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

Abstract An experimental investigation of a scraped surface heat exchanger (SSHE) was undertaken using visual observations and the electrochemical technique in order to, firstly, study the transition between laminar and vortex flows and, secondly, evaluate the wall shear rates. Visual observations and the electrochemical 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 annular space in the same conditions. In a SSHE, the formation of the vortices is thwarted by the rotation of the blades (and by the clips of the blades) and the transition between laminar and vortex flows occurs at Ta gc ≈80 ( Ta gc ≈45 in the annular space without blades). Local 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 negligible influence in comparison with that of blade scrapings. Wall shear rates due to blade scraping can reach up to 40 000 s −1 . Under these conditions, the lowest value of the clearance between the edge of the blades and the stator is about 50 × 10 −6 m.