Francine Fayolle

Wall shear rate in the Taylor–Couette–Poiseuille flow at low axial Reynolds number

Abstract An experimental investigation was done by the use of visual observations and the electrochemical technique in order to study the appearance of hydrodynamic instabilities at low Re axg in the gap between two coaxial cylinders, with radius ratio R r / R s =0.615 and aspect ratio L / e =24. A motor drove the inner cylinder and the outer cylinder was fixed. A Newtonian fluid (Emkarox HV45) and two non-Newtonian fluids (aqueous solutions of guar and CMC) have been used. The analysis of the evolution of the size of the Taylor vortices is carried out for Re axg Re axg Ta g ≈80, and a stretching phase for Ta g >80. For higher Re axg , the contraction phase vanished and the vortices progressively stretched with increasing Ta g . A dimensionless representation was also proposed for wall friction generalized to non-Newtonian fluids following the Ostwald law.

Determination of the volume fraction submitted to high shear in a scraped surface heat exchanger

Abstract This works deals with the mechanical treatment realised in a surface scraped heat exchanger (SSHE) and consequence on products. In this type of apparatus, the presence of high shear rates is demonstrated and added to the thermal treatment. An innovative method was developed to measure the quantity of product that is subjected to high shear in the SSHE. This method used a starch suspension, which was sensitive to mechanical treatment. The starch granule diameter increased with the shear rate applied to the suspension. The increase of granule diameter measured by laser granulometry yielded the volume fraction of stressed starch. This method indicated that granules started to swell at a rotational speed of 6.5 rps and that 37% of starch is stressed at 10 rps. These results are explained by the role of the blades and confirm the presence of high wall shear rates. It was shown that starch granules were stressed and that Taylor vortices did not generate sufficient high shear rates in a SSHE to modify the starch granules. The mechanical stress was correlated to the shear rates applied. The starch granules became sensitive to shear treatment at 20,000 s −1 and this value was exceeded in the SSHE by the rotation of blades.