Flow characteristics of last-segment axial turbine adopted in compressed air energy storage system under low load and windage conditions

During the discharge process of the compressed air energy storage (CAES) system, the decline in storage capacity of the air tank and the demand for flexible operation of the system may cause the last-segment axial turbine (LSAT) to operate under low load conditions. It is particularly important to investigate the LSAT internal flow field and its influence on performance, especially under windage conditions. In this study, the performance and internal flow of the LSAT coupling chamber under low load conditions are analyzed by full annuls computational fluid dynamic simulation which is validated by experiments. Analysis of the total pressure nonuniformity on chamber outlet, R1 and R2 inlet indicates that the inlet chamber has little influence on the flow of the second rotor row. The nonuniformity coefficient of R1 and R2 inlet first rises, and then decreases under low load conditions, and the main nonuniform distribution mode of total pressure changes from circumferential to radial. At the condition of pi(tt) = 1.32, the windage state appears above 80% span of R2, resulting in the decrease of stage 2 power output is only 0.47 kJ/kg. Then the range of windage state extends to the blade root as pi(tt) continues to decline, which causes negative power output of the whole turbine under conditions with pi(tt) < 1.13. The main three-dimensional flow structures in the rotor rows under windage conditions are also revealed. Flow separation on the blade pressure side and near the suction side of the trailing edge can be seen under conditions with pi(tt) < 1.32, while the back flow region can be captured only when pi(tt) <= 1.03. All the flow separation and back flow regions will gradually expand with the further decrease of pi(tt). In addition, the back flow region will merge with the suction flow separation region at a 30%-45% span of the rotor blade.