The mode of heat transfer which is governed by Fourier’s law. The Fourier’s law states that the heat flux or transfer of energy from higher temperature to lower temperature is directly proportional to temperature gradient. The proportionality constant is called thermal conductivity and is property of a material through which heat is being transferred. It is denoted by letter ‘k’.

Convection is the transfer of energy governed by Newton’s law of cooling. This law states that heat flux is directly proportional to temperature difference. The proportionality constant is called the heat transfer coefficient. It is denoted by letter ‘h’. In natural convection the heat transfer takes place due to density differences and the heat transfer coefficient is a function of material properties and geometry under consideration.

Radiation mode of heat transfer is due to propagation of electromagnetic waves. Radiation is governed by Stefan-Boltzman law which states that the heat flux is directly proportional to fourth power of temperature. The proportionality constant is called the Stefan Boltzman constant and is denoted by the letter ‘σ’.

This mode of heat transfer involves phase change or formation of vapor bubbles when heat transfer takes place between metal surface and the fluid. This happens when the fluid is at its saturated temperature and extra heat is supplied to the fluid. The boiling process can be demonstrated by different regimes namely natural convection boiling, nucleate boiling, film boiling. The entire process can be represented by a graph of heat flux vs temperature difference and is called the boiling curve.