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crometers for bacteria or even larger for droplets containing
a high number of germs, so that, both laws have to be em-
ployed depending on the studied microorganisms.
Hence, Table 1 reported the computation results for the
flow field characteristics, whereas Figure 2 evidenced the
probability pattern of spherical particles with diameters
ranging from10 nmto 10 µmto hit the photocatalytic surface,
in photoreactors with annular spaces ranging from 6 to 46
mm, at a constant flow input (5 m3
/h). Thus, this describes
the behavior of most of the microorganisms, classified in
different aerodynamic groups, fromviruses usuallywith sizes
in the 10 100 nm range to bacteria and bacterial spores in
the 0.5 3 µm range. Dust or droplets composed of more
than one AMO should be considered as larger particles. The
decreases of the residuals for the continuity and the velocity
field components to values lower than 10 3
are usually
considered as a good convergence indicator for the simula-
tions (25). One should add that the reactor could be
considered in a first approximation as being close to a plug
flow reactor, due to residence times ofmicroorganisms with
a narrow distribution.
Figure 2A shows that the probability formicroorganisms
to hit the photoactive surface of an annular photoreactor
remains very low, mostly under 10%, whatever the annular
space. This was especially true for submicronic particles, for
example, viruses or small bacteria, which actually follow the
main stream. This hit probability increasedwhen increasing
the passage time in the photoreactor, which is proportional
to the open section, and thus to the annular space (Table 1).
This observation was in agreement with the fact that larger
the reactor, slower the particles, so that the probability they
hit the photocatalytic boundary increased. One should also
note that larger themicroorganism, higher the hit probability
because of the higher inertia which allows more important
direction changes. This behavior resulted from the fact that
small size particles (e.g., nanosize viruses compared to
bacteria or bacterial spores) will follow the mean air flow
and thuswill be less impacted on the photocatalytic surface.
For a given annular space, the hit probability increasedwhen
increasing the particle diameter, so that a hit probability of
about 35% could be obtained for 10 µm diameter large-size
microorganisms at low passage times. However, the il-
lumination of the photoactive surface quickly decreaseswith
the increase in the external radius of the annulus, so that,
for an equal hit probability, the shortest annular space should
be preferred, since the illuminated surface remains more
active. Another possibility for enhancing the hit probability
through the use of a large photoreactorwith very lowinternal
speed would consist in the use of an external illumination.
Unfortunately, even if the efficiency of this configuration
was already reported, it would require the use ofmany light
sources for surrounding the reactor, leading to very restrictive
extra costs.
Bactericidal Efficiency of a Annular Photoreactor. The
simulations showed that the hit probability of microorgan-
isms increasedwith the size of the annular space, and reached
a plateau for annular spaces larger than 25 30 mm (mainly
for particles with diameter lower than 5 µm, that is, usual
viruses, bacteria and bacterial spores). Indeed, large annular
spaces lowered the flow velocity inside the reactor and
increased the residence time; on the other hand, too large
annular spaces decreased the hit probability on the biocidal
surface since the surface-to-volume ratio decreased. Since