Modes of Heat Transfer – Conduction, Convection & Radiation – Mechanical Engineering

# Modes of Heat Transfer – Conduction, Convection & Radiation

Heat is a form of energy which transfers between bodies which are kept under thermal interactions. When a temperature difference occurs between two bodies or a body with its surroundings, heat transfer occurs. In this article, we are going to deal with the different modes of heat transfer. Heat transfer occurs basically in three modes:

1.      Conduction

2.      Convection and

CONDUCTION:

Conduction is the mode of heat transfer 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. In conduction, there is no noticeable movement of molecules. You might be think that then how this heat transfer occurs? The heat transfer occurs here by the two mechanisms happen.

1.      By the transfer of free electrons. (Good conductors like metals have a plenty of free electrons to make conductive heat transfer.

2.      The atoms and molecules having energy will pass those energy they have with their adjacent atoms or molecules by means of lattice vibrations.

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Now we can think how this conduction occurs in gases and liquids. In the cases of gases, the molecules having energy in the form of kinetic energy and during their random movements, they exchange their momentum and energy by colliding with others. By doing so, the first molecule loses the energy while the second one gains it. This is how energy is transferred in the case of gases.

In the case of liquids also, the working is similar to that of gases. Here, the only difference is that, the molecules in liquids are more closely packed and hence inter molecular forces came into action in the case of liquids.

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.

CONVECTION:

Conductive heat transfer occurs within a fluid itself and it is carried out by transfer of one fraction of the fluid to the remaining portion. Hence unlike conduction, transfer of molecules occurs during convection. Since movement of particles constitutes convection, it is the macro form of heat transfer. Also convection is only [possible in fluids where the particles can moved easily and the rate of convective heat transfer depends on the rate of flow to a great extend. Convection can be of two types:

1.      Natural convection: In this type of convection, the movement of particles which constitutes convection occurs by the variation in densities of the fluids. As we already know, as temperature increases, the density decreases and this variation in density will force the fluid to move through the volume. This cause convection to occur.

2.      Forced Convection: The difference between natural convection and forced convection is that in forced convection, a work is done to make movement in the fluid. This is done using a pump or blower.

Newton’s Low Of Cooling:

Q = hA(Ts-T∞)

Where: Ts is the surface temperature

T∞ is the fluid temperature

h is the heat transfer coefficient

Radiation is the third mode of heat transfer. This mode of heat transfer didn’t require any medium to occur. Every matter having a temperature above absolute zero will emit energy in the form of electromagnetic waves and called radiation. It is the same way the energy of the Sun reach us. The key features about radiation are it do not require any medium and also laws of reflection is applicable for radiation.

Stefan- Boltzman Law:

Q = A∑Ts

Where: Ts is the absolute temperature of surface

∑ is the proportionality constant.