What do you understand by free convection boundary layer and how does it affect heat transfer ?

When a fluid flows across a solid surface, a disturbed flow region extends out from the solid surface/liquid interface, called the boundary layer. Characteristics of fluid flow in boundary layer adjacent to the solid surface have a domnant effect upon the conavective heat transfer. The motion of the fluid in free convective boundary layer is caused by buoyance effects resulting from fluid density variations associated with the boundary layer temperature gradients.