Determination of the volume fraction submitted to high shear in a scraped surface heat exchanger

摘要

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.