Determination of heterogeneities in a scraped surface heat exchanger using electrochemical sensors

Abstract

An experimental investigation of a scaled-down model of an industrial exchanger, using an electrochemical technique, was undertaken in order to show the presence of hydrodynamic heterogeneities at low axial Reynolds number. Heterogeneities were revealed in the bowls with respect to the generalised Taylor number as the result of the perturbations added to the flow by blade rotation at both ends of the exchanger. Shear heterogeneities associated to flow visualisations were correlated to temperature heterogeneities observed in the bowls. Shear fluctuations were revealed in the scraped part describing two distinctive zones at low rotation speed caused by varying viscosity in the flow field. A complex spiral flow was observed by flow visualisation characterising a mass transfer evolution comprised between these two distinctive zones at low Taylor number.of the microelectrode (m) d h hydraulic diameter (m) d h = (d s -d r ) d r rotor diameter (m) d s stator diameter (m) e gap (m) F Faraday constant (A s mol )1 ) K consistency coefficient of the product (Ostwald law (Pa s n )) L stator length (m) n flow behaviour index of the product (Ostwald law (-)) N rotation speed (rps) n L number of blades (-) Q flow rate (m 3 s )1 ) Re ax ¼ &U d d h axial Reynolds number (-) Re axg ¼ &U 2Àn d d n d K generalised axial Reynolds number (-) Re r ¼ &Nd 2 r Rotational Reynolds number (-) R r rotor radius (m) R s stator radius (m) S wall shear rate (s )1 ) Sc exchange surface (m 2 ) T temperature (°C) Ta ¼ ffiffiffiffiffiffiffiffiffiffi R s ÀR r R r q &d h 2 ðR r Þ Taylor number (-) Ta g ¼ ffiffiffiffiffiffiffiffiffiffi R s ÀR r R r q &d n h 2 n ðR r Þ 2Àn K generalised Taylor number (-) Ta c , Ta gc critical values of Taylor number (-) U d mean axial velocity in the annular space (m s )1 ) z number of electrons (-) q density (kg m )3 ) g dynamic viscosity (Pa s) cshear rate (s )1 ) X rotating speed of the rotor (rad. s )1 ); X = 2pN s shear stress (Pa)

CFD analysis of the flow pattern and local shear rate in a scraped surface heat exchanger

“…This zone is mainly localized in the space between the blade and the stator wall, where the shear rates are very important compared with the remainder of the exchanger. Fayolle et al [27] considered that two layers of the fluids must coexist: one layer with very low viscosity close to the wall, and another one with higher viscosity near the rotor, and that these two layers behave like two immiscible fluids. Numerical results shown in Fig.…”

Development of a time temperature integrator for quantification of thermal treatment in scraped surface heat exchangers

“…In order to record the thermocouples signal, a rotary collector (Litton EC3848) was installed at the tip of the rotor. A similar system has already been used to collect currents from microelectrodes in order to measure shear rates (Mabit, Loisel, Fayolle & Legrand, 2003;Fayolle et al, 2005), and was successfully used for temperature measurements in this study.…”