Modelling the heat treatment of a starch suspension inside a tubular exchanger: the influence of food product transformation on residence time distributions

摘要

An experimentally based thermo-kinetic model of starch granule swelling is coupled with a fluid flow / heat transfer numerical model, allowing a tool for studying the evolution of a starch suspension in laminar flow inside a tubular heat exchanger. Both granule swelling and suspension viscosity are the highest near the heating wall, where fluid particles move slower and reach higher temperatures. Fluid particles running at the exchanger’s axis have the highest velocity; when heat transfer and food product transformation are both considered, the fastest particles spend only 43 % of the mean residence time for leaving the domain. Such a value is significantly lower than for an isothermal flow without transformation where the lowest residence time is 50 % of the mean value. This result indicates that the two-way coupling of fluid flow, heat transfer and transformation has to be considered in this case. The evolution of food product properties is also studied with the help of two hypothetical trajectories of fluid particles released at the inlet, the first at 1.0 mm and the second at 0.5 mm from the heating wall. When both fluid particles reach the temperature 66°C (i.e. when the first particle reaches the domain outlet), the mean diameter of starch granules for the second fluid particle was about 10 % greater, and the suspension viscosity was about 45 % greater, than the respective values for the first one. Hence the temperature evolution along the fluid particle trajectories must be properly assessed when the food product viscosity depends on the degree of transformation.