is very complicated and any user who requires accurate heat transfer values is advised to refer to quality reference documents or use specialised software.
When a hot surface us surrounded by an area which is colder energy in the form of heat will be transferred from the hot surface to the cooler area. The rate of this transfer is depended on the temperature difference and the process will continue until both the surface and the surroundings are at the same temperature. This process in called heat transfer and takes place by one or more of the following methods
Conduction takes place in solids, liquids, and gases. Solids offer the least resistance to transfer of heat by conduction. Conduction requires physical contact between material through which the heat is transferred. A materials temperature is related to the motion of the constituent molecules. The conduction process involves the molecule moving at higher velocities transferring their kinetic energy to the adjacent molecures which have lower kinetic energy.
Convection results in a gas or liquid. The fluid adjacent to a hot surface heats up as a result of conduction. The density of this fluid is reduced and it therefore rises to be replaced by a colder fluid of higher density. This process continues resulting in convective flow producing an enhanced transfer of heat throughout the fluid.
The transfer of heat energy by radiation can occur in a vacuum , unlike conduction and convection. Heat radiation is the same form of wave energy transfer as light, radio, and x-ray wave energy. The rate of emmission of heat energy is related to the temperature difference, the distance between the surfaces, and the emissivity of the surfaces. Bright reflective surfaces have the lowest emissivity values.
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