Complex flow behaviors of concentrated alumina suspensions in pressure-driven flow

Abstract

A new approach was taken to understand the flow behavior of concentrated particle suspensions in pressure-driven capillary flow. The flow of concentrated alumina suspensions in a slit channel was visualized depending on dispersion states, coagulated system and well-dispersed system, with modified capillary rheometer. The complex flow behaviors of suspensions including the flow development in solid-liquid transition process were quantitatively described in terms of flow profile and pressure fluctuation. The suspensions showed complex flow behaviors

unique solid-liquid transition and shear banding. In the solid-liquid transition, there was no-flow at first and continuous change of flow profile was observed with time. At low shear rates in particular, the suspensions exhibited shear banded flow profile which could be divided into 3 regions: the region with low flow rate near the wall, the region with rapid increase of flow velocity to maximum, and the region of velocity plateau. The banding in pressure-driven flow was different from that in Couette flow. The banding of concentrated alumina suspensions was unique in that sluggish velocity profile was pronounced and two inflection points in velocity profile was exhibited. Based on both flow visualization and measurement of shear stress, it was found that the shear banded flow profile in pressure driven slit channel flow was strongly correlated with shear stress. For the coagulated alumina suspension, the shear stress showed an N-curve that included a region of stress decrease with an increase in shear rate followed by a monotonic increase. Depending on the region in the stress curve, the flow profile changed from a shear banded profile with no-flow or sluggish flow rate closed to the channel wall and plug flow at channel center to a plug-like flow profile similar to the Newtonian fluid. In addition, it was observed that the transient flow behavior over time at high shear rate in liquid state experienced all of the steady state flow profiles at lower shear rates in solid-liquid transition. During the solid-liquid transition, the flow profile was found to be shear banded, and the pressure profile did not reach a steady state but fluctuated with a characteristic time period. As the solid concentration increased for the coagulated system, the flow profile in the solid-liquid transition region (stress-decay region) exhibited more complex flow behavior of very large no-flow region along with an asymmetric flow and its fluctuations. With the FT analysis of pressure fluctuation and flow fluctuation, it was determined that pressure fluctuation had the same time period with the flow profile fluctuation and the flow profile change in the slit channel had close relationship with the pressure change, which is meaning of that the pressure fluctuation is caused by the complex flow behavior inside the channel. In contrast with coagulated suspension, the well-dispersed suspension with low solid concentration showed only a monotonic increase of shear stress in the range of shear rates we could measure, indicating that the suspension was in liquid state. The flow profile was plug-like, and the pressure was fluctuating without any characteristic time period. However, the well-dispersed suspensions with higher solid concentration exhibited complex flow behaviors similar to coagulated suspension such as unique solid-liquid transition and shear banded flow profile. In this study, shear banding of concentrated alumina suspensions in slit channel flow was visualized and quantitatively analyzed. We expect this approach can be an effective method to understand the flow behavior of particulate suspensions in the pressure-driven flow which is typical in industrial processing.