All matter is made up of molecules and atoms. These atoms are always in diï¬€erent types of motion (translation, rotational, vibrational). The motion of atoms and molecules creates heat or thermal energy. All matter has this thermal energy. The more motion the atoms or molecules have the more heat or thermal energy they will have. Heat transfer is the exchange of thermal energy between physical systems. The rate of heat transfer is dependent on the temperatures of the systems and the properties of the intervening medium through which the heat is transferred. The three fundamental modes of heat transfer are conduction, convection and radiation. Heat transfer, the ï¬‚ow of energy in the form of heat, is a process by which a system changes its internal energy, hence is of vital use in applications of the First Law of Thermodynamics. Conduction is also known as diï¬€usion, not to be confused with diï¬€usion related to the mixing of constituents of a ï¬‚uid. Heat energy transferred between a surface and a moving ï¬‚uid at diï¬€erent temperatures is known as convection. In reality this is a combination of diï¬€usion and bulk motion of molecules. Near the surface the ï¬‚uid velocity is low, and diï¬€usion dominates. Away from the surface, bulk motion increases the inï¬‚uence and dominates. Natural convection is caused by buoyancy forces due to density diï¬€erences caused by temperature variations in the ï¬‚uid. At heating the density change in the boundary layer will cause the ï¬‚uid to rise and be replaced by cooler ï¬‚uid that also will heat and rise. This continues phenomena is called free or natural convection. Conduction as heat transfer takes place if there is a temperature gradient in a solid or stationary ï¬‚uid medium. With conduction energy transfers from more energetic to less energetic molecules when neighboring molecules collide. Heat ï¬‚ows in direction of decreasing temperatures since higher temperatures are associated with higher molecular energy. This book emphasizes on the principles of convection and conduction heat transfer.