Convective Heat Transfer

Recently I have been reading about ‘Energy’, especially the heat and light energies and the transfer of both these forms of energies which basically sustains life on this planet. This post discusses a mode of heat transfer, i.e., convection which explains many of the everyday phenomena around us.

Heat is transferred by three basic modes:

• Conduction
• Convection
•  Radiation

Conduction of heat occurs through a thermally conductive material such as metal, or between two thermally conductive materials in contact. Radiation is the transfer of heat from one body to another when they are not in contact. Here heat is transmitted through space in the form of electromagnetic radiation in the infra-red region.

Convection is the method of heat transfer via the mass motion of fluid; liquid or gas. When fluid is heated, it moves up from the source of heat to cooler areas carrying the heat energy and its space is filled by cooler fluid from the surroundings which in turn gets heated up and starts moving up, continuing the process and transferring heat energy from the source of heat to the surroundings.

Convection explains many of the day-to-day phenomena such as:

• Boiling of water
• Fire heating up the surroundings
• Wind
• Cloud formation, thunderstorms, etc.

 Let us looking into few of them to understand heat transfer process.

Convection in Liquid

We know that heat is the result of vibration of atoms and molecules in an object, often bumping into each other when triggered by an energy source.

Consider boiling of water. Water is heated from the bottom of a pan, close to the heat source. It should be noted that the pan gets heated up via conduction; first the bottom of the pan is heated up by the source (which subsequently heats up the water above it) and the heat is transferred to the rest of the pan via conduction. When water at the bottom gets heated, the particles move faster and move farther apart making it less dense.

 When particles move farther apart, their volume increase. Since mass does not change for a given volume of water, the density decreases.

 The less dense water rises up to float over the cooler, denser water. As the warm water rises up, its place is taken by the cooler water from the surroundings which then gets heated up and rises up. By then, the warm water which has already risen up to the top, gets cooled by the surroundings and moves down with the new warmer water displacing it. This eventually leads to a continuous circular motion of water in the pan known as convection current.

 Note that the cooling of water when it reaches the top of the pan is also via convection. The warm water at the top transfers heat to the air above it which rises up, and that space is filled by cooler air which then gets heated up and rises as well, providing a continuous supply of heat transfer upwards. This continues till the heating stops and water in the pan cools down completely.

As you place your hand well over a boiling pan of water, you can feel the heat rising.

Newtion's law of cooling

The rate of heat loss of a body is proportional to the difference in temperature between the body and the surroundings.

Example: Cooling of a hot object when placed in a cooler surrounding.

Atmospheric Convection

Atmospheric convection results in wind, cloud formation, thunderstorms (moist convection), etc.

1. Wind

Wind is the movement of air from a region of high pressure to a region of lower pressure. Low pressure systems are the result of the uneven heating of the Earth's surface.

For example, land and water bodies are heated unevenly during day and night. That explains the directions of land breeze and sea breeze often observed in the coastal areas.

Day time:

During day time, land gets heated up faster. This causes the air above the land to heat up and rise up creating a low pressure system there. The air from the cooler areas (sea/ocean) fills that space which again gets heated up and rises up and repeating the process we have a circulation of air from sea to land. That is, during day time, the wind blows from sea to land.

Night time:

During night time, the land gets cooled down faster than the water body. This means that the water body is warmer than the land which causes the air above the water to heat up and rise up. Then a low pressure system is created above the water, causing air from land to move to that space which in turn gets heated up and rises up causing an air circulation from land to sea. That is, during night time, the direction of wind changes. 

2. Cloud formation and Thunderstorms

Cloud formation and thunderstorms are the results of moist convection. This happens in areas where there is moisture in air.

When the Sun's rays heat up the land, it causes the air above it to get heated up as well. This warm air rises up accompanied by the moisture in air. As it moves up, it starts getting cooled down as the atmospheric temperature drops. When it reaches a certain altitude, it gets saturated and the moisture starts to condense to tiny droplets and expand.

As it moves further up, temperature further drops and the condensation and expansion continues. These condensed tiny droplets are suspended in air and when they come together, clouds are formed.

 In tropical areas, where there is a high content of moisture in air, the cloud formation is accompanied by rain when the accumulated water droplets become heavy and fall under gravity.We also know that the Sun's rays heat up water bodies, causing vapour to rise. This vapour in air is what we call humidity (moisture).

 Thunderstorms occur in tropical areas with high humidity. During the cloud formation, the air cools and condenses to tiny droplets. During the condensation process, the air releases heat inside the cloud causing it to get warmer as compared to its surroundings. This keeps the air unstable, increases the buoyancy of air and convection continues. This results in the building up of clouds vertically upwards. When it reaches a certain altitude where the temperature is below freezing, large raindrops, ice crystals and hail are formed.

When these ice crystals and hail are heavy enough, they fall under gravity as precipitation. This causes a cool drift of air downwards. When this collides with the warm moist updrift, lighting and thunder will occur.

And what causes lightning? We will discuss that in another post!!!