As a fluid is heated, it expands while remaining the same mass. Therefore, its density changes as it is heated.
Density determines the pull that gravity has on a substance.
This varying gravity force pulls the fluid apart and this is called free convection. In this section we will be examining the free convection involved in the flow of a fluid between two plates of different temperatures. Because this fluid is heated differently depending on where it is between the plates, its density varies and gravity pulls at the fluid differently depending on its orientation between the plates. The fluid near the hot plate rises and the fluid near the cooled plate descends as can be seen in the velocity profile of the following figure. There should be an equal volume on liquid flowing in each direction even though the velocity profile may seem slightly lop sided.