A hybrid aircraft engine, where a combustion chamber with multiple detonation tubes precedes a turbine stage, is being studied at the NASA Glenn Research Center. These engines, known as pulse detonation engines (PDEs), operate with constant-volume combustion, rather than the constant-pressure combustion that is currently used in gas turbine engines, and have been predicted to be more efficient than current aeronautical propulsion systems. However, the structural integrity of the turbine blades for PDEs needs to assessed.
High-momentum fluid exiting from the detonation tubes results in large input pressure fluctuations for the downstream turbine rotor blades. These fluctuations may excite the natural modes of the blades, resulting in multiple resonance conditions. Forced vibration at or near these resonant conditions is the primary contributing factor to high-cycle fatigue (HCF) failures. In order for the design to operate safely, the turbine must avoid all the resonant conditions. However, it may not be possible to avoid these multiple engine-order crossings in a hybrid-PDE turbine, so the magnitude of the stresses and displacements must be predicted at multiple resonant conditions to ensure safe operation.
The objectives of this study were to predict the unsteady aeroelastic response on the turbine rotor blades and assess the safety of the rotor blades. To accomplish this, researchers used a finite-volume-based Navier-Stokes solver developed at Mississippi State University (MSU-TURBO) to calculate the flow field around the turbine stage. The unsteady solution is an improved model in which the detonation tubes are included as part of the computational domain. The solution was written in blocks, and a postprocessor assembled the parts of the blocks that make up the rotor grid.
The unsteady pressures were Fourier analyzed to obtain the frequencies and the magnitudes of the corresponding mean, and the unsteady pressures. The pressures were then converted to grid nodal forces for use by the structural analysis program.
Campbell diagram showing multiple engine orders close to the first, second, and third modes.
Long description of figure 1.
A free-vibration analysis used ANSYS structural analysis software (ANSYS, Inc.) to obtain the mode shapes and modal frequencies of the rotor blade at six rotating speeds, resulting in the preceding Campbell diagram. The diagram plots the modal frequencies with rotational speed, for various engine orders, modified for a PDE application. It indicates that the response has to be analyzed near the first three modal frequencies for multiple engine orders at the rotor operating speed. The mode superposition method was used to predict the structural response. Six modes were used in the modal summation, and the analysis was repeated with various values of structural damping.
The following graph below shows the response and stress obtained with varying damping for the loading condition corresponding to the 36th engine order. The response obtained at 6170 Hz, close to the third-mode frequency, is shown. Just 0.3-percent damping reduced the stresses and blade displacements by about 75 percent. This amount of damping is typically present in rotor blades because of the friction at the blade root-disk interface. The analysis showed that flightweight turbine blades can be built to survive the pulse detonation loadings in a hybrid PDE engine.
Normalized response and stress variation with damping for engine order 36.
Long description of figure 2.
The present study was performed under a NASA grant to University of Toledo researchers in collaboration with Glenn researchers. This work was supported by the Low-Emissions, Alternative Power Project, under the Constant Volume Combustion Cycle Engine subproject, Leo Burkardt, manager.
University of Toledo contact:Last updated: December 17, 2007
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