Thermal Food Processing

Heat and Mass Transfer in Thermal Food Processing 59
they are cheap in terms of computation time. Predictions by general codes based
on the k-e model are often very different from experimental data. Because the shape
of many food products is very complex, the experimental determination of heat
transfer coefficients remains at the time quicker and much more reliable than
predictions. The calculation based on the current CFD codes has to be used with
caution, and more research is needed to improve near-wall modeling, particularly
around blunt bodies placed in a turbulent flow. A full treatment of turbulence would
require more complex models, such as large eddy simulations (LESs) and Reynolds
stress models (RSMs). However, LES models require large computing resources
and are not of use as general-purpose tools. Because the RSM accounts for the
effects of streamline curvature, swirl, rotation, and rapid changes in strain rate in
a more rigorous manner than the k-e models, it has greater potential to give accurate
predictions for complex flows. However, the fidelity of RSM predictions is still
limited by the closure assumptions used to model various terms in the exact transport
equations for the Reynolds stresses. The modeling of the pressure strain and
dissipation rate terms is particularly challenging. Therefore, the RSM with additional
computational expense might not always yield results that are clearly superior
to simpler models in all cases of flows. The mathematical expressions of turbulence
models may be quite complicated, and they contain adjustable constants that need
to be determined as best-fit values from experimental data. Therefore, any application
of a turbulence model should not go beyond the data range. Besides, the current
turbulence models can be used to guide the development of other models through
comparative studies.
2.4.2 JUDGMENT OF ASSUMPTIONS IN MODELS
Accurate modeling of a thermal process of foods is complex. For simplification
and saving of computational time, some assumptions made in the modeling are
necessary. Most assumptions come from the geometrical dimension and shape,
surface heat, and mass transfer coefficients, food materials properties, and volume
change during thermal processes. Before simulation, whether or not to use a
model of coupled heat and mass transfer or coupled heat transfer and fluid flow
should also be determined.
Sensitivity analysis can make a judgment for the acceptability of an assumption
in the modeling. Some research has been carried out to investigate the sensitivity
of variables of interest, such as temperature on operating conditions of a thermal
system and thermal properties of foods. 99–107 Findings from the research include
that the time- and location-dependent variations in operating conditions, such as
variable temperature and surface heat transfer coefficient, cause the detachment of
the thermal and geometric centers during processing of foods. 99 For simulating a
thermal process with a low heat transfer coefficient, small deviations in the coefficient
may result in large deviations in the core temperature of foods. 100,107 The
disturbances of different means, but with the same scale of fluctuation in processing
the medium temperature, resulted in comparable center temperature variation. 103
For a typical sterilization process, it was found that thermal-physical properties