Analysis of the runner behavior during the start-up sequence in a bulb turbine model

An experimental analysis of the start-up sequence of a bulb turbine was performed in a closed loop circuit, and analyses of global performances were conducted under three start-up conditions. In order to conduct a deeper analysis of the runner behavior, a runner blade was equipped with 26 sensors, which were used to evaluate the transient pressure field using an interpolation method. By checking the global performances of all the conditions, the flow rate evolution follows the GVO evolution only for the two slowest GVO test cases. Additionally, the use of defined dimensionless numbers allowed for some degree of universal evolution to be revealed, and for the peak of torque to be characterized. The pressure on the runner blades was also investigated. Although the runner operates like an impulse turbine at the beginning of the start-up sequence, its blades act like more airfoils when the torque reaches its peak. Moreover, the fluctuations at the end of the start-up sequence suggests that the stress on the blade could be more important than the stress observed on the driven shaft. Furthermore, local phenomena, such as suction on the pressure side of the runner blade near the shroud, were also observed on the pressure distribution, which is incongruent with the theoretical inlet velocity field estimated from global performances. These observations suggest the presence of a backflow and a cavitation pocket. Moreover, a flow instability probably occurs at low GVO speed and produces some torque fluctuations around the maximum torque value.