Thermal Properties : Basic principal of heat transfer

Holman had proposed a difinition of heat transfer as “science which seeks to predict the energy transfer which may take place between material bodies as a result of temperature difference”. The energy transfer is known as heat . There are three mode of heat transfer : conduction, convection, and radiation.

1. Conduction heat transfer : When a single body is subjected to a temperature gradient heat will be transferred from the hight temperature region to the low temperature region. This mode of heat transfer is call “conduction” and from Fourier’s law for steady state condition, the heat transfer rate per unit area is proportional to the normal temperature gradient, as shown in equation 1

when dQ/dt is the heat transfer rate and partial derivative of temperature respective to x is the temperature gradient in the direction of the heat flow. The constant k is called the thermal conductivity of the material which is define as the heat transport in a material per unit temperature gradient per unit area between two isothermal planes. In case of unsteady state (transient) conduction, where the temperature at any point in a body varies with both time and position, the basic Fourier equation becomes a patial differential :

where C and r are specific heat and density, respectively. The expression k/(rC) is called “thermal diffusivity” of the material and is represented by a .A hight value of a could either result from a high value of thermal conductivity (k), which would indicate a rapid energy transfer rate, or from a low value of the thermal heat capacity rC . The low value of heat capacity indicate that less of the energy moving through the material would be absorbed and used to raise the temperature of the material thus more energy would be available for further transfer. Therefore, the larger the value of a , the faster will heat diffuse through the material.