Analysis of Experimental Time-Dependent Blade Surface Pressures From an Oscillating Turbine Cascade With the Influence-Coefficient Technique

Abstract

A two-dimensional section of the last stage of a steam turbine has been investigated experimentally in an annular non-rotating cascade facility as regards to its steady-state and time-dependent aerodynamic characteristics at design and off-design conditions. The unsteady experimental data obtained with the blades vibrating in the “travelling wave” mode indicate that one of the main reasons for the flutter susceptibility of the cascade lies in the high expansion and following shock wave close to the blade suction surface leading edge and the corresponding high unsteady loading. The decomposition of the experimental data into unsteady aerodynamic influence coefficients validates this conclusion and gives also that another reason for the flutter susceptibility can be found in the fact that the cascade is overlapped for a part of the blade surface where the local flow velocities are close to sonic. The unsteady aerodynamic influence coefficients show that the instability arises because of the time dependent aerodynamic coupling effects between, essentially, the reference blade and its immediate suction surface and, to a lesser extent, pressure surface neighbors.

Periodicity and Repetivity of Unsteady Measurements of an Annular Turbine Cascade at Off Design Flow Conditions

“…Once the results in the traveling wave mode have been obtained, a data reduction into influence coefficients is done (Fransson, 1990;Szóchenyi and Girault, 1983;Hanamura et al, 1980). The influence coefficients are calculated for a range of interblade phase angles (i.e.…”

“…The excitation control system had difficulties to keep a pre-set amplitude and phase relation of the blades. Yet, the minimum aerodynamical damping for the cascade is at 306° interblade phase angle (Boles, A.; Fransson, T. H.;Kbrbacher, H.;1991). At this interblade phase angle the vibration mode was kept without difficulties.…”