Measurement and Evaluation of Drag Coefficient for Settling of Spherical Particles in Pseudoplastic Fluids

G. Matijašić and A. Glasnović

Abstract
The drag coefficient is an important hydrodynamic characteristic of the motion of particles in both Newtonian and non-Newtonian fluids and the possibility of its evaluation based on mathematical equations is very significant. Development of mathematical model for estimating drag coefficient values is primarily based on measurement accuracy. The experiments were run using w = 1 – 4 % carboxymethylcellulose (CMC) aqueous solutions with different rheological properties which were determined by viscosity measurements. Approximately 200 experimental results were obtained using spherical particles of different materials and diameters. Measurements of its falling velocities were carried out in a glass tube. An optical method of measurement was developed for this purpose. The laboratory device contains three printed boards. Two are identical and each consists of 7 photodiode/photoreceiver pairs, while the third is used for connecting with a computer. Photodiodes emit 2 mm wide IR rays towards photoreceivers and both have TTL levels as output. Output signals from these two boards are connected with RS flip-flop on the third board, and its output is connected to the computer. This technique enables time measurement accuracy of 0.02 s. By comparison of experimental drag coefficient values with those obtained using proposed model for pseudoplastic fluids, the value of mean relative deviation is 25 %. Our proposed mathematical model simplifies the correlation of correction factor vs. flow behaviour index and achieves better results for a wide range of Reynolds number (RePS < 1000) giving 15 % mean relative deviation.

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Keywords
Drag coefficient, power law fluid, optical method