Heat convection can occur in liquids and gases when the molecules move freely and independently. This occurs as a result of cubical expansion or
in density. This initiates movement by natural means with the warmer ﬂuid rising and the cooler ﬂuid dropping, due to the effects of gravity upon it. This type of movement is termed free convection and occurs, for example, over radiators and natural draught convectors. See Figure 3.1.