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The hydraulic diameter, D H, is a commonly used term when handling flow in non-circular tubes and channels. Using this term, one can calculate many things in the same way as for a round tube. When the cross-section is uniform along the tube or channel length, it is defined as [1] [2] =, where
For example, it is used to calculate flow through circular and non-circular tubes in order to examine flow conditions (i.e., the Reynolds number). In those cases, the characteristic length is the diameter of the pipe or, in case of non-circular tubes, its hydraulic diameter D h {\displaystyle D_{h}} :
In this expression for Reynolds number, the characteristic length D is taken to be the hydraulic diameter of the pipe, which, for a cylindrical pipe flowing full, equals the inside diameter. In Figures 1 and 2 of friction factor versus Reynolds number, the regime Re < 2000 demonstrates laminar flow; the friction factor is well represented by ...
For a circular pipe, the hydraulic diameter is exactly equal to the inside pipe diameter: D H = D . {\displaystyle D_{\text{H}}=D.} For an annular duct, such as the outer channel in a tube-in-tube heat exchanger , the hydraulic diameter can be shown algebraically to reduce to
The Reynolds number Re is taken to be Re = V D / ν, where V is the mean velocity of fluid flow, D is the pipe diameter, and where ν is the kinematic viscosity μ / ρ, with μ the fluid's Dynamic viscosity, and ρ the fluid's density. The pipe's relative roughness ε / D, where ε is the pipe's effective roughness height and D the pipe ...
D H is the diameter in round tubes or hydraulic diameter in arbitrary cross-section ducts L is the length Re is the Reynolds number and Pr is the Prandtl number. This number is useful in determining the thermally developing flow entrance length in ducts.