Types of Heat Transfer

Heat Transfer is a mechanism in thermal engineering that involves the exchange, transfer and generation of heat energy between any physical bodies. It also involves moving or transferring atomic particles or molecules from one position to another. In this article, we will discuss the mechanism of heat transfer and the various types of heat transfer in detail.

Heat transfer can be classified into the following four types of mechanisms:

• Thermal radiation

• Thermal conduction

• Thermal convection

• Vapourisation

What is thermal radiation?

Thermal radiation involves the transfer of heat energy by the emission of electromagnetic waves. Unlike the other heat transfer mechanisms, this process does not need any medium for the heat transfer to occur, and the object must have a temperature above zero degrees. The rate at which a body is radiating or absorbing these radiations depends entirely on the type and nature of the surface.

Some objects which are good emitters can also be good absorbers (according to Kirchhoff’s Law of Radiation). If a surface is blackened, it can act as both an excellent emitter and a good absorber. If the surface is silvered, it is considered a poor emitter and absorber. When an object absorbs all the radiant energy that falls on it, it is called a blackbody. It will act as a perfect absorber and also a perfect emitter.

The concept of thermal heat transfer involving thermal radiation can be easily understood with the figure shown below.

Fig1. Thermal radiation mechanism

What is conduction?

Conduction can be further classified into the following three categories:

• Thermal conduction

• Electrical conduction

• Sound conduction 

Thermal conduction

This type of conduction is the mechanism where the energy is transferred from a warm substance to a colder substance through any direct contact. For example, someone touching the handle of a hot metal handle of a pan. The following figure can easily understand the mechanism of thermal conduction,

Fig2. Thermal conduction mechanism

When one end of a conductor (a good conductor) or a wire is heated gradually, the molecules start vibrating and move from the conductor’s warmer part to the cooler part, as shown in the figure above. In this way, heat transfer takes place through thermal conduction.

Electrical conduction

Here, the electrically-charged particles’ transfer with the help of a medium takes place. For example, the power lines or cables transmitting electricity. 

Sound conduction (also called acoustic conduction) is the process through which the sound waves are transferred through a medium, like the vibrations from loud music that passes through a wall.

What is thermal convection?

Thermal convection is the transfer of heat from one point or place to another through the movement of a liquid or fluid, or plasma. Here, the warmer part of the fluid will move up, and the cooler part of the fluid will move down. 

Like conduction and radiation, convection is one of three basic methods of heat transfer.

What is vapourisation?

Vapourisation is the conversion of a solid or liquid substance into a vaporous or gaseous state. Vapourisation can be further classified into:

• Evaporation

• Boiling

Boiling

If bubbles are formed within a liquid when heating, then the vapourisation process is called boiling. Heat is supplied to any solid or liquid to initiate vapourisation. 

Boiling is the liquid phase’s transition to a gaseous phase. Due to the heating of the liquid, bubbles are formed due to vapour at the bottom of the liquid’s surface. Boiling occurs in a condition where the substance’s equilibrium vapour pressure is greater than or equal to the pressure exerted by the environment. 

Boiling temperature or boiling point is the point in temperature where the boiling process starts. The boiling point will vary accordingly with the pressure of the environment. This can be easily understood with the help of the diagram below.

Fig.3. Boiling and evaporation mechanism

When a liquid is converted into a gaseous state (in the form of bubbles) by heating its top surface, then it is called evaporation. If the liquid is converted to gas by heating its bottom surface, it is called boiling. In condensation, vapour can be converted into a liquid or solid state through condensation, and it is the reverse of the vapourisation process. Heat will be transferred from the condensing vapour to the surroundings in this process. 

Conclusion

In this article, we have discussed the concept of heat transfer and the different types of heat transfer mechanisms in detail. Heat is transferred through conduction, convection and radiation. This is a critical question asked in several important physics exams.