Convection

The matter can transfer heat. Heat transfer always takes place from a high-temperature body to a low-temperature body. In formal ways, heat transfer can be defined as the movement of heat across the system due to a temperature difference between the system and surroundings. Heat is sometimes also referred to as flux. Heat transfer can occur by various means like :

• conduction
• convection
• radiation

Conduction is defined as a process that involves the transfer or glow of heat from objects having high temperatures to objects having low temperatures.

Radiation can be defined as a process where heat transfer occurs without the involvement of a medium.

Convection is defined as the movement of fluid molecules from high temperature to low temperature.

The convection takes place by advection or diffusion or maybe both. It does not take place in most of the solids because neither proper diffusion nor bulk current flows can take place. Diffusion of heat can take place in rigid solids by a method known as thermal conduction. The process of heat transfer between a solid and fluid in contact is called convective heat transfer. When heat needs to be transferred through a barrier between two systems, convection is carried out on both sides. The convective heat transfer takes place by thermal diffusion ( where the random motion of fluid molecules occurs) or by advection (where matter or heat is transferred by large scale motion in current).

Define convection

Convection can be defined as the heat transfer by the bulk movement of molecules within liquids and gases. It may be natural or forced.

• Forced or assisted convection
• natural or free convection

Forced convection:

Forced convection occurs when a flow of fluid is induced by an external force such as a pump or fan. Such examples are the use of the heater, geysers for instant heating of water or the use of a fan on hot days in summers.

The forced convection is related to Newton’s law of cooling, which is given by:

P = dQ/dt = hA(T−T0)

where P = dQ/ dt is the heat transfer rate 

h = convection heat transfer coefficient

A = exposed surface area

T = Temperature of an immersed object

T0 = temperature of fluid under convection

The value of the convection heat transfer coefficient depends upon 

• viscosity
• density
• specific heat capacity
• thermal conductivity

Natural or free convection:

Free convection is a result of the difference in buoyant force due to different temperature variations that occur in the fluid. While heating, the density change in the border layer will force the fluid to rise and be replaced by cooler fluid that will also heat and rise. This phenomenon is known as natural convection.

Convective heat transfer coefficient:

The constant of proportionality is known as the convective heat transfer coefficient, which is represented by ‘h’. Formally it can be defined as

The rate of heat transfer between a solid surface and fluid per unit surface area per unit temperature difference.

h = q / ΔT

where q = local heat flux density

 h = heat transfer coefficient

ΔT = temperature difference

The convective heat transfer coefficient depends upon the physical properties of the fluid and the physical conditions.

It is not a property of the fluid. Rather it is an experimentally derived parameter whose value depends upon surface geometry, nature of the fluid, property of fluid etc.

The convective heat transfer coefficient of laminar airflow is less than the convective heat transfer coefficient of turbulent flow. This is because the turbulent flow has a thinner stagnant fluid layer film on its heat transfer surface. This stagnant fluid layer plays an important role in convective heat transfer. A fluid completely stops and assumes a zero relative velocity; this is known as the no-slip condition, and at the surface, the energy transfer occurs due to conduction. The same phenomenon is observed in the case of temperature. The temperature of the fluid at the surface and the surface will have the same temperature at the point of contact. This is known as the temperature jump condition and plays a crucial role in the nucleate boiling theory.

Convection equation 

When there is a rise in the temperature of a liquid, its volume also increases by the same factor. This is referred to as displacement. The rate of convection can be calculated by the equation:

Q = hc. A . ( Ts – Tf )

where Q = heat transferred per unit time

 hc = coefficient of convective heat transfer 

A = area of heat transfer

Ts = surface temperature

Tf = fluid temperature

Convection examples

There are several examples of convection observed in our daily life. These are;

• Boiling water: when the water starts boiling, the heat is transferred from the burner to the pot leading to heat water at the bottom. This hot water rises above, and as a result, the cold water moves down, causing a circular motion.
• Melting ice: The melting of ice is a result of the movement of hot air into ice.
• Steam coming out from a hot cup of coffee: The steam that comes out from a hot cup of coffee is an indication of heat rising in the air. 
• Frozen food: it is observable that the frozen food defrosts quickly when placed under running water as compared to when just simply placed in water. This is because the running water quickly transfers the heat into frozen food leading to defrosting it.
• Air conditioners: The Air conditioner works on a similar principle. The warm heat inside the room is removed, and cooler air is drawn in. This is an example of forced convection.
• Hot air balloon: The heater inside a hot air balloon heats the air forcing the sir to move upwards. This results in the balloon rising because the hot air gets trapped inside.
• The chimney effect: is the movement of air inside and outside of the buildings due to buoyancy (different intensity between the air inside and the air outside). The buoyant force increases to greater heights.
•  Oceanic circulation: The convection causes the oceans to circulate continuously. The warm water at the equator moves towards the pole, and the cool water towards the pole moves to the equator.
• Squall lines: It is a type of convective thunderstorm. It is accompanied by high winds and blowing rains and can form lines of thunderstorms.

Conclusion:

Convection is defined as the heat transfer by a macroscopic mass movement. It can be natural or can be forced. It generally transfers heat at a faster rate than conduction. Convection is driven by the flow of large scales of matter. However, convection is somehow more complicated than conduction. In context to geology, convection can be defined as the slow movement of material beneath the earth’s crust. In context to meteorology, convection is defined as a transfer of heat and other atmospheric properties in an upward direction.