Heat transfer is the energy transfer caused by the temperature differences.
The three modes are –
- Conduction
- Convection
- Radiation
Conduction

In this, the material medium is required and heat transfer occurs due to solid bodies and still fluids, without bulk motion of the medium.
Convection

In this, the material medium is required and heat transfer occurs due to bulk motion of the medium. There are two types of convections namely –
- Natural convection
- Forced convection (external effort is made)
Radiation

This is basically an electromagnetic wave which is produced by any material at any temperature. It does not require any medium for flow.