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 cooler and denser ones descend.
An example of this is the closed room in the image, which is immediately updated as soon as the doors or windows are opened, as the warm air inside escapes even through the slits, giving way to fresh air outside, which is lower.
Types of convection
Natural and forced convection
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, fluid molecules move more or less randomly and heat transmission is slow.
On the other hand, 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.
How is heat transferred by convection?
A simple mathematical model of convection heat transfer is Newton’s law of cooling. Consider a warm surface of area A, surrounded by cooler air so that the temperature difference is small.
Let’s call the transferred heat Q and the time t. The rate at which heat is transferred is dQ / dt or derivative of the function Q(t) with respect to time.
As heat is thermal energy, its units in the International System are joules (J), so the transfer rate is in joules/second, which are watts or watts (W).
This rate is directly proportional to the temperature difference between the hot object and the medium, denoted as ΔT, and also to the surface area A of the object:
ΔT = Temperature at the surface of the object – Temperature away from the object
The proportionality constant is called h , which is the convection heat transfer coefficient and is determined experimentally. Its units in the International System (SI) are W / m 2 . K, but it is common to find it in degrees Celsius or centigrade.
It is important to note that this coefficient is not a property of the fluid as it depends on several variables such as surface geometry, fluid velocity and other characteristics.
Combining all of this, Newton’s law of cooling mathematically takes this form:
dQ / dt = hA ΔT
Application of Newton’s law of cooling
One person stands in the middle of a room at 20°C, through which a light breeze blows. What is the rate of heat that a person transmits to the environment by convection? Suppose the exposed surface area is 1.6 m 2 and the skin surface temperature is 29°C.
Fact : the convection heat transfer coefficient in this case is 6 W / m 2 . °C
The person can transmit heat to the air around them as they are moving when the breeze blows. To find the transfer rate dQ / dt, simply replace the values in Newton’s equation with cooling:
dQ / dt = 6 W / m 2 . ºC x 1.6 m 2 x (29 ºC – 20 ºC) = 86.4 W.
Warm your hands over a fire
It is common to warm your hands by moving them closer to a fire or a hot toaster, as the air around the heat source heats up and expands, rising, as it is less dense. As you circulate, this warm air surrounds and warms your hands.
air flow on the coast
On the coast, the sea is colder than the land; therefore, the air above the earth heats up and rises, while the cooler air arrives and settles in the space left by that other when it ascends.
This is called a convection cell and it’s the reason you feel cooler when you look out to sea and the breeze blows against your face on a hot day. At night, the opposite happens, the cool breeze comes from the land.
the water cycle
Natural convection occurs in the air on the ocean shores, through the hydrological cycle, in which water heats up and evaporates thanks to solar radiation. The water vapor thus formed rises, cools, and condenses into clouds, whose masses rise and rise by convection.
As the size of the water droplets increases, there comes a time when the water precipitates as rain, solid or liquid, depending on the temperature.
Boil water in a bowl
When water is placed in the kettle or pan, the layers closest to the bottom are heated first, as the flame or heat from the burner is closest. Then water expands and its density decreases; therefore, it increases and the cooler water takes its place at the bottom of the container.
In this way, all layers circulate quickly and the entire body of water is heated. This is a good example of advection.
Convection in air masses, together with the Earth’s rotational motion, produces winds, as cold air moves and circulates under warm air, creating various currents called convection currents.
Water behaves in the same way as air in the atmosphere. Warmer waters are almost always close to the surface, while cooler waters are deeper.
It occurs in the molten core of the planet’s interior, where it combines with the Earth’s rotational motion, generating electrical currents that give rise to the Earth’s magnetic field.
Transmission of energy within the stars
Stars like the Sun are huge spheres of gas. Convection is an efficient mechanism for transporting energy there, as gas molecules are free enough to move between areas within stars.
The air conditioner is placed close to the ceiling of the rooms, so that the cooler, denser air descends and cools down more quickly near the floor.
It is a device that allows the transmission of heat from one fluid to another and is the operating principle of air conditioners and automobile engine cooling mechanisms, for example.
Thermal insulation in buildings
They are made by combining sheets of insulating material and adding air bubbles to the interior.
Also called cooling towers, they are used to discharge the heat produced by nuclear power plants, oil refineries and other industrial installations into the air, instead of landing or watering.