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Experimental and numerical investigations ofmixing in raceway ponds for algae cultivation
Matteo Prussi a,*, Marco Buffi b, David Casini b, David Chiaramonti b,Francesco Martelli b, Mauro Carnevale c, Mario R. Tredici d,Liliana Rodolfi d,e
a RE-CORD/DISPAA, University of Florence, Italyb CREAR/RE-CORD, University of Florence, Italyc Centre of Vibration Engineering of Mechanical Engineering Dept., Imperial College, London, UKd DISPAA/Department of Agrifood and Environmental Sciences, University of Florence, Italye Fotosintetica & Microbiologica S.r.l., Florence, Italy
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Aim of the paper is to discuss the appropriateness of using
Re number to define the level of mixing in a commercial size
algae pond. The study does not suggest direct ways to increase
pond productivity, as mixing enhancement is only one of the
parameters involved, but highlighting the fact that the com-
mon on-site approach to algae cultivation and pond man-
agement leaves rooms of improvements for water velocity
levels and mixing.
In this work a statistical approach, based on the evaluation
of the z-distribution of the algae cells, has been proposed as
numerical parameter to assess the mixing. In order to calcu-
late the z-displacement of each cell, a 3-Dmultiphase CFD tool
has been used. The procedure has been validated thanks to
the experimental data measured in a 20 m2 RWP. A portion of
a commercial scale 500 m2 pond has been also modeled and
the flow field calculated. The actual trajectory of an algae
particle has been compared to the initial particle z-distribu-
tion so to assess the level of vertical mixing, here intended as
vertical particles displacement.
Results clearly show that in the straight part of the pond
the verticalmixing ismodest and the algae tend to settle to the
bottom. Only the bends are sites where the algae can sub-
stantially change their trajectories, due to the vortexes
generated by flow separation, moving from the bottom to the
top and vice-versa.
Although the velocity considered in the evaluation
(25 cm s�1) is high in terms of energy consumption (paddle
wheel engine and other devices), poor vertical mixing can be
expected if adequate technical solutions to improve mixing
are not taken. For instance, the suppression of the vortexes
along the curves by baffles, typically adopted in commercial
RWPs, has the positive effect of reducing energy consumption
and algae settling in the recirculation zone (curve). Never-
theless, if vertical mixing is linked to the large vortex struc-
tures associated with head losses, this solution has the
drawback to further reduce the global mixing in the pond.
These conclusions also limit the usefulness of the Re
(constant along the pond sections) as a unique parameter to
estimate the real level of vertical mixing in a large pond.
Further work is necessary to define the best compromise
between high vertical mixing, low sedimentation and low
energy inputs. In view of using algae for energy (and/or
biofuels production) the energy saving must be carefully
taken into account and for this the paper is suggesting to
change the approach on mixing evaluation, on the base of a
more detailed statistical analysis of the various parts of
pond.
Acknowledgments
The author would like to acknowledge F&M for the use of their
facilities.
This research work has been carried out also thanks to the
Italian research project MAMBO (MicroAlgae, startingMaterial
for BioOil) launched in June 2009 under NovaolS.r.l. coordi-
nation, with the financial support of Cereal Docks S.p.A., DP
LubrificantiS.r.l., EcoilS.r.l., Fox PetroliS.p.A, NovaolS.r.l., Oil B
S.r.l. and OxemS.p.A., and in collaboration with the Italian
Biodiesel Manufacturers Association Assocostieri.
Nomenclature
ADV acoustic Doppler velocimeter
CAPEX capital expenditures
CFD computer fluid dynamic
Dh hydraulic diameter
FD drag force
F2r Froude number
Gi body force related to the gravity
H free surface altitude
nt number of time steps
OPEX operational expenditures
p,P pressure
RANS Reynolds average NaviereStokes
Re Reynolds number
RWP raceway pond
Sij deformation tensor
t time
ui the three-dimensional velocity field
V average section velocity
VoF volume of fluid
xi space
Greek symbols
r0 density of water
m0 dynamic viscosity of water
rg density of air
mg dynamic viscosity of air
4 fraction value of air/fluid
tij sub grid-scale stress tensor
r dimensionless variable densities
m dimensionless variable viscosities
n kinematic viscosity
l density ratio in VoF-method
h viscosity ration in VoF-method
r e f e r e n c e s
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