Heat transfer is the transition of thermal energy, or simply heat, from a hotter object to a cooler object. There are three main modes of heat transfer.
Convection is usually the dominant form of heat transfer in liquids and gases. Convection comprises the combined effects of conduction and fluid flow. In convection, enthalpy transfer occurs by the movement of hot or cold portions of the fluid/gas together with heat transfer by conduction.
Radiation is the only form of heat transfer that can occur in the absence of any form of medium (i.e., in a vacuum). Thermal radiation is based on the emission of electromagnetic radiation, which carries energy away from the surface. At the same time, the surface is constantly bombarded by radiation from the surroundings, resulting in the transfer of energy to the surface.
Q = -λ A ; q = -λ T2 -T1
Conduction is the most significant means of heat transfer in a solid. On a microscopic scale, conduction occurs as hot, rapidly moving or vibrating atoms
and molecules interact with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring atoms. The free movement of electrons also contributes to conductive heat transfer. To quantify the ease with which a particular medium conducts, the thermal conductivity, also known as the conduction coefficient, λ, has been employed. The thermal conductivity λ is defined as the quantity of heat, Q, transmitted in time (t) through a thickness (x), in a direction normal to a surface of area (A), due to a temperature difference (∆T).
Our previous article Common Types of Insulation Materials we encourage you to read our article.