Convection Heat transfer by convection occurs in a fluid by the mixing of one portion of the fluid with another portion due to gross movements of the mass of fluid. The actual process of energy transfer from one fluid particle or molecule to another is still one of conduction, but the energy may be transported from one point in space to another by the displacement of the fluid itself. Convection can be further subdivided into free convection and forced convection. If the fluid is made to flow by an external agent such as a fan or pump, the process is called “forced convection”. If the fluid motion is caused by density differences which are created by the temperature differences existing in the fluid mass, the process is termed “free convection”, or “natural convection”. The motion of the water molecules in a pan heated on a stove is an example of a free convection process. The important heat transfer problems of condensing and boiling are also examples of convection, involving the additional complication of a latent heat exchange. It is virtually impossible to observe pure heat conduction in a fluid because as soon as a temperature difference is imposed on a fluid, natural convection currents will occur due to the resulting density differences