NUMERICAL AND EXPERIMENTAL COMPARISON OF FORCED RESPONSE OF FREE-STANDING AND SINGLE -CONNECTED LAST STAGE BLADES

In this paper a comparison is made between numerical forced response simulations and experimental measurements for two popular Steam Turbine Last Stage Blade (LSB) architectures: a free standing LSB and a single-connected LSB with a mid span wing (also known as a `snubber'). These variants share the same aerodynamic design in operation, i.e. they have the same 'operating' geometry. The focus of this study is the level of vibration response induced on the blades by resonance with a non-synchronous excitation. This can reduce the maximum condenser pressure safely attainable by a Steam Turbine during low volume flow (LVF) operation. The study develops a common method to effectively represent the LVF excitation in FEM harmonic analysis for both free standing and single connected LSB's. This is valuable to LSB designers during the initial phase of a new development (when an assessment of the LVF capability is required before. experimental measurements have been taken). In addition, the method can be used to evaluate the suitability of an existing LSB design to a revised environment, as it is often the case in retrofit applications.