(Layer 1, 4 and 8). Note that although all computational models are
evaluated for the same air speeds, the pressure drop over each
model differs, with much lower values when a gap is present.
Although this boundary condition implies that the
flow rate per
meter squared at the inlet is the same, the total
flow rate increases
with gap width (Table 1). A comparison of different gap widths at
the same air speed does not entirely correspond to reality as the
working point of the fans will also change if gaps are present, due
to the change of the system (resistance) curve of the container's
airflow circuit. The aim of the present study was, however,
primarily to gain elementary insight in the magnitude of the
impact of gaps on the cooling rate.
From Fig. 8, the SECT clearly increases with increasing gap
width for all air speeds. A complex dependency of the SECT on the
gap width and the air speed is found, which differs for each layer of
boxes. The large impact of such gaps on the fruit cooling rate
stresses the importance of minimising such airflow short-circuits
between pallets, since otherwise a large amount of refrigerated air
will bypass the produce in this way. The presence of gaps, however,
improved cooling uniformity (i.e., differences in SECT) between
different layers on the pallet.
4. Discussion
Despite being a promising alternative cold-chain protocol, as it
avoids pre-cooling of fruit prior to loading in refrigerated
containers, ambient loading is very challenging due to the many
practical restrictions inherent to the method, amongst others: (1)
the airflow rate and installed cooling capacity in a reefer container
are limited and are prescribed by the container type; (2) vertical
airflow induces a longer pathway for air through the fruit,
compared to horizontal pre-cooling over a single pallet; (3) box
design and stacking on the pallet are often not optimised for
vertical airflow and a wooden pallet base also blocks a significant
amount of vent holes; (4) limited accessibility during loading for
closing airflow short-circuits between pallets. Below, the limitations
of the current computational model are highlighted, the
feasibility of ambient loading is discussed in light of the
findings of
the present study and possible ways forward to optimise the
method are given.
4.1. Computational modelling of container cooling
To assess the cooling process of fruit within a refrigerated
container, CFD was shown to exhibit unique advantages, such as
the ability to evaluate the more relevant volume-averaged fruit
temperature instead of that in the centre of the fruit but also the
cooling of each individual fruit in a box, thus the intra-box
heterogeneity. Such heterogeneity within a single box could
explain the variation of chilling injury that is normally found
within a box after commercial export when applying a cold
disinfestation protocol. CFD is thus clearly a valuable tool for
evaluation and optimisation of cooling processes of fruit in reefer
containers, and nicely complements experimental work.
However, some model simplifications were made in the
simulations to limit the computational cost. The most important
one is the use of an idealised stacking pattern on the pallet,
implying perfect vertical channelling of airflow since there is no
blockage of vent holes by subsequent boxes and by the wooden
pallet base (for the
first layer of boxes). As such, the reported SECTs
are somewhat too optimistic and will differ to some extent from
reality. A next step could be to model an entire pallet (Fig. 2b),
including short circuits between the pallets, which would however