5
Transport Phenomena
The transport of momentum, heat and mass, or in simpler terms the
phenomena of flow, convection, heat conduction, and diffusion, are
primarily studied by process engineers and in some aspects also by
rheologists. Important though these topics are for the food technologist,
they are not the subject of this book. However, some basic concepts are
needed in various chapters; this includes aspects of rheology and
hydrodynamics. Furthermore, transport phenomena inside solidlike foods
often are rather intricate, and this subject is also introduced in this chapter.
5.1 FLOW AND VISCOSITY
Rheologists study what happens with a system, be it an amount of fluid or a
piece of solid material, when work (¼mechanical energy) is applied to it.
Remembering that work equals force times distance, we come to the
definition:
Rheologyis the study of the relations between theforceacting on a
material, its concomitantdeformation, and thetime scaleinvolved.
To keep the relations simple, rheologists tend to usestressrather than
force; it is defined as the force divided by the area over which it is acting (S.I.