What is a hydraulic diameter, how is it defined and what is its value or usefulness in solving real heat transfer problems involving internal flow situations? Solution For a circular tube the hydraulic diameter is simply the diameter of the tube. For a fully filled duct or pipe whose cross section is a regular polygon, the hydraulic diameter is equivalent to the diameter of a circle inscribed within the wetted perimeter. According to similarity theory, any geometrical parameter of the channel can be taken as an independent dimension. However, it is advisable to choose an independent dimension that takes into account the hydrodynamic characteristics of fluid flow. Therefore, as an independent dimension of channels with internal flow, use is made of an equivalent hydraulic diameter. .