Comparative study on the performance, flexibility, and flow characteristics of very low head turbines: Numerical and experimental insights into original model and its optimized variant

This study presents a comprehensive comparative analysis of the performance and flow characteristics of two Very Low Head (VLH) turbine models: an original and an optimized model. The original turbine is created utilizing an established algorithm for designing existing models, which was subsequently implemented to suit the experimental setup. The turbines were installed in an open channel within the Marine and Hydrokinetic Energies laboratory at the University of Tehran. Key parameters, including rotational speed, flow rate, head, and output power, were measured to evaluate the performance of the turbines. Characteristic curves for dimensionless parameters, including efficiency, head coefficient, and power coefficient against the flow coefficient were plotted and compared for both turbine models based on experimental findings. The results indicate a higher efficiency of the optimized turbine (79.51 %) compared to the efficiency of the original turbine (74.88 %) at their best efficiency points. Moreover, the optimized turbine exhibits greater output power across all operational conditions than the original model, reaching 38.27 W during BEP, while the power output of the original turbine at its BEP was 31.73 W, according to the experimental tests. Additionally, experimental data shows that the optimized turbine achieves a peak power output of 46.79 W, whereas the original turbine reaches 41.26 W. To gain insight into the complex flow physics, 3D transient computational fluid dynamic simulations (CFD) were carried out by Ansys CFX software. CFD simulations were carried out by using the homogeneous two-phase model for capturing free-surface flow, the SST turbulence model, and the Transient rotor/stator approach for modeling the flow physics at the rotating and stationary interfaces. While generally aligned with experimental results, the CFD simulations showed slight discrepancies, with a 7.69 % and 6.01 % higher turbine output power for the original and optimized turbines, respectively, compared to experimental data. Numerical analyses further underscored the performance gains achieved with the optimized model. Specifically, the optimized turbine exhibited an output power of 40.57 W at BEP compared to the 34.17 W of the original model. Furthermore, CFD results indicate that at this point, the efficiency of the optimized turbine is 84.31 %, whereas the original turbine's efficiency stands at 80.64 %. Factors such as mechanical losses, leaks, and manufacturing tolerances overlooked in simulations, along with limitations of the homogeneous model neglecting certain phase interaction losses, contribute to disparities between CFD simulations and experimental results. This study includes a comprehensive investigation utilizing the geometric attributes and fundamental principles of turbomachinery to elucidate the performance variations between turbine models.