Thermal conduction, convection & radiation are the modes of heat transfer and it is a nice topic that will be discussed here. Here, we will learn all the details about heat, heat transfer, and modes of heat transfer.
Heat is an energy that can be transferred from one body to another body and it is driven by the temperature. Heat always transfers from the body which maintained at high temperature to the body which is kept at the lower temperature.
Now, how the transfer happens and what are the modes of heat transfer, we will learn in this session.
Before discussing thermal conduction, convection, radiation heat transfer, we need to learn the basics of heat transfer.
Heat transfer means the transfer of energy from two different bodies kept at different temperatures.
Example of Heat Transfer
Do you like tea or coffee in the morning? I like to have a cup of coffee. When coffee is made, it is at a very high temperature. So, we cannot immediately take it as it is very hot.
What do we do to make the coffee? We simply wait a little time at the normal room temperature or keep the coffee in front of a fan. Gradually, coffee becomes a little less temperature and we can take.
Explanation of Heat Transfer
When coffee is made, it consists of high heat content and temperature high, say, 70 deg. C. So, we cannot take it. The normal temperature of the atmosphere is about 30 deg. C (for example).
When we keep it sometimes in the room, heat is released into the atmosphere. Coffee released the temperature to the atmosphere, and it comes at the same to the atmosphere, like 30 deg. C. This release of heat means a transfer happens between coffee and the atmosphere.
This transfer is called heat transfer.
Are discussing the heat transfer, hence, the unit of heat transfer should be known,
Relationship, 1 Calorie = 4.186 J, 1000 watt = 1 kW
We have learned the basic of heat transfer, that means heat can transfer from one object to another object or one object to surrounding to any objects.
There are three modes of heat transfer,
When two solids of different temperatures come into contact, heat transfer happens from high-temperature solid to low-temperature solid.
Conduction is defined as the transfer of heat by the vibration or the agitation of molecules instead of motion.
It depends on a few factors,
Heat transfer occurs due to direct contact of matter. As per “Fourier’s Law“, we know that,
Rate of heat transfer pro to A (T2-T1)/d
Q/t = kA (T2=T1) / d
q = U A (T2-T1) (1)
U = k / d
= Coefficient of Heat Transfer (W/(m2K), Btu/(ft2 h oF)
dT = T2 – T1
= temperature gradient – difference – over the material (oC, oF)
Based on the equation as well as example, the following factors are affecting the rate of conduction,
If you take a metal plate and heat it in an oven, what will happen? The metal plate will be heated.
If the area increases, contact surfaces will increase.
If the temperature difference is more, heat flow will be more.
Thermal conductivity is one of the main properties associated with heat transfer. It is property, by which we can understand the heat transfer rate. If the value of thermal conductivity is more, heat transfer will be more.
Various metals have different thermal conductivity.
|Material||Thermal Conductivity (W/m-K)|
|Wood||0.15 to 0.2|
Convection heat transfer happens in the liquids or gases if there is any temperature difference. In this method, heat transfer happens partly by molecule movements or mass transfer.
It may be between solid and moving fluids also.
Convection is defined as the method of heat transfer by the movement of molecules or transfer of the mass in liquids or gasses
from a region of higher temperature to a region of lower temperature. Convection heat transfer occurs partly due to the actual movement of molecules or due to the mass transfer.
The convection rate depends on the following factors,
Let us see the expression for convection heat transfer. We will see here, heat transfer between a solid and fluid in motion.
The rate of convection is calculated as,
|Q = hc ∙ A ∙ (Ts – Tf)|
This law is known as Newton’s Law of Cooling.
Natural or Free Convection
Radiation heat transfer happens when heat transfer happens between two bodies, which are not in contact.
Radiation is defined as the method of heat transfer by which heat is transferred from a body to another body, which doesn’t have any contact.
The light, as well as the heat, comes from the sun to the earth, however, there is no medium or only air. This is an example of radiation.
In the micro oven, radiation is the main process to make the food hot.
Thermal radiation is expressed by Stefan-Boltzmann law. It is as follows,
|q = e ∙ σ ∙ A· (Tr – Tc)4|
We have learned the basics of conduction heat transfer, convection heat transfers, and radiation heat transfer,
|Heat transfer happens between objects when they are in contact||Heat transfer happens with the fluid itself.||Heat transfer happens by electromagnetic waves|
|It is a very slow process||Faster||Very fast|
|Transfer due to temperature difference.||Transfer happens due to density difference or buoyancy||It is by waves.|
|A hot solid transfers heat by conduction||Boiling milk in a container transfer heat by convection||Sun provides thermal radiation to the earth.|
We have learned the basics of modes of heat transfer, its basic, understanding, simple mathematical derivations.
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