Flow regimes and wall shear rates determination within a scraped surface heat exchanger Eric Dumont a,b , Francine Fayolle a, * , Jack Legrand b a Ecole Nationale Sup erieure dÕIng enieurs des Techniques Agricoles et Alimentaires, D epartement G enie des Proc ed es Alimentaires, Rue de la G eraudi  ere, BP 82225, 44322 Nantes Cedex 3, France b Laboratoire de G enie des Proc ed es, UPRES EA 1152, Universit e de Nantes, CRTT, BP 406, 44602 Saint Nazaire Cedex, France Received 30 July 1999; accepted 6 March 2000 Abstract An experimental investigation of a scraped surface heat exchanger (SSHE) was undertaken using visual observations and the electrochemical technique in order to, ®rstly, study the transition between laminar and vortex ¯ows 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 ¯ow patterns in a SSHE are noticeably dierent 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 ¯ows 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 in¯uence 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. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Scraped surface heat exchanger; Flow patterns; Vortex ¯ow; Microelectrode; Visual analysis; Wall shear rates 1. Introduction Scraped surface heat exchangers (SSHE), mainly used in the food industry, allow highly viscous ¯uids with complex rheology (cream cheese, fruit concentrate, ice cream, among others) to be processed. The ®rst advan- tage of these exchangers lies in the rotation of a shaft equipped with blades which periodically scrape the ex- change surface in order to prevent crust formation and to promote heat transfer (Fig. 1). The second one lies in the geometry of the inlet and outlet bowls. In this type of exchanger, the ¯ow is the result of the superposition of a Poiseuille ¯ow in an annular space and a Couette ¯ow, on which perturbations created by the blades are added. This ¯ow pattern is particularly complex and has only been super®cially studied directly (Trommelen & Beek, 1971a; Naimi, 1989; Burmester, Winch & Russel, 1996). Most of the works on the subject tend to model the geometry of the SSHE as an annular space without blades, where the hydrodynamics are well known (H arr od, 1986; Abichandani, Sarma & Heldman, 1987). Depending on the rotational speed of the rotor, two dierent ¯ow regimes can be shown: laminar or vortex ¯ow. Without blades, the change between these two regimes is due to the appearance of vortices at a critical value of generalised Taylor number (Ta gc ) varying with the radius ratio (R r /R s ; Esser & Grossmann, 1996), in- creasing with the axial ¯ow rate (Re axg ; DiPrima, 1960) and depending on the rheological properties of the product used (Wronski & Jastrzebski, 1990). From the vast literature on non-Newtonian annular ¯ow (Giord- ano, Giordano, Prazeres & Cooney, 1998), it can be expected that plug ¯ow conditions may occur in a SSHE when the ¯ow is vortical but close to the laminar ¯ow regime (H arr od, 1990a). H arr od (1990b) experimentally veri®ed that the most favourable ¯ow pattern in a SSHE occurs at these conditions. H arr od (1990a) also indicates that the most favourable ¯ow pattern is obtained by a slight increase of the rotational speed. Thus, it is im- portant to distinguish between laminar and vortex ¯ow Journal of Food Engineering 45 (2000) 195±207 www.elsevier.com/locate/jfoodeng * Corresponding author. Tel.: +33-2-51-78-54-80; fax: +33-2-51-78- 54-67. E-mail address: fayolle@enitiaa-nantes.fr (F. Fayolle). 0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 5 6 - X