INTRODUCTION & MODES OF HEAT TRANSFER
HEAT TRANSFER
INTRODUCTION
It is well known that, energy transform one form to the other, when a device using that energy is put in use. A computers system accept electrically energy and during its functioning it release a consideration amount of heat energy. Thus the heat generated must be dissipated to the surrounding air, so as to maintain components like processor with in a specified range of temperature. The temperature gradient between the processor and the surrounding air temperate makes the heat transfer. So the study of heat transfer mechanism is very important to understand the heat transfer rates, temperature distribution, and the optimization of the heat sink.
MODES OF HEAT TRANSFER
Modes of heat transfer are
- Conduction
- Convection
- Radiation
Heat transmission, in majority of real situations, occur as a result of combination of there modes of heat transfer. Heat always flow in the direction of lower temperature. The above three modes are similar in that a temperature differential must exist and the heat exchange is I the direction of decreasing temperature.
CONDUCTION
“conduction’ is the transfer of heat from one part of a substance to another part of the same substance, or from one substance to another to another in physical contact with it, without appreciable displacement of molecules forming the substance.
in solids, the heart is conducted by the following two mechanism:
- By lattice vibration
- By transport of free electrons
In case of gases, the mechanism of heat conduction is simple. The kinetic energy of a molecule is a function of temperature. These molecules are in a continuous random motion exchanging energy and momentum. When a molecule from the high temperature region collides with a molecule from the low temperature region, it loses energy by collision.
In liquids, the mechanism of heat is nearer to that of gases, However the molecules are closely spaced and inter molecular forces come into play.
CONVECTION
“convection” is the transfer of heat with in a fluid by mixing of one portion of the fluid with another.
- Convection is possible only in a fluid medium and is directly linked with transport of medium it self.
- Convection constitute the microform, of the heat transfer since macroscopic particle of the fluid moving in space cause the heart exchange.
- The effectiveness of heat transfer by convection depends largely upon the mixing motion of the fluid.
Free or natural convection occurs when the fluid circulates by virtue of the natural differences in densities of hot and clod fluids; the denser portions of the fluid movie downward because of the greater force of gravity, as compared with the force on the less dense.
Forced convection is one in which the work is done to blow or pump the fluid.
RADIATION
“radiation” is the transfer of heat through space or matter by means other than conduction or convection.
Radiation heat is though of as electro magnetic wave or quanta as emanation of the same nature as light and radio waves. All bodies radiate heat; so a transfer of heat by radiation occurs because hot body emits more heat than it receives and a cold body receives more heat than it emits. Radiant energy no medium for propagation and will pass through vacuum.
The properties of radiant heat in general, are similar to those of light. Some of the properties are:
- It does not require the presence of material medium for its transmission.
- Radiant heat can be reflected from the surface and obeys the ordinary law of reflection.
- Its travel with velocity of light.
- Like light, it shows interference, diffraction, and polarization etc.,
- If follow the law of inverse square.
The wave length of heat radiation is longer than that light, hence they are visible to the eye.