Numerical study on high-fidelity flow field around vanes of a Francis turbine

Abstract

Current Francis turbines are encountering vibration issues, potentially attributed to the flow instabilities around the guide and stay vane cascades. To explore the impact of the flow field on the vibration mechanism, the current study implements a high-fidelity spectral element method to predict intricate turbulent activities and performs cascade models based on a high-head prototype turbine that experiences severe vibration at an attack angle of 30°. The findings reveal significant effects of the narrow guide vane passage on pressure distribution at the low head, while the combination of flow velocity and the passage width induces the highest pressure magnitude at the high head. Favorable pressure gradients and incoming flow alternation cause elongated vortical structures with an approximate length of the guide vane (1.46 m), forming on the pressure side of the guide vanes and at the entrances of the guide vane passages, respectively. At the high head, intense flow separation induces a high-stress region (−30 < u′v′¯ < −20), connecting the downstream half of the stay vane with the stagnation point of the guide vanes at the attack angle of 39.5°. The operating condition with the attack angle of 30° and high head exhibits a larger flow velocity compared to the smaller attack angle and features a narrower guide vane passage than the larger attack angle, causing the largest fluctuating energy (K> 0.14) with 4.5 times the cross-sectional area of the guide vane in the vaneless region and the most unfavorable flow instabilities around the cascades.