Free convection refers to the natural movement of fluid caused by density differences resulting from temperature variations. In free convection, the fluid motion is driven solely by buoyancy forces. On the other hand, forced convection involves the fluid motion being induced by an external force, such as a fan or a pump.
The depth of the boundary layer is influenced by the flow velocity near the surface. In the case of free convection, the flow velocity is relatively low compared to forced convection. As a result, the thermal boundary layer extends to a greater depth in free convection because the slower-moving fluid near the surface has more time to transfer heat through conduction.
In forced convection, the higher flow velocities near the surface enhance heat transfer through convection. The faster-moving fluid effectively carries heat away from the surface, reducing the thickness of the thermal boundary layer. Consequently, the depth of the boundary layer is generally smaller in forced convection compared to free convection.
It's important to note that the specific circumstances of a particular convection scenario can influence the depth of the boundary layer. Factors such as fluid properties, surface conditions, and geometry play a role in determining the characteristics of the boundary layer.