The convection it is one of the three mechanisms in which heat is transferred from a warmer area to a colder one. 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 so that this mechanism can take place..
The faster the movement of the fluid in question, the faster the transfer of thermal energy between zones of different temperatures. This occurs continuously with atmospheric air masses: buoyancy ensures that the warmer and less dense ones rise while the colder and denser ones descend.
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 under.
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Convection can be natural or forced. In the first case the fluid moves by itself, as when opening the room door, while in the second it is forced by a fan or a pump, for example.
There can also be two variants: diffusion Y advection. In diffusion, the fluid molecules move more or less randomly and heat transfer is slow..
On the other hand, in advection, a good amount of fluid mass moves, 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.
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 temperature difference is small.
Let us call the heat transferred Q and the 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 TO of the object:
ΔT = Temperature on the surface of the object - Temperature away from the object
The constant of proportionality is called h, what is he convection heat transfer coefficient and is determined experimentally. Its units in the International System (SI) are W / mtwo. 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:
dQ / dt = hA ΔT
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 mtwo and the surface temperature of the skin is 29 ºC.
Fact: the convection heat transfer coefficient in this case is 6 W / mtwo. ºC
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 / mtwo. ºC x 1.6 mtwo x (29 ° C - 20 ° C) = 86.4 W.
It is common to warm your hands by bringing them close to a campfire or hot toaster, as the air surrounding the heat source in turn heats up and expands, rising because it is less dense. As it circulates, this warm air envelops and warms the hands.
On the coast, the sea is colder than the land, so the air above the land heats up and rises, while the coldest air arrives and settles in the space left by this other when ascending..
This is called convection cell And it's the reason why it feels 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 land.
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..
As the size of the water drops increase, there comes a time when the water precipitates in the form of solid or liquid rain, depending on the temperature.
When the water is placed in the kettle or saucepan, the layers closest to the bottom are heated first, since 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..
In this way all the layers circulate rapidly and the entire body of water heats up. This is a good example of advection.
Convection in air masses, together with the earth's rotational movement produces winds, since cold air moves and circulates underneath hot air, creating various currents called convection currents.
Water behaves in a similar way to how air does in the atmosphere. Warmer waters are almost always near the surface, while colder waters are deeper.
It occurs in the molten core of the planet's interior, where it combines with the Earth's rotational movement, generating electric currents that give rise to the Earth's magnetic field..
Stars like the Sun are huge spheres of gas. Convection is an efficient energy transport mechanism there, since the gaseous molecules have sufficient freedom to move between areas of the interior of the stars..
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.
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 car engine, for example.
They are made by combining sheets of insulating material and adding air bubbles inside.
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 on the ground or in the water..
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