Equivalent Diameters of a Nonspherical Particle

An equivalent diameter of a particle is usually defined in relation to a specific sizing method developed on the basis of a certain equivalency criterion. Several equivalent diameters of a spherical particle commonly employed are discussed in the following sections. 

A sieve diameter is defined as the width of the minimum square aperture through which the particle will pass. A common sizing device associated with this definition is a series of sieves with square woven meshes. Two sieve standards, i.e., Tyler Standard and American 

Society for Testing and Materials (ASTM) Standard, are widely used they are introduced in 1.2.2.1. 

Martin s diameter and Feret s diameter of a particle depend on the particle orientation under which the measurement is made. Thus, obtaining a statistically significant measurement for these diameters requires a large number of randomly sampled particles which are measured in an arbitrarily fixed orientation. Since Martin s diameter, Feret s diameter, and projected area diameter are based on the two-dimensional image of the particles, they are generally used in optical and electron microscopy. The principles of microscopy as a sizing method are discussed in 1.2.2.2. 

3 Surface Diameter, Volume Diameter, and Sauter s Diameter The surface diameter, ds, volume diameter, d, and Sauter s diameter, d, are defined such that each of them reflects a three-dimensional geometric characteristic of an individual particle. A surface diameter is given as the diameter of a sphere having the same surface area as the particle, which is expressed by 

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Grashof number for mass transfer L is a characteristic dimension, i.e., the diameter of a spherical particle, or the equivalent diameter of a nonspherical particle, etc. v is the kinematic viscosity D is the binary diffusion coefficient U is the linear velocity of the gas stream flowing past the particle (measured outside the boundary layer surrounding the particle) g is the acceleration due to gravity is a characteristic concentration difference, and... 

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