What happens when you leave a bowl of hot soup on a table? It gets cold after some time due to the loss of heat to the surrounding air. And that’s (transfer of heat) what we call convection.
There are three types of heat transfer: radiation, conduction, and convection. In radiation, heat is transferred in the form of electromagnetic waves. In conduction, heat travels between bodies that are physically connected.
Convection, however, is the most complex form of heat transfer because it involves the bulk movement of molecules within fluids such as liquids and gases. In simple terms, convection can only happen in liquids and gases.
Convective heat and mass transfer (in fluids) take place via two mechanisms:
- Diffusion: the net movement of molecules from a region of higher concentration to a region of lower concentration.
- Advection: transport of matter or heat by the bulk motion of currents in the fluid.
Neither of the processes takes place in rigid solids. However, convective heat transfer may occur in soft solids or mixtures where solid particles can travel from one position to another.
Convective heat transfer can be divided into two categories: natural and forced.
In free or natural convection, currents are produced only by temperature-derived density differences in the fluid. Whereas, in forced convection, currents are due to an external factor such as a fan or pump. The faster the fluid moves, the quicker the rate of convection.
Now that you have a general idea of what convection actually is let’s move to the various examples and applications. Below we’ve listed plenty of examples of convective heat transfer that you see in your everyday life.
Table of Contents
14. Boiling Water
Type: Forced Convection
When you put the kettle on the stove, it heats the water from the bottom. The molecules near the bottom gain kinetic energy and become less dense. Since the hot water at the bottom is less dense than the cold water above it, the hot water starts rising to the surface while the cold water sinks.
The cold water at the bottom then gets hot and less dense than the water above it, so it rises to the surface. The process repeats again and again, and it all happens due to the temperature difference between the bottom and top of the kettle. The movements of water molecules are convection currents.
13. Heat Sink
Type: Forced Convection
The heat sink is a passive heat exchanger that improves the rate of convective heat transfer. It is widely used in electronic equipment. For example, a conventional CPU/GPU uses a heat sink in conjunction with a fan to keep the operating temperature within tolerable limits.
Generally, a heat sink consists of a base and fins that extend outwards. Its performance is affected by various factors such as air velocity, fin design, and material used to build the heat sink.
12. Land and Sea Breeze
Image credit: Freepik
Type: Natural Convection
The formation of land and sea breeze is one of the classic examples of convection. The Sun heats up both land and sea surface during the day. But since the land has less heat-absorbing capacity than the sea, its surface temperature increases, heating up the air around it. The warm (less dense) air begins to rise, and low pressure is created.
At the same time, a high-pressure area develops (with cold, dense air) over the top of the sea. Due to the difference in pressure, air flows from the sea to the land, and the cold air that comes along is called a sea breeze.
The process gets reversed during the night. The land cools down quicker than the sea, decreasing the air temperature around it and creating a high-pressure situation. Now the cold air flows from the land to the sea, and this is what we call the land breeze.
In both cases, the heat transfers through air molecules.
11. Chimney Effect
Type: Forced Convection
When air flows in and out of the buildings, flue gas stacks, or other similar structures, due to buoyancy, it is called the chimney effect. It happens due to indoor and outdoor temperature/moisture differences. The high-density cold air always pushes the low-density hot gases up.
The taller the structure and higher the thermal difference, the more the buoyancy force and thus the chimney effect. Many skyscrapers and cooling towers utilize the same principle to achieve natural ventilation and infiltration.
10. Convection Oven
An industrial convection oven used in the airplane manufacturing industry
Type: Forced Convection
Convection ovens use convection mechanism to cook food faster than conventional ovens. They have fans to circulate hot air around the food, which enables food to cook more evenly at lower temperatures and in less time. Industrial convection ovens are used for manufacturing many products, including non-food items.
9. Melting Of Ice
Type: Natural Convection
Convection plays a major role in the process of reducing ice thickness. As warm air blows over the surface of the ice, it increases the temperature of the ice’s outer layer. The hotter the air and the faster it blows, the less time ice will take to melt.
8. Radiator
Type: Forced Convection
A Radiator transfers thermal energy from one medium to another. Despite the name, most radiators use convection (instead of thermal radiation) to transfer the bulk of their heat. They are mostly used in buildings, automobiles, and electronics.
In a room heating system, for example, hot water or sometimes steam is generated in the internal coils. As the water heats the coil, the air next to the radiator warms and raises. Once the heated air rises, cool air can be drawn into and through the radiator from underneath. This airflow generates vertical currents that distribute warm air throughout the room.
7. Refrigerator
Type: Forced Convection
A refrigerator contains a thermally insulated compartment and a heat pump that moves heat from the inside of the fridge to its external environment. It uses convection to circulate cold air around the food.
The freezer compartment cools air at the top. As air sinks, it is replaced by warmer air rising from below. This circulating air carries away heat from all items in the fridge.
6. Cumulus and Cumulonimbus Clouds
Cumulus clouds
Type: Natural Convection
Cumulus and Cumulonimbus are two different types of clouds that form and grow via convection. They are formed from water vapor carried by powerful upward air currents.
Since convection clouds tend to form quickly in the rising columns of air, they are optically dense. The surfaces of tiny droplets in these clouds scatter the sunlight more than the clouds containing fewer or larger droplets. This is why these clouds often look dark grey on the sides away from the Sun and bright white on the sides facing it.
Read: 10 Basic Types of Clouds According To Their Altitude Levels
5. Blood Circulation
Type: Forced Convection
Humans and other mammals employ convection to regulate body temperature. The heart pumps blood throughout the body at an average rate of 5 liters per minute, and the heat produced by the body cells is transferred to air (or water) flowing over the skin.
If skin temperature is lower than that of the surrounding air temperature, the body gains heat by convection and conduction. But, if skin temperature is higher, the body loses heat by convection and conduction. In surface tissues, where the rate of blood circulation is higher, heat transfer is mainly convective.
4. Marangoni Effect
Experimental demonstration of Marangoni Effect | Wikimedia
Type: Natural Convection
The Marangoni effect is the convection of fluids due to a gradient of the surface tension. Surface tension depends on the temperature or composition of the compounds. In the former case, the effect is called thermo-capillary convection.
The presence of a gradient in surface tension naturally causes the liquid to flow away from regions of low surface tension. This happens because the liquid with a high surface tension pulls more strongly on nearby liquid than one with low surface tension.
3. The Motion of Earth’s Mantle
Whole-mantle convection | Wikimedia
Type: Natural Convection
The movement of Earth’s solid silicate mantle resulted from convection currents, which carry heat from the interior to the planet’s surface, is called Earth convection.
More specifically, mantle convection is driven by three fundamental processes:
- Heat loss from the core (20%)
- Internal heating from radioactive decay (80%)
- Cooling from above (sinking of lithospheric slabs)
It causes the movements of lithospheric plates, surface volcanic activity, magmatism, earthquakes, as well as most of the tectonic and geological processes manifested in the crust. As the rate of heat production reduces, the planet cools, and ultimately convection slows or halts altogether.
Read: 4 Different Layers Of The Earth | Explained
2. Stars Have Convection Zone
Type: Natural Convection
Various turbulent activities, occurring inside the star, cause energy to move outward. When gases heat up (due to energy flux from deeper inside the star) to temperatures that are higher than the surrounding gas, they rise, expand, and cool. Once they thermalize, they stop rising.
Generally, low mass stars have radiative cores and convective envelopes, whereas high mass stars have convective cores and radiative envelopes. The Sun’s convection zone, for example, is the outermost layer of the interior, which extends from a depth of 200,000 kilometers up to the visible surface. This means the energy is transferred through convection in this region.
1. Accretion Disks Of Black Holes
Type: Natural Convection
This is quite an unusual example, but research and simulations show that convection of dust and gas occurs in the black holes‘ accretion disks at speeds close to that of light. The connective motions are developed by the dissipation of intense gravitational and rotational energies in the accretion flows.
