Control of the structure and sooting characteristics of a coflow laminar methane/air diffusion flame using a central air jet: An experimental and numerical study

Abstract

Multi-port co-annular burners are widely used in practice to achieve low NOx and soot emissions from combustion devices. However, there is lack of fundamental studies on the structure and flame regime under different flow conditions. A conventional laminar axisymmetric coflow diffusion flame burner was modified by introducing a central air jet inside the fuel tube to investigate how the central air jet velocity affects the structure and sooting characteristics of a coflow methane/air diffusion flame. The modified burner produces a double flame structure: an inner inverse diffusion flame or an inner partially premixed flame and an outer normal diffusion flame. Experiments were conducted to observe the effect of the central air jet velocity on the appearance of the flame. At a given and relatively low central air jet flow rate the inner flame can be either a partially premixed one or an inverse diffusion one, depending on how the central air jet flow rate is adjusted. The overall flame structure and sooting characteristics can be controlled effectively by varying the flow rate of the central air jet. Detailed numerical calculations were conducted using GRI-Mech 3.0 without the NOx chemistry and a simplified soot model. Numerical results reproduce the experimental observations and provide detailed information on the flow field, temperature, and species concentration distributions. The central air jet is an effective aerodynamic means to control the flame size, structure, and sooting characteristics.