Additional information for the announcement of:
PhD Student: CFD for Hydropower Lifetime Analysis
https://www.chalmers.se/en/about-chalmers/work-with-us/vacancies/?rmpage=job&rmjob=13949&rmlang=UK
Taken from the original project proposal (leading to the funding):
Aim:
To give guidance on safe and cost-efficient operation of hydropower plants, to predict maintenance intervals and remaining lifetime of the machines, and to give input to cost estimations due to off-design and transient operation.
To numerically study how the flow in water turbines during different operation sequences influences the lifetime of the machines, aiming at guidance for avoiding operation that reduces the lifetime.
Objectives:
Utilize and further develop computational fluid dynamics methods (in OpenFOAM) for improved knowledge and understanding of the flow in turbines during off-design and transient operation.
Quantify the fluid flow forces acting on the wet components of the turbines during off-design operation and during transient operation, under realistic conditions.
Numerically determine unsteady stresses in runner blades and guide vanes during off-design operation and during transient operation, and the transfer of those stresses to mounting points and their structures (while changing blade angles).
Address how cavitation may influence the forces on the turbine components during off-design operation and transient operation.
Address how to connect the efforts in the project with efforts in other projects in SVC (the Swedish Center for Sustainable Hydropower).
Background to the research question:
The fast growth of intermittent renewable energy from wind and solar power, as well as the varying electricity demands from the market, make the hydropower frequency/power regulation become increasingly important to stabilize our electrical grid. Additionally, climate change may alter the boundary conditions for hydropower due to new variations in water availability in the dams. Hydropower operation needs to adapt to these changes, while keeping its availability and safety at a reasonably low cost. Major questions in the hydropower industry are how to safely operate the turbines during the transients that are needed due to the continuously changing request for electric power, which effect this has on the hydropower plants, which costs it is associated with, how to plan for efficient additional maintenance due to the new circumstances, and which lifetime that is expected for different components. This requires a detailed understanding of the fluid flow in the turbines while operating at off-design conditions and during transients. With increased knowledge it is possible to adapt the operating sequences to maintain availability and safety, to predict actual costs of providing these auxiliary services, and to plan maintenance and estimate remaining lifetime. These aspects have been highlighted by the Swedish hydropower industry as some of the most important for the future of hydropower in a rapidly changing renewable energy system.
The present project will employ and further develop computational fluid dynamics (CFD) methods for determining flow-induced forces in the turbines under realistic conditions. The CFD approach will take advantage of the methods that have been developed the last decades, to study how different operating sequences affect the flow in the turbines and how the flow-induced forces affect the wetted structural parts of the machine. The flow-induced forces influence the stresses and strains in the blades, the mounting points of the blades (which for adaption of blade angles is prone to excessive wear during transients), and the complex hydraulic-mechanical-electrical coupling in the rotor system. The forces determined by CFD can be used to determine the stresses in the blades, as well as the transfer of the forces to the mounting points and to their structures. This gives the boundary conditions for studies of rotor dynamics, wear and fatigue.
The CFD approach needs methods that can handle off-design and transient operation of water turbines. This includes boundary conditions that can take into account the upstream and downstream system, including variations in elevations in head. Such a boundary condition has recently been designed, implemented and published by our group, for the use in pumped hydro storage. It includes flow-dependent losses in pipes and bends, flow-losses in opening and closing gates, and varying water surface elevations both upstream and downstream. Being developed for pumped hydro, it also allows the machine to seamlessly shift from turbine mode to pump mode, while the flow rate is given by the solution due to the instantaneous head of the system. Numerical simulations of transient operation of water turbines also requires methods to continuously change the angles of the guide vanes, and for Kaplan turbines also the angles of the runner blades. Such methods have successfully been developed and tested by our group during recent years, and although there is always need for improvement they are ready for extensive use for studies of transients in water turbines. Although detailed cavitation simulations requires extreme resolution and computational time, we intend to use the methods we have previously evaluated, using somewhat coarse resolution to determine qualitative effects of cavitation on the flow-induced forces. The flow-induced forces on the wetted surfaces is a direct output from the simulations, which can be transferred to structural solvers that gives the stresses and (at least one-way coupled) strains in the blades and the transfer of the forces to the mounting points and to their structures.
Implementation and expected result:
Identify the most representative operations as base-line cases:
Different steady off-design and transient sequences have different flow features, which contribute to degraded lifetime in different ways. A few highly relevant representative base-line cases must first be chosen, for the initial studies in the project and the development to be done in the project. Here it is also of interest to choose cases that may eventually have experimental validation data in some form. Some further development of the CFD code may be required for particular cases.
Quantify unsteady flow-induced forces during base-line cases:
The flow-induced forces are in principle given as part of the solution. However, it remains to be decided how the forces should be gathered in a way that make them applicable for transfer of those forces to the structures. It is not necessarily the case that the flow simulation can use exactly the same geometry as a structural solver, so a direct transfer may not be possible. Special considerations may also have to be made due to blades that continuously change their angles.
Determine unsteady stresses in runner blades, guide vanes, mounting points and structures:
The unsteady flow-induced forces need to be transferred to a software (or another module in the same software) that uses them as input to determine the stresses and strains in the blades. The solution of the stresses and strains in the blades determine the transfer of the forces to particular regions of the blades, and further to the mounting points. For blades that change their angles during transient operation, this gives input to studies of wear during transient operation. It also gives input to the forces on the control structures.
Address effects on forces due to cavitation:
Presence of cavitation alters the flow paths, limits the pressure forces in certain regions, and may induce shedding. Although it is not possible to resolve all of the highly complex physics of cavitation and cavitation erosion for engineering applications, such as the flow in water turbines, the available cavitation models may be used to give a qualitative estimation of the effects of cavitation. The studies of flow-induced forces, stresses and strains may be repeated both under non-cavitating and cavitating conditions to qualitatively determine the effects of cavitation.
Connect to other efforts in SVC:
Many other projects in SVC perform studies that are related to turbine lifetime. The present project will actively communicate with the other researchers in SVC, to determine how this project can contribute to other projects, and vice-versa.
Organization:
A new PhD student will be employed. Prof. Håkan Nilsson will coordinate the work and act as main supervisor. Prof. Rickard Bensow at the division of Marine Technology will contribute with knowledge in cavitation and act as examiner. The industry will be involved by providing validation data and taking part in discussions and evaluations.
Additional notes:
The project consists of doing CFD simulations with OpenFOAM to determine the unsteady loading of water turbines during transient operation. The loads will be used to determine the stresses on different parts of the turbine, which can be used as input to lifetime assessment. The determination of the stresses should ideally be done by the PhD student, but there might also be a possibility to do this in collaboration with our industrial partners. Scaling from model scale to full scale may be part of the project, since this is something requested from the industry. Some development (coding) of OpenFOAM may be required. A LOT of highly demanding CFD simulations must be made by the PhD student, and the results must be validated and analysed thoroughly with advanced methods.
Some examples of related papers from our group:
https://research.chalmers.se/publication/546171
https://research.chalmers.se/publication/544594
https://research.chalmers.se/publication/535409
https://research.chalmers.se/publication/529014
https://research.chalmers.se/publication/524767
https://research.chalmers.se/publication/527096