Suppression Dynamics of a Laminar Oscillating Diffusion Flame with Co-flow Air

摘要

The present paper reports an experimental investigation of co-flow air velocity effects on the flickering behaviour of non-lifted laminar methane diffusion flame. Chemiluminescence, high speed photography accompanied with digital image processing techniques have been used to study the change in global flame shape, the instability initiation point, the frequency and magnitude of the flame oscillation. It has been observed that the flame dynamics and combustion characteristics of co-flow diffusion flame are strongly affected by the co-flow air velocity. The oscillation frequency was observed to increase linearly with the co-flow velocity, whilst, the frequency amplitude was observed to continuously decrease. When the co-flow velocity has reached a certain value the buoyancy driven flame oscillation was completely suppressed. The high speed imaging has revealed that the co-flow of air is able to push the location of instability initiation point beyond the visible flame to create a very steady laminar flow region in the reaction zone. It is observed that the oscillation magnitude and wavelength decrease continuously as the co-flow air increases. The average oscillating flame height behaviour, however, was observed to be bimodal. It was initially enhanced by the co-flow air then starts to decrease towards the stabilised level. This height was observed to remain almost constant after stabilisation, despite further increase at air flow rate. The mean pixel intensity (MPI) values, in a full data-range of high speed images, coupling with the standard deviation (s) of MPI are found to be general indicators of the flame flickering suppression.