## What is Thermal Convection mode of Heat Transfer?

Thermal convection is the process of heat transfer in fluids by which heat flows from the region of high temperature to the region of lower temperature by the actual movement of the material particles.

• All fluids either liquids or gases are heated by the convection process.
• In convection process, buoyancy and gravity play an important role.

### Convection Currents

Consider about a liquid being heated in a flask. A bit of K Mn O_4 salt is added at the bottom of the flask for clear understanding and judging of the movement of liquid particles.

The liquid in the flask near the bottom surface will receive heat first. It get warmer and less dense and thus become lighter. This lighter liquid rises in upward direction due to buoyancy. The liquid from colder regions on top surface being dense and heavier flows downward to occupy the vacant space created beneath. In this way a current of movement of liquid particles is set up in the liquid mass as shown in figure. This is called convection current.

These convection currents continue till there is any temperature difference within the liquid mass. When, the temperature of whole liquid body attains the same temperature, convection currents stops.

Thus in convection, molecules of liquid mass are the carrier of heat energy to flow in the liquid body from one region to another.

Convection process is of two types –

1. Natural convection – In this mode of heat transfer, currents are set up by natural phenomenon of atmospheric temperature and pressure.
2. Forced convection – In this mode of heat transfer, currents are set up by external energy source such as heat pump or fan, blower etc.

### Natural convection

If the material particles move due to the difference in density, the process of heat transfer is called natural or free convection.

It is a natural phenomenon which arises due to unequal heating of liquid body. When a liquid is heated in a flask, the convection currents set up in the liquid mass is natural convection currents as shown in figure.

In natural convection, the more heated and less dense parts of the fluid rises and replaced by cooler parts. Natural convection is responsible for different types of winds in the atmosphere.

Examples of Natural Convection –

1. Land and sea breezes.
3. Monsoon etc.

### Forced convection

If the heated material particles are forced to move by an agency like pump or blower, the process of heat transfer is called forced convection.

The fluid body is made to circulate by external source of energy.

Examples of Forced Convection –

1. Air conditioning.
2. Central heating system.
3. Brisk stirring etc.

The process of the transfer of heat from one place to another place without heating the intervening medium is called radiation.

Properties of thermal radiations are as follows –

1. The electromagnetic waves emitted by a body by virtue of its temperature is called thermal radiation or radiant energy. All bodies having temperature above ( 0 \degree K )  emit thermal radiation continuously.
2. These are electromagnetic waves having wave length ranging from ( 1 \mu m ) to ( 100 \mu m ) . These are also called infrared waves.
3. Medium is not required for the propagation of these radiations.
4. Like light waves, thermal radiations travel in straight line. These radiations obey the laws of reflection and refraction like light waves.
5. Thermal radiation also shows the phenomena of interference, diffraction and polarization.
6. Spectrum of this radiation can not be obtained with the help of glass prism because it absorbs heat radiation. Spectrum is obtained by quartz or rock salt prism because these materials do not absorb heat radiations.
7. Thermal radiation produces heat when they are absorbed by a body.

### Effects of boundary on Radiation

When thermal radiations fall on a surface of separation, they partly reflected, partly absorbed and partly transmitted.

Let, ( Q ) be the amount of radiant energy incident on a body. Suppose, the part ( R )  is reflected ( A ) is absorbed and ( T )  is transmitted. Then –

( R + A + T ) = Q

Or, \quad \left [ \left ( \frac {R}{Q} \right ) + \left ( \frac {A}{Q} \right ) + \left ( \frac {T}{Q} \right ) \right ] = 1

Or, \quad ( r + a + t ) = 1

Therefore, \quad \text {Reflectance} + \text {Absorptance} + \text {Transmittance} = 1

### Reflectance or Reflective Power

Reflective Power of a radiant body is defined as the ratio of the amount of thermal energy reflected by a body in a certain time to the total amount of thermal energy incident upon the body in the same time.

If, ( Q ) be the amount of total radiant energy incident on a body ( R )  is the reflected part of that energy, then –

Reflectance or reflective power  \quad r = \left ( \frac {R}{Q} \right )

### Absorptance or Absorptive Power

Absorptive Power is defined as the ratio of the amount of thermal energy absorbed by a body in a certain time to the total amount of thermal energy incident upon the body in the same time.

If ( Q ) be the amount of total radiant energy incident on a body ( A )  is the absorbed part of that energy, then –

Absorptance or absorptive power \quad a = \left ( \frac {A}{Q} \right )

### Transmittance or Transmittive Power

Transmittive Power is defined as the ratio of the amount of thermal energy transmitted by a body in a certain time to the total amount of thermal energy incident upon the body in the same time.

If ( Q ) be the amount of total radiant energy incident on a body ( T )  is the transmitted part of that energy, then –

Transmittance or transmittive power \quad t = \left ( \frac {T}{Q} \right )