Have you ever paid attention to how the air around a steaming cup of Espresso feels warmer or how a soft breeze can make you feel cooler on a hot summer day? These everyday experiences we go through are examples of a natural phenomenon known as convection.
Convection influences how heat is transferred and how fluids like air and water move and circulate. From how warm air moves around in our homes to the circulation of ocean currents, convection is an intriguing process that shapes our surroundings and affects our comfort.
Meaning Of Convection
Convection is a process that facilitates the transfer of heat from one region to another by the movement of a fluid, such as water or air, due to temperature differences. This movement of particles in the fluid helps to transport heat energy from hotter regions to cooler ones.
The harder and denser fluid then takes its place. This process is repeated as the new part also gets heated and rises to be replaced by the colder upper layer. This is how heat is transferred through convection.
Types Of Convection
Convection is a natural phenomenon that can be observed and experienced by humans. It can be classified into two main categories, namely:
Natural convection, also called free convection, is a form of heat transfer that occurs when fluids such as gases and liquids are heated or cooled by non-human means.
The movement of the fluid is solely driven by variations in density caused by temperature gradients and not by any external force.
In other words, when convection is caused by the buoyancy force resulting from differences in temperature-induced densities.
E.g. sea, land and Mountain breeze.
Forced convection is a method of transferring heat in which an external source, such as a pump or fan, causes a fluid or gas to move over a solid surface.
This is distinct from natural convection, which occurs due to the density changes in a fluid caused by temperature differences. Forced convection is highly effective in transporting large amounts of heat energy.
E.g. Air conditioning systems, Automotive radiators, convection ovens and Aerospace applications.
Newton’s law of cooling is connected to forced convection which states that;
“The rate of heat exchange between an object and its surroundings is proportional to the difference in temperature between the object and its surroundings”.
And it is expressed as:
q = h * A * ΔT
- q represents the heat transfer rate through forced convection,
- h is the convective heat transfer coefficient,
- A is the surface area through which heat is transferred,
- ΔT denotes the temperature difference between the surface and the surrounding fluid.
Examples Of Convection
Sea And Land Breezes
Land and sea breezes are weather patterns commonly observed in coastal regions. A land breeze is characterized by the movement of wind from the land towards a nearby body of water, while a sea breeze involves wind blowing from the water onto the land.
These phenomena occur due to temperature variations between the land and water surfaces.
In daylight hours, the land heats up more rapidly than the sea, making the air above the land warmer and less dense. As a result, the heated air rises, allowing cooler and denser air from the sea to flow in and create a sea breeze.
Conversely, the land cools down more quickly during the night, causing the air above it to become cooler and denser. This prompts the wind to flow from the land towards the sea, forming a land breeze.
Atmospheric circulation pertains to the extensive motion of air across the Earth’s surface, primarily influenced by variations in pressure. It involves forming broad wind systems organized into east-west belts encircling the planet.
The sun’s unequal heating of the Earth’s atmosphere is responsible for this process, as the equator receives more solar energy than the poles.
Consequently, warm air ascends at the equator, while cooler air descends at the poles. These movements generate convection currents, propelling the global air circulation in the atmosphere.
Convection ovens use a fan to evenly and rapidly circulate hot air around the food during cooking, ensuring uniform heating. Similarly, cooking methods such as boiling, grilling, and frying involve convection.
When heat is applied to a pot of water on a stove, the lower portion becomes hotter and less dense. As a result, the heated water rises to the surface.
In contrast, cooler and denser water descends to the bottom, establishing convection currents facilitating heat transfer throughout the pot.