UNSTEADY-FLOW FIELD DUE TO NOZZLE WAKE INTERACTION WITH THE ROTOR IN AN AXIAL-FLOW TURBINE .1. ROTOR PASSAGE FLOW-FIELD

The flow field in turbine rotor passages is complex with unsteadiness caused by the aerodynamic interaction of the nozzle and rotor flow fie lds. The two-dimensional steady and unsteady flow field at midspan in an axial flow turbine rotor has been investigated experimentally using an LDV with emphasis on the interaction of the nozzle wake with the r otor flow field. The flow field in the rotor passage is presented in P art I, while the flow field downstream of the rotor is presented in Pa rt II. Measurements were acquired at 37 axial locations from just upst ream of the rotor to one chord downstream of the rotor. The time-avera ged flow field and the unsteadiness caused by the wake have been captu red. As the nozzle wake travels through the rotor flow field, the nozz le wake becomes distorted with the region of the nozzle wake near the rotor suction surface moving faster than the region near the rotor pre ssure surface, resulting in a highly distorted wake. The wake is found to be spread out along the rotor pressure surface, as it convects dow nstream of midchord. The magnitude of the nozzle wake velocity defect grows until close to midchord, after which it decreases. High values o f unresolved unsteadiness were observed at the rotor leading edge. Thi s is due to the large flow gradients near the leading edge and the int eraction of the nozzle wake with the rotor leading edge. High values o f unresolved unsteadiness were also observed near the rotor pressure s urface. This increase in unresolved unsteadiness is caused by the inte raction of the nozzle wake with the pow near the rotor pressure surfac e.