We have repeatedly used the term heat (Q) and heat exchanges between bodies without discussing the details of the phenomena by which heat is transferred.
Heat transfer is a name of general process applied to any device that effects a transfer of heat from one substance to another.
The various devices that employ the principles of heat transfer are:
(i) Steam boilers
(ii) Surface condensers, evaporators, closed feed water heaters and the automobile radiator etc.
Whenever a temperature gradient exists within a system, or when two systems at different temperatures are brought into contact, energy is transferred. The process by which the heat energy transport takes place is known as heat transfer.
Modes of Heat Transfer:
Heat transfer can be defined as the transmission of energy from one region to another as a result of a temperature difference between them. Since temperature differences exist all over the universe, the phenomena of heat flow over the universe, the phenomena of heat flow are as universal as those associated with gravitational attractions. Unlike gravity, however, heat flow is governed not by a unique relationship, but rather by a combination of various independent laws of physics.
The literature of heat transfer generally recognises three distinct modes of heat transmission:
(i) Conduction
(ii) Radiation and
(iii) Convection
Strictly speaking, only conduction and radiation should be classified as heat transfer processes, because only these two mechanisms depend for their operation on the mere existence a temperature difference. The last of the three, convection, does not strictly comply with the definition of heat transfer because it depends for its operation on mechanical mass transport also. But since convection also accomplishes transmission of energy from regions of higher temperatures to regions of lower temperatures, the term “heat transfer by convection”, has been generally accepted.
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