Comparisons of the dynamic responses of diffusion flames subjected to acoustic disturbances in the fuel and air lines

In this paper, the dynamic responses of a laminar diffusion flame subjected to acoustic disturbances in the fuel line (ADF) and surrounded air flow (ADA) were experimentally studied. Experiments were conducted at different frequencies by keeping the velocity perturbation amplitudes for both ADF and ADA the same. Chemiluminescence data (CH* and OH*) measured by an intensified high-speed camera and density perturbations measured by a Mach–Zehnder interferometer (MZI) were used to identify the flame dynamic responses in detail. Results show that both ADF and ADA can induce flame oscillations by stimulating disturbances in the fuel flow but in different manners. The perturbations led by the ADF propagate along the flame front with a hydrodynamic wavelength, while those driven by the ADA are more likely to periodically squeeze and extend the flame region from the lateral interfaces, leading to a global periodic motion along the longitudinal direction. Both types of perturbation are difficult to propagate downstream due to the damping of flow perturbation. In addition, the flame oscillation patterns driven by the ADA remain the same under different disturbance frequencies. When the diffusion flame is simultaneously subjected to both ADF and ADA, the perturbations of them are mutually coupled at this time, but the oscillation patterns of the flame are dominated by the ADF. Due to the different mechanisms of two disturbances, the oscillation patterns after coupling are related to the disturbance frequency. Based on the above results, a model of the diffusion flame oscillation subjected to ADF and ADA was proposed.