Flow characterisation of diffusion flame under non-resonant acoustic excitation

An experimental investigation has been carried out to study the flow characteristics of a diffusion flame under acoustic excitations. High speed direct imaging, high speed schlieren visualisation and phase-locked PIV techniques were applied to investigate the flow field induced by acoustic excitation in a tube and its effect on the flame. Non-resonant frequencies which are between the first and the second resonant frequency of the test chamber were selected in the experiments. Results indicate that the flame pattern and luminosity of the acoustically excited diffusion flames were very different in each excitation case. It was observed that the lower excitation frequency of 90 Hz has the strongest effect on the flame dynamics. Both the fuel jet flow and ambient air motion were strongly affected. At the excitation frequency of 150 Hz, the external excitation effect on the surrounding area was low and the buoyancy-driven flickering started to dominate the flame oscillation behaviour. Under the higher acoustic excitation frequency of 200 Hz, only the fuel jet was slightly affected by external forcing. The frequency analysis also showed that flame/acoustic interaction results in a nonlinear coupling effect between the buoyancy driven instability and the acoustic disturbance.