In natural or free convection heat transfer, fluid flow takes place when there is a difference in temperature between the body and the fluid. The difference in density between the cold and hot fluids in the region causes the hot fluid to move in the upward direction. The force causing this flow is known as buoyancy or the buoyant force. Natural convection heat transfer coefficients are usually very low. The mechanism of natural convection heat transfer is applied in many fields of engineering for cooling purposes. Heat transfer by free convection from electric transformers, rectifiers, pipes carrying refrigerants and steam, walls of furnaces and walls of air-conditioned houses are some examples of this phenomenon.