Experimental investigation of coflow effect on the ignition process of a methane jet diffusion flame

The coflow air effect on the ignition process of a methane jet diffusion flame has been investigated using high speed colour/schlieren imaging and image processing techniques experimentally. The methane flow rate is kept at constant (Re = 55.4), while the coflow air flow rate changes (Re from 171 to 5985), creating a wide range of the air/fuel velocity ratios varying from 0.36 to 12.5. Special digital image processing techniques are applied to visualise the weak blue flame and weak yellow flame, which is often difficult to view in the presence of the bright orange diffusion flame, during ignition process. The processed images have shown clearly that a sooty diffusion flame is initially formed inside a blue flame pocket at low air velocities. When the coflow air flow rate exceeds 75 l/min, only blue flame can be observed. The equivalence ratio of blue flame has been evaluated based on colour characteristics, which is close to 1 during the ignition process for all the cases. Moreover, the fuel flow, flame and hot gas interactions with the cold air flow are investigated by visualising the schlieren images. It is found that a hot gas bulge is formed due to the excessive fuel exiting before ignition and a hot laminar central jet is formed with the help of coflow effect. The hot gas bulge tip and bottom moving velocities are found to increase with the coflow air flow rates. Besides flow visualisation based on high-speed schlieren imaging sequences, the velocity fields during ignition process have been evaluated quantitatively using optical flow method.