Different Forms of Heat Transfer

For any method of heat transfer to take place, a temperature difference is necessary between two faces of a solid body, or at the boundaries of a gas or vapor. Flear transfer will take place only from a high-temperature source to a lower-temperature sink and is an irreversible process unless acted upon by another agency, as is the case with the refrigeration process. 

Heat transfer may occur by one or more of three different modes  

Conduction takes place at a solid, liquid, or vapor boundary through the collisions of molecules, without mass transfer taking place. The process of heat conduction is analogous to that of electrical conduction, and similar concepts and calculation methods apply. The thermal conductivity of matter is a physical property and is its ability to conduct heat. Thermal conduction is a function of both the temperature and the properties of the material. The system is often considered as being homogeneous, and the thermal conductivity is considered constant. Thermal conductivity, A, W m, is defined using Fourier s law. 

The minus sign in the equation denotes that the heat flow is positive in the direction of decreasing temperature. 

Integration of Eq. (4.154) with constant heat conductivity gives 

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