This is the characteristic that distinguishes convection from conduction and radiation, because in convection there is always a net displacement of masses. On the other hand, radiation does not require a material medium to propagate itself and as for transmission by conduction, it is due to successive collisions between atoms and molecules, without net movement of matter .
However, at the level of the atmosphere and the oceans, it is easy for displacements of large masses of air and water to occur. That is why convection is the predominant energy transfer mechanism in these media, and it is the one that largely determines the Earth’s climate. example of convection heat transfer
In a home kitchen you can see heat transfer mechanisms up close. Simply put water to heat in a saucepan. The portion of fluid that is closest to the burner flame heats up, its density decreases, and it rises. Its place is taken by colder water, which descends to the bottom of the pot.
Types of convection example of convection heat transfer
When a portion of fluid is heated, its molecules move faster and away from each other. For this reason, the fluid at a higher temperature becomes less dense and is capable of rising by flotation, carrying the heat with it.
Then a cooler mass of fluid takes the place left by these rising molecules and this continuous exchange generates so-called convection currents . example of convection heat transfer
This can be achieved in two ways: by natural (free) convection or by forced convection. Also, both forms of convection are present in central heating systems or in solar power plants.
What each one consists of is explained below:
Natural and forced convection
In this mechanism, heat flows only thanks to the fact that the difference in temperature of the fluid in question, in the presence of Earth’s gravity, produces a change in density, which causes the hottest portion to float up and the cooler portions to descend. Without gravity there is no natural convection.
There is a simple experiment in the laboratory that allows you to visualize these naturally convective currents as they form in water. example of convection heat transfer
A glass pipe bent into a square or rectangular shape and a colorant that makes updrafts visible is required. This is usually potassium permanganate, which stains water purple or drops of some kind of ink. example of convection heat transfer
Now one of the lower corners of the pipe is heated and the density of the portion of water just above the flame decreases and rises, being replaced by a portion of cooler water.
This process of continuous exchange between hot and cold water generates a convection current in an anticlockwise direction, which is observed thanks to the violet dye, as shown in the figure above. example of convection heat transfer
The fluid can also be forced to circulate to transmit heat, rather than allowing convection currents to occur naturally due to the difference in densities.
When convection occurs thanks to external media that propels the fluid, such as a fan or a pump, it is forced convection. The fluid can be forced to flow through a pipe, as in the central heating systems of houses, the radiator of a car or in a more open space, thanks to a fan blade. example of convection heat transfer
Examples of heat transfer by convection
Central heating systems example of convection heat transfer
The central heating system in a home makes use of convective heat transfer in water.
For this, hot water must be circulated through pipes under the floor, from a central boiler. In this way the water transfers heat to the radiators or heaters and from these the heat passes to the rooms, while the cold water returns again to the heating boiler to repeat the cycle.
As can be seen, both natural and forced convection are present in the central heating mechanism.
Radiators, stoves and fireplaces
Heat sources such as radiators heat the air around them and it rises, while the air at the top descends, generating convective air currents in the heated room. example of convection heat transfer
Cook: boil and fry example of convection heat transfer
Every time food is boiled in water or dipped in oil for frying, it is cooked by heat transferred by convection.
In pasteurization, milk and other liquid foods are heated to high temperatures for certain periods of time, depending on the pasteurization variant used. This is done in order to eliminate bacteria and increase the durability of the product.
Convection is the main heat transfer mechanism in these cases, although other mechanisms, such as conduction, are not excluded. example of convection heat transfer
The winds example of convection heat transfer
Convection currents in the atmosphere cause the winds. These currents are formed due to many factors, including the fact that the Earth’s surface heats unevenly. example of convection heat transfer
For example, during the day the beach warms up more than sea water, so the buoyancy causes the air above the beach to rise and the cooler air, coming from the sea, takes its place.
But at night the process happens in reverse, since the beach loses heat faster than the water and the warmer air is directed towards the sea. That is why in a night fire on the beach, the smoke moves towards the sea, while if the fire is made during the day, the smoke moves towards the land.
Earth’s magnetic field example of convection heat transfer
Earth is made up of layers, and the core has an outer, high-temperature layer that is not solidified. The movement of the planet creates convection currents in this fluid, which are believed to be responsible for the Earth’s magnetic field.
Magnetic fields are due to the presence of moving electrical charges. The ions and charged particles present in the outer core are capable of generating this field, since the planetary movements cause these particles to behave similarly to that of small currents (closed circuits) of current.
Scientists have found a correlation between the intensity of the magnetic field and the speed of the planet’s rotation. Venus’s weak magnetic field is believed to be due to its rotational speed being slower than that of Jupiter , whose magnetic field is much stronger. example of convection heat transfer