Hydraulic and heated equivalent diameters are approximations to account for different flow geometries in thermal-hydraulic analyses. Most of the empirical models used in single- and two-phase flow heat transfer are based on experiments in heated tubes and extrapolated to complex geometries by means of the equivalent diameters. For heat transfer calculations, as a general rule, the heated equivalent diameter must be used for bundle geometries and the hydraulic equivalent diameter for annulus geometries. The use of both diameters in different correlations is discussed and clarified in this technical note.
