Numerical study of opposed non-premixed jet flames of methane in a coaxial narrow air tube

In a previous study, an opposed non-premixed jet flame (ONPJF) in a coaxial narrow air tube was described, and various flame structures and their flame extinction limits were evaluated experimentally. In this study, flame structures and flow variation near the ONPJF of methane in a narrow air tube were numerically investigated using a one-step reaction model. Boundary conditions were examined for various tube sizes: cold or adiabatic walls and no-slip or slip walls. The flame extinction limits were numerically evaluated and classified into three modes: a higher air limit (HA-limit), a lower fuel limit (LF-limit), and a lower air limit (LA-limit). The HA-limits were determined by flame stretch, while the LF-limits and LA-limits were determined by the thermal quenching effect. The extinction mechanism at the LA-limit was investigated in detail. The structural transition was observed, and an enclosed edge flame structure was observed at the LA-limits. These results will be helpful in understanding the overall behavior of opposed non-premixed flames in narrow spaces, and in designing small-scale combustors with better stabilization performance.