AS-Manufactured Modeling of a Mistuned Turbine Engine Compressor Evaluated Against Experimental Approaches (2019)

As-manufactured rotors behave quite differently than nominal, as-designed rotors due to small geometric and material property deviations in the rotor, referred to as mistuning. Traditional integrally bladed rotor (IBR) modeling approaches assume each blade is identical. State-of-the-art IBR dynamic response predictions can be accomplished using asmanufactured models (AMM) generated via optical topography measurements and mesh morphing. As-manufactured models account for geometric deviations occurring through the machining process, material deviations and field wear, allowing each blade to respond differently. Rotor designs are intended to avoid resonance crossings throughout an engine’s operating range, but total avoidance is challenging. This has led to conservative designs as well as heavily instrumented rig and engine testing to attempt to reduce future HCF issues, debiting aircraft performance while increasing development costs. Therefore, it is vital that accurate modeling approaches predict the forced response of resonance crossings to capture mistuning phenomenon and to place safety instrumentation appropriately. Safe engine operation is ensured by setting safety limits on rotor airfoil mounted strain gages that monitor the dynamic response of the component. Traditionally, strain gage limits are generated utilizing geometry obtained from an “as-designed nominal model where finite element analysis is used to compute the static and modal stresses. Predicted modal stresses of the cyclic analysis are used to optimize strain gage locations to ensure modal observational coverage, modal identification, and maximum vibrational stress for each mode. Strain gage limits are then produced for these optimal strain gage locations on the tuned finite element model. The described nominal geometry based process is subject to errors associated with airfoil mode shape variations caused by manufacturing deviations. This work develops a new process based on as-manufactured geometry measurements that obtains more accurate strain gage limits. It is shown that, due to the variability of blade-to-blade geometry, strain gage limits can vary significantly between blades. This is demonstrated by analyzing a mistuned IBR on a sector by sector basis. The developed approach has the capability to more accurately place gages on responsive blades to ensure safe engine operation during testing. Although blade mounted strain gages are vital during rig and engine development to ensure safe engine operation, they also enable a change in dynamics of IBRs. The mistuning of a 20 bladed IBR is evaluated via analytical methods, benchtop testing, and using a rotating compressor research facility. The resonant response of the IBR at various modes and harmonic excitations is investigated in this work. Two AMM finite element models (FEM) are created of a 20 bladed IBR. One FEM has no strain gages present, where the second FEM includes strain gages on six blades. Traditionally, strain gages and lead wires are treated as the same material property as the IBR itself. It will be shown that the inclusion of strain gages in AMMs using this method changes the IBRs predicted mistuning. An alternative AMM approach is developed that changes the material properties of the finite elements attributed to the strain gages. The predicted mistuning for each AMM is accomplished using the Fundamental Mistuning Model (FMM ID), where the predicted mistuning will be compared to both Traveling Wave Excitation (TWE) experiments and a rotating compressor rig. Findings show mistuning predictions of the non-strain gaged AMM compare far better to the experiments compared to the inclusion of the strain gages in the AMM. Additionally, altering material properties of the strain gages in the AMM improves mistuning prediction compared to treating the strain gages as the parent IBR material. The work herein supports the recommendations that AMM should be acquired using clean, non-strain gaged rotors or the material properties of strain gaged elements need to be altered to more accurately model the component. This body of work ultimately shows the importance and ability to use AMM approaches to significantly increase the fidelity of understanding of turbine engines from both a component and system level.