Numerical study on the influence of variable-speed operation on flow stability during the load rejection process of Francis turbines

Hydropower systems play a crucial role in enhancing grid flexibility. In order to improve the operational stability of hydroturbines during load-rejection conditions, this study utilizes dynamic meshing and separated vortex simulation methods to perform numerical analyses on the load-rejection processes of the Francis-99 hydroturbine in both fixed-speed and variable-speed modes. This investigation systematically analyzes the internal flow characteristics associated with three regulation schemes: fixed-speed guide vane closure (Scheme 1), variable-speed coordinated regulation (Scheme 2), and high-initial-speed fixed-speed closure (Scheme 3). The characteristics of pressure pulsations for all three schemes were analyzed utilizing the Hilbert-Huang transform(HHT) method. The final results indicate that, in comparison with the fixed-speed scheme, Scheme 2 effectively mitigates the abrupt flow reduction induced by guide vane closure. This approach significantly suppresses water hammer effects and reduces pressure pulsation amplitude, while simultaneously enhancing pressure field uniformity and transient stability, the total entropy production decreased by 4.68%. Merely increasing the initial rotational speed (Scheme 3), while elevating the overall pressure energy level, exacerbates the flow angle mismatch, leading to stronger low-frequency pulsations and periodic spectral fluctuations, thus posing greater challenges to operational stability. Furthermore, variations in rotational speed significantly influence the dominant frequency and energy distribution of pressure pulsations in the impeller inlet region, while exerting lesser effects on pulsations in the draft tube region, which are primarily determined by blade passing frequency. This investigation elucidates the mechanism through which active speed regulation enhances transient hydraulic performance, thereby providing a foundation for optimizing the control of variable-speed turbines.