Heat transfer by convection - What is Convection?- eSaral

Heat transfer by convection

The Process in which Heat transfer occurs between a fluid that is in contact with a surface called heat transfer by convection.

What is Convection?

Convection requires a medium and is the process in which heat is transferred from one place to other by the actual movement of a heated substance (usually fluid).

The type of convection which results from difference in densities is called natural convection (for example, a fluid in a container heated through its bottom). However, if a heated fluid is forced to move by a blower, fan, or pump, the process is called forced convection.

The rate of heat convection from an object is proportional to the temperature difference

$(\Delta \theta)$ between the object and convective fluid and the area of contact A, i.e., $\left[\frac{\mathrm{dQ}}{\mathrm{dt}}\right]$

$=\mathrm{hA} \Delta \theta$ where, h represents a constant of proportionality called convection coefficient

and depends on the properties of fluid such as density, viscosity, specific heat and thermal conductivity, etc.

Phenomena Based on convection :

(i) Land and sea breezes -

The heat from the sun is absorbed more rapidly be land than by sea-water. Moreover, the specific heat of land is low as compared to that of sea-water. Consequently, the rise in temperature of land is higher as compared to that of sea-water. To sum-up, land is hotter than the sea during day time. As a result of this, the colder air over the sea blows towards the land. This is called sea-breeze.

At right, air blows from land towards sea. This is called land breeze.

(ii) Formation of trade winds –

The surface of Earth near the equator gets heated strongly. So, the air in contact with the surface of Earth at the expands and rises upwards. As a result of this, a low pressure is created at the equator.

At the poles, the air in the upper atmosphere gets cooled and comes down. So, a high pressure is created at the poles.

Due to difference of pressures at the poles and equator, the air at the poles moves towards the equator, rises up, moves the poles and so on. In this way, a wind is formed in the atmosphere.

The rotation of the Earth also affects the motion of the wind. Due to anti-clockwise rotation of Earth the warm wind blowing from equator to north drifts towards east.

The steady wind blowing from north-earth to equator, near the surface of Earth, is called trade wind.

(iii) Monosoons –

In summer, the peninsular mass of central Asia becomes more strongly heated than the waters of the Indian Ocean. This is due to the fact that the specific heat of water is much higher than that of the soil and rocks.

Hot air from the heated land mass rises up and moves towards the Indian ocean. Air filled with moisture flows over the Indian ocean on the south towards heated landmass. When obstructed by mountains, the moist air rushes upwards to a great height. In the process, it gets cooled. Consequently, the moisture condenses and falls as rain.

(iv) Ventilation –

Ventilator of exhaust fan in a room help of remove impure and warm air from a room. The fresh air from outside blows into the room. This is all due to the convection current set up in the room.

(v) To regulate temperature in the human body –

Heat transfer in the human body involves a combination of mechanisms. These together maintain a remarkably uniform temperature in the human body inspite of large changes in environmental conditions.
The chief internal mechanism is forced convection. The heart serves as the pump and the blood as the circulating fluid.
Some important points :

• Natural convection takes place from bottom to top while forced convection in any direction.
• In case of natural convection, convection currents move warm air upwards and cool air downwards. This is why heating is done from base, while cooling from the top.
• Natural convection is not possible in a gravity free region such as a freely falling lift or an orbiting satellite.
• Natural convection plays an important role in ventilation, in changing climate and weather and in forming land and sea breezes and trade winds.
• The forced convection of blood in our body by a pump (heart) helps in keeping the temperature of body constant.

So, that's all from our side. I hope you understand the Heat transfer by convection. For a better understanding of this chapter, please check the detailed notes of the Heat Transfer. If you want more Free Learning Videos and Study Material Then don't forget to download the eSaral App.