Study of the Geometrical Effects of Impeller on the Flow Field in Hybrid Mixing Process for Manufacturing Nanocomposites

Abstract

Abstract For liquid-based processing of nanocomposite materials, mechanical stirring and ultrasonic processing are two different physical processes that are widely used for mixing of nanomaterials in a liquid medium such as molten metals or polymer solutions. The acoustic cavitation bubble effect generated during ultrasonic processing is highly advantageous in breaking apart aggregates or clusters of nanomaterials in the liquid, but the extent of this effect is confined within a limited volume of liquid adjacent to the source of the ultrasound energy. On the other hand, mechanical stirring using rotary impellers can achieve flow circulation in much larger volumes of liquids, aided by agitation of a liquid medium. However, the shear forces are not sufficient to overcome the inter-particle attraction of nanomaterials. In this work, a computational model of the mechanical stirring process was implemented to study the turbulent flow characteristics generated by a rotating radial flow impeller that can accommodate an ultrasonic probe concentrically located inside a hollow impeller shaft. Systematic investigations were performed using statistical analysis and design of experiments (DoE) to determine the influence of the different geometric parameters of the impeller, such as blade length, blade height, blade thickness, hub thickness, and number of blades, on the turbulent flow characteristics of the flow field in the mixing vessel with a large volume of liquid. The hybrid mixing process combined mechanical stirring and ultrasonic processing with the determined geometric configuration of the impeller has been used to prepare aqueous suspensions of carbon nanofibers (CNFs), and the experimental observations exhibited prolonged stability of the suspensions, compared to that of suspensions separately prepared through the mechanical stirring or the ultrasonic processing. This research provides a route towards overcoming the scientific and engineering challenges associated with scaling up the liquid-based manufacturing of high performance lightweight nanocomposite materials to sufficiently large quantities for use in practical industrial applications.