A pilot scale flow loop was used to investigate conditions for the onset of retention aid polymer degradation during dosage into the approach flow. A 1700 ppm cationic-polyacrylamide (cPAM) solution was dosed concentrically into a turbulent pipe flow consisting of either pure water or a 2 % softwood pulp suspension. Retention polymer degradation was characterized by changes in the percent turbulent drag reduction induced by the retention aid additives, which was determined by measurement of the pressure differential downstream the dosage point. Dosage was made concentrically through a long brass pipe with a fixed inflow nozzle diameter (8 mm) followed by a sudden contraction to a reduced outflow nozzle diameter. Three different outlet diameters were investigated, namely 3 mm, 1.5 mm, and 1 mm, each representing an increase in the elongational strain inside the nozzle. Estimates of the shear stress inside the dosage nozzle were made with two-phase, computational fluid dynamics (CFD) simulations of the dosage flow. The dosage velocity, i.e. the ratio of inner (dosed) flow to outer (stock) flow was varied in order to investigate shear effects outside of the dosage nozzle. Shear estimates outside the dosage nozzle were made using pulsed ultrasound Doppler velocimetry (PUDV) measurements.
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