Hydraulic diameter and equivalent diameter

Generally speaking, for non-circular pipes, since the shape of their flow cross-section affects both the distribution of fluid velocity and the flow resistance, in turbulent flow conditions a diameter equivalent to that of a circular pipe can be used as a substitute for the non-circular cross-section channel; this is where the concept of hydraulic radius comes into play. For a circular tube, the hydraulic radius is R = D/4; accordingly, the hydraulic radius for a non-circular tube is also R. To facilitate calculations, the concept of equivalent diameter De is introduced during design for pipes of different shapes; in calculations, it serves the same purpose as D for circular pipes. For non-circular pipes, their equivalent diameter De = 4R.

Thank you, OP! I’m not sure if equivalent diameter and hydraulic diameter are the same concept?

Definition: The diameter of a circular pipe with an equal hydraulic radius is defined as the equivalent diameter of the non-circular pipe. On the effective cross-section of the overall flow, the length at which the fluid comes into contact with the solid wall surface is called the wet perimeter, denoted by the letter L. The ratio of the effective cross-sectional area A of the flow to the wet perimeter L is defined as the hydraulic radius, denoted by the letter R. R = A/L. For pipes with a circular cross-section, the geometric diameter can be expressed in terms of the hydraulic radius as follows: A = 1/4*πd^2 and L = πd. Therefore, R = A/L = 1/4*d, which implies that d = 4R; in other words, the geometric diameter of a pipe with a circular cross-section is 4 times the hydraulic radius. 　　By analogy with circular pipes, the equivalent diameter D of pipes with non-circular cross-sections can also be expressed as 4 times the hydraulic radius: D = 4A/L = 4R

Hydraulic radius and equivalent diameter in fluid mechanics. Wet perimeter: The perimeter of the portion on the effective cross-section where the fluid is in contact with the solid wall surface. Hydraulic radius: the ratio of the effective cross-sectional area to the wet perimeter. Equivalent diameter: 4 times the hydraulic radius