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When processing highly viscous fluids in scraped surface heat exchangers, it is often difficult to follow the real heat flux received by each part of the product. Only average temperatures are measured, and the process cannot always guarantee a correct thermal treatment. During a pasteurisation process, the average temperature is around 80 °C, but some parts of the product may not stay long enough at this temperature compared to the average residence time. The aim of this study is to develop a tracer in order to follow the thermal treatment of Newtonian or non Newtonian viscous fluids in an experimental scraped surface heat exchanger in conditions similar to a pasteurisation treatment. A natural coloured tracer (Betanin, from red beets) which thermal degradation is well known is used as time temperature integrator. Thermal degradation of the tracer is determined experimentally and compared with a dispersion RTD model. First results show that Betanin degradation diluted in viscous fluids follows a classical first order reaction in a batch system. However, this degradation is enhanced when mechanical treatments occur in a well identified shearing channel as well as in the SSHE, for shear rates higher than 20 000 s − 1 . The major concern was then to identify the origin of this enhancement. Temperature increase due to viscous friction was measured in the SSHE and compared with the prediction conversion factors. It was shown that thermal treatment received by the Betanin was highly dependent on the mechanical treatment delivered during the thermal treatment. Industrial relevance When processing highly viscous fluids in scraped surface heat exchangers, it is often difficult to follow the real heat flux received by each part of the product, especially in industrial apparatus. Only average temperatures are measured, and the process cannot always guarantee a correct thermal treatment. During a pasteurisation process, the average temperature is around 80 °C, but some part of the product may not stay long enough at this temperature compared to the average residence time. The aim of this study is to develop a tracer in order to follow the thermal treatment in an experimental scraped surface heat exchanger in conditions similar to a pasteurisation treatment. When this tracer is found to be appropriate, it will be used directly in industrial equipments, in order to help industrials in: first, understanding their processes; verifying that their actual operating conditions are feasible for a correct thermal treatment; if not, in optimising these operating conditions. Betanin, representative of sensitive food products treated in a scraped surface heat exchanger was found to be, not only sensitive to temperature but also to mechanical treatment. This rather unexpected result has to be taken into account very carefully by industrials in order to insure a content product quality, whereas the mechanical treatment undergone by the product can lead to an accentuated degradation of thermally sensitive products.

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.

Viscous products treated in a scraped surface heat exchanger receive a thermal treatment as well as a mechanical treatment. The new method developed by Mabit et al. [J. Food Engrg. 57 (2) (2003) 165] using starch as a marker for the shearing treatment was performed in order to show the influence of blades movement and flow pattern on shearing conditions. Two different flow regimes with respect to the generalised Taylor Number Ta g were investigated: a laminar regime with Taylor vortices and a weakly turbulent one. The blade rotation without scraping in the annulus space can be sufficient to apply a shear treatment above 20,000 s −1 . The product is submitted to the highest shear treatment for high rotational speed with a laminar regime with Taylor vortices. Blades scraping creates high wall shear rates, Taylor vortices improve radial mixing and renew product to be treated at the wall surface. The quantity of stressed product is directly influenced by the flow behaviour and the product residence time in the exchanger.

The electrodiffusion technique was performed in order to investigate the shear rate on a scraped surface heat exchanger. Microelectrodes were placed inside: the walls of the outer cylinder; the inlet and outlet bowls; the rotor and the blades. Highly viscous Newtonian fluid (Emkarox HV45 solutions) and non-Newtonian model fluid (aqueous solutions of CMC) were used. The electrodiffusion method allowed us to measure wall shear rates. Maximum shear rate was observed at the scraping surface and caused by blades scraping, high shear rate was also measured on the leading edge of the blades. In the other parts of the exchanger, shear rate remained low but the development of Taylor vortices completely modified the scraped surface heat exchangers behaviour inside the surface of the bowls. A dimensionless representation of the friction factor was established for the inner and outer wall surface of the exchanger.

This works deals with the mechanical treatment realised in a surface scraped heat exchanger (SSHE) and consequence on products. In this type of apparatus, the presence of high shear rates is demonstrated and added to the thermal treatment. An innovative method was developed to measure the quantity of product that is subjected to high shear in the SSHE. This method used a starch suspension, which was sensitive to mechanical treatment. The starch granule diameter increased with the shear rate applied to the suspension. The increase of granule diameter measured by laser granulometry yielded the volume fraction of stressed starch. This method indicated that granules started to swell at a rotational speed of 6.5 rps and that 37% of starch is stressed at 10 rps. These results are explained by the role of the blades and confirm the presence of high wall shear rates. It was shown that starch granules were stressed and that Taylor vortices did not generate sufficient high shear rates in a SSHE to modify the starch granules. The mechanical stress was correlated to the shear rates applied. The starch granules became sensitive to shear treatment at 20,000 s−1 and this value was exceeded in the SSHE by the rotation of blades.