Stagger Angle and Pitch-Chord Ratio Effects on Secondary Flows Downstream of a Turbine Cascade at Several Off-Design Conditions

Abstract

The paper presents the results of an extensive measurement program addressed to investigate the influence of a large number of parameters on the development of secondary flows downstream of linear turbine cascades. Tests have been performed on linear cascades manufactured in order to obtain a large performances database referred to new 50 per cent reaction steam turbine stages. Measurements have been carried out for three different incidence angles (I = −24, 0, +24 degrees), for three pitch-chord ratios (S/C = 0.6, 0.8, 1.0) and for six values of the isentropic downstream Mach number (M2is = 0.3, 0.5, 0.65, 0.8, 0.9, 1.0). The paper also presents the analysis about the influence of the contemporary variation of the abovementioned parameters on the downstream flow features. Moreover, particular attention is addressed to the role of stagger angle variation by considering two different conditions: Δγ = −4 and Δγ = +2 degrees, measured from the design value. Experiments have been obtained by traversing a miniaturized five holes probe in a plane located at 50 per cent of the axial chord downstream of the trailing edge. Results show that pitch-chord ratio is the parameter mostly influencing the profile aerodynamic efficiency, but also incidence strongly influences the spanwise distribution of discharge angle and secondary losses. Besides, the expansion ratio throughout the cascade has a sensitive influence on secondary flows only when the combination of other test parameters already evidences strong secondary effects. Measurements point out that also a small change in stagger angle may surprisingly affect blade performances and 3D flow field.