Numerical analysis of the dynamic characteristics of the bulb turbine during decreased and increased load transition processes

Hydraulic stations often undertake the tasks of peak and frequency regulation of the power grid, due to the excellent performance and relatively low costs. Thus, operation conditions vary frequently in the turbine. These rapid changes in operating parameters within a short time can seriously affect the stable operation of the power station. In this study, the dynamic characteristics test of a prototype tubular turbine was implemented by the dynamic grid technology, in order to consider the gravity of free surface and water gravity during the transient process of load decrease and load increase under the same output range. The research results showed that: The flow distribution in the unit was different at the same output. The reason was that the different initial flow states were caused by the initial operating conditions in the transient process of load increase and decrease. In the process of load decrease, the center area of the draft tube inlet was affected by the water discharge cone. A low-velocity zone of the same diameter occurred as the water discharge cone. The area of this low-velocity zone was further expanded as the main flow flows downstream into the diffusion section of the draft tube. There were outstanding vortexes and refluxes at the outlet of the draft tube. The starting condition of the load increase process was a partial load condition with a low flow state. The flow state in the unit was more turbulent in the whole load increase process, compared with the load decrease process. The low-velocity areas in the tailpipe in the load increase process were outstandingly larger than those in the load decrease process for the same output. The vortex begins at the inlet area of the tailpipe and spreads into most of the entire tailpipe area. This vortex state in the draft tube posed a great impact on the stability of the turbine. The vortex area and intensity in the draft tube during the load decrease process were significantly smaller than those during the load increase process. Even there was no exception at the vortex scale in the draft tube. After that, the positive and negative back pressure difference between the load-increasing and load-decreasing process was outstandingly larger than that of the load-increasing process under the same output. The pressure of the blade head and suction surface in the load-decreasing process was much smaller than that of the negative pressure state in the load-increasing process, leading easy to cavitation. The water pressure pulsation in the turbine was mainly composed of 0.1f_n low-frequency pressure pulsation caused by the vortex rope of the draft tube, and 3f_n high-frequency pressure pulsation caused by the rotation of the runner. The amplitude of the pressure pulsation during the load increase process was much larger than that during the load reduction process. The main vibration area was concentrated on the runner of the tubular turbine for the combined action of the two pressure pulsations. Water pressure pulsation was transmitted to the runner area and then coupled with the cantilever beam structure of the runner, leading to the vibration of the runner, which in turn exacerbated the vibration of the water body. And the amplitude of pressure pulsation in the load-increasing process was much larger than that in the load-decreasing process, leading to the low stability of turbine operation in the load-increasing process. The findings can provide a strong reference to design and operate the tubular turbine, particularly for the multi-energy complementary system on the operation requirements.