Experimental investigations on the jet flow field propagation characteristics and ignition mechanisms of stoichiometric CH4/O2/N2 premixed turbulent jet flames

Abstract

The jet flow field (JFF) and flame propagation of stoichiometric CH4/O2/N2 mixtures under variable thermodynamic conditions were investigated experimentally using Schlieren and natural flame luminosity imaging techniques. A two-color method by wavelength integration was used to characterize the temperature field. The turbulent flame velocity (ST), Damköhler number (Da), Reynolds number (Re), and jet flame regime were explored by a combination of experimental data and theoretical calculations. The flame morphology shows that the propagation process can be subdivided into three stages: first, the JFF enters the main combustion chamber, then the flame propagates in the JFF, and finally, the JFF disappears after the flame front and the JFF front coincide. The vortex motion changes the thin reaction zone and the local high temperature region distribution. The normal jet shifts to the Mach disk jet at higher oxygen content. The ignition location changes from the jet tail to the shear layer. The top secondary vortex is more likely to generate at higher ambient temperatures, which enhances the ignition probability on the flame top. In addition, the degree of orifice blockage determines the jet flame type. Since the initial jet flame is more affected by orifice blockage, the variation in ST is strongly influenced by the change in the length-scale ratio. As the jet flame gradually moves away from the orifice, ST is mainly governed by turbulence intensity. Finally, a strong correlation between the Re number and Da number of the jet flame was found.