The structural effects of the unsteady loading of a pulse detonation tube on a downstream turbine were investigated at the NASA Glenn Research Center as a possible basis for a hybrid aircraft engine in which the combustor would be replaced by a set of pulse detonation tubes arranged in a circumferential array. Such hybrid engines may increase fuel efficiency. Combustion would occur under constant-volume conditions rather than under the constant-pressure conditions of current conventional combustors. Also, the combustion process would consist of a sequence of detonations, rather than the deflagration that characterizes current combustors. Consequently, the flow downstream of the pulse detonation tubes is expected to have a large unsteady component. The objective of this study was to analyze the unsteady aerodynamic loading on turbine blades to evaluate its structural effects, such as increased vibration amplitudes or increased dynamic stresses, which can result in premature high-cycle fatigue failures.
This study used numerical flow-field simulation to calculate the aerodynamic loading history on turbine blades resulting from the unsteady flow created by the upstream pulse detonation tubes. A Reynolds-averaged Navier-Stokes code named TURBO was used to model the unsteady flow through the turbine, and the pulses from the detonation tubes were introduced into the flow domain through upstream boundary conditions. The flow inside the pulse detonation tubes was not modeled as part of this effort. Instead, the conditions at the exit of the tube were obtained from a separate one-dimensional calculation.
The unsteady flowfield through the turbine stage was calculated as part offal study that addressed aeroacoustics, unsteady aerodynamics, aeroelastics, performance, and heat transfer. The TURBO code was modified to generate a time history of the pressure on the blade surfaces, and the blade passage grids were partitioned to enable rapid computations using the parallel TURBO code on a computer cluster. Accordingly, the time history of pressure for individual grid blocks (that comprise the blade surfaces of a turbine blade) had to be merged and then decomposed into Fourier components. The Fourier components with frequencies near the natural frequencies of the blade were the ones of most interest since these can cause resonant response vibrations in the blades. Large vibration amplitudes lead to large dynamic stresses that can shorten the high-cycle fatigue life.
On the structural side, a finite element model was created for the turbine blade. The structural analysis packages NESSUS (Southwest Research Institute, San Antonio, TX) and ANSYS (ANSYS, Inc., Canonsburg, PA) were used to calculate the structural dynamic characteristics of the blade. Then, the unsteady aerodynamic loading was mapped from the TURBO computational grid onto the finite element mesh and was prescribed as a forcing function (or excitation force). Calculations performed with and without the pulse detonation tubes upstream of the turbine enabled the effect of the tubes to be identified. Initial calculations only modeled the spatial distortion in total pressure and total temperature coming into the turbine vane. The detonation tubes actually produced a spatially nonuniform flow and a time nonuniform flow, but the spatial nonuniformities did not have a strong effect on the rotor blades. The unsteady flow calculations are being updated with an improved model in which the detonation tubes are included as part of the computational domain and provide an initial pulse in the flowfield that propagates through the turbine stage. This work was supported by the Constant Volume Combustion Cycle Engine Subproject (Leo Burkardt, manager) of the Low Emissions Alternative Propulsion Project.Glenn contact: Dr. Milind A. Bakhle, 216-433-6037, Milind.A.Bakhle@nasa.gov
Last updated: October 11, 2006
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