Flow interaction from the exit cavity of an axial turbine blade row labyrinth seal

The structure of labyrinth cavity flow has been experimentally investigated in a three fun axial turbine labyrinth seal (four cavities). The geometry corresponds to a generic steam turbine rotor shroud. The relative wall moti on has not been modeled. The measurements were made with specially develope d low-blockage pneumatic probes and extensive wall pressure mapping. Instea d of the classical picture of a circumferentially uniform leakage sheet exi ting from the last labyrinth clearance, entering the channel, and uniformly spreading over the downstream channel wall, the results reveal uneven flow and the existence of high circumferential velocity within the entire exit cavity. The circumferential momentum is brought into the cavity by swirling fluid from the main channel, This fluid penetrates the cavity, and breaks up the leakage sheet into individual jets spaced according to the blade pas sages. This gives rise to strong local cross flows that may also considerab ly disturb the performance of a downstream blade row.