Combustion Dynamics Behavior in a Single-Element Lean Direct Injection (LDI) Gas Turbine Combustor

Abstract

A concurrent computational and experimental study of self-excited combustion dynamics in a model configuration of a lean direct injection (LDI) gas turbine combustor are described. Incoming air temperature and equivalence ratio were varied. Simulation at low equivalence ratio compared better with measurement and thus this condition was selected for a more detailed study of the underlying combustion dynamics mechanisms. First, hydrodynamic modes are investigated by conducting the simulation with an acousticallyopen combustor so that acoustic effects on the flow field are minimized. The Vortex Breakdown Bubble (VBB) proves to be an important flow structure that can easily interact with the acoustic field to sustain instability. Second, detailed cycle studies of the acoustically closed combustor simulation reveals enhanced mixing and vaporization of the JP-8 fuel spray due to acoustic compression wave. Dynamic Mode Decomposition (DMD) analysis is used to identify the coupling between axial acoustics and the vortex breakdown bubble in the lower frequency region. Presence of another important hydrodynamic mode, the Precessing Vortex Core (PVC) is also identified from the DMD analysis. The possibility of nonlinear coupling between the acoustics and PVC modes is indicated.

Parametric investigation of combustion instabilities in a single-element lean direct injection combustor

“…Another observation is the presence of high amplitude modes in the 6-8 kHz frequency range observed in all configurations at higher equivalence ratios ( > 0.5). Concurrent computational results of the experiment 24 indicate the presence of a PVC hydrodynamic instability coupled with heat release in this frequency range, especially near the dump plane of the combustor and in the diverging section of the venturi.…”

“…This frequency range corresponds to the 4L mode in the combustion chamber for the longer chamber configurations and 2L mode for the shorter combustion chamber. Simulations performed by Huang et al 23,24 and Gejji et al 28 show the presence of a hydrodynamic mode related to the VBB in the chamber in the 1200-1500 Hz range. A prominent feature of swirl stabilized combustion is a central recirculation zone (CRZ) and an outer recirculation zone (ORZ) that is filled with hot combustion products, which serves as a continuous source of ignition for the fresh injected reactants and plays a critical role in flame stabilization.…”

“…The highest instability amplitudes are observed at frequencies that correspond to natural acoustic modes of the chamber as well as align with hydrodynamic VBB mode in the combustor. 23,24 In the longer chamber, since the fundamental longitudinal mode frequency is lower ($350 Hz), a large number of modes that correspond to the harmonics of this frequency are observed in the pressure spectrum. For the shorter chamber, the fundamental longitudinal mode frequency of the combustion chamber is higher ($700 Hz), hence, smaller number of harmonics are seen in power spectrum.…”

“…For the higher frequency modes in the 6-8 kHz range, the pressure signals were not in phase (140-160 ) with each other indicating the pressure mode are not purely longitudinal. Dynamic mode decomposition of pressure and velocity field at the frequencies using companion hybrid RANS-LES investigations by Huang et al 23,24 indicate presence of a PVC instability in this frequency range.…”

“…The diameter at the throat of the venturi is 15.5 mm. The combustion chamber has a 50.8 mm inner diameter and its length can be varied in discrete increments from 0.28 m to 1.08 m. To reduce heat transfer to the chamber walls in the combustion zone and approach an adiabatic wall boundary condition employed in a concurrent computational modeling effort, [23][24][25] thermal barrier coating is applied to the first 100 mm of the combustion chamber and the diverging section of the venturi where the maximum heat release occurs in the combustor. This allows better comparison with computational results where adiabatic boundary conditions are commonly used for the combustor walls.…”