Inserção de geradores de vórtices longitudinais para a mitigação do vórtice de corda em uma turbina hidráulica do tipo Francis
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Data
2023-02-15
Autores
Museti, Daniel Marcos
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Universidade Estadual Paulista (Unesp)
Resumo
No Brasil a energia hidrelétrica tem sido usada como a principal fonte para a geração de eletricidade, fornecendo energia continuamente à rede elétrica. Paralelamente, a demanda por fontes de energias renováveis vem substancialmente crescendo nos últimos anos em decorrência das crescentes preocupações com as mudanças climáticas devido ao uso excessivo de combustíveis fósseis. Algumas destas energias renováveis como a energia solar e eólica, por exemplo, são consideradas como fontes de energias intermitentes, podendo gerar instabilidades na rede elétrica de distribuição. Além da produção de eletricidade, as turbinas hidráulicas fornecem estabilidade à rede devido à sua rápida resposta às mudanças na demanda e flexibilidade para operar em uma variedade de condições operacionais, tornando-a ideal para uso como modulador de rede, evitando a falta ou a sobrecarga no fornecimento de eletricidade. Neste sentido, com o avanço da tecnologia de modelagem computacional (CFD), o projeto de turbinas hidráulicas tem permitido a avaliação do escoamento sob diferentes condições de operação, identificando, ainda na fase de projeto, fenômenos como o vórtice de corda no tubo de descarga, que são responsáveis por muitas características operacionais indesejáveis, incluindo redução de eficiência, ruído, vibrações, variações na potência de saída, e pulsações de pressão no tubo de descarga e conduto forçado. Esta ferramenta numérica não era uma realidade para as usinas hidrelétricas instaladas no Brasil em meados da década de 70, onde o desenvolvimento era essencialmente empírico e analítico. Desta forma, o objetivo deste trabalho é investigar a possibilidade de mitigação do vórtice de corda em uma turbina hidráulica benchmarking do tipo Francis apresentada no Workshop Francis-99, considerando a inserção de dispositivos passivos do tipo geradores de vórtices sobre o perfil da pá do rotor. Foi adotada uma modelagem considerando a periodicidade do rotor e palhetas direcionadoras móveis para reduzir o custo computacional. O escoamento é considerado incompressível e turbulento, e uma abordagem inicial foi feita considerando o regime estacionário e posteriormente a configuração mais promissora na mitigação do vórtice de corda foi submetida ao regime transitório para comparação. As condições de operação PL (Partial Load), BEP (Best Efficiency Point) e HL (High-Load) foram investigadas. A inserção de geradores de vórtices sobre as pás de um rotor de uma turbina Francis permitiu a mitigação do vórtice de corda no tubo de descarga. A mitigação do vórtice de corda com a inserção de geradores de vórtices provocou redução na eficiência da turbina hidráulica em todas as configurações e condições, mas com a análise dos parâmetros e resultados foi possível minimizar essa queda na eficiência e, para a maioria dos casos, obter um ligeiro acréscimo no torque. Os resultados qualitativos no regime transitório mostram que as mudanças no vórtice de corda foram sutis quando comparadas aos resultados do regime estacionário, permitindo concluir que para uma análise inicial de mitigação do vórtice de corda o regime estacionário se mostra razoavelmente satisfatório. Apesar da inserção dos geradores de vórtices no rotor ocasionarem uma perda na eficiência, estes dispositivos passivos modificam a dinâmica do escoamento mitigando o vórtice de corda e, possivelmente, melhorando a estabilidade operacional da turbina hidráulica, podendo assim evitar a necessidade de uma completa substituição das turbinas.
In Brazil, hydroelectric power has been used as the main source for the generation of electricity, continuously supplying energy to the electrical grid. At the same time, the demand for renewable energy sources has been growing substantially in recent years as a result of growing concerns about climate change due to the excessive use of fossil fuels. Some of these renewable energies such as solar and wind energy, for example, are considered intermittent sources of energy, which can generate instabilities in the electrical distribution network. In addition to producing electricity, hydraulic turbines provide grid stability due to their quick response to changes in demand and flexibility to operate in a variety of operating conditions, making them ideal for use as a grid modulator, preventing fault or overload in the supply of electricity. In this sense, with the advancement of computer modeling technology (CFD), the design of hydraulic turbines has allowed the evaluation of the flow under different operating conditions, identifying, even in the design phase, phenomena such as the vortex rope in the draft tube, which are responsible for many undesirable operating characteristics, including reduced efficiency, noise, vibrations, variations in output power, and pressure pulsations in the draft tube and penstock. This numerical tool was not a reality for the hydroelectric plants installed in Brazil in the mid-1970s, where development was essentially empirical and analytical. Thus, the objective of this work is to investigate the possibility of mitigating the vortex rope in a Francis-type benchmarking hydraulic turbine presented at the Francis-99 Workshop, considering the insertion of passive devices such as vortex generators on the rotor blade profile. The rotor periodicity and moving vanes were adopted to reduce the computational cost. The flow is considered incompressible and turbulent and an initial approach was made considering the steady state and then the most promising configuration in string vortex mitigation was submitted to the transient regime for comparison. The operating conditions PL (Partial Load), BEP (Best Efficiency Point) and HL (High-Load) were investigated. The insertion of vortex generators on the rotor blades of a Francis turbine allowed the mitigation of the rope vortex in the discharge tube. The mitigation of the string vortex with the insertion of vortex generators caused a reduction in the efficiency of the hydraulic turbine in all configurations and conditions, but with the analysis of the parameters and results it was possible to minimize this drop in efficiency and for most cases to obtain a slight increase in torque. The qualitative results in the transient regime show that the changes in the chord vortex were subtle when compared to the steady-state results, allowing to conclude that for an initial analysis of chord vortex mitigation the steady-state regime is reasonably satisfactory. Although the insertion of vortex generators in the rotor caused a loss in efficiency, it allowed a change in the flow sufficient to mitigate the rope vortex and possibly improve the operational stability of the turbine and consequently of the plant, thus avoiding the need for a complete replacement of the turbines.
In Brazil, hydroelectric power has been used as the main source for the generation of electricity, continuously supplying energy to the electrical grid. At the same time, the demand for renewable energy sources has been growing substantially in recent years as a result of growing concerns about climate change due to the excessive use of fossil fuels. Some of these renewable energies such as solar and wind energy, for example, are considered intermittent sources of energy, which can generate instabilities in the electrical distribution network. In addition to producing electricity, hydraulic turbines provide grid stability due to their quick response to changes in demand and flexibility to operate in a variety of operating conditions, making them ideal for use as a grid modulator, preventing fault or overload in the supply of electricity. In this sense, with the advancement of computer modeling technology (CFD), the design of hydraulic turbines has allowed the evaluation of the flow under different operating conditions, identifying, even in the design phase, phenomena such as the vortex rope in the draft tube, which are responsible for many undesirable operating characteristics, including reduced efficiency, noise, vibrations, variations in output power, and pressure pulsations in the draft tube and penstock. This numerical tool was not a reality for the hydroelectric plants installed in Brazil in the mid-1970s, where development was essentially empirical and analytical. Thus, the objective of this work is to investigate the possibility of mitigating the vortex rope in a Francis-type benchmarking hydraulic turbine presented at the Francis-99 Workshop, considering the insertion of passive devices such as vortex generators on the rotor blade profile. The rotor periodicity and moving vanes were adopted to reduce the computational cost. The flow is considered incompressible and turbulent and an initial approach was made considering the steady state and then the most promising configuration in string vortex mitigation was submitted to the transient regime for comparison. The operating conditions PL (Partial Load), BEP (Best Efficiency Point) and HL (High-Load) were investigated. The insertion of vortex generators on the rotor blades of a Francis turbine allowed the mitigation of the rope vortex in the discharge tube. The mitigation of the string vortex with the insertion of vortex generators caused a reduction in the efficiency of the hydraulic turbine in all configurations and conditions, but with the analysis of the parameters and results it was possible to minimize this drop in efficiency and for most cases to obtain a slight increase in torque. The qualitative results in the transient regime show that the changes in the chord vortex were subtle when compared to the steady-state results, allowing to conclude that for an initial analysis of chord vortex mitigation the steady-state regime is reasonably satisfactory. Although the insertion of vortex generators in the rotor caused a loss in efficiency, it allowed a change in the flow sufficient to mitigate the rope vortex and possibly improve the operational stability of the turbine and consequently of the plant, thus avoiding the need for a complete replacement of the turbines.
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Palavras-chave
Turbina hidráulica Francis, Vórtice de corda, Geradores de vórtices, CFD, Francis hydraulic turbine, Vortex rope, Vortex generators