Onset of global instability in a premixed annular V-flame
arxiv(2024)
摘要
We investigate self-excited axisymmetric oscillations of a lean premixed
methane–air V-flame in a laminar annular jet. The flame is anchored near the
rim of the centrebody, forming an inverted cone, while the strongest vorticity
is concentrated along the outer shear layer of the annular jet. Consequently,
the reaction and vorticity dynamics are largely separated, except where they
coalesce near the flame tip. The global eigenmodes corresponding to the
linearised reacting flow equations around the steady base state are computed in
an axisymmetric setting. We identify an arc branch of eigenmodes exhibiting
strong oscillations at the flame tip. The associated eigenvalues are robust
with respect to domain truncation and numerical discretisation, and they become
destabilised as the Reynolds number increases. The frequency of the leading
eigenmode is found to correspond to the Lagrangian disturbance advection time
from the nozzle outlet to the flame tip. This linear result suggests a
non-local feedback mechanism consistent with the scenario of `intrinsic
thermoacoustic instability'. Nonlinear time-resolved simulation further reveals
notable hysteresis phenomena in the subcritical regime prior to instability.
Hence, even when the flame is linearly stable, perturbations of sufficient
amplitude can trigger limit-cycle oscillations and higher-dimensional dynamics
sustained by nonlinear feedback. Notably, linear analysis of the subcritical
time-averaged limit-cycle state yields eigenvalues that do not match the
nonlinear periodic oscillation frequencies, emphasising the essential role of
nonlinear harmonic interactions in the system dynamics.
更多查看译文
AI 理解论文
溯源树
样例
生成溯源树,研究论文发展脉络
Chat Paper
正在生成论文摘要