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catalytic surface. This hypothesis corresponded to a 100%
inactivation efficiency of the biocidal surface, so that the
computation described the motion of particles, but not the
photocatalytic aspects of the reactors (i.e., 100%inactivation
efficiency toward a microorganism if it impacts on the
illuminated surface, at a given and constantUV-A irradiation
rate). A Lagrangian approachwas used, it focuses on particle
tracks, rather than on a control volume like the Eulerian
method. The systemwas considered as isothermal, the flow
governed by the ideal gas law and the AMOs were assumed
not to significantly influence the flow characteristics due to
very low AMO concentrations. The velocity of the flow at the
photoreactor inlet and the initial velocity of the particles
were normal to the inlet boundary, and the outlet boundary
was set at atmospheric pressure.Hence, this simplifiedmodel
did not incorporate the effects of parameters related to the
motion of the air flow, such as the rotation or the shape of
fan bladeswhen they are present.However, Sahle-Demessie
et al. (23) detailed that these parameters have a significant
influence on the process efficiency and should be included
in the investigation of more realistic reactors. Moreover,
thanks to the axisymetry of this geometry, this 3D problemin the Cartesian frame could be replaced by a 2D one using
a cylindrical frame. Details were reported in Table 1.
The mass and the momentum conservation equations
were approximated using the Reynolds Averaged Navier
Stokes equations, the most widely used model since it can
model all turbulence scales (24). The Reynolds numbers
reported in Table 1 were calculated by considering the
hydraulic diameter for an annular duct as being the difference
between both inner and outer diameters of the annular
reactor. Used for the simulations, they indicated a flow
globally ranged in a laminar to transitional regime.However,
in order to accuratelymodel the photocatalytic treatment of
a stream, it is necessary to emphasize on the phenomena
occurring in the boundary layer, near the photocatalytic
surface. This near-wall zone is particularly complex because
of the no-slip condition and the high shear stress at the wall
which result inlarge gradients.Thus, the shear stress transport
k-ω turbulence model (SSTKW) was used to model the
continuous phase, since it is an empiricalmodelwell adapted
for wall-bounded flows. The solution control was based on
the coupled pressure velocity mode, and used explicit
relaxation factors set at 0.75 for both momentum and
pressure, second order for the pressure, and also third order
muscle for themomentum, the turbulent kinetic energy and
the specific dissipation rate. Themotion of the particles was
solved by integrating the force balance equation (written in
the x direction), which equates the particle inertia with the
forces acting on the particle, taking into account the drag
and the Archimedes forces as well as an additional accelera-
tion (e.g., Brownian forces). By contrast to particles having
an aerodynamic diameter larger than 1 µm, it should be noted
that working with particles smaller than 1 µm required to
incorporate the Cunningham’s correction factor to the drag
forces per unit particle mass expression derived from the
Stockes law. The Cunninghamcorrection factor rapidly and
asymptotically decreases down to 1 with increasing the
particle diameter under normal pressure. Details relative to
that section are reported in SI S5 and in ref (24). Microor-
ganisms have variousmorphologies and sizes, ranging from
some 10s of nanometers for small viruses, to a few mi-