Abstract
Optimizing blade geometry is one of the key methods for improving cross-flow turbine performance without adding additional complexity to their design. While symmetrical airfoils have conventionally been preferred for their versatility at both positive and negative angles of attack, the curved blade path leads to a “virtual” camber of the foil, resulting from changes in local angle of attack. This leads to the question: what direction should we camber a blade to improve the efficiency of a cross-flow turbine? Concave-in, opposite to the virtual camber, to achieve hydrodynamics more similar to a symmetrical blade, or concave-out where the virtual camber is accentuated. Each of these would optimize different regions of power generation, with outward camber improving max power in the upstream while inward would result in reduced upstream performance but improve reattachment in the downstream reducing the losses of the turbine. Prior work has applied multiple streamtube and CFD methods which inherently struggle to model the complexities of dynamic stall and induction that are critical to cross-turbines. As a result, we do not know which direction of camber is likely to be most advantageous and whether a general set of rules can be identified for cross-flow turbine foil geometry across the design space.
This work experimentally tested the performance of a one- and two-bladed cross-flow turbine of 0.47 chord-to-radius ratio with symmetrical, NACA 0018 blades, and compared it to positively (concave-out) and negatively cambered (concave-in), NACA 2418 bladed turbines that have a similar amount of camber to the virtually induced camber of the turbine chosen. Additionally, the in-rotor and near-wake flow fields were captured using particle image velocimetry (PIV) to link the hydrodynamics and performance. Experiments were conducted in the Alice C. Tyler flume at a blockage of 9.5% and under conditions of constant Froude and Reynolds numbers. From the collected data, we identify the impacts of camber on turbine phase and time-averaged performance as well as turbine loading and the effects on the in-rotor flow field.
The symmetrical airfoil had the highest time-averaged peak performance for the two-bladed turbine, however at higher tip-speed ratios (TSR) and for a one-bladed turbine, the highest-performing configuration was the negatively-cambered blade, which decreases performance in the upstream power stroke while improving flow recovery and detrimental performance in the downstream sweep. Meanwhile, at very low TSR the positively cambered blade performs the best, suggesting that the impact of improving downstream flow recovery is less at lower tip-speeds and that it is more important to improve upstream power production. Testing the cambered blades across different preset pitch angles also showed consistent relative behavior. It is clear that camber and performance are closely linked to power production and flow recovery in a complex manner that leads to no clear winner in all regimes. But those turbines that operate at higher TSR might be benefitted more by helping upstream performance. More work is needed to explore the influence of the position and amount of maximum camber on overall performance.