Simulation of flow induced noise in process of pump-turbine load rejection

The grid market is redistributed with significantly increase of the exploitation of unpredictable renewable energy, such as wind and solar energy sources, however, the ways of electricity generation by both wind and solar energy depend on environment which is extremely unstable. For the sake of balancing electricity generated by renewable energy, pumped storage power stations are experiencing a thriving process. As the core of pumped storage power station, the stable operation of the pump turbine is extremely important, especially for pump-turbine working at transient conditions. In order to study pressure fluctuating characteristics and its' influence on flow-induced noise, a continuous unsteady simulation was carried out in pump-turbine guide vane closing process under generating mode. In this article, wall sliding mesh was used to realize guide vane continuous motion, which ensured that the mesh quality at any moment was larger than 60% compared with the initial mesh quality, meanwhile, DES turbulent model was adopted in all calculations due to its good performance in many industrial cases. The whole pump-turbine model was meshed with structured mesh by commercial software ICEM, and five different mesh sizes were used in mesh sensitivity validation, with the size of 14 million selected finally. On the other hand, a test was performed by the team of Giorgio Pavesi to prove this model in open test facility in Padova University, the entire model validation was carried out according to ISO standards, and relative parameters were measured based on IEC standards. Commercial software ANYSYS CFX 16.2 was used to realize all simulating calculations with 8 computer cores, one month was taken to finish this calculation. The flow field calculating results were analyzed in frequency and time-frequency domains, including mass flow, pressure, and torque et al., in the meantime, the pressure on the surfaces of blades was regarded as flow-induced noise source to study sound field. The solution obtained from flow field illustrates that pressure fluctuating amplitudes at guide vane outlet is more than twice compared to the relative value at guide vane inlet location, the main reason is flow in the vaneless space that is close to runner is affected by rotor-rotor interaction. In addition, pressure pulsations at runner outlet arrive at peaks when two vortexes appear in draft tube with two different rotating directions. As for frequency domain characteristics, both strauhal number St=0.051 and St=1 are captured, whereas the spectrum of those pressure fluctuations that are close to guide vane outlet is 10 times of the relative value at guide vane inlet, which explains that rotor-rotor interaction has a stronger influence on flow field than rotor-stator interaction. Some rules are found by analyzing flow-induced noise in sound field, the analysis illustrates that flow-induced noise radiation level is related to both pressure fluctuating and shell natural frequency captured in exterior acoustic field, the shape of sound distribution is like "∞" and sound level distributions in different directions and faces are symmetrical, this explains that the blade noise radiation has obvious dipole characteristics. Furthermore, at the first and second-order blade passage frequencies, the effect of flow rate on the radiation performance of noise is stronger under larger flow conditions during guide vane closure, which becomes weaker under smaller flow conditions in the first half of the guide vane closure, as for the second half phase of guide vane closure, the results are exactly opposite to the previous phenomena. Moreover, flow-induced noise radiation is consistent with fluid characteristics during pump-turbine load rejection. Consequently, to improve pressure fluctuating characteristics can reduce flow-induced noise.