Convection traditionally refers to heat transfer associated with a fluid which is in motion. When the fluid motion is caused by an external agency, for example, a fan or a “wind,” the convection is termed as forced convection. In free convection, in contrast, fluid motion is predominantly due to buoyancy forces caused by density differences in the fluid; the density difference is generally due to variations of temperature or variations of concentration of species or both. © 2013 CRC Press/Taylor & Francis Group, LLC.