Consider the flow of a fluid at a uniform temperature of $T_∞$over an isothermal flat plate at temperature$T_s$. The fluid particles in the layer adjacent to the surface will reach thermal equilibrium with the plate and assume the surface temperature$T_s$. These fluid particles will then exchange energy with the particles in the adjoining-fluid layer, and so on. As a result, a temperature profile will develop in the flow field that ranges from $T_sat$ the surface to $T_∞$ sufficiently far from the surface. The flow region over the surface in which the temperature variation in the direction normal to the surface is significant is the thermal boundary layer.

The thickness of the thermal boundary layer $δ_tat$ any location along the surface is defined as the distance from the surface atwhich the temperature difference$T-T_s$ equals0.99($T_∞-T_s$).

Thermal Boundary layer on a flat plate (the fluid is hotter than the plate surface)

The thickness of the thermal boundary layer increases in the flow direction, since the effects of heat transfer are felt at greater distances from the surface further down stream.

The convection heat transfer rate anywhere along the surface is directly related to the temperature gradient at that location. Therefore, the shape of the temperature profile in the thermal boundary layer dictates the convection heat transfer between a solid surface and the fluid flowing over it. In flow over a heated (or cooled) surface, both velocity and thermal boundary layers will develop simultaneously. Noting that the fluid velocity will have a strong influence on the temperature profile, the development of the velocity boundary layer relative to the thermal boundary layer will have a strong effect on the convection heat transfer.