This paper presents results of a program to investigate the magnitude and parametric variations of rotordynamic forces which arise in high power turbines due to blade-tip leakage effects. Five different unshrouded turbine configurations and one configuration shrouded with a labyrinth seal were tested with static offsets of the turbine shaft. The forces along and perpendicular to the offset were measured directly with a rotating dynamometer. Exploration of casing pressure and flow velocity distributions was used to investigate the force-generating mechanisms. For unshrouded turbines, the cross-forces originate mainly from the classical Alford mechanism (nonuniform work extraction due to varying blade efficiency with tip gap) while the direct forces arise mainly from a slightly skewed pressure pattern. The Alford coefficient for cross-force was found to vary between 2.4 and 4.0, while the similar direct force coefficient varied from 1.5 to 3.5. The cross-forces are found to increase substantially when the gap is reduced from 3.0% to 1.9% of blade height, probably due to viscous blade-tip effects. The forces also increase when the hub gap between stator and rotor decreases. The force coefficient decreases with operating flow coefficient In the case of the shrouded turbine, most of the forces arise from nonuniform seal pressures. This includes about 80% of the transverse forces. The rest appears to come from uneven work extraction (Alford mechanism). Their level is about 50% higher than in the unshrouded cases.
展开▼
