Shift of the Maximum Efficiency Point Induced by Manufacturing Tolerances in the Radial Clearance of Archimedes Screw Turbines: A Transient Multiphase CFD Analysis

Archimedes screw turbines (ASTs) are a well-established technology for low-head micro-hydropower generation, whose performance is highly sensitive to the radial clearance between the helical blade and the casing. In engineering practice, manufacturing and assembly tolerances frequently result in realized (as-built) clearances larger than the optimal design (as-designed) value; however, the literature lacks a quantitative assessment of how this discrepancy affects the performance curve and the position of the best efficiency point (BEP). This study numerically investigates a 1.0 m3/s, 2.1 m head AST in a closed tubular configuration, adopting a 15.00 mm radial clearance that exceeds the analytical optimum (6.36 mm) by 136% as a worst-case construction scenario, representative of both relaxed manufacturing tolerances and severe diametral wear. A three-dimensional, transient, multiphase CFD model (RANS SST k–ω with a volume-of-fluid free-surface formulation) is applied over six flow-rate conditions (0.55–2.00 m3/s) at constant rotational speed, with mesh independence verified using the Grid Convergence Index. At the nominal design flow rate, the overall efficiency decreases to 65.0%, 18 percentage points below the leakage-limited prediction of the one-dimensional model for the same clearance (83.3%) and 28 points below the ideal design (93.0%), i.e., a penalty nearly three times the one-dimensional estimate. The fitted best efficiency point shifts to 1.44±0.27 m3/s (95% CI), approximately 44% above the design flow rate, with an interpolated maximum efficiency of 66.0%; the efficiency plateau (η≥ηmax−1 pp) spans 1.16–1.72 m3/s and excludes the design flow rate, whereas the fixed-speed one-dimensional model predicts a shift of only +10% by volumetric compensation of the leakage. The discrepancy with the analytical model is attributed to two mechanisms: turbulent dissipation of the leakage jet and, more importantly, the internal nature of overfilling in a closed casing, where excess flow recirculates above the shaft and is retained by the machine rather than being treated as a terminal loss. The discharge also exhibits pulsatile behavior at the rotational frequency (0.52 Hz), with a relative amplitude that decreases from 43% to 29% of the mean torque over the simulated range, with a secondary local maximum near the best efficiency point, carrying direct implications for shaft fatigue design. The study is an exploratory numerical investigation whose predictions constitute testable hypotheses for experimental validation; these findings demonstrate that radial-clearance manufacturing tolerances require an operational reclassification of the intake flow rate and provide criteria for incorporating clearance effects into the design and long-term planning of low-head micro-hydropower projects.