Convection occurs when heat is transferred through a gas or liquid by the hotter material moving into a cooler area. Discover what convection really is and review several examples of this phenomenon.
What is Convection heat transfer?
Convection is the process of heat transfer by the bulk movement of molecules within fluids such as gases and liquids. The initial heat transfer between the object and the fluid takes place through conduction, but the bulk heat transfer happens due to the motion of the fluid.
- Convection is the process of thermal energy exchange in fluids via the motion of matter within them.A bulk transfer of molecules within the fluid occurs.
- It occurs in both gases and liquids and leads to a cyclical effect.
- Both natural and forced convective heat transfer exist.
- Modes of heat transfer: conduction, convection, and radiation
Consider a liquid warmed over a heated surface. As the lower levels of the fluid increase in temperature, they become less dense. The heated layer then rises due to buoyancy and is replaced by colder fluid. The new lower layer then becomes heated, rising and replacing the cooler, denser upper layer. This causes convection’s cyclical effect.
How is Heat Transferred through Convection?
When a fluid is heated from below, thermal expansion takes place. The lower layers of the fluid, which are hotter, become less dense. We know that colder fluid is denser. Due to buoyancy, the less dense, hotter part of the fluid rises up.
And the colder, denser fluid replaces it. This process is repeated when this part also gets heated and rises up to be replaced by the colder upper layer. This is how the heat is transferred through convection.
Types of Convection heat transfer
There are two types of convection: natural and forced.
#1. Natural convection
This is caused by buoyant forces due to a difference in density connected to temperature differentials. When the fluid comes in contact with a hot surface or atmosphere, the molecules are separated and scattered, reducing the fluid’s density.
Natural convection examples:
- Hot air rising above a fire.
- Ice melting.
- Sea breeze or land breeze caused by a difference in pressure.
- Blood circulation in warm-blooded animals.
#2. Forced convection.
This is when the fluid undergoes forced flow from an outside source such as a water heater, pump, or fan.
Forced convection examples:
- Air conditioning.
- Car radiators.
- Water heaters/boilers.
- Convection ovens.
Examples of convection in Real-Life
As we already discussed there are two types of convection now let’s see what are the example of convection. In this section, we divided all the examples into two sections according to their types.
#1. Boiling Water.
This is the most common and relatable household example of convection.
For instance, when water is heated in a kettle, thermal expansion takes place. The lower layer of the water which is hotter becomes less dense and moves upward due to buoyancy.
The cooler/upper layer of the waterfalls below and gets heated as well. The process is repeated until the entire water in the kettle is heated up.
#2. Air-Conditioner.
On a hot summer day, air-conditioners are used constantly. The process of cooling air in air-conditioners employs the very principle of convection.
The cold air is released by the air-conditioners. Now, this cold air is denser than the warm air, and, hence, it sinks. The warm air, being less dense, rises and is drawn in by the air-conditioner. As a result, a convection current is set up and the room is cooled.
#3. Radiator.
A motor radiator is a mechanical device that cools the hot liquid circulating through the block of an internal combustion engine.
In automobiles, this liquid is called coolant. Hot coolant finds its way to the radiator through radiator tubes. Cool air from the environment also passes through the fins of the radiator as you drive.
The cool air passing through the radiator fins helps cool the hot coolant. The temperature of the coolant drops gradually after which it runs through the engine again to pick up more heat and travels back to the radiator.
The heat is transferred from the hot coolant to the cool air through convection and the process continues so the nearby heated air is transmitted to the environment through radiation.
So, we can identify two modes of heat transfer around a radiator, convection and radiation. Convection transfers the heat from the coolant to the surrounding air through the radiator fins,
while radiation transmits the heated surrounding air to the environment. This brings us to another important question.
Since this mechanical device involves two modes of heat transfer (convection and radiation), why are they called car radiators instead of car convectors?
Well, it is called radiators instead of convectors, because of the fins surrounding the mechanical device. Mechanical fins are extended surfaces that enhance radiation heat transfer, simply by increasing the surface area of the device. Heat lost through car radiators is a forced type of convection.
#4. Refrigerator.
The working principle employed by refrigerators is very similar to that of air-conditioners. The freezer, in the case of refrigerators, is placed at the top.
As mentioned above, the warm air, being less dense, rises and, hence, is cooled down by the freezer. Now, this cool air, being denser, sinks and, therefore, keeps the lower part of the fridge cool.
#5. Hot Air Popper.
The hot air popper which is used to make popcorn also utilises the principle of convection. The hot air popper has a fan, vent, and a heating element. When the popper is turned on, the fan is employed to blow air on the heating element through the vent.
The heating element, in turn, warms the air; which then rises. Above the heating element, the popcorn kernels are placed. The kernels are heated when the hot air rises; therefore, resulting in the popping of the kernels.
#6. Hot Beverage.
The simplest example of convection is a steaming beverage. You may have observed steam coming out of a cup of hot tea or coffee. Due to the heat of the fluid, the warm air rises up. This warm air is the steam.
#7. Hot Air Balloon.
A heater inside a hot air balloon heats the air, causing the air to move upward. This causes the balloon to rise because the hot air gets trapped inside. When the pilot wants to descend, the individual will release some of the hot air. Cool air takes its place, causing the balloon to lower.
#8. Air-Cooled Engines.
Engines in vehicles like cars are cooled by water jackets. Prolonged running of the engines leads to the warming of the water in the water jacket/water pipes that surround the engine. To keep the engine running, the water must be cooled down.
When the water heats up, it starts to flow in the pipes which are present around the engine. When the warm water flows through these pipes, it is cooled down by the fans.
These fans are also present in the pipes. Once the water cools down, it flows back into the engine; hence, obeying the very principle of convection and cooling the engine down.
#9. Convection Oven.
Who doesn’t like cakes and cookies? But do you know most of the ovens utilize the principle of convection? In the case of convection ovens, forced convection is employed. With heating, the molecules present in the air also get heated up and start moving. The food inside the oven is cooked due to this warm air.
#10. Blood Circulation in Warm-Blooded Mammals.
You might be surprised to know that warm-blooded animals employ convection to regulate the temperatures of the body. The human heart is a pump and blood circulation in the human body is an example of forced convection. The heat which is generated by the cells in the body is transferred to air or water which is flowing over the skin.
#11. Land and Sea Breeze.
This type of convection occurs naturally, it’s either called natural convection or free convection.
In the daytime, the land surface is heated up making it warmer than a nearby sea. The heated air molecule moves up and it’s replaced by cooler air from the sea resulting in what we call “Sea Breeze.”
This repetitive circular motion creates a convective current. At night, the reverse is the case. The air above the sea is warmer compared to that of the land.
As a result of convection, the warm air from the sea rises and is replaced by the cooler air from the land resulting in what we term also as “Land Breeze.”
Now the question is Why is land and sea breeze called natural convection?
Land and sea breeze is called natural convection because convective current moves air molecules across the land and sea intermittently depending on their respective temperature difference. This convective process is a natural process completely devoid of any form of human intervention.
#12. Rainfall & Thunderstorms.
Warm water from the oceans rises in the air and turns into saturated water drops that form clouds. When this process continues, the smaller clouds collide with each other, and bigger clouds are formed. Upon reaching the final growth stage, cumulonimbus clouds or thunderstorms are formed.
#13. Melting Of Ice.
The melting of ice is yet another example of convection. The temperature of the surface or boundary of ice increases as warm air blows over the surface; or water, which is at a higher temperature as compared to the ice, flows underneath it.
As the temperature of the surface or boundary of ice alters, the ice melts. In a similar fashion, a frozen material thaws when kept in water.
#14. Convective Clouds.
When the air has a lot of moisture, the convection currents will carry that moisture up into the sky to form convective clouds. When droplets build up in the clouds sufficiently, the result will be precipitation in the form of a convective thunderstorm.
#15. Squall Lines.
A squall line is a type of convective thunderstorm. This type of convective event produces a line of thunderstorms accompanied by high wind and blowing rain.
#16. Supercell.
A supercell is a more severe form of a convective thunderstorm. This type of storm typically lasts for an extended period (an hour or longer) and has a high chance of forming dangerous tornadoes.
#17. Mantle Convection.
The rocky mantle of the Earth moves slowly because of the convection currents that transfer heat from the interior of the Earth up to the surface. This is the reason the tectonic plates move gradually around the Earth.
Hot material is added at the growing edges of a plate and then it cools. At the consumption edges, the material becomes dense by contracting from the heat and sinks into the Earth at an ocean trench. This triggers the formation of volcanoes.
#18. Gravitational Convection.
Because freshwater is buoyant in salt water, dry salt diffuses downward into wet soil. This is an example of gravitational convection.
#19. Oceanic Circulation.
Convection causes the oceans to continually circulate globally. Warm water around the equator circulates towards the poles and the cooler water at the poles moves towards the equator.
These different examples of convection show how convection occurs throughout many human-made and natural occurrences. Now that you are familiar with examples of convection, consider expanding your knowledge of related scientific phenomena.