 # Heat Transfer By Convection

## What is Convection?

Convection is the process of heat transfer in which transfer of heat energy occurs by the mass movement of molecules of the fluids like gases and liquids. Gases and liquids are not a good conductor of heat under normal conditions, but they can easily transfer heat.

Heat transfer through convection occurs other through diffusion or advection or both. Convection does not take place in solids, because no movement of its constituent particles occurs. The diffusion of heat occurs in solids, and it is called thermal conduction.

The process in which heat transfer occurs between a surface and a liquid or a fluid that is in contact with the surface is called convection heat transfer. Convection plays a major role while transferring heat from one liquid to another liquid through a barrier. Heat transfer by convection either occurs due to thermal diffusion (motion of fluid molecule) or advection, in which heat is transferred by the bulk motion of heat currents in the fluid.

### Newton's Law of Cooling

Under normal conditions, the heat transfer due to convection is directly proportional to the temperature difference between the parts. Newton's law of cooling has expressed this phenomenon, and the law states that:

The heat transfer rate of a body due to convection is directly proportional to the temperature difference between the body and its surroundings. The temperature difference should be small, and the nature of the radiating surface remains the same. This is the convection heat transfer equation:

P = $\frac{{dq }}{dt}$ = hA(T - T0)

Where

P = $\frac{{dq }}{dt}$ is the rate of heat transfer

h is the coefficient of the convection heat transfer

A is the surface area of the body

T is the temperature of the object

T0 is the temperature of the fluid or liquid which is subjected to convection

Heat-transfer coefficient h depends on the following factors:

Density, Thermal conductivity, Viscosity, coefficient of convective heat transfer, and specific heat capacity

The constant of proportionality mentioned in the above formula is an important parameter and is called a coefficient of convective heat transfer 'h'. It is defined as:

The rate at which heat transfer occurs between a solid and a fluid part per unit surface area and for the unit temperature difference:

𝒉 = $\frac{{q}}{ΔT}$

Where

q is the local heat flux density [W.m-2]

H is the heat transfer coefficient [W.m-2.K]

ΔT is the temperature difference [K]

1. Give some examples of convection heat transfer

Some of the major convection heat transfer example is:

Heat transfer due to convection occurs both naturally and artificially. The most common natural phenomena of convection are sea breeze and land breeze.

Sea breeze: It occurs during the daytime—the heat radiations from the sun heat both the land and the sea surface. Due to the large heat capacity of water, the sea absorbs a significant portion of the incident heat radiation, but it gets heated up very slowly as compared to land. Due to this, the temperature of the land surface rises, further raising the temperature of the air above the lab surface. Warm air expands because it has less density, thereby creating low pressure above the land surface. Meanwhile, high pressure exists over the sea. This difference makes the air move from sea to land. This process is known as a sea breeze.

Land breeze: This occurs during night time. This process is completely opposite from the sea breeze. When the sun sets, the land surface and the sea surface starts cooling down. The land surface cools quickly, whereas the sea surface doesn't lose its heat much easier. This increases the temperature of the air above the sea surface, thereby creating low pressure. At the same time, high air pressure is present above the land surface, and hence air moves from the land surface towards the sea. This phenomenon is called a land breeze.

2. What are the types of convection?

Ans Types of Convection

The two types of convection are:

● Natural convection

● Forced convection

Natural convection: When convection occurs due to the difference in the densities of air caused by a temperature difference, it's called natural convection.

The oceanic wind is an example of natural convection.

Forced convection: The convection occurs when some external source like fans and pumps are used to create a temperature difference to conduct heat transfer for the convection. It's called forced convection.

Water heaters, fans, and geysers are the major applications of the forced convection. In these appliances, heat transfer occurs due to the difference in temperature and air pressure.

3. What is the method of convection heat transfer?

Convection is a type of heat transfer between a solid surface and a liquid or gas in motion. The convection of heat is done by the compound effects of fluid motion and conduction. If the fluid motion is faster, the process of heat convection will be better, because a better fluid motion enhances the transfer of particles between the solid surface and the fluid. In the absence of any fluid motion, heat transfer occurs through the process of conduction through the vibrations of the molecules.

Let us consider the cooling of a hot object by blowing air above its surface. Energy is initially transmitted to the layer of air adjacent to the object. After cooling the surface of the object, this energy is then gradually transferred throughout the surface through the process of convection. In this manner, conduction and convection, both work together to make the surface cool.

The process of convection, which involves the passage of a fluid over a surface through some external source, is called forced convection. Alternatively, when the fluid flow occurs due to forces induced by a variation in density and consequently, a difference in temperature, it is called natural convection.

The process of heat transfer involves a change in the fluid phase, is also considered to be convection because fluid motion occurs during this process. The rise of vapor bubbles while boiling water is due to this. The falling of liquid droplets during the condensation process is also an example of this type of heat convection.