Innovative study of co-axial normal and inverse diffusion flames

Abstract

An experimental work was performed to investigate the co-axial normal/inverse diffusion flames at both unbound and confined conditions. Three novel burner designs were employed such that initially co-axial air and fuel streams issuing from an inverse flame burner port were combusted within a confinement and then mixed with an outer atmosphere thus involving an outer normal flame. Strongly turbulent conditions were enforced through flow restriction, introducing cross-flow opposing jets, extending the aerodynamic expanding zones, and jet splitting. Such parametric variation was coupled with changes in the flow areas and relative velocities. With the vertically straight design, decreasing the covering plate punching diameter or its separation gap above the supplying air tube significantly decreased the exhausted unburned hydrocarbon concentrations. Sufficiently high air jet velocities led to lifted flames with an associated fuel entrainment. The peak flame temperature reduction to around 1300 °C highlighted the decrease in the maximum NOx concentrations. Enhanced qualitative/quantitative flame characteristics were also obtained upon employing an expanding interior wall design. The fuel/air mixing and recirculation were significantly enhanced by extending the confining wall such that the combustion efficiency was optimized by the flow dynamics of the inverse flame inner air supply. The maximum unburned hydrocarbon and carbon monoxide concentrations were 0.57 per cent and 800 ppm, respectively. In the third design, the mixing rates were increased by splitting the fuel jet so that the enlarged flame envelope increased the combustion efficiency at the proper relative jet velocities. The turbulence characteristics were computationally addressed by highlighting the role of jet shear layers and recirculation in augmenting the combustion efficiency and stability.