Identification and Mitigation of Unsteady Pressure Fluctuations in a Gas Turbine Testing Facility
Date of Award
Aeroacoustics, Aerodynamics, CFD, Gas turbine testing, Lobed mixer
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As governmental demands for reduced thrust specific fuel consumption drive the trend of turbofan engine bypass ratios higher, gas turbine testing facilities are subjected to higher mass flow rates. As a result, very high amplitude, low frequency unsteady pressure fluctuations have been observed, which have deleterious effects on downstream facility components. This dissertation identifies the mechanisms responsible for the generation of these unsteady pressure fluctuations, whether they are the result of convection or propagation through the facility, and possible abatement strategies. A high-fidelity, scale resolving computational framework is used to accurately capture broadband and tonal turbulence and is applied to a scale model facility with flow driven by an ejector nozzle instead of an engine. Far downstream pressure fluctuations are identified to be strongly correlated to turbulence generated in the free shear layer between ejector nozzle outflow and surrounding entrained flow. The natural frequency of the Kelvin-Helmholtz instability in the shear layer is close enough to the natural frequency of a diamond-shaped (2,2 diag) mode in the square cross-section chamber that houses the ejector nozzle that it leads to locked-in aeroacoustic excitation. Strong tonal behaviour is registered in the entire facility as a result, and the low-frequency broadband content is more energetic than for flow conditions where the lock-in does not occur. These fluctuations are shown to convect far downstream with minimal decay due to the high convective Mach number of the flow and energy addition by a secondary shear layer. The use of a symmetry plane for high-fidelity computations of negligible swirl is assessed to reduce the computational cost of investigating mitigation strategies. Provided that the excited aeroacoustic mode is not altered by the symmetry plane, a half domain computation captures the full-domain, far-downstream low-frequency sound pressure level within 0.6 dB. Mass injection into the shear layer is investigated as a method for mixing enhancement. This active flow control is ineffective at reducing large pressure fluctuations due to the requirement of injecting at a location where the instabilities have already grown into distinct vortices. Passive flow control by a lobed mixer placed inside the mixing duct downstream of the ejector nozzle, however, is effective at mixing large vortical structures, reducing the amplitude of pressure fluctuations on downstream components. The critical parameters in determining device effectiveness are axial placement, which sets the potential for further downstream turbulence development, and eccentricity, which exhibits increased levels of turbulence kinetic energy as vorticity undergoes an axis switching phenomenon by means of self-induction. The furthest downstream placed device is able to reduce low frequency, broadband pressure fluctuations by up to 4.5 dB.
Hill, David, "Identification and Mitigation of Unsteady Pressure Fluctuations in a Gas Turbine Testing Facility" (2022). Electronic Theses and Dissertations. 8918.