Numerical and experimental investigations of Newtonian and non-Newtonian flow in Annular Gaps with Spraper Blades

Stranzinger M.

This work presents numerical and experimental investigations of narrow annular gap flows using an apparatus with an inner cylinder as rotor, with two attached scraper blades, and an outer cylinder actin g as stator (with a rotor/stator cylinder ratio of 0.8). Applications in industry are Barrow gap – scraped surface beat exchangers (NG-SSHE) or narrow gap – shear crystallizers (NC-SC) developed by the Laboratory of Food Process Engineering at ETH Ziirich. Narrow annular gap apparatus with wall scraping tools are the basis of a newtype of continuous reactor principle for the mechanical and thelmaI treatment of fluid systems. This principle combines the advantages of narrow mechanical stress distribution, narrow residence time distribution, optimized “thin layer” heat and mass transfer.A finite volume method (FVM; the numerical part) and a digital-particle image velocimetry method (D-PIV; the experimental part) are employed in the investigation. A variety of rotor velocities represented by a rotational Reynolds number (parameter Re) and a variety of scraper blade angles between 30º and 150º (parameter ) are considered for Newtonian and inelastic non-Newtonian shear-thinning fluids (modelled with power-law exponents between n=0.2 and 1.0).Two-ditnensional numerical flow simulations in a characteristic axial cross sectional plane are compared to experimentally induced two-dimensional velocity fields measured with DPIV in an annular gap reactor (AGR, developed by the Laboratory of Food Process Engineering at ETH Z;iirich) as a function of the chosen parameters Re, and n. The comparisons are in good agreement. A numerical particle tracking method (NPT) is used to further analyze the flow fields produced from the simulations.Based on the collected flow fields, for various process parameters Re, and n, the mechanisms which influence the macroscopic flow structuring behaviour of viscous fluid systems related to food applications are discussed. These include the hydrodynamic pressure, secondary flows, non-Newtonian viscosity, shear stress, energy dissipation and anisotropic flow structuring deformation behaviour due to pure elongational and pure shear strain (E.e, flow structuring deformations, flow structuring energies and local flowstructuring efficiencies).