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Heat Transfer Types : Conduction vs Convection vs Radiation

Jan 11, 2023
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Heat transfer is a quite common phenomenon that takes place in most situations. All matter which is made of molecules and atoms is capable of undergoing heat transfer. The motion of atoms varies frequently.

Transfer of heat occurs when there is the movement of atoms and molecules. It is also responsible for creating thermal energy. Every matter contains this energy. The heat energy of the particle will be more when the movement of atoms and molecules is more. 

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This article will talk about the transfer of heat in general while closely examining its subtopics.

What is Heat Transfer?

It is nothing but the transfer of heat from one medium to another. It is defined as the heat movement throughout the body due to irregular temperatures between the system and its surroundings. 

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The temperature difference is the main reason heat transfer occurs from one point to the other.

Types of Heat Transfer

The heat is capable of travelling from one place to another in various ways possible. The following are the three different types :

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  1. Conduction
  2. Convection
  3. Radiation

These are also called modes of heat transfer. It has been a common fact that can occur from the higher system to the lower system. In simple words, it starts from a hotter region to a colder region. After it finally reaches the colder region (lower system), the whole system will possess almost the same temperature everywhere if the medium is an even object like a steel rod or a square plate.

Conduction Of Heat Transfer 

Conduction generally means the process of transfer of energy or heat from one particle to another while being in direct contact. Likewise, conduction is the transmission of heat from one particle to another. Here, both the particles are obliged to be in contact for this process to occur.

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Furthermore, thermal energy transfers from a higher kinetic energy region to a lower one. Here, the particles that travel at high-speed clash with the ones that are slower. Doing so will enhance the speed and kinetic energy of the slower-moving particles. It involves physical contact between each particle.

In addition, high-speed particles have higher temperatures while slow-speed particles have lower temperatures. Therefore, that is how heat is transferred in conduction. The other name for conduction is heat conduction or thermal conduction.

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Conduction Equation

The following is the expression for calculating the conduction rate:

Q = [K A (Thot – TCold)] / d

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where,

K indicates the object’s thermal conductivity

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Q means heat transfer per unit time

A indicates the heat transfer area of the region

d is the body’s thickness

That  means a hot region’s temperature

TCold means a cold region’s temperature

When it comes to conduction, metal objects are capable of conducting heat better than other objects. 

Examples of Conduction

Below mentioned are a few examples of conduction:

  • Conduction of heat through beach sand: During summertime, the beach’s sand can be very hot. It is because the conduction of heat takes place through the sand. Also, sand is a very good heat conductor.
  • Transfer of heat from hand to ice cube: Imagine placing an ice cube on your hand. What will happen? Due to the presence of heat in the body, the ice cube will start to melt.
  • Ironing of clothes: Here, heat from the iron box gets transferred to the clothes and helps them regain their original and neat shape. It is a very good example of conduction heat transfer.

Convection Heat Transfer

In convection heat transfer, the movement of fluid molecules takes place. This flow starts from a higher temperature area and ends at a lower temperature region.

Convection Equation

The volume of the liquid is subjected to increase if its temperature rises. This increase in volume will be the same factor as the temperature. This effect is called displacement. 

The following is the expression to calculate the convection rate:

Q = hc A (Ts – Tf)

where,

Q is the transfer of heat per unit volume

A indicates the area of heat transfer

hc is the coefficient used in convective heat transfer

Ts is the temperature of the surface

Tf is the temperature of the fluid

Examples of Convection

To understand the concept in detail, let us take a look at the following real-time examples:

  • In warm-blooded animals, the circulation of blood occurs with the help of convection. Doing so will regulate their body temperature.
  • Warm water around the equator will move towards the poles, where cold water is present. Likewise, cold water in the poles will make its way toward warm water on the equator.
  • While boiling the water, denser molecules move towards the bottom, whereas thinner molecules move to the top of the water surface. As a result, a circulation motion is formed, and the molecules get heated.

Radiation Heat Transfer

Radiant heat transfer can be seen in various forms in our lives. We call radiant heat thermal radiation. Electromagnetic wave emission is the main reason for the generation of thermal radiation. These waves are capable of carrying the energy away from the body that is performing emission.

Moreover, radiation occurs through a transparent medium or vacuum, which can be liquid or solid. Molecules in matter move randomly, and as a result, thermal radiation is formed. There are two phenomena responsible for the emission of EM radiation. They are the movement of charged protons and electrons. 

Furthermore, the thermocouple is an instrument used to measure the radiation heat transfer. This device’s main purpose is to measure the temperature. However, while measuring the temperature of heat transfer, some errors might take place. These errors can be calibrated for better accuracy and usage in such cases.

Radiation Equation

There will be a reduction in the emitted spectra radiation if the temperature increases. As a result, the emitted radiation’s wavelengths are shorter. We can use the Stefan-Boltzmann law to calculate thermal radiation:

P = e σ A (Tr – Tc)4

where = 5.67×10-8 Wm-2  K-4

P denotes the net power of the radiation

A represents the area of radiation

Tr is the temperature of the radiator

Tc indicates the temperature of the surroundings

e indicates emissivity 

σ is the Stefan constant 

Examples of Radiation

Take a look at the below-mentioned examples to understand radiation heat transfer better:

  • An ideal example of radiation is the emission of Ultraviolet rays from the sun.
  • Another good example would be the emission of heat radiation from the microwave oven.
  • Apart from that, the release of alpha particles, especially during the decay of Thorium 234 and Uranium 238, is another good example of radiation heat transfer.

Conduction vs Convection vs Radiation

The following is the differentiation table that distinguishes conduction, convection and radiation:

ConductionConvectionRadiation
The transfer of heat occurs because of the dissimilarity in temperature.The transfer of heat occurs because of the dissimilarity in density.The transfer of heat occurs in all the bodies with a temperature greater than 0 K.
Transfer of heat takes place between bodies and objects which are in direct contact with one another.Transfer of heat occurs within the specific fluid.Transfer of heat takes place through EM waves. It does not involve any particle.
Conduction does not follow the law of refraction and reflection.Convection does not follow the law of refraction and reflection.Radiation follows the law of refraction and reflection.
The occurrence of heat transfer is done through a solid object. This object is heated.Transfer of heat takes place through intermediate objects. An example would be heat transfer between water and air.In radiation, the transfer of heat takes place through EM waves.
Conduction heat transfer is slow.Convective heat transfer is faster than conduction.Radiation heat transfer is the fastest of all.

Conclusion

All in all, heat transfer is a very important phenomenon in thermal studies. It acts as a base for all the thermal subjects such as thermodynamics, thermal engineering, etc. 

From this blog, we have comprehensively understood some heat transfer concepts such as types, conduction, convection, radiation and all their equations and examples.

Frequently Asked Questions

1. What is the fourth type of heat transfer?

A. The fourth type of heat transfer is evaporative cooling. It is a technique used in atomic physics. Its other name is adiabatic cooling. This technique works due to the principle of water evaporation, where air can be cooled down to a comfortable temperature. It generally is a cooling and ventilation technique and uses water as its refrigerant.

2. What are the three types of heat transfer? 

A. Heat transfer is of three major types. They are as follows:

  • Conduction
  • Convection
  • Radiation

Conduction heat transfer takes place in solids or fluids. On the other hand, convection occurs within the fluids. Lastly, radiation heat transfer takes place through EM waves.

3. What are the types of convection heat transfer?

A. The types of convection heat transfer are as follows:

  • Air – free
  • Air – forced
  • Boiling water
  • Liquids – forced

 

 

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