Characterization of Shear Thinning Flow Within a Scraped Surface Heat Exchanger

Residence time distribution analysis and flow visualization were used to characterize the flow behaviour of both a model shear thinning fluid (Carbopol) and a food stream (ice cream) in a commercial scale scraped surface heat exchanger (SSHE). Different flow behaviour was observed in the isothermal model system compared to that for ice cream, due to a radial gradient of temperature, and therefore viscosity, in the ice cream system which produced more axial dispersion. Nevertheless, the model solution showed similar relative changes in flow behaviour with changing operating conditions thus representing a viable technique for SSHE optimization. Experiments with the model system revealed that the degree of shear thinning is a more important factor in determining the residence time distribution than the magnitude of the apparent viscosity. This is due to the axial velocity profile across the annular gap becoming flatter as the flow behaviour index decreases, causing less axial flow dispersion. It was also found that gaps between blades created disturbances in the flow pattern, which contributed to axial dispersion. This effect increased with increasing rotor speed. Information such as that obtained in this work can be used to optimize the processing of food products in SSHEs.