On the Instability of Spheres Settling through a Vertical Pipe Filled with HPG

An extensive set of experiments were performed to determine the stability boundary for the longitudinal oscillation of steel spheres settling through Hydroxy Propyl Guar (HPG) contained in long vertical tubes, a phenomenon first reported by Mollinger, et al. (۱۹۹۹). Results are reported in d/D-pH parameter space, where d is the sphere diameter and D is the internal diameter of a tube. Longitudinal oscillations, which owe their existence to the self-healing nature of the gel, occur over the region ۰.۴۸ < d/D < ۰.۸۶ and ۸.۳۱ < pH < ۸.۸۲. We discover that the upper pH limit of this stability domain is linked to the HPG gel point. During the course of experimentation, additional instabilities in the form of high-frequency transverse oscillations or helical motions were discovered and documented. The pure transverse oscillations found in the domain ۸.۰۳۵ < pH < ۸.۳۱ and ۰.۹ < d/D < ۰.۶ result from the elastic nature of the guar in this region. For ۶.۲۵ < pH < ۸.۰۳۵ transverse oscillations or stable descents are interspersed between bands of tight helical motion at large d/D and wide helical motion at low d/D. At the lowest value pH = ۶.۲۵ the guar is a Newtonian fluid, yet the spiral motion persists in small bands of d/D. We have verified, in the literature and through additional experiments using distilled water as the Newtonian liquid that the spiral motion of spheres falling inside vertical tubes does indeed exist.