SUMMARYBulk volume expands or contracts when solid particles of fairly uniform size flow out of a container by gravity and are loaded into another container. Expansion or contraction depends on the initial void fraction of the bulk volume. In this study, only expansion of the bulk volume was studied. The expansion was measured after wheat and spherical Lucite particles flowed separately through a plain tube and through a motionless mixer. It was found that expansion of the bulk volume was independent both of the number of helices in the mixer and the length of the plain tube. Passing wheat particles through the mixer expanded their bulk 3–4 less than did passing them through a plain tube of the same distance. Using 3/16‐in. particles, the mixer gave 3 less bulk volume expansion. For a given weight range of particles (using the expanded volume in the plain tube as a basis) the mixer reduced the expanded volume of wheat 20 and reduced more than 40 of the expanded volume of 3/16‐in. Lucite particles. The degree of mixedness, a measure of radial mixing, increased exponentially as the number of helices in a motionless mixer increased. Use of four helices for mixing 3/16‐in. Lucite particles, six helices for mixing 1/8‐in. particles and six helices for mixing red and white wheat appears to be optimum for the operating conditions in t
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