On the Effect of Axial Spacing Between Rotor and Stator Onto the Blade Vibrations of a Low Pressure Turbine Stage at Engine Relevant Operating Conditions

Abstract

In order to achieve the ACARE targets regarding reduction of emissions, it is essential to reduce fuel consumption drastically. Reducing engine weight is supporting this target and one option to reduce weight is to reduce the overall engine length (shorter shafts, nacelle). However, to achieve a reduction in engine length, the spacing between stator and rotor can be minimised, thus changing the rotor blade excitation. Related to the axial spacing, a number of excitation mechanisms with respect to the rotor blading must already be considered during the design process. Based on these facts several setups have been investigated at different engine relevant operating points and axial spacing between the stator and rotor in the subsonic test turbine facility (STTF-AAAI) at the Institute for Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. In order to avoid upstream effects of supporting struts, these struts are located far downstream of the stage which is under investigation. For rotor blade vibration measurements, a novel telemetry system in combination with strain gauges is applied. To the best of the author’s knowledge, the present paper is the first report of blade vibration measurements within a rotating system in the area of low pressure turbines under engine relevant operating conditions. In addition, aerodynamic measurements including unsteady flow measurements have been conducted, but will not be presented in this paper. By analysing the flow field, aerodynamic excitation mechanisms can be identified and assigned to the blade vibration. However, this is not presented in this paper. Within this paper, the flow fields are analysed in both upstream and downstream of the turbine stage, visualised for two axial gaps and then compared to the forced response of the blading. Detailed structural dynamic investigations show critical modes during the operation which are identified by the telemetry measurements as well. Finally the influence of the axial spacing regarding the rotor blade excitation and vibration can be elaborated and is prepared to get a better understanding of basic mechanisms. The paper shows that reducing axial spacing is a promising option for reducing engine weight, but aeroelasticity must be carefully taken into account.