Thermal boundary layer

Here are the facts : the fluid particles' temperature in direct contact with the solid is equal to the solid surface's temperature, whereas this is not the case in the middle of the flow (where generally temperature is minimal when the fluid cools the solid). Temperature won't go suddenly from its value at the surface to its minimum, but it will be achieved gradually, and this occurs in this boundary layer. Here is where the effects of the fluid's thermal conductivity take place.

The following picture shows a laminar thermal boundary layer, it was obtained by differential interferometry (the heated fluid has a density and an refraction index which varies).

Laminar thermal boundary layer, photo from the book " An album of fluid motion, Van Dyke, The Parabolic Press "