Electrodiffusional wall shear rate analysis in scraped surface heat exchanger

Abstract

An electrochemical technique was used to measure local and instantaneous wall shear rates in a scraped surface heat exchanger (SSHE). The inner cylinder, equipped with two blades symmetrically opposed, was driven by a stepping motor and the outer cylinder was fixed. Highly viscous Newtonian fluids (Emkarox HV45 solutions) and non-Newtonian model fluids (aqueous solutions of guar and CMC) were used. The reliability of the electrochemical technique was validated for these fluids in an annular space (SSHE without blades). The evolution of the limiting diffusion current was given by the probes in a scraping situation. In the general case of scraping, the flow was unsteady, and very fast large fluctuations of the limiting diffusion current were measured. The shear rate at the wall of the stator of the SSHE appeared fully controlled by the rotation of the blades. Then, electrodiffusion measurements were interpreted using three different wall shear rate calculating procedures. Wall shear rate in SSHE appeared ten to 100 times more important than in annulus, and the lowest values of the clearance between the edge of the blades and the stator was about 50 × 10−6 m.