Structure and soot formation characteristics of a double coflow methane diffusion flame

An experimental and numerical study was conducted to investigate the effect of an air jet introduced in the centre of the fuel jet on the structure and soot formation characteristics of a laminar methane/air diffusion flame established on an axisymmetric coflow burner at atmospheric pressure. Under the conditions of this study such a burner arrangement in general results in a double flame configuration: an inner partially premixed flame surrounded by an outer normal diffusion flame. Experimentally it was observed that the flame luminosity and the visible flame height decrease significantly with increasing the central air jet flow rate. At sufficiently high central air jet flow rates, the flame becomes essentially blue. Numerically the double diffusion flame was modeled by solving the elliptic conservation equations in axisymmetric cylindrical coordinates with a standard control volume method along with detailed chemistry and thermal and transport properties. Soot formation was accounted for using a semi-empirical acetylene based model. Thermal radiation was also included in the numerical simulation using the discrete-ordinates method along with a 9-band non-grey radiative property model. Overall, the numerical model qualitatively reproduced the effects of the central air jet on the visible appearance of the double diffusion flame observed experimentally.