Parametric analysis of archimedes screw turbine for micro hydropower generation using CFD

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.