当量直径 equivalent diameter

the equivalent diameter of an irregularly-shaped object is the diameter of a sphere of equivalent volume.
According to the IUPAC definition, the equivalent diameter of a non-spherical particle is equal to a diameter of a spherical particle that exhibits identical property (e.g., aerodynamic, hydrodynamic, optical, electrical) to that of the investigated non-spherical particle. For particles in non-turbulent motion, the equivalent diameter is identical to the diameter encountered in the Stokes' law.

这是在英语专有名词字典上查的。。。
Specialty Expressions: equivalent diameter
Definition
In sedimentation analysis, the diameter assigned to a nonspherical particle. It is numerically equal to the diameter of a spherical particle having the same density and velocity of fall.
Equivalent diameter Mining
1: In sedimentation analysis of particle size, the diameter assigned to a non-spherical particle; that is, the diameter of a spherical particle of the same density and velocity of fall. Sometimes referred to as the equivalent spherical diameter.
2: A. The diameter of a hypothetical sphere composed of material having the same specific gravity as that of the actual particle and of such size that it will settle in a given liquid at the same terminal velocity as the actual particle. Also called equivalent size. Equivalent diameter (equivalent size) Energy
The diameter of a hypothetical sphere composed of material having the same specific gravity as that of the actual soil particle and of such size that it will settle in a given liquid at the same terminal velocity as the actual soil particle.

equivalent diameter
The equivalent diameter is the diameter of a circular duct or pipe that gives the same pressure loss as an equivalent rectangular duct or pipe.

直径就是圆的最长的