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Modes Of Heat Transfer



Heat is a form of energy which transfers or flows takes place between bodies which are kept under thermal interactions. According to Second Law Of Thermodynamics, heat transfer or flows from a body at higher temperature to a lower body temperature.When a temperature difference occurs between two bodies or a body with its surroundings, heat transfer occurs. Following are the different modes of heat transfer. Heat transfer occurs basically in three modes:
1.Conduction
2.Convection and
3.Radiation.


1.Conduction:
 

Heat Transfer due to conduction takes place between one particle of the body to another in the direction of fall of temperature.It means the Heat  transfer will occurs from one part of a substance to another part of within the substance itself or with another substance which is placed in physical contact.If a substance heated up at one end and then heat will transfer to other end by Conduction.



Fourier Law of Conduction:




Where: Q is the heat flow rate by conduction

K is the thermal conductivity of the material

A is the cross sectional area normal to direction of heat flow and

dT/dx is the temperature gradient of the section.


2. Convection:


Convection is a mode of heat transfer in which Thermal Energy is transferred from hot places to cold places.when liquid or gases heated, expansion takes place.Due to these it became lighter and rise up.Its place is taken by colder or dense fluid.This results in a continuous circulation pattern.Water boiling in a pan is a best example of these convection currents. And a atmosphere is also a best example for convection. The earth's surface is warmed by the sun, the warm air rises and cool air moves in.
Convection heat transfer is further classified into two types:


i.Forced Convection
ii.Natural Convection


Forced convection is a type of heat transfer in which fluids motion is generated by external source, in order to increase the heat transfer. This forcing can be done with a blowers,ceiling fan, a pump, and other. In these process heat energy can be transferred very efficiently.


Natural convection is type of heat transfer in which the fluids motion is not generated by any type of external force.Example Cold air cools the egg. Warm air heats the cool drink.





Newton’s Low Of Cooling:




3.Radiation.


Radiation is a method of heat transfer in which the heat transfer takes place from one body to another without effecting the medium through which heat travels.Radiation takes place either through vacuum or through a transparent medium.].In radiative mode, heat transfer takes place through photons present in the electromagnetic waves and it is called Radiation. The random movement of atoms and molecules in heated substances results in emission of electromagnetic waves which carry the heat to be transferred. Best example is heat transferred from sun to earth.

The radiative heat transfer is works on the principle of Stephen- Boltzman law. According to these law the emissive power of a black body per unit area and time is directly proportional to the fourth power of the absolute temperature.


Stefan- Boltzman Law:






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