Forced Response Vibrations of a Low Pressure Turbine Due to Circumferential Temperature Distortions

Abstract

This paper presents the results of an experimental and analysis program which investigated the forced response of a low pressure turbine (LPT) blade due to circumferential temperature distortions which emanated from the combustor and propagated through the entire turbine system. Two-per-rev temperature variations of 350°C were measured at the LPT inlet and 300°C at the LPT exit. Total pressure fluctuations of 4% of the steady value were generated in the turbine system due to these temperature distortions. These distortions generated Entropic and vortical waves which gave excitations large enough to give unsteady loadings and stresses on the 5^th stage LPT blade sufficient to cause HCF failures for a disk/blade/tip-shroud system mode crossing. Using the measured temperature and pressure variations at the aft of the LPT, an analysis system was developed for predicting the stresses on the LPT blade. The predicted stress at the strain gage locations were within 20%–30% of the measured values, thus demonstrating the capability of determining the impact of circumferential staging of combustor fuel nozzles on turbine system blading vibrations. Therefore, in designing turbine blading, mode crossings of counts from combustor staging should be avoided or analysis similar to the above should be performed to ensure acceptable tolerances on blade vibrations.