dc.description.abstract |
Archimedes Screw Turbine (AST) is a growing technology in micro-hydropower
generation, specifically suited for sites having very low hydraulic heads of less than
5m. However, mechanical efficiencies of installed ASTs are limited to a range of 60%
to 80%. The main objective of this research was to numerically investigate how both
the mechanical power and mechanical efficiency of the AST are impacted by varying
related parameters: angle of inclination/screw length (β/L) and number of blades/pitch
(N/S). Specific objectives were: to develop a 3D CAD model of an AST using typical
parameters and dimensions and hence simulate flow through the machine; to determine
the combined effect of each of the two sets of related parameters (β/L and N/S) on
torque, mechanical power and efficiency of an AST and finally to establish optimal
values of AST’s parameters based on the results obtained from the second objective.
3D geometry of a reference AST of dimensions N=4, β=24.5 o , L=617 mm, external
diameter (D o )=381 mm, internal diameter (D i )=168 mm and pitch (S)=381 mm was
developed using two CAD software packages: Solidworks and Design modeler.
Tetrahedral and hex dominant mesh types were applied for the rotating and stationary
parts respectively. CFX 2019 R1, which is an ANSYS-based CFD (Computational
Fluid Dynamics) code, was used to numerically analyze the flow in order to determine
pressure field and generated torque from which both mechanical power and efficiency
of the turbine were computed. The flow was modeled as transient, multiphase (water
and air) and turbulent, hence K-ε turbulence method was used to solve Reynolds
averaged Navier-Stokes equations. Pressure was found to increase radially outwards to
a maximum value (1447 Pa for reference screw) at the tip of blade. Torque oscillated
about mean value (4.04 Nm for reference screw) with an amplitude that decreased with
decreasing rotational speed. Decrease in inclination angle (β) and corresponding
increase in screw length (L) led to increase in torque, mechanical power and
mechanical efficiency. Highest values of both average and peak efficiencies were
83.4% and 89.4% respectively, produced by the screw inclined at 10 o . Increase in
number of blades (N) and corresponding increase in pitch (S) did not show a clear
pattern on efficiency, but the screw geometry having 4 blades produced the highest
average efficiency of 71.9% at a bucket width ratio (Wbr) of 0.17. Thus, optimal
parameters were N = 4, W br = 0.17, smallest tested value of β (10 o ) and n in the range
of 30 rpm to 40 rpm. Numerical results were validated using data from experimental
study of AST by Simmons et al. (2019). Peak efficiency from the numerical results
closely estimated that from experimental data, especially for rotational speed not
exceeding 45 rpm. In conclusion: slower rotational speeds reduce the amplitude of
torque fluctuations; designs based on related parameters (β/L and N/S) were found to
improve both the mechanical power and efficiency of the AST (for example, the
highest average mechanical efficiency improved by 15.7% from 67.7% for designs
based on β to 83.4% for designs based on β /L); further, AST designs based on β /L had
more impact on AST’s efficiency than those based on N/S which had an improvement
on average mechanical efficiency of 4.2% from 67.7% for designs based on N to 71.9%
for designs based on N /S. The following areas were recommended for further research:
tests on related parameters at different values of diameter ratio, D r (this study used D r =
0.44); effect of torsional yielding and sagging on maximum screw length; and lastly,
since scaling for AST has not been established, tests on the related parameters using
full-size prototype ASTs should be conducted. |
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