Many peoples are confusing with this term” Boundary Layer”, since most of the text books definitions are like this, The boundary layer is defined as ” the layer in which -local velocity reaches 99% of free stream velocity for velocity boundary layer or the local temperature reaches 99% of free stream temperature for thermal boundary layer. Let us explain boundary layer completely here.
Before explanation, if the boundary layer is exist in any surface it should be either velocity or thermal boundary layer, since they are the common types of boundary layer.
How velocity boundary layer forms?
The boundary layer is formed when the fluid is viscous, viscous fluid means it has the resistance to flow i.e viscosity. When the viscous fluid flows over the surface, the layer of fluid closer to the surface sticks to the surface, sticks means it sticks partially to the solid surface and flows, hence the layer of fluid sticks to the surface experiences the inability to flow because of that sticking [ viscous] property, it tends to reduction in velocity of that particular layer not to the all layers of fluid flow. At this point velocity is about zero.
Now first layer of fluid flow is stopped to flow partially and the second layer( the flow layer above the layer which sticks to the surface i.e 1st layer) tries to flow over the first layer. Since both layer of fluid is viscous now the second layer sticks to the first layer partially, so 2nd layer experiences the restriction to flow from the first layer.But this restriction is some what lesser than the restriction to flow between 1st layer and surface, hence the second layer has velocity greater than first layer. The phenomenon that happened between 1st and 2nd layer will occur between the layers 2and3, 3and4……..n-1 and n layers. At the end the nth layer will have greater velocity than n-1 layer. After the nth layer there would not be any restriction to flow i.e the restriction is zero, at that point the velocity will be maximum than all the layers beneath the level of nth layer.
After the nth layer, since there is no restriction to flow, the fluid layers will flow freely, hence they have maximum velocity.Now from the point at zero velocity layer to the point at 99% of free stream velocity layer there is a region, that region is called as velocity boundary layer or velocity boundary thickness.
How thermal boundary layer forms?
As we stated above, the resistance exist between the layers and surface, there will be a friction, hence heat is generated. Now when the fluid flows over the surface, it receives the heat from the surface if it is cooler than surface and it losses the heat to the surface if it is hotter than surface.
We know that, when the fluid’s temperature increases its density decreases, under this principle, when the fluid layer exchanges the heat its density also changes. Let us assume the fluid is cooler and the surface is hotter. When this fluid flows over the surface, its receives heat from the surface, hence its density decreases, and it moves to top. now the adjacent one has low temperature and high density than the later one, so it will come down i.e adjacent to the hot surface and receives heat, this same phenomenon takes place between surface and layers. Now the first layer heated will be at top most position than all layers since it has high temperature. In one condition the first layer temperature will reach free stream temperature as it exchanges the heat between layer to layer while going to top position , after that its temperature will not change. Hence there is a region with low temperature to high temperature between surface and free stream, this region is called as thermal boundary layer or thermal boundary layer thickness. if you take fluid is hotter and surface is cooler, the heat exchange will be reversed and same region will be there.