Long-term experimental study on gravitational sedimentation of water aluminum oxide nanofluid at different volumetric concentrations

The stability of nanofluids is critical in engineering applications. The sedimentation of the nanoparticles in the base fluid limits the stability of the nanofluid. By measuring the absorbance of a visible laser through water aluminum oxide nanofluids it is possible to observe the evolution of the sedimentation process. A simple experimental setup consisting of a thin closed test cavity (0.8 mm or 1.5 mm thickness) filled with nanofluid and a set of laser diodes-photodiodes pairs was used in the experiments to determine the absorbance and the local volumetric concentration. The evolution of the local volumetric concentration of the nanofluid was measured at 13 height positions along the test cavity. Five initial volumetric concentrations of aluminum oxide (Al2O3; 0.5%, 1.0%, 1.5%, 2.0%, 3.0% and 4.0%) were considered. Data and digital photos were acquired for a total time lapse of 260 days. The results were independent of the container thickness. The same general trend was consistent for all the initial volumetric concentrations, i.e., the sedimentation rate decreases in time. The time evolution of the volumetric concentration was compared with results from a modified Mason-Weaver model. The comparison was possible by setting a variable sedimentation velocity in the model, suggesting that the sedimentation is affected by the dynamics of nanoparticle clusters with diverse sizes produced by agglomeration.