Flow Patterns of an Annular Viscoplastic Jet

The global flow field characteristics of a submerged annular viscoplastic jet were investigated within the steady laminar flow regime. The governing mass and momentum conservation equations, along with the Bingham rheological model, were numerically solved using a finite-difference scheme. Central and outer recirculation zones typically characterize the flow of a Newtonian annular jet. However, the performed parametric study revealed new and unique-to-viscoplastic-fluids flow features. The following flow patterns were observed and classified according to the characteristics of the central and outer recirculation zones: 1) for small yield numbers, central and outer recirculation zones exist. The extent of the outer zone and recirculation intensity of both zones were found to substantially diminish with the yield number; 2) at a high enough yield number, a stagnant, attached-to-the-wall region replaces the outer recirculation zone while a central, yet weaker one, still exists; 3) a further increase in the yield number results in the replacement of the weak central recirculation zone with a stagnant one and the elimination of flow recirculation throughout the whole flow field. The annular jet was found to decay faster and dissipate more rapidly when increasing the inner-to-outer annular nozzle diameter ratio.