PAT – Pump as Turbine Optimization

PAT-Characheristics

Comparison Prediction methodology (PM Proposed Method)

Research activities

• Numerical simulation of centrifugal turbopump in the four quadrant.
  

• Multi-objective optimization of PAT.
  

• Experimental tests on PAT in both pump and turbine mode.

• Time-frequency, wavelet analysis and Omega vortex method will be used to study the pressure pulsation characteristics of PAT to identify and reduce both strong and weak vortex structures
  

Projects

In order to reduce greenhouse gas emissions, countries around the world have been developing hydropower as a mature renewable energy technology. At the same time, small hydropower stations have become a focus of attention due to their environmental friendliness, low operating costs, and low reservoir costs. In practical applications, pumps as turbines (PAT) are widely used to reduce equipment costs. Among pumps with different specific speeds, centrifugal pumps return on investment due to the wider ranger of flow rate, lower manufacturing cost and easier maintenance. The purpose of this study is to capture the morphology of vortices and unsteadyness using FFT, Wavelet and Omega vortex identification technique at part load, Shut-Down process and runaway transition.

Funding

This research is supported by the NexGenerationEU project Prot. 2022MFSN7R

Tools

Facility – Pump Facility

Mesh Tools – ICEM

CFD Tools – ANSYS CFX

Post Processing Tools – Wavelet, Ω-method, ANSYS

Contacts

PhD Giorgio Pavesi

Associate professor
Group: TURBOMACHINERY

+39 049 827 6768

giorgio.pavesi@unipd.it

Orcid Profile

MSc Tingyun Yin

Assistant professor
Group: TURBOMACHINERY

+39 049 827 6700

tingyun.yin@unipd.it

Orcid Profile

Publications

2025

Shen, Jiantao; Jia, Xuanwen; Cheng, Li; Jiao, Weixuan; Zhang, Bowen; Pavesi, Giorgio

Investigation into the coupling mechanism of tailwater vortex dynamics and cavitation during pump-as-turbine operations Journal Article

In: Physics of Fluids, vol. 37, iss. 10, 2025, ISSN: 10897666.

Abstract | Links | BibTeX | Tags: Cavitation, PAT, Pump as Turbine

@article{Shen2025b,

title = {Investigation into the coupling mechanism of tailwater vortex dynamics and cavitation during pump-as-turbine operations},

author = {Jiantao Shen and Xuanwen Jia and Li Cheng and Weixuan Jiao and Bowen Zhang and Giorgio Pavesi},

doi = {10.1063/5.0287323},

issn = {10897666},

year  = {2025},

date = {2025-01-01},

urldate = {2025-01-01},

journal = {Physics of Fluids},

volume = {37},

issue = {10},

publisher = {American Institute of Physics},

abstract = {In order to investigate the coupling mechanism between tailwater vortex and cavitation bubbles (CB) during the operation of a low-head pump-as-turbine (PAT), the method of combining experiment and numerical simulation is used to quantify the vortex dynamics characteristics in combination with the vorticity transport equation under high flow conditions for PAT mode. The results show that the decrease in Thoma number significantly regulates the symbiotic evolution of tailwater vortex and CB: prolonging the residence time of CB in the draft tube (DT) and changing its evolution mode, resulting in the extension of vortex rope (VR) generation period, length contraction, and the increase in breaking vortex in DT. The peak volume of CB is 7 times that in the rotor region, squeezing the channel vortex and the wake vortex, weakening its contribution to VR. Vortex dynamics shows that the relative vortex stretching term is the core driving force of vorticity, which causes velocity gradient distortion and VR high-frequency oscillation synchronously with vertical vorticity. The baroclinic torque term (BT) only generates pulse contribution in the early stage of CB collapse. Under critical cavitation, BT converts the cavity collapse energy into vortex energy through density-pressure gradient coupling, which expands the vortex core radius to 0.03 m, increases the circulation peak to 2.3 m2/s, and shifts outward by 27.3%, resulting in vortex energy diffusion and high-frequency oscillation of the flow field. This study provides a theoretical basis for cavitation suppression and operational optimization of low-head PAT.},

key = {Cavitation, PAT, Pump as Turbine, tailwater vortex},

keywords = {Cavitation, PAT, Pump as Turbine},

pubstate = {published},

tppubtype = {article}

}

Close

In order to investigate the coupling mechanism between tailwater vortex and cavitation bubbles (CB) during the operation of a low-head pump-as-turbine (PAT), the method of combining experiment and numerical simulation is used to quantify the vortex dynamics characteristics in combination with the vorticity transport equation under high flow conditions for PAT mode. The results show that the decrease in Thoma number significantly regulates the symbiotic evolution of tailwater vortex and CB: prolonging the residence time of CB in the draft tube (DT) and changing its evolution mode, resulting in the extension of vortex rope (VR) generation period, length contraction, and the increase in breaking vortex in DT. The peak volume of CB is 7 times that in the rotor region, squeezing the channel vortex and the wake vortex, weakening its contribution to VR. Vortex dynamics shows that the relative vortex stretching term is the core driving force of vorticity, which causes velocity gradient distortion and VR high-frequency oscillation synchronously with vertical vorticity. The baroclinic torque term (BT) only generates pulse contribution in the early stage of CB collapse. Under critical cavitation, BT converts the cavity collapse energy into vortex energy through density-pressure gradient coupling, which expands the vortex core radius to 0.03 m, increases the circulation peak to 2.3 m2/s, and shifts outward by 27.3%, resulting in vortex energy diffusion and high-frequency oscillation of the flow field. This study provides a theoretical basis for cavitation suppression and operational optimization of low-head PAT.

Close

• doi:10.1063/5.0287323

Close

2024

Zhang, Chenying; Pavesi, Giorgio; Pei, Ji; Wang, Wenjie; Yuan, Shouqi; Shen, Jiawei

Unstable flow analysis of transient process in the pump as turbine during turbine mode caused by pump power failure Journal Article

In: Physics of Fluids, vol. 36, iss. 11, 2024, ISSN: 10897666.

Abstract | Links | BibTeX | Tags: PAT, Power Failure, Pump as Turbine, Runaway

@article{Zhang2024c,

title = {Unstable flow analysis of transient process in the pump as turbine during turbine mode caused by pump power failure},

author = {Chenying Zhang and Giorgio Pavesi and Ji Pei and Wenjie Wang and Shouqi Yuan and Jiawei Shen},

doi = {10.1063/5.0233491},

issn = {10897666},

year  = {2024},

date = {2024-01-01},

urldate = {2024-01-01},

journal = {Physics of Fluids},

volume = {36},

issue = {11},

publisher = {American Institute of Physics},

abstract = {Mixed-flow pump as turbines (PATs) serve as pivotal components within energy micro-grids, facilitating energy conversion and storage. However, the emergence of pressure pulsations in these systems can markedly affect their stability and efficiency, especially in pump power failure. To simulate the power-off transition accurately, the commercial computational fluid dynamics code ANSYS CFX® is integrated by a Fortran program through ANSYS parametric design language in the transient simulation to enable real-time iterative calculations of angular momentum equations for mixed-flow PAT at varying speeds. This study integrates the analysis of radial forces, vortices, and flow lines to elucidate the sudden changes in pressure pulsations observed during the transition stages. Specifically, significant fluctuations in the amplitude of pressure pulsations at the volute tongue were found for various initial flow rates, which correlated closely with changes in radial forces. The sudden increase and nonuniform distribution of radial forces emerged as the main factors of these fluctuations. In addition, the study reveals that the intensity of pressure fluctuations evidenced by wavelet time-frequency analysis on the pressure surface of the blade significantly exceeds that on the back surface of the blade. Furthermore, in the flow characteristics inside the draft tube, the pressure pulsation signals are mainly concentrated in the low-frequency region and are accompanied by the presence of a double-helix structure. These results provide an important reference for further understanding of the operating mechanism of the mixed-flow pump as a turbine, which helps to optimize the design and improve the performance.},

keywords = {PAT, Power Failure, Pump as Turbine, Runaway},

pubstate = {published},

tppubtype = {article}

}

Close

Mixed-flow pump as turbines (PATs) serve as pivotal components within energy micro-grids, facilitating energy conversion and storage. However, the emergence of pressure pulsations in these systems can markedly affect their stability and efficiency, especially in pump power failure. To simulate the power-off transition accurately, the commercial computational fluid dynamics code ANSYS CFX® is integrated by a Fortran program through ANSYS parametric design language in the transient simulation to enable real-time iterative calculations of angular momentum equations for mixed-flow PAT at varying speeds. This study integrates the analysis of radial forces, vortices, and flow lines to elucidate the sudden changes in pressure pulsations observed during the transition stages. Specifically, significant fluctuations in the amplitude of pressure pulsations at the volute tongue were found for various initial flow rates, which correlated closely with changes in radial forces. The sudden increase and nonuniform distribution of radial forces emerged as the main factors of these fluctuations. In addition, the study reveals that the intensity of pressure fluctuations evidenced by wavelet time-frequency analysis on the pressure surface of the blade significantly exceeds that on the back surface of the blade. Furthermore, in the flow characteristics inside the draft tube, the pressure pulsation signals are mainly concentrated in the low-frequency region and are accompanied by the presence of a double-helix structure. These results provide an important reference for further understanding of the operating mechanism of the mixed-flow pump as a turbine, which helps to optimize the design and improve the performance.

Close

• doi:10.1063/5.0233491

Close

2016

Cavazzini, Giovanna; Covi, Alberto; Pavesi, Giorgio; Ardizzon, Guido

Analysis of the Unstable Behavior of a Pump-Turbine in Turbine Mode: Fluid-Dynamical and Spectral Characterization of the S-shape Characteristic Journal Article

In: Journal of Fluids Engineering, vol. 138, iss. 2, pp. 021105, 2016, ISSN: 0098-2202.

Abstract | Links | BibTeX | Tags: Francis-type Reversible Turbine, PAT

@article{Cavazzini2015e,

title = {Analysis of the Unstable Behavior of a Pump-Turbine in Turbine Mode: Fluid-Dynamical and Spectral Characterization of the S-shape Characteristic},

author = {Giovanna Cavazzini and Alberto Covi and Giorgio Pavesi and Guido Ardizzon},

url = {http://fluidsengineering.asmedigitalcollection.asme.org/article.aspx?doi=10.1115/1.4031368},

doi = {10.1115/1.4031368},

issn = {0098-2202},

year  = {2016},

date = {2016-01-01},

urldate = {2016-01-01},

journal = {Journal of Fluids Engineering},

volume = {138},

issue = {2},

pages = {021105},

abstract = {The most common mechanical equipment adopted in the new generation of pumped-hydro power plants is represented by reversible pump-turbines (RPT), required to rapidly switch between pumping and generating modes in order to balance the frequent changes in electricity production and consumption caused by unpredictable renewable energy sources. As a consequence, pump-turbines are required to extend their operation under off-design conditions in unstable operating areas. The paper presents a numerical analysis of the unstable behavior of a pump-turbine operating in turbine mode near the no-load condition. To study in depth the unsteady phenomena which lead to the S-shape of the turbine characteristic, a load rejection scenario at constant and large guide vane opening (GVO) was numerically analyzed by running through the flow-speed characteristic up to the turbine brake region. The flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating stall (RS) (65.1% of the runner rotation frequency) during the turbine brake operation. These phenomena were characterized by frequency and time-frequency analyses of several numerical signals (static pressure, blade torque, mass flow rate in blade passages). The influence of the development of these unsteady phenomena on the pump-turbine performance in a turbine operation was also analyzed, and the potential causes that generated the S-shaped characteristic curve were also investigated.},

keywords = {Francis-type Reversible Turbine, PAT},

pubstate = {published},

tppubtype = {article}

}

Close

The most common mechanical equipment adopted in the new generation of pumped-hydro power plants is represented by reversible pump-turbines (RPT), required to rapidly switch between pumping and generating modes in order to balance the frequent changes in electricity production and consumption caused by unpredictable renewable energy sources. As a consequence, pump-turbines are required to extend their operation under off-design conditions in unstable operating areas. The paper presents a numerical analysis of the unstable behavior of a pump-turbine operating in turbine mode near the no-load condition. To study in depth the unsteady phenomena which lead to the S-shape of the turbine characteristic, a load rejection scenario at constant and large guide vane opening (GVO) was numerically analyzed by running through the flow-speed characteristic up to the turbine brake region. The flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating stall (RS) (65.1% of the runner rotation frequency) during the turbine brake operation. These phenomena were characterized by frequency and time-frequency analyses of several numerical signals (static pressure, blade torque, mass flow rate in blade passages). The influence of the development of these unsteady phenomena on the pump-turbine performance in a turbine operation was also analyzed, and the potential causes that generated the S-shaped characteristic curve were also investigated.

Close

• http://fluidsengineering.asmedigitalcollection.asme.org/article.aspx?doi=10.1115[...]
• doi:10.1115/1.4031368

Close

Pavesi, Giorgio; Cavazzini, Giovanna; Ardizzon, Guido

Numerical Simulation of a Pump-Turbine Transient Load Following Process in Pump Mode Proceedings Article

In: ISROMAC, researchgate.net, 2016, (Query date: 2017-05-06).

Abstract | Links | BibTeX | Tags: Francis Turbine, Francis-type Reversible Turbine, PAT, Variable Speed

@inproceedings{pop00006,

title = {Numerical Simulation of a Pump-Turbine Transient Load Following Process in Pump Mode},

author = {Giorgio Pavesi and Giovanna Cavazzini and Guido Ardizzon},

url = {https://www.researchgate.net/profile/Giorgio_Pavesi/publication/308522782_ISROMAC_2016_International_Sym-posium_on_Transport_Phenomena_and_Dynamics_of_Rotating_Machinery/links/57e630b608aedcd5d1a44471.pdf},

year  = {2016},

date = {2016-01-01},

urldate = {2016-01-01},

booktitle = {ISROMAC},

publisher = {researchgate.net},

abstract = {This paper presents the simulation of the dynamic behaviour of variable speed pump-turbine. A power reduction scenario at constant wicket gate opening was numerically analysed from 100% to 93% rpm corresponding to a power reduction from full load to about 70% with a ramp rate of 1.5% per second. The flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating partial stall during the pump power reduction. These phenomena were characterized by frequency and time-frequency analyses of several numerical signals (pressure, blade torque, flow rate in blade passages). The unsteady pattern in return channel strengthened emphasizing its characteristic frequency with the rotational velocity decreasing posium on reaching a maximum and then disappearing. At lower rotational speed, the flow field into the Transport wickets gates channel start to manifest a full three-dimensional flow structure. This disturbance was related to the boundary layer separation and stall and it was noticed by a specific frequency},

note = {Query date: 2017-05-06},

keywords = {Francis Turbine, Francis-type Reversible Turbine, PAT, Variable Speed},

pubstate = {published},

tppubtype = {inproceedings}

}

Close

This paper presents the simulation of the dynamic behaviour of variable speed pump-turbine. A power reduction scenario at constant wicket gate opening was numerically analysed from 100% to 93% rpm corresponding to a power reduction from full load to about 70% with a ramp rate of 1.5% per second. The flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating partial stall during the pump power reduction. These phenomena were characterized by frequency and time-frequency analyses of several numerical signals (pressure, blade torque, flow rate in blade passages). The unsteady pattern in return channel strengthened emphasizing its characteristic frequency with the rotational velocity decreasing posium on reaching a maximum and then disappearing. At lower rotational speed, the flow field into the Transport wickets gates channel start to manifest a full three-dimensional flow structure. This disturbance was related to the boundary layer separation and stall and it was noticed by a specific frequency

Close

• https://www.researchgate.net/profile/Giorgio_Pavesi/publication/308522782_ISROMA[...]

Close

2015

Pavesi, Giorgio; Yang, Jun; Cavazzini, Giovanna; Ardizzon, Guido

Experimental analysis of instability phenomena in a high-head reversible pump-turbine at large partial flow condition Proceedings Article

In: 11th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2015, pp. 1-2, euroturbo.eu, 2015, ISSN: 24104833.

Abstract | Links | BibTeX | Tags: Flow instability, Francis Turbine, Francis-type Reversible Turbine, Hump instability, PAT, Pump as Turbine

@inproceedings{pop00010,

title = {Experimental analysis of instability phenomena in a high-head reversible pump-turbine at large partial flow condition},

author = {Giorgio Pavesi and Jun Yang and Giovanna Cavazzini and Guido Ardizzon},

url = {http://www.euroturbo.eu/paper/ETC2015-060.pdf},

issn = {24104833},

year  = {2015},

date = {2015-01-01},

urldate = {2015-01-01},

booktitle = {11th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2015},

pages = {1-2},

publisher = {euroturbo.eu},

abstract = {Growing environmental concerns, and the need for better power balancing and frequency control have increased attention in renewable energy sources, such as, the reversible pump-turbine which can provide both power generation and energy storage. Pump-turbine operation along the hump-shaped curve can lead to unusual increases in water pressure pulsations, which lead to machine vibrations. Measurements of wall pressure in the stators were performed together with high-speed flow visualizations. Starting from the best efficiency point (BEP) and by decreasing the flowrate, a significant increase of the pressure fluctuations was observed mainly in the wicket gates channels. The analyses in frequency and time-frequency domains showed a rise of low frequency components. High-speed movies revealed a quite uniform flow pattern in the guide vanes channels at the normal operating range, whereas, the flow was highly disturbed by rotating stall passage at part load. The situation was more critical in the dump flow rate range, where backflow and vortices in the guide vanes channels developed during the stall cell passage.},

keywords = {Flow instability, Francis Turbine, Francis-type Reversible Turbine, Hump instability, PAT, Pump as Turbine},

pubstate = {published},

tppubtype = {inproceedings}

}

Close

Growing environmental concerns, and the need for better power balancing and frequency control have increased attention in renewable energy sources, such as, the reversible pump-turbine which can provide both power generation and energy storage. Pump-turbine operation along the hump-shaped curve can lead to unusual increases in water pressure pulsations, which lead to machine vibrations. Measurements of wall pressure in the stators were performed together with high-speed flow visualizations. Starting from the best efficiency point (BEP) and by decreasing the flowrate, a significant increase of the pressure fluctuations was observed mainly in the wicket gates channels. The analyses in frequency and time-frequency domains showed a rise of low frequency components. High-speed movies revealed a quite uniform flow pattern in the guide vanes channels at the normal operating range, whereas, the flow was highly disturbed by rotating stall passage at part load. The situation was more critical in the dump flow rate range, where backflow and vortices in the guide vanes channels developed during the stall cell passage.

Close

• http://www.euroturbo.eu/paper/ETC2015-060.pdf

Close

2000

Ventrone, Giuseppe; Ardizzon, Guido; Pavesi, Giorgio

Direct and reverse flow conditions in radial flow hydraulic turbomachines Journal Article

In: Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, vol. 214, iss. 6, pp. 635-644, 2000, ISSN: 09576509.

Links | BibTeX | Tags: PAT, Pump, Pump as Turbine, Turbine

@article{Ventrone2000,

title = {Direct and reverse flow conditions in radial flow hydraulic turbomachines},

author = {Giuseppe Ventrone and Guido Ardizzon and Giorgio Pavesi},

url = {http://journals.sagepub.com/doi/10.1243/0957650001538155},

doi = {10.1243/0957650001538155},

issn = {09576509},

year  = {2000},

date = {2000-01-01},

journal = {Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy},

volume = {214},

issue = {6},

pages = {635-644},

keywords = {PAT, Pump, Pump as Turbine, Turbine},

pubstate = {published},

tppubtype = {article}

}

Close

• http://journals.sagepub.com/doi/10.1243/0957650001538155
• doi:10.1243/0957650001538155

Close

1999

Ardizzon, Guido; Pavesi, Giorgio

Funzionamento Inverso di una Pompa Centrifuga. Previsione delle Prestazioni e Confronto con i Dati Sperimentali Proceedings Article

In: 54° Congresso Nazionale ATI, pp. 1383-1395, SGE - Padova, 1999.

Abstract | BibTeX | Tags: PAT, Pump as Turbine

@inproceedings{Ardizzon1999a,

title = {Funzionamento Inverso di una Pompa Centrifuga. Previsione delle Prestazioni e Confronto con i Dati Sperimentali},

author = {Guido Ardizzon and Giorgio Pavesi},

year  = {1999},

date = {1999-01-01},

urldate = {1999-01-01},

booktitle = {54° Congresso Nazionale ATI},

pages = {1383-1395},

publisher = {SGE - Padova},

abstract = {Direct and reverse operation of radial flow turbomachines is examined and a method suggested for the evaluation of reverse operation when performance characteristics in the direct mode are known. The theoretical results are checked against those obtained from tests on a number of machine configurations and compared with those predicted by current empirical formulations.},

keywords = {PAT, Pump as Turbine},

pubstate = {published},

tppubtype = {inproceedings}

}

Close

Direct and reverse operation of radial flow turbomachines is examined and a method suggested for the evaluation of reverse operation when performance characteristics in the direct mode are known. The theoretical results are checked against those obtained from tests on a number of machine configurations and compared with those predicted by current empirical formulations.

Close

1998

Ventrone, Giuseppe; Ardizzon, Guido; Pavesi, Giorgio

Flow Configurations in Centrifugal Pumps Used as Turbines Proceedings Article

In: 2nd International Minihydro Conference, pp. 5-8, 1998.

Abstract | BibTeX | Tags: PAT, Pump as Turbine

@inproceedings{VentroneG.1998,

title = {Flow Configurations in Centrifugal Pumps Used as Turbines},

author = {Giuseppe Ventrone and Guido Ardizzon and Giorgio Pavesi},

year  = {1998},

date = {1998-01-01},

urldate = {1998-01-01},

booktitle = {2nd International Minihydro Conference},

issue = {september},

pages = {5-8},

abstract = {Direct and reverse operation of hydraulic machines can be interrelated by means of a theoretical expression that is instrumental in the prediction of turbine H-Q values from pump performance curves. Tests have been carried out on four machine configurations in order to verify the validity of the procedure. Results furnished by present method are finally compared with those predicted by current empirical relationships.},

keywords = {PAT, Pump as Turbine},

pubstate = {published},

tppubtype = {inproceedings}

}

Close

Direct and reverse operation of hydraulic machines can be interrelated by means of a theoretical expression that is instrumental in the prediction of turbine H-Q values from pump performance curves. Tests have been carried out on four machine configurations in order to verify the validity of the procedure. Results furnished by present method are finally compared with those predicted by current empirical relationships.

Close