Heat Transfer Convection - Definition, Examples, Types, FAQs

The process of heat transfer by convection involves the transportation of thermal energy within the fluids (liquids or gases) through molecular movement. It plays such an important part in our everyday life that we feel sensations, such as the warmth of a heater or the cool air from a ceiling fan, quite easily. Be it boiling water on a gas stove or movements of air in the atmosphere that creates climatic changes, convection is always in operation, making sure that energy is effectively spread around us.

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This Story also Contains

1. What is Convection Heat Transfer?
2. Heat Transfer By Convection
3. Solved Examples Based On Heat Transfer By Convection

What is Convection Heat Transfer?

Convection is the process of heat transfer through a fluid (liquid or gas) caused by the movement of the fluid itself. It occurs when warmer, less dense portions of the fluid rise, and cooler, denser portions sink, creating a circulation pattern. This transfer of heat can happen naturally due to temperature differences (natural convection) or be forced by external means, such as a fan or pump (forced convection).

Examples of Convection

1. A radiator is a device that emits warm air from the top and draws cooler air in from the bottom. A steaming cup of hot tea - When you drink a cup of hot tea, the steam you observe signals that heat is being transported into the air. Ice melts because heat from the air is transferred to the ice.
2. When water boils, the molecules that are denser sink to the bottom, and the molecules that are less dense rise, resulting in a circular motion of the molecules, which heats the water.
3. Warm water goes towards the poles as it approaches the equator, while cooler water moves towards the equator.
4. Warm-blooded animals use convection to circulate their blood, which helps to regulate their body temperature.

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Heat Transfer By Convection

Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids. Convection is of two types -

1. Natural Convection

The main cause of this is the difference of densities at two places and is a consequence of gravity because on account of gravity, the hot light particles rise up (density becomes less) and cold heavy particles (density becomes high) try setting down. It mostly occurs on heating a liquid/fluid.

2. Forced Convection

If a fluid is forced(by means of a fan or draught or any external means) to move to take up heat from a hot body then the convection process is called forced convection. In this case, Newton's law of cooling holds good. According to which rate of loss of heat from a hot body due to moving fluid is directly proportional to the surface area of body and excess temperature of body over its surroundings.

Application of Convection

1. Convection currents in the water pipes cool car engines. Water is an excellent conductor of heat away from the engine and into the radiator.
2. Convection currents are what cause land and sea winds.
3. Convection currents are rising air over land that glider pilots employ to maintain their gliders in the air.
4. Air conditioners are placed towards the room's ceiling to allow for the formation of convection currents. Cool, dry air is blown into the room by the air conditioner. Cool air sinks because it is denser. Warm air rises because of is less dense. The air circulated, and the temperature of the air fell to the ideal level over time.

Convection Examples

Sea breeze: This phenomenon occurs during the day. The sun heats up both the sea surface and land. As the sea has a greater heating capacity, it absorbs much of the sun's energy but gets warmed up much slower than the land. As a result, the temperature above the land rises and heats the air in the atmosphere above it. Warm air is less dense, and hence, it expands, creating a low-pressure area over the land near the coast. Meanwhile, there is relatively high pressure over the sea. The difference in air pressure causes the air to flow from sea to land. The sudden gush of wind felt due to this is known as the sea breeze.
Land Breeze: This phenomenon occurs during the night when the situation reverses. As the sun sets, the land and sea start cooling down. The land quickly loses heat when compared to water due to the differences in heat capacity. Consequently, the temperature of the sea is relatively higher, which creates low air pressure there. This sets up a flow of cool breeze offshore, known as the land breeze.

What Does the Term "Thermal Convection" Mean?

Heat convection is the physical movement of a fluid (liquid, gas, or plasma) from one area to another to transfer thermal energy. In liquids and gases, heat convection is frequently the primary mechanism of energy transmission. Convection is one of three primary ways of heat transport, along with conduction and radiation.

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Solved Examples Based On Heat Transfer By Convection

Example 1: It is hotter for the same distance over the top of fire than it is on the side of it, mainly because

1) Air conducts heat upwards

2) Heat is radiated upwards

3) Conversion takes more heat upward.

4) Conversion, conduction and radiation all contribute significantly to transferring heat upwards.

Solution:

Convection: Mode of transfer of heat through migration of material particles of the medium.

Convection significantly transfers heat upwards. The top of a fire is hotter than its sides because hot air rises, creating a column of intense heat directly above the flames, while the heat is more dispersed on the sides.

Hence, the answer is the option (2).

Example 2: The layer of the atmosphere is heated through

1) Convection

2) Conduction

3) Radiation

4) 2 & 3 both

Solution:

Natural Convection

This arises due to the difference in densities at the two places.

wherein

The heating of the atmosphere is mainly because of natural convection.

Hence, the answer is the option (1).

Example 3: Heating of a room using a blower is an example of

1) Conduction

2) Natural convection

3) Forced convection

4) Radiation

Solution:

Forced Convection

If a fluid is forced to move to take up heat from a hot body.

wherein

So, heat transfer with the help of a blower is an example of heat transfer through forced convection.

Hence, the answer is the option (3).

Example 4: The rate of heat transfer in forced convection is

1) proportional to $(\theta -{\theta }_0)$
2) proportional to ${(\theta -{\theta }_{\mathrm{o}})}^2$
3) proportional to ${(\theta -{\theta }_{\mathrm{o}})}^3$
4) proportional to ${(\theta -{\theta }_{\mathrm{o}})}^4$

Solution:

Forced Convection

$$Q_{t^{\ast }}(\theta -{\theta }_0)\Rightarrow Q_t=hA(\theta -{\theta }_0)$$
Where,
$\mathrm{h}=$ Constant of proportional
$\theta =$ Temperature of body
${\theta }_0=$ Temperature of surrounding

Hence, the answer is the option (1).