Francis turbines develop an unfavorable flow pattern in the draft tube under off-design conditions when operating as an energy regulator in hydro-wind-solar integrated systems. An adjustable hydrofoil (ADRO) system was newly tested in a high-head Francis draft tube cone model with promising experimental results. This work reproduces numerically the work to elucidate the mechanism and potential in mitigating spiral vortex breakdown. Two part-load operations are investigated, corresponding to 58% and 70% of the designed flow rate. The numerical simulations are validated against experimental results, including global performance, pressure pulsation, and velocity profile with and without ADRO, demonstrating high fidelity. With the ADRO, the vortex rope-induced pressure pulsation amplitude is effectively reduced by 50%, consistent with experimental observations. The observed reduction in vortex rope intensity is associated with a significant decrease in the swirl number, driven by a lowered mean tangential velocity in the ADRO region. The residual vortex rope structures exhibit intermittent amplitude and frequency dependent on the operation investigated, indicating the need to radially adjust the ADRO to further decrease the swirl number and thus the pressure pulsation completely. The revealed mitigating mechanism is useful to inspire the invention of new active wall-mounted control measures for draft tube flows.
Xing et al. (Fri,) studied this question.
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