Flow regimes and wall shear rates determination within a scraped surface heat exchanger

An experimental investigation of a scraped surface heat exchanger (SSHE) wa s undertaken using visual observations and the electrochemical technique in order to, firstly, study the transition between laminar and vortex flows a nd, secondly, evaluate the wall shear rates. Visual observations and the el ectrochemical technique had undergone preliminary testing in a SSHE without blades, a well-known reference annulus. Then, visual observations and wall shear rates was compared in both geometries. It was established that flow patterns in a SSHE are noticeably different from those observed in an annul ar space in the same conditions. In a SSHE, the formation of the vortices i s thwarted by the rotation of the blades land by the clips of the blades) a nd the transition between laminar and vortex flows occurs at Ta-gc approxim ate to 80 (Ta-gc approximate to 45 in the annular space without blades). Lo cal measurements of the shear rate at the tube wall of the SSHE showed that it is fully controlled by the rotation of the blades, Vortices have a negl igible influence in comparison with that of blade scrapings. Wall shear rat es due to blade scraping can reach up to 40 000 s(-1). Under these conditio ns, the lowest value of the clearance between the edge of the blades and th e stator is about 50 x 10(-6) m. (C) 2000 Elsevier Science Ltd. All rights reserved.