管内下方伝播火炎における高次音響モードに対するパラメトリック不安定性の実験的観察

抄録

We present experimental investigation of parametric instability in downward propagating C₂H₄/O₂/CO₂ flames. Parametric instability of fundamental mode is observed for S_L: 27.5 ~ 30 cm/s. Parametric instability of first harmonic transitioning to fundamental mode is observed for S_L: 32.5 ~ 42.5 cm/s. Parametric instability of second harmonic transitioning to first harmonic and further to fundamental mode is observed for S_L: 45 ~ 50 cm/s. Higher modes of parametric instability transitioned successively to lower modes as the flame propagated downward. The transition of parametric instability to lower modes can be understood by studying theoretical variation of acoustic structure functions along the length of tube for velocity coupling. Flame position of maximum pressure amplitude under each acoustic mode corresponds to maximum of acoustic structure functions for corresponding mode. Wavenumber of corrugated flame structure in parametric instability depends on acoustic mode generated in tube. Higher mode instability makes larger wavenumber compared to lower mode instability.