Energy is unarguably the key factor for today's economic and social development within nations Electricity as one of many energy forms is a critical input to developing countries in the struggle to the national self-satisfaction in all domains Rural electricity supply involved institutions have recently recommended the pump as turbine (PAT)-based micro hydropower plant (MHP) schemes for remote off-grid electrification, mostly from their economic advantages However, from different published research findings, PAT-based MHP is not only simple and economically feasible, but has presented bottlenecks in the move to its full understanding Moreover, compared to other clean energy technologies, PAT technology has not found much literature in academic published researches, thus contributing to its limited understanding within the community Therefore, the PAT literature availability is one way to level up its understanding, which can be helpful to academic and professional communities In the present study, a state-of-the-art review on the two most challenging PAT aspects, namely PAT performance prediction and PAT flow stability aspects are presented In the presented literature, the selected energy sources history leading to the actual MHP global adoption was first briefly explained, followed by an intensive literature on PAT operations, where details about PAT selection and performance prediction were provided Finally, the PAT flow stability aspects where pump-turbine S-shape and Saddle-type characteristics constitute the main focus, were discussed It is worth an attention to mention that the words “pump-turbine”, “Pump as turbine”, and “reversible pump turbine”; are equally used throughout the whole literature It is within the authors wish that this paper can scale up the reader's PAT technology understanding, thus serving awareness in the same

Investigation on pump as turbine (PAT) technical aspects for micro hydropower schemes: A state-of-the-art review

Numerical simulation of hysteresis characteristic in the hump region of a pump-turbine model

https://repository.tudelft.nl/islandora/object/uuid%3Aaf74378b-cc66-41c3-8770-0a771d42b055/datastream/OBJ/download

An energy conservative method to predict the erosive aggressiveness of collapsing cavitating structures and cavitating flows from numerical simulations

This study presents a novel physical model to convert the potential energy contained in vaporous cavitation into local surface impact power and an acoustic pressure signature caused by the violent collapse of these cavities in a liquid. The model builds on an analytical representation of the solid angle projection approach by Leclercq et al. [“Numerical cavitation intensity on a hydrofoil for 3D homogeneous unsteady viscous flows,” Int. J. Fluid Mach. Syst. 10, 254–263 (2017)]. It is applied as a runtime post-processing tool in numerical simulations of cavitating flows. In the present study, the model is inspected in light of the time accurate energy balance during the cavity collapse. Analytical considerations show that the potential cavity energy is first converted into kinetic energy in the surrounding liquid [D. Obreschkow et al., “Cavitation bubble dynamics inside liquid drops in microgravity,” Phys. Rev. Lett. 97, 094502 (2006)] and focused in space before the conversion into shock wave energy takes place. To this end, the physical model is complemented by an energy conservative transport function that can focus the potential cavity energy into the collapse center before it is converted into acoustic power. The formulation of the energy focusing equation is based on a Eulerian representation of the flow. The improved model is shown to provide physical results for the acoustic wall pressure obtained from the numerical simulation of a close-wall vapor bubble cloud collapse.This study presents a novel physical model to convert the potential energy contained in vaporous cavitation into local surface impact power and an acoustic pressure signature caused by the violent collapse of these cavities in a liquid. The model builds on an analytical representation of the solid angle projection approach by Leclercq et al. [“Numerical cavitation intensity on a hydrofoil for 3D homogeneous unsteady viscous flows,” Int. J. Fluid Mach. Syst. 10, 254–263 (2017)]. It is applied as a runtime post-processing tool in numerical simulations of cavitating flows. In the present study, the model is inspected in light of the time accurate energy balance during the cavity collapse. Analytical considerations show that the potential cavity energy is first converted into kinetic energy in the surrounding liquid [D. Obreschkow et al., “Cavitation bubble dynamics inside liquid drops in microgravity,” Phys. Rev. Lett. 97, 094502 (2006)] and focused in space before the conversion into shock wave energy takes...

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On the relevance of kinematics for cavitation implosion loads

Numerical analysis of non-axisymmetric flow characteristic for a pump-turbine impeller at pump off-design condition

The cavitation erosion remains an industrial issue for many applications. This paper deals with the cavitation intensity, which can be described as the fluid mechanical loading leading to cavitation damage. The estimation of this quantity is a challenging problem both in terms of modeling the cavitating flow and predicting the erosion due to cavitation. For this purpose, a numerical methodology was proposed to estimate cavitation intensity from 3D unsteady cavitating flow simulations. CFD calculations were carried out using Code_Saturne, which enables U-RANS equations resolution for a homogeneous fluid mixture using the Merkle"s model, coupled to a - turbulence model with the Reboud"s correction. A post-process cavitation intensity prediction model was developed based on pressure and void fraction derivatives. This model is applied on a flow around a hydrofoil using different physical (inlet velocities) and numerical (meshes and time steps) parameters. The article presents the cavitation intensity model as well as the comparison of this model with experimental results. The numerical predictions of cavitation damage are in good agreement with experimental results obtained by pitting test.

https://www.jstage.jst.go.jp/article/ijfms/10/3/10_254/_pdf

Numerical Cavitation Intensity on a Hydrofoil for 3D Homogeneous Unsteady Viscous Flows

Flexibility and energy storage seem to be the main challenges of the energy industry at the present time. Pumped Storage Power Plants (PSP), using reversible pump-turbines, are among the most cost-efficient solutions to answer these needs. To provide a rapid adjustment to the electrical grid, pump-turbines are subjects of quick switching between pumping and generating modes and to extended operation under off-design conditions. To maintain the stability of the grid, the continuous operating area of reversible pump-turbines must be free of hydraulic instabilities. Two main sources of pumping mode instabilities are the presence of the cavitation and the rotating stall, both occurring at the part load. Presence of cavitation can lead into vibrations, loss of performance and sometimes erosion. Moreover, due to rotating stall that can be observed as periodic occurrence and decay of recirculation zones in the distributor regions, the machine can be exposed to uncontrollable shift between the operating points with the significant discharge modification and the drop of the efficiency. Both phenomena are very complex, three-dimensional and demanding for the investigation. Especially rotating stall in the pump-turbines is poorly addressed in the literature. First objective of the presented PhD study has been to develop the cost-efficient numerical methodology in order to enable the accurate prediction and analysis of the off-design part load phenomena. The investigations have been made on the reduce-scaled high head pump-turbine design (nq = 27rpm) provided by Alstom Hydro. Steady and unsteady numerical calculations have been performed using code FINE/Turbo with barotropic cavitation model implemented and developed before in the laboratory. Some of the numerical results have been compared to the experimental data. Cavitating flow analysis has been made for various flow rates and wide range of cavitation levels. Flow investigation has been focused on the cavitation influence on the flow behavior and on the performance of the machine. Main analyses include incipient cavitation values, head drop curves and cavitation forms prediction for wide ranges of flow rates and NPSH values. Special attention has been put on the interaction between cavitation forms and the performance drop (hump zone) caused by the rotating stall. Cavitation results showed good agreement with the provided experimental data. Second part of the thesis has been focused on the prediction and analysis of the rotating stall flow patterns. Computationally fast steady simulations has been presented and used to predict stable and unstable operating regions. The analyses have been done on 4 different guide vanes openings and 2 guide vanes geometries. In order to get detailed information about the unsteady flow patterns related to the rotating stall, more exact unsteady simulations have been performed. Local flow study has been done to describe in details the governing mechanisms of the rotating stall. The analyses enable the investigations of the rotating stall frequencies, number of stalled cells and the intensity of the rotating stall. Moreover, the unsteady calculations give very good prediction of the pump-turbine performance for both, stable and unstable operating regions. Numerical results give very good qualitative and quantitative agreement with the available experimental data. The approach appears to be very reliable, robust and precise. Even though the numerical results (rotating stall frequencies, number of cells...) on the actual geometry should be confirmed experimentally, author believes that the methodology could be used on any other pump-turbine (or centrifugal pump) geometry. Moreover, the simulations can be used industrially to study the effects of the guide vanes geometries, guide vanes opening angles and influence of the gap between the impeller and the distributor in order to reduce or even eliminate the negative effects of the rotating stall.

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Numerical Study of Pump-Turbine Instabilities Under Pumping Mode Off-Design Conditions

As a part of complex works aiming at the evaluation of the pump’s dynamic transfer matrix, this paper presents an estimation method of the speed of sound in water and water/air flows using three pressure transducer measurements. The experimental study was carried out at the CREMHyG acoustic test rig, for a void ratio varying from 0 to 1 % and for amplitudes of speed of sound from 100 to 1400 m/s. To estimate the speed of sound in this large range of amplitude, a new post-treatment approach was developed, based on the least mean squares method. Experimental results obtained were compared with existing theoretical models, and a very good agreement was observed. The post-processing appeared fast, robust and accurate for all the mono- and diphasic flows analyzed. The results presented in this paper can be applied, for instance, in acoustic characterization of the hydraulic systems, mainly in the case of space rocket turbopump applications.

A new process to estimate the speed of sound using three-sensor method

The cavitation erosion remains an industrial issue. In this paper, we deal with the cavitation intensity which can be described as the aggressiveness - or erosive capacity - of a cavitating flow. The estimation of this intensity is a challenging problem both in terms of modelling the cavitating flow and predicting the erosion due to cavitation. For this purpose, a model was proposed to estimate cavitation intensity from 3D unsteady cavitating flow simulations. An intensity model based on pressure and void fraction derivatives was developped and applied to a NACA 65012 hydrofoil tested at LMH-EPFL (Ecole Polytechnique Federale de Lausanne) [1]. 2D and 3D unsteady cavitating simulations were performed using a homogeneous model with void fraction transport equation included in Code_Saturne with cavitating module [2]. The article presents a description of the numerical code and the physical approach considered. Comparisons between 2D and 3D simulations, as well as between numerical and experimental results obtained by pitting tests, are analyzed in the paper.

https://iopscience.iop.org/article/10.1088/1755-1315/49/9/092007/pdf

Numerical investigations on cavitation intensity for 3D homogeneous unsteady viscous flows

/pdf/assessment-of-cavitating-flow-aggressiveness-on-a-hydrofoil-2unw0swjli.pdf

Regiane Fortes-Patella

Papers

Numerical Cavitation Intensity on a Hydrofoil for 3D Homogeneous Unsteady Viscous Flows

Numerical Study of Pump-Turbine Instabilities Under Pumping Mode Off-Design Conditions

A new process to estimate the speed of sound using three-sensor method

Numerical investigations on cavitation intensity for 3D homogeneous unsteady viscous flows

Assessment of Cavitating Flow Aggressiveness on a Hydrofoil: Experimental and Numerical Approaches