MEASUREMENT OF TURBINE BLADE-TIP ROTORDYNAMIC EXCITATION FORCES

This paper presents results of a program to investigate the magnitude, origin, and parametric variations of destabilizing forces that arise in high power turbines due to blade-tip leakage effects. Five differen t unshrouded turbine configurations and one configuration shrouded wit h a labyrinth seal were tested with static offsets of the turbine shaf t. The forces along and perpendicular to the offset were measured dire ctly with a dynamometer, and were also inferred from velocity triangle s and pressure distributions obtained from detailed flow surveys. Thes e two routes yielded values in fair agreement in all cases. For unshro uded turbines, the cross-forces are seen to originate mainly (similar to 2/3) from the classical Alford mechanism (nonuniform work extractio n due to varying blade efficiency with tip gap) and about 1/3 from a s lightly skewed hub pressure pattern. The direct forces arise mainly (3 /4) from this pressure pattern, with the rest due to a slight skewness of the Alford mechanism. The pressure nonuniformity (lower pressures near the widest gap) is seen to arise from a large-scale redistributio n of the flow as it approaches the eccentric turbine. The cross-forces are found to increase substantially when the gap is reduced from 3.0 to 1.9 percent of blade height, probably due to viscous blade-tip effe cts. The forces also increase when the hub gap between stator and roto r decreases. The force coefficient decreases with operating flow coeff icient. In the case of the shrouded turbine, most of the forces arise from nonuniform seal pressures. This includes about 80 percent of the transverse forces. The rest appears to come from uneven work extractio n (Alford mechanism). Their level is about 50 percent higher than in t he unshrouded cases.