Increased utilization of hydraulic turbines as a regulatory tool for electrical grid stabilization forces some turbines to operate away from their design point, thus increasing wear and tear. In here, rotating vortex rope (RVR) mitigation by means of radial insertion of cylindrical rods in the draft tube is investigated experimentally on a 10 MW Kaplan turbine operating as a propeller. Pressure measurements in the draft tube, runner chamber and spiral casing, along with strain and acceleration measurements on the turbine shaft are performed to scrutinize the effectiveness of the mitigation system. Three part-load operating points are investigated, corresponding to 83%, 72%, and 68% of the guide vane servo stroke relative to the best-efficiency point opening. It is shown that the mitigation system dampens the harmful effects of the vortex rope at all operating points, especially at lower part-load conditions. Specifically, the pressure amplitude of the RVR fundamental mode inside the runner chamber reduces by 84%, 63%, and 73% at the three investigated operating points, relative to the amplitudes without mitigation. On the turbine shaft, the fundamental mode of the axial thrust oscillations at the RVR frequency reduces by 65%, 83%, and 95%. It is shown that the mitigation does not come with a high cost on the turbine time-averaged efficiency over the course of the measurements, the penalty being 2%, 2.5%, and 3.2% at the protrusion length where optimal mitigation is achieved at each operating point. For high-head machines, the penalty is expected to be lower since the relative importance of the draft tube diminishes for higher heads.

Under off-design and transient operations of hydraulic turbines, these machines are subjected to harmful pressure fluctuations originating from the presence of vortical flow. These oscillations increase stress-induced fatigue damage on the turbine runner, shortening turbine life and reducing its reliability. This study investigates how cylindrical rods in the draft tube affect the runner blade strains and their consequent fatigue damage during transient and off-design steady-state operations. Different part-load conditions of an axial model turbine and two transients between speed-no-load and best efficiency point were experimentally studied using time-resolved pressure and runner blade strain measurements. The proposed adjustable flow control technique effectively reduced the runner damage, particularly at lower loads where reductions as high as 70% were obtained. In addition, draft tube pressure data were used for fatigue estimation, and a correlation with blade stress damage was observed at lower loads where high-amplitude load cycles occurred. The results showed that different protrusion lengths should be used function of the prevailing operating condition to obtain optimal damage reductions with lower efficiency penalties. Therefore, the proposed technique can provide an adjustable solution that mitigates off-design pressure oscillations and their consequent damage while limiting efficiency losses.

The global demand for electricity generation has been increasing over the recent decades and is expected to grow steadily. Therefore, activating new energy sources to improve the present capacity of electricity production is inevitable. In addition, the reduction of greenhouse gas emissions is another growing concern that is increasingly promoted. Consequently, the integration of clean sources of energy into the electrical grid is a necessity. Renewable energy sources such as wind and solar appear as practical and easy-to-harvest solutions that are clean and sustainable. Thus, their penetration into the electrical grid is both encouraged and pursued on a global scale. However, these sources are intermittent and have a slow regulation response, and an electric network predominantly comprising such sources faces challenges in adapting to the market’s fluctuating demand. As a result, a rapid-response auxiliary source is required in such networks to balance the grid output and guarantee a stable supply of electricity. Hydropower is an ideal and clean alternative that can adopt this regulation role due to its short response time. The shift in hydropower implementation towards this new role requires a broader range of operations with more frequent transitions between the design and off-design conditions. However, current hydro turbines are designed to operate at a limited range of the highest efficiency, termed the best efficiency point (BEP). When operating away from the BEP, hydro turbines confront adverse flow-induced phenomena such as vortex breakdown that can induce pressure pulsations and periodic loadings. These oscillations can cause power swings and aggravated wear rates on turbine compartments through increased fatigue. Part-load (PL) turbine operation is a condition where the precession of a rotating vortex rope (RVR) in the turbine diffuser induces harmful oscillations. With prolonged turbine PL operations, these machines are expected to face a shortened life span and increased repair cycles. Therefore, the need for practical flow control methods to reduce the pressure pulsations under PL is growing.

The present thesis aims to introduce and investigate the concept of protrusion-based methods to mitigate PL pressure pulsations. The latter is attempted by perturbing the flow in the turbine with the radial insertion of solid bodies into the draft tube. The proposed geometries include cylindrical rods and flat plates. The impact of cylindrical rods has been examined on multiple scales of axial turbines, including a downscaled model turbine, a model turbine, and a prototype. These effects were observed with different measurement tools depending on the investigated turbine. The obtained resultspresented in this work for rod protrusion experiments include turbine operation parameters, timeresolved pressure data, strain data, flow visualization, and efficiency. As an improvement to the rod protrusion concept, the flat plates were tested on the downscaled model turbine. The results from these measurements consist of turbine operation parameters, time-resolved pressure data from the draft tube and vaneless space, and efficiency.

Investigated under different PL conditions, the rods could effectively mitigate the RVR-induced pressure pulsations at upper and lower PL conditions. The obtained mitigation rates under these conditions reached as high as 80%. In addition, this method proved effective in reducing flow induced fatigue at lower PL and even speed-no-load (SNL) conditions. However, rod protrusion entailed mixed results under the PL conditions where the RVR induced the strongest periodic oscillations. Moreover, the rods caused a maximum efficiency drop of approximately 3% of the BEP efficiency. More importantly, for all the investigated scales, under the conditions where the rods appeared effective, an optimum protrusion length was found where the most significant mitigation occurred. On the other hand, the investigation of plates on the downscaled model showed complete mitigation of the RVR-induced pressure pulsations under the entire turbine PL range, and the turbine efficiency was even improved under lower PL conditions.

The investigation of both protrusion-based methods verified the need for an adjustable mitigation technique that can adapt to variable flow conditions under different PL ranges. Protrusion-based flow control systems can be incorporated as a new degree of freedom in the existing turbines and modify their operational maps. Thus, depending on the turbine operating condition, the solid bodies can be protruded at a given length or retracted completely to reduce the flow-induced adverse effects with marginal efficiency penalties. Hence, the turbine can operate at an extended range with fewer consequences, which will be a key requirement for hydropower in the future.

The present paper investigates the rotating vortex rope (RVR) mitigation on an axial turbine model by the radial protrusion of four cylindrical rods into the draft tube. RVR mitigation is of particular interest due to the unfavorable pressure pulsations it induces in the hydraulic circuit that can affect turbine life and performance. The protrusion lengths, which were the same among the four rods, were varied according to a predefined sequence. The experiments were performed under four part-load regimes ranging from upper part load to deep part load. Time-resolved pressure measurements were conducted at two sections on the draft tube wall along with high-speed videography and efficiency measurement to investigate the effect of the mitigation technique on the RVR characteristics and turbine performance. The recorded pressure data were decomposed and studied through spectral analyses, phase-averaging, and statistical analyses of the RVR frequency and peak-to-peak pressure amplitude distributions. The results showed different levels of pressure amplitude mitigation ranging from approximately 10% to 85% depending on the operating condition, protrusion length, and the method of analysis. The hydraulic efficiency of the turbine decreased by a maximum of 3.5% that of the best efficiency point (BEP) with the implementation of the mitigation technique. The variations in the obtained mitigation levels and efficiencies depending on protrusion length and operating condition indicate the need for the implementation of a feedback-loop controller. Thus, the protrusion length can be actively optimized based on the desired mitigation target. 

The implementation of hydropower to stabilize electrical grids dictates more frequent off-design operations of these renewable energy resources. Flow instabilities under such conditions reduce the efficiency of hydro turbines. Part-load operation is particularly detrimental since the development of a rotating vortical structure termed rotating vortex rope (RVR) in the draft tube leads to periodic pressure pulsations that jeopardize turbine performance. This paper experimentally explores a novel solution involving the protrusion of flat plates into the turbine draft tube. Three flat plates equally separated by 120° were vertically installed on the draft tube wall. The plates were protruded up to 83% of the draft tube local radius under four different part-load conditions. Their impact was observed through time-resolved pressure measurements in the draft tube and vaneless space, as well as efficiency measurements. The results demonstrated successful RVR mitigation, achieving a maximum 85% reduction in pressure oscillation amplitudes. Protruding flat plates disrupted RVR periodicity and coherence, confining its orbit to the draft tube center. This approach proved particularly effective at lower part-load conditions, enhancing turbine hydraulic efficiency by increasing torque extraction. Reducing the adverse effects under part load, the proposed method appears promising in extending the operational range of hydraulic turbines.

Francis turbines are now widely used to support the integration of renewable and intermittent energy sources such as solar and wind power. Consequently, these turbines often operate away from their best efficiency point (BEP). Such operations cause detrimental pressure fluctuations in the runner and draft tube, leading to early fatigue failures. To address these harmful flow conditions and extend the operating range of Francis turbines, a mitigation system was developed and tested on a large-scale, high-head 200 MW Francis turbine. The system consists of four circular rods placed in the draft tube with variable radial protrusion lengths, adjustable using linear actuators. Pressure, accelerometer, and vibration sensors installed on the turbine allowed quantification of the rod system performance. The results demonstrate the system’s capability to reduce pressure pulsations by up to 80 % in terms of maximum pressure amplitude and 100 % in terms of fatigue cycle in both low and high-frequency ranges, up to ten times the runner frequency, based on pressure analysis. The optimal rod protrusion ranges from 5 to 20 % of the runner outlet diameter function of the operating load. The impact of the rods’ protrusion on the turbine structure appears negligible from the accelerometer measurements performed on the draft tube and spiral casing. The hydraulic efficiency is reduced by up to 1 %. These findings are significant across a wide range of part-load operations, from 40 % to 60 % load, indicating the potential to extend the operational range of existing Francis turbines. The research presented here is a novel attempt to enhance the existing Francis turbines with a new degree of freedom using protruding rods.

The present work examines the effects of the radial protrusion of four cylindrical rods at different lengths within the flow field of a down-scaled turbine draft tube under part-load operating conditions. Four rods were placed on the same plane 90 degrees apart. The protrusion length was varied from zero to approximately 90 % of the draft tube radius. Time-resolved pressure measurements were performed to quantify the effect of the rod protrusion, using two pressure sensors at the same vertical level 180 degrees apart. Such sensor configuration enabled the decomposition of the signals into rotating and plunging components of the rotating vortex rope (RVR). The results show that different levels of mitigation are achieved for the rotating and plunging components depending on the protrusion length. The effects on the plunging component differ from the ones on the rotating component. The RVR plunging pressure pulsations slightly increase with the initial rod protrusion and then significantly drop after a certain length. On the contrary, the rotating component of the pressure pulsation amplitudes immediately decreases with the onset of rod protrusion. However, an optimum length is obtained in both cases where the highest mitigation occurs before reaching the maximum protrusion. This observation falls in line with the previous investigations conducted for oscillatory rod protrusions, further approving the point that a closed-loop controller should accompany the mitigation technique to achieve optimum mitigation.

In this study, the flow in the conical section of the draft tube of a propeller turbine has been investigated at the best efficiency point and part-load operating conditions using 2D and stereoscopic 3D particle image velocimetry. Since the flow in the turbine is periodic, it is necessary to study the mean flow field rather than the instantaneous one to identify the flow characteristics from a statistical standpoint. However, the statistical convergence of the obtained mean velocity is questionable. Thus, the current work proposes a methodology for investigating the convergence of mean velocity profiles based on the central limit theorem. The methodology is applied to the best efficiency point and part-load results. The results show that 3D PIV results have lower uncertainty than 2D PIV results because measuring the tangential velocity component affects uncertainty, only measured in 3D PIV. The uncertainty difference is more significant, especially in part-load operation, due to the presence of the rotating vortex rope, and therefore a more accurate measurement is necessary to produce a reliable mean flow field. Furthermore, the convergence of the mean velocity profile is faster, with lower uncertainty for best efficiency point results since, at the part-load condition, the tangential velocity component of the flow is higher. In addition, the converged mean velocity profiles show a backflow region with minor rotation in the center, surrounded by a high rotational axial flow during the part-load operation of the turbine.