Mohamed Nabil Noui-Mehidi

Characterisation of Isolated Mixing Regions in a Stirred Vessel

An experimental study of isolated mixing regions (IMRs) in a cylindrical vessel stirred by three different impellers (a six-blade Rusthon turbine, a four-blade Lightnin A315 impeller and an axial CPE RTF4 impeller) has shown the existence of IMRs in all three systems at low Reynolds numbers. Calculations of the Re ranges in which IMRs exist, torque measurements and flow visualizations enabled the analysis of the effects of impeller type and the flow structures when IMRs were present. Particular attention was given to the case of the Rushton turbine, where digital image analysis revealed that the mechanism of IMR disappearance can be described as a period-doubling locus in the physical space, in which IMRs completely disappear at a critical Re, resulting in global mixing.

A numerical study of Taylor vortex flow in a finite length tapered annulus

The transient evolution and steady state analysis of Taylor vortex flow in a tapered annulus was conducted by numerical experiments in the case where the inner cylinder was rotated and the outer one fixed. The gap between the cylinders was linearly tapered from a supercritical value at the upper base to the critical value at the lower base. The wavelength adjustment depended on the variation of Reynolds number in the spatially ramped gap. The axisymmetric conservative governing equations were solved by the use of an simplified marker and cell (SMAC) algorithm. A coordinate transformation function allowed us to numerically solve the problem in a rectangular computational domain. The results have shown that Taylor vortices growth was sensitive to the spatial ramp of the gap even with tapering angle values less than one degree. The interaction between the inflow and outflow boundaries could be clearly seen as the taper angle was increased. The investigation of the transient dynamics related to the flow system also revealed a characteristic dependence on the taper angle.