This paper describes a method for determining the sequence of transient model test experiments in a closed loop test
rig, using previous 1D simulations. An existent power plant with reversible pump turbine in the Austrian Alps serves as baseline. First, real measurement data of power jumps are reproduced in 1D simulations. These simulation results were transferred to the scaled model size via similarity laws. Of particular importance is the choice of model to prototype speed ratio whose influence has been studied more closely. Subsequently, the transient processes are simulated in a closed loop test rig. The simulations are controlled by a temporal power variation of the service pump. An iteration loop with optimizer has adapted the power in order to achieve similar conditions as in the prototype simulation. Due to the good reproducibility of the power jumps in the closed loop model test rig more demanding processes are investigated in the next step. A fast transition with load rejection and guide vane closing has been simulated, as it is state of the art. Furthermore, it was assumed that this power plant also has a full size frequency converter for fast transition from pump mode to turbine mode and vice versa applying linear speed variation. The findings of the pump behavior as well as the test rig behavior can then be used for the transient model test experiments.
Hydraulic machines are designed to operate in flow conditions close to the best efficiency point. However, to respond to the increasing demand for flexibility mainly due to the integration of renewable energy in the electric grid, the operating range of Francis turbines has to be extended towards smaller discharge levels without restriction. When Francis turbines are operated typically between 30% and 60% of the rated output power, the flow field is characterized by the appearance of inter blade vortices in the runner. In these off design operating conditions and due to these phenomena, dynamic stresses level can increase, and potentially lead to fatigue damage of the mechanical structure of the machine. The objective of this paper is to present investigations on the dynamic behaviour of the inter blade vortices and their impact on the runner by using numerical simulations. Computations were performed with different turbulence modelling approaches to assess their relevance and reliability: Reynolds Averaged Navier Stokes (RANS) and Large
Eddy Simulation (LES). Computations aimed to better understand the emergence condition of the inter blade vortices. The analysis showed that vortices can be generated due to poor inlet adaptation at part load, however other vortices can also be due to a local backflow in the runner. The competition between these both phenomena leads to various topologies of the inter blade vortices. The numerical results were compared to experimental visualizations performed on scaled model as well as to previous numerical studies results. The impact of these inter blade vortices on the runner were also investigated by considering the pressure fluctuations induced on the blades. The dynamic loading on the blade has to be known in order to evaluate the lifetime of the runner by mechanical analysis. Different operating conditions have been simulated to understand how the pressure fluctuations depend on the operating conditions. The localization of the pressure fluctuations and their consequences on the frequency signature of the torque fluctuations have been analyzed. This article is presenting a part of the work presented at the 29th IAHR Symposium on Hydraulic Machinery and Systems, Kyoto, 2018 , and presents another vortex topology and a comparison of LES results of several operating conditions.
Reaction turbines, of which Francis turbines, constitute a large proportion of low and medium head turbines installed in hydropower plants. Managing these machines represents a real challenge in terms of efficiency, competitiveness and demands on the energy market. Turbines runner blades exhibit loss of performance from damage due to several reasons. One common source of damage is erosion due to the cavitation phenomenon. Indeed, at a given operating region, rapid changes of velocity can create bubbles in the water flow due to local low pressures. When cavitation bubbles reach pressure recovery, they collapse and may induce wear or erosion in these regions. Even if this phenomenon has been intensively studied in the past decades, cavitation erosion is not fully understood as it is driven by several parameters such as flow dynamic, turbine design, environment, or material properties. Some of these parameters can be studied in laboratory to compare materials resistance between each other. This article aims to model the cavitation by a stochastic model using erosion experimental data observed in the laboratory. The benefit of such models is to consider both the uncertainties and natural fluctuations of the phenomenon. With the proposed framework, the study will highlight the differences observed in cavitation erosion experiments of two common materials used to manufacture Francis’s runners. This study is the first step in a project aiming at the prediction of turbines mass loss due to cavitation erosion on actual operating Francis turbines.
Undershot cross-flow turbines are applicable to shallow open channel flows and are characterized by a simple structure free of guide vanes or a casing. Since these water turbines operate in a free surface flow field, the optimum inlet blade angle and outlet blade angle are considered to vary with the flow rate in the open channel; however, guidelines for setting these angles are yet to be established. In this study, a method to improve the performance of an undershot cross-flow water turbine based on shock loss reduction was proposed. First, by varying the blade angle of the straight blades of the turbine, the inlet blade angle and outlet blade angle which reduced shock loss in the first-stage inlet region and second-stage inlet region were investigated through free surface flow analysis. From the results, it was shown that by taking the inlet and outlet blade angles which minimize the first-stage and second-stage shock losses for straight blades and applying them to curved blades, shock loss is reduced, thereby improving turbine efficiency and turbine output.
Low efficiency is an urgent problem to be solved for vortex pumps due to large number of vortices and backflows. This paper concentrates on the performance improvement of vortex pumps caused by changing the indent distance of the impeller. First, a complete numerical model was established by selecting the optimal length of inlet and outlet and appropriate mesh quantity. Then, the reliability of this model was validated by comparing simulation results with experimental data. Based on this model, the internal flows of vortex pumps with different impeller indent distances were observed in simulations and the fitted curves of head and efficiency were created. Finally, the cause of the performance improvement was demonstrated by analyzing the flow field in vortex pumps.
To improve the energy conversion efficiency and cavitation performance of the ultra-low specific-speed centrifugal pump (ULSSCP), the impeller-volute interaction has been studied. Blade outlet setting angle (β2), wrap angle (φ), volute inlet width (b3) and throat area (St) were addressed as the design parameters. The entropy production at 0.5 Q0 and NPSHc (critical net positive suction head) at 1.5 Q0 were selected as the target to characterize the energy loss and cavitation performance. The L9 (34) orthogonal matrix was established via Taguchi method. Results show that the contribution ratio of φ on S/NS is the most vital, followed by b3 and St, while the influence of β2 is relatively small, and the design parameter combination with β2=19°, φ=220°, b3=12mm and St=190mm2 is the best choice for the lowest entropy production at 0.5Q0 and NPSHc. At last, the optimization design reduces the loss greatly before and after cavitation by alleviation of vortex generation and backflow intensity.
Ultra-small axial flow hydraulic turbines, which are of the size of your palm, are a type of turbine that can be applied to the low heads of existing pipelines and open channels. However, due to their compact size, they are more likely to malfunction in case of foreign body contamination. In our study, we observed the passage of foreign bodies through an ultra-small axial flow hydraulic turbine and their encounter with the blocking mechanism of the turbine. We selected polyethylene ropes of varying lengths with a wire diameter of 5 mm to serve as foreign bodies. By varying the length of the rope, we were able to visually observe the movement of the foreign body. The turbine’s blocking mechanism can be broadly classified as guide vanes or runners. In the case of runner, blocking occurs when foreign bodies are bent and are caught at the leading edge of the blade. The passage rate through the hydraulic turbine is largely dependent on the passage rate at the runner section, which decreases proportionally with the length of the foreign body and the rotational speed of the blades.