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Originally Posted by **gizmo** Really? Why? Is it due to the increased turbulence at the boundary reducing the boundary layer? |

I don't have the book handy to quote the equation, but when you work through the math for the overall heat transfer coefficient for a system, it is directly proportional to the bulk velocity of the fluid. Most of these equations are, again, empirical combinations of various dimensionless numbers (Nusselt and Prandtl and viscosity correction usually).

In terms of boundary layer conditions, at higher flow rates, you get either a transitional or turbulent boundary layer at a shorter distance from the last obstacle (at which point the boundary layer disappears and you have little bits of laminar bulk flow). Obviously the best mixing, hence best heat transfer, occurs when you have a fully turbulent bulk flow and fully turbulent boundary layer. This is rather more detail than we need.

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Originally Posted by **gizmo** Empirical answers are still useful for getting a feel for the nature of the problem, though. Thanks for posting this. This is good info. |

My entire profession is a collection of one empirical result after another