convection

Convective heat transfer often referred to simply as convection, is the transfer of heat from one place to another by the movement of fluids. Convection is usually the dominant form of heat transfer in liquids and gases.

Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. Cooler liquid or gas then takes the place of the warmer areas which have risen higher. This results in a continuous circulation pattern.

Convection is a heat transfer process. When currents are produced, the matter is moved from one location to another. So, this is also a mass transfer process.

Convection is the flow of heat through a bulk, macroscopic movement of matter from a hot region to a cool region, as opposed to the microscopic transfer of heat between atoms involved with conduction.

Suppose we consider heating a local region of the air. As this air heats, the molecules spread out, causing this region to become less dense than the surrounding, unheated air. Being less dense than the surrounding cooler air, the hot air will subsequently rise due to buoyant forces – this movement of hot air into a cooler region is then said to transfer heat by convection.

Water boiling in a pan is a good example of the transfer of heat by convection. When the stove is first turned on heat is transferred first by conduction between the elements through the bottom of the pot to the water. However, eventually, the water starts bubbling, these bubbles are actually local regions of hot water rising to the surface, thereby transferring heat from the hot water at the bottom to the cooler water at the top by convection. At the same time, the cooler, more dense water at the top will sink to the bottom, where it is subsequently heated.

Another good example of convection is in the atmosphere. The earth’s surface is warmed by the sun, the warm air rises and cool air moves in.

Convection that occurs naturally is called natural convection or free convection. If a fluid is circulated using a fan or a pump, it’s called forced convection. The cell formed by convection currents is called a convection cell or Benard cell.

Another important example of convection currents is the creation of breezes over land masses next to large bodies of water. Water has a larger heat capacity than land and subsequently holds heat better. It, therefore, takes longer to change its temperature, either upward or downward. Thus, during the day the air above the water will be cooler than that over the land. This creates a low-pressure area over the land, relative to the high-pressure area over the water, and subsequently one finds breezes blowing from the water to the land. On the other hand, during the night water cools off more slowly than the land, and the air above the water is slightly warmer than over the land. This creates a low-pressure area over the water relative to the high-pressure area over the land, and breezes will blow from the land to the water.

Types of heat convection

Heat convection is of three types – natural, forced, and mixed.  

• Forced convection - In one case, heat may be carried passively by fluid motion, which would occur even without the heating process a heat transfer process termed loosely as forced convection.
• Natural convection - In the other case, heating itself may cause the fluid to move via expansion and buoyancy forces, while simultaneously causing heat to be transported by this motion - a process loosely known as natural convection or free convection.
• Mixed (combined) convection is a combination of forced and free convection which is the general case of convection when a flow is determined simultaneously by both an outer forcing system (i.e., outer energy supply to the fluid-streamlined body system) and inner volumetric (mass) forces, viz., by the non-uniform density distribution of a fluid medium in a gravity field. 

Some everyday examples of natural convection

1. Boiling water – Boiling water in a bowl also operates on the convection principle. When the water starts getting heated, the water molecules expand and move in the pot. Thus, heat is transferred to other parts of the pot and the cold water starts sinking while the warm water rises.
2. A simple example of convection currents is warm air rising toward the ceiling or attic of a house. Warm air is less dense than cool air, so it rises.
3. The wind is an example of a convection current. Sunlight or reflected light radiates heat, setting up a temperature difference that causes the air to move. Shady or moist areas are cooler, or able to absorb heat, adding to the effect. Convection currents are part of what drives the global circulation of the Earth’s atmosphere.
4. Steaming cup of hot 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 steam.
5. Ice melting – Heat moves to the ice from the air. This causes the melting from a solid to a liquid.
6. Hot air balloon – A heater inside the balloon heats the air and so the air moves upward. This causes the balloon to rise because the hot air gets trapped inside. When the pilot wants to descend, he releases some of the hot air and cool air takes its place, causing the balloon to lower.
7. Frozen material thawing – Frozen food thaws more quickly under cold running water than if it is placed in water. The action of the running water transfers heat into the food faster.
8. Thunderstorm - Warm water from the oceans rises up 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.

Convection in meteorology and geology

1. 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.
2. Oceanic circulation – Warm water around the equator circulates towards the poles and the cooler water at the poles moves towards the equator.
3. The stack or chimney effect – This is the movement of air in and out of buildings, flues, or other objects because of buoyancy. In this case, buoyancy refers to the different densities in the air between the air inside and the air outside. The buoyancy force increases due to the greater height of the structure and a greater difference between the heat level of the inside and outside air.
4. Convection of a star - A star has a convection zone where energy is moved by convection. Outside the core is a radiation zone where plasma moves. A convection current forms when the plasma rises and the cooled plasma descends.
5. Gravitational convection - This shows when dry salt diffuses downward into wet soil because freshwater is buoyant in saltwater.
6. Convection currents are evident in the sun. The granules seen in the sun's photosphere are the tops of convection cells. In the case of the sun and other stars, the fluid is plasma rather than a liquid or gas.

Forced convection

This is where an external device such as a fan, pump, or suction device is used to facilitate convection.

Here are some examples of forced convection:

1. Radiator - In the radiator, the heating element is placed at the bottom of the machine. Thus, the warm air from this heating element is replaced by cold air.
2. Refrigerator – The freezer unit is placed at the top. The reason behind this is that the warm air inside the refrigerator will rise up but the colder air in the freezer region will sink and keep the lower portion of the refrigerator warm.
3. Air conditioner – The cooling unit in an air conditioner is placed at the top. Thus, warm air rises up to the cooling unit, it is replaced by cold air, and the room is cooled.
4. Hot air popper – It has a fan, a heating element, and a vent. When the popper is powered on, the fan blows air on the heating element via the vent. The air becomes warm and thus rises. Popcorn kernels are placed just above the heating element. The hot air rises and the popcorn kernels are heated. This is how we get our delicious popcorn.
5. Convection oven – In a convection oven, the principle of forced convection is used. The air in the compartment is forced to heat by using heating elements. Due to this heating, the air molecules expand and move. The food inside is cooked due to this warm air.
6. Air-cooled engine – Air-cooled engines are cooled by convection currents in their water pipes. The engine, on running for a long time, gets heated. The heat that is dissipated needs to be cooled so as to keep the engine running. The engine is covered by a water jacket that is heated. Due to this heating, warm water flows through the pipes encircling the engine. These pipes have fans due to which the warm water is cooled. This warm water, by the convection principle, sinks down, thus cooling the engine.