The pressure drop was measured for air suspensions of spherical paricles and two types of fibrous partcles, all having mean diameters of between 8 and 20 μm, in the same apparatus and at the same operating conditions. The pipe diameter was 50.42 mm. A Reynolds number range of 61,000 to 114,000 and loading ratio range of 0.06 to 0.30 were investigated. Air-sphere suspensions showed drag reduction, in agreement with published results. Fairly uniform fibers of aspect ratio = 75 gave no drag change and drag increases resulted with “random cut” fibers of mean aspect ratio = 31, contrary to the published results for water-fiber suspensions. A relative humidity effect was also shown for fibrous particles. The flocculation of the fibers may be an important variable to address in future experiments.

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