Re: Laminar layer heat transfer

>I have a pipe with water flowing across the length of the pipe, and I want
>to calculate the thickness of the laminar layer (on the outside surface of
the
>pipe).

Why do you want to do this? It makes a difference whether you're doing it for a heat transfer exercise or something else. If the pipe is in cross flow, there won't be a laminar sublayer over half the pipe circumference because the flow will have separated. If you're looking for film coefficients, you don't actually need the thickness of the sublayer since that's built into the film coefficient formulation.

It's fair to say that there really is no such thing as the 'thickness' of a boundary layer, since the velocity distribution is continuous. For laminar boundary layers a common convention is defining the thickness as the point where the local speed is 99% of the free stream value

If you're looking for the boundary layer thickness in laminar flow the thickness (Blasius solution) is
thickness = 4.96x/sqrt(Rex) where x is the streamwise distance from the leading edge and Rex is the Reynolds number based on the distance from the leading edge. Strictly speaking the Blasius solution applies to a flat plate but it'll be accurate enough if the water flow is truly lengthwise on the pipe.

A common assumption for turbulent boundary layers is that the velocity variation from the wall goes as a power of the distance from the wall. The resulting thickness is
thickness = 0.1285x(Rex)^(1/7). This works for Reynolds numbers above 6E6. For lower Reynolds numbers the relationship is 0.376xRex^(1/5)

It gets complicated from here. _Fluid Dynamics and Heat Transfer_ by Knudsen and Katz is my bible on the topic. There's a chapter on the laminar sublayer, too, if you're a glutton for punishment.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw@skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)