How Spatial Constraints Govern Aerodynamic Performance of Archimedes Spiral Wind Turbines: A CFD-Based Comparative Analysis

Archimedes Spiral Wind Turbines (ASWTs) are suitable for small-scale urban wind applications, but changing the blade angle can also change the rotor dimensions under different geometric constraints. This study examines these coupled effects by comparing five ASWT configurations with blade angles of 30°, 45°, and 60° under fixed-diameter (Fixed D) and fixed-axial-length (Fixed L) conditions. Steady three-dimensional Reynolds-averaged Navier–Stokes simulations using the Multiple Reference Frame method were conducted to evaluate the power coefficient (Cp), torque coefficient (Ct), and mid-plane pressure and velocity fields. The numerical setup was assessed through grid-independence and reference-data comparisons. Under the Fixed D constraint, the 60° configuration achieved the highest Cp of 0.2915 at a tip-speed ratio (λ) of 1.9, whereas the 30° configuration reached a maximum Cp of 0.1792 at λ = 1.0. Under the fixed L constraint, the corresponding Cp values were 0.2869 at λ = 2.5 for the 60° configuration and 0.1713 at λ = 0.8 for the 30° configuration. The baseline 45° configuration achieved a maximum Cp of 0.2444 at λ = 1.5. The torque coefficient decreased with increasing λ for all configurations, while the pressure and velocity fields differed between the two constraints at the same blade angle. These results indicate that blade angle should be evaluated together with rotor diameter, axial length, and the installation envelope, because a larger rotor or swept area does not necessarily produce a proportional increase in normalized aerodynamic efficiency.