4 - Dimensional Analysis of Processes Influenced by the Variability of Physical Properties

Abstract

Food processes involve matter (fluid, emulsion, suspension, etc.), which is placed in equipment in order to be transformed. In numerous cases, one (or several) physical properties of the matters change between the inlet and the outlet of the equipment. There is also very often a spatial and/or temporal distribution of these physical properties within the equipment insofar as they are dependent on various potential fields (composition, temperature, etc.). This dependence has a significant effect on the process and must be taken into account. For instance:–the variation of viscosity with temperature which can be observed within an exchanger during thermal processing. This variation in viscosity with temperature modifies the heat transfer performance of the process when compared to a fluid whose thermo-dependence is negligible;–the variation of apparent viscosity with shear rate for non-Newtonian fluids. The distribution of apparent viscosity significantly influences the velocity field within the equipment compared to that obtained when viscosity is constant (Newtonian fluids). For example, this is the case for the flow of fluids presenting a yield stress in a stirred tank: a formation of caverns is thus observed, which does not occur during the agitation of Newtonian fluids.