A coupled numerical approach is investigated for predicting combustion instability limit cycle characteristics
when the combustor contains a long flame. The test case is the ORACLES combustor, with a
turbulent premixed flame a metre long: it exhibits limit cycle oscillations at ∼ 50 Hz and normalised
velocity amplitude ahead of the flame of ∼ 0.29. The approach obtains the flame response to acoustic
excitation using Large Eddy Simulations (LES), and couples this with a low-order wave-based network
representation for the acoustic waves within the combustor. The flame cannot be treated as acoustically
compact; the spatial distribution of both its response and the subsequent effect on the acoustics must
be accounted for. The long flame is uniformly segmented axially, each segment being much shorter than
the flow wavelengths at play. A series of “local” flame describing functions, one for the heat release rate
response within each segment to velocity forcing at a fixed reference location, are extracted from the LES.
These use the Computational Fluid Dynamics toolbox, OpenFOAM, with an incompressible approximation
for the flow-field and combustion modelled using the Partially Stirred Reactor model with a global onestep
reaction mechanism. For coupling with the low-order acoustic network modelling, compact acoustic
jump conditions are derived and applied across each flame segment, while between flame segments,
wave propagation occurs. Limit cycle predictions from the proposed coupled method agree well with
those predicted using the continuous 1-D linearised Euler equations, validating the flame segmentation
implementation. Limit cycle predictions (frequency 51.6 Hz and amplitude 0.38) also agree well with experimental
measurements, validating the low-order coupled method as a prediction tool for combustors
with long flames. A sensitivity analysis shows that the predicted limit cycle amplitude decreases rapidly
when acoustic losses at boundaries are accounted for, and increases if combustor heat losses downstream
of the flame are accounted for. This motivates more accurate determination of combustor boundary and
temperature behaviour for thermoacoustic predictions.