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Convection heat transfer




We have already studied about conductive heat transfer. In this page we will learn about heat convection.

What is Heat convection?

In heat convection, heat is transferred from one place to another by the actual motion of heated particles. Some examples where heat transfer takes place by convection are
  • Hot air blower
  • Hot water heating system
  • Land breezes, sea breezes etc.

Heat Convection Definition:-

The process by which heat is transmitted from one point to another due to the actual bodily movement of the heated particles of the substance is called convection
Convection is prominent in case of liquids and gases. This is because the molecules of liquids and gases are free to move.
Convection
How convection heat transfer works
Note That:-
  1. Transfer of heat by convection cannot take place in solids because molecules in solids are fixed and cannot move about freely.
  2. Transfer of heat by convection cannot take place in a vacuum because there are no particles of any kind in a vacuum which can move and transfer heat.

Heat convection explanation

  • The fluids (liquids and gases) are heated by this process. The fluid on getting heated becomes lighter in density and therefore rises upwards, while colder fluids move down to take its place. Thus currents are set up in the heated fluid. These are called convection current.
  • Fluids, in general, are a poor conductor of heat. They cannot conduct heat by convection. This because unlike solids their molecules are not held together tightly. Due to this, they cannot transfer vibrational energy between them as we have read in conduction heat transfer that faster vibrating molecules transfer heat energy to the neighboring molecules
  • Now is fluids they transfer heat by convection. Both conduction and convection are different processes. So how long a fluid would take to heat depends on its specific heat capacity at constant volume. If a fluid has more specific heat then it would take longer for this fluid to warm up.

Natural convection heat transfer

If the material moves due to the difference in density then the process of heat transfer is called natural of free convection
It arises due to unequal heating of the fluid. In this case, heated and less dense molecules fluids rise and are replaced by the colder molecules. Natural convection is responsible for different types of winds in the atmosphere.

Forced convection heat transfer

If a heated material is forced to move by an agency like a pump or a blower, the process of heat transfer is called forced convection.
Air-conditioning, central heating system, heating a liquid by brisk stirring are examples of forced convection heat transfer.


Convection heat transfer examples or convection examples

Some of the phenomena which are based on thermal or heat convection are
  1. Convection plays an important role in ventilation and in heating and cooling systems of the houses.
    Convection Example
    Convection Example :- Convection plus Ventilation in a house.

    Here from the figure you can see how heat is being transferred through convection.

  2. Regulating human body temperature. In the human body, the heart acts as a pump that circulates blood through different parts of a body. This transfers heat by forced convection. This maintains remarkable constant body temperature.
  3. Formation of trade winds:- surface of the earth near equator gets heated strongly. As a result of this, the air in contact with this product northward wind in the northern hemisphere and southward wind in the southern hemisphere.
    The earth’s surface at equator expands and rises upwards. Thus equator becomes a region of low pressure. On the other hand, at poles, the air in the atmosphere gets extremely cold and comes down thereby making poles a region of high pressure at poles and equator.
    • The air at pole descends
    • Then it moves towards the equator
    • There it gets heated and raises up.
    • It now blows towards the pole and so on.
    This process results in the formation of wind in the atmosphere. This motion of wind is further affected by the rotation of the earth. Due to rotation of the earth on its axis from west to east, the air close to equator has a speed of 1600 km/hr. Towards the east, and is zero close to the poles.
    As a result, the actual direction of the wind in the northern hemisphere is northeast and in the southern hemisphere, southwest. These winds are called trade winds.
    Apart from this convection in the air is one of the seasons of occurrence of monsoons in India. Convection plays an equally important role in the formation of land and sea breezes.

Question :- can we boil water inside an earth satellite?
Answer: - No, heat transfer by convection is based on the fact that a liquid becomes lighter on becoming hot and rises up. In the condition of weightlessness, this-this is not possible. So the transfer of heat by convection is not possible in a satellite.




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