CONVECTION: CHARACTERISTICS, EXAMPLES, APPLICATIONS - PHYSICAL - 2025

The convection is one of the three mechanisms that heat is transferred from one zone to another warmer cooler. It takes place because of the movement of the mass of a fluid, which can be a liquid or a gas. In any case, a material medium is always required for this mechanism to take place.

The faster the movement of the fluid in question, the faster the transfer of thermal energy between zones of different temperatures. This happens continuously with atmospheric air masses: buoyancy ensures that the warmer and less dense ones rise while the colder and denser ones descend.

Figure 1. A room is cooled by opening the door, since the hot air (red arrow) and less dense rises and escapes from it. Source: Wikimedia Commons. Genieclimatique / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

An example of this is the closed room in the image, which is immediately refreshed as soon as the doors or windows are opened, since the hot air from inside escapes even through the cracks, giving way to the fresh air from outside that remains more down.

Types of convection

Natural and forced convection

Figure 2. Examples of forced convection and natural convection. Source: Cengel, Y. Thermodynamics.

Convection can be natural or forced. In the first case, the fluid moves by itself, as when opening the door to the room, while in the second it is forced by a fan or a pump, for example.

Diffusion and advection

There can also be two variants: diffusion and advection. In diffusion, the molecules of the fluid move more or less randomly and the transmission of heat is slow.

In contrast, advection moves a good amount of fluid mass, which can be achieved by forcing convection with a fan, for example. But the advantage of advection is that it is much faster than diffusion.

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A simple mathematical model of convective heat transfer is Newton's law of cooling. Consider a hot surface of area A, surrounded by cooler air, such that the difference in temperature is small.

Let us call the heat transferred Q and time t. The rate at which heat is transferred is dQ / dt or derived from the function Q (t) with respect to time.

Since heat is thermal energy, its units in the International System are joules (J), therefore the transfer rate comes in joules / second, which are watts or watts (W).

This rate is directly proportional to the difference in temperature between the hot object and the medium, denoted as ΔT and also to the surface area A of the object:

The constant of proportionality is called h, which is the coefficient of heat transfer by convection and is determined experimentally. Its units in the International System (SI) are W / m ². K, but it is common to find it in terms of degrees Celsius or centigrade.

It is important to note that this coefficient is not a fluid property, since it depends on several variables, such as the geometry of the surface, the speed of the fluid and other characteristics.

Combining all of the above, mathematically Newton's law of cooling takes this form:

Application of Newton's law of cooling

A person stands in the middle of a room 20 ° C, through which a slight breeze blows. What is the rate of heat that the person transmits to the environment by convection? Assume that the exposed surface area is 1.6 m ² and the surface temperature of the skin is 29 ºC.

Fact: the convection heat transfer coefficient in this case is 6 W / m ². ºC

Solution

The person can transmit heat to the air around them, since it is in motion when the breeze blows. To find the transfer rate dQ / dt, simply plug the values ​​into Newton's equation for cooling:

dQ / dt = 6 W / m ². ºC x 1.6 m ² x (29 ºC - 20 ° C) = 86.4 W.

Examples of convection

Warm your hands over a campfire

It is common to warm your hands by bringing them close to a campfire or hot toaster, as the air around the heat source in turn heats up and expands, rising because it is less dense. As it circulates, this hot air envelops and warms your hands.

Figure 3. One way to warm your hands is through the convection current created in the air by the fire. Source: Pxfuel.

Air flow on the coast

On the coast, the sea is colder than the land, so the air above the land heats up and rises, while the colder air arrives and settles in the space left by this other when ascending.

This is called a convection cell and is the reason why it feels cooler when looking out to sea and the breeze blows against your face on a hot day. At night the opposite happens, the cool breeze comes from land.

The water cycle

Natural convection occurs in the air of the oceanic coasts, through the hydrological cycle, in which water is heated and evaporated thanks to solar radiation. The water vapor thus formed rises, cools and condenses into clouds, the masses of which increase and rise by convection.

By increasing the size of the water droplets, there comes a time when the water precipitates in the form of rain, solid or liquid, depending on the temperature.

Boil water in a container

When the water is placed in the kettle or saucepan, the layers closest to the bottom are heated first, as the flame or heat from the burner is closest. Then the water expands and its density decreases, therefore it rises and the colder water takes its place at the bottom of the container.

Figure 4. Water heating by convection. Source: wikimedia Commons. User: Oni Lukos / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/).

In this way all the layers circulate rapidly and the entire body of water heats up. This is a good example of advection.

Wind generation

Convection in air masses, along with the earth's rotational motion, produces winds, as cold air moves and circulates underneath hot air, creating various currents called convection currents.

Ocean currents

Water behaves in a similar way to how air does in the atmosphere. Warmer waters are almost always near the surface, while cooler waters are deeper.

Dynamo effect

It occurs in the molten core inside the planet, where it combines with the Earth's rotational movement, generating electrical currents that give rise to the Earth's magnetic field.

Transmission of energy inside the stars

Stars like the Sun are huge spheres of gas. Convection is an efficient energy transport mechanism there, since gaseous molecules have sufficient freedom to move between areas of the interior of stars.

Applications of convection

air conditioners

The air conditioner is placed near the ceiling of the rooms, so that the cooled air, which is denser, descends and cools closer to the floor quickly.

Heat exchangers

It is a device that allows the transmission of heat from one fluid to another and is the principle of operation of air conditioners and cooling mechanisms of the automobile engine, for example.

Thermal insulators in buildings

They are made by combining sheets of insulating material and adding air bubbles inside.

cooling towers

Also called cooling towers, they serve to dispose of the heat produced by nuclear power plants, oil refineries and other various industrial facilities into the air, instead of to the ground or to the water.

References

1. Giambattista, A. 2010. Physics. 2nd. Ed. McGraw Hill.
2. Gómez, E. Conduction, convection and radiation. Recovered from: eltamiz.com.
3. Natahenao. Heat applications. Recovered from: natahenao.wordpress.com.
4. Serway, R. Physics for Science and Engineering. Volume 1. 7th. Ed. Cengage Learning.
5. Wikipedia. Convection. Recovered from: en.wikipedia.org.
6. Wikipedia. Convection thermique. Recovered from: fr.wikipedia.org.