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Unsteady Rotor-Stator Interaction in a Four-Stage Axial Turbine: An Experimental Investigation


 This paper presents an experimental investigation of the transient rotor-stator interactions (RSI) in a four-stage axial flow turbine at design as well as off-design conditions. The pressure fluctuations recorded on the surface of a stator vane in stage 3 of the machine are phase-averaged to the rotor position and decomposed into the main contributing periodic components. The origins of the different fluctuations are identified by matching their frequency to the individual blade counts of the different blade rows which, in turn, allows to get insight into their interaction mechanisms and their inter-blade-row propagation mechanisms. Dependent on the operating point, the magnitude, and local distribution of the pressure fluctuation, components change significantly and components originating from different upstream and downstream blade rows become dominant. For operating points close to the design point both directly neighboring blade rows R2 and R3 are the main contributors to the RSI affecting the pressure fluctuations on stator vane 3 at mid span. For the intermediate off-design operating conditions the effect of the downstream blade row R3 diminishes and instead the blade row R1 further upstream beyond the directly neighboring R2 reaches magnitudes similar to that of R2. For strong off-design conditions the influence of the downstream blade row R3 diminishes further and instead of pressure fluctuations corresponding to the blade passing frequency of the immediate upstream blade row R2, a pressure fluctuation corresponding to the difference in blade counts of both upstream blade rows becomes the dominant component.