Flame stability is an important trait of practical combustors that impacts combustion emissions and the ability of the combustor to operate effectively with changing fuel and oxidizer compositions and flow rates. Fuel and oxidizer in combustion systems are rarely perfectly premixed or perfectly segregated as they enter the flame zone. Combustion models must accurately predict the behavior of such mixed-mode combustors. However, few experiments have been performed on mixed-mode turbulent combustion in burners with well-defined boundary conditions critical for quantitatively testing models and better understanding the fundamental behavior of such flames.
To address these needs, Sydney University has developed a variant of the well-known Sydney/Sandia piloted jet burner. This new burner (Figure 1) introduces a retractable central tube (d = 4 mm) within the main tube (D = 7.5 mm) of the piloted burner. Experiments at Sydney showed that flame stability, as measured by the blowoff velocity, could be enhanced by as much as 40% compared to the homogeneous jet flame when the central tube was retracted by an optimal distance and with fuel delivered through the central tube and air through the main tube’s annulus.
Collaborating with visiting Professor Assaad Masri and student Shaun Meares from Sydney University, Sandians Gaetano Magnotti, Robert Barlow, and Bob Harmon (all in Sandia’s Reacting Flow Research Dept.) have conducted line-imaged Raman/Rayleigh/CO-laser-induced fluorescence measurements of temperature and all major species in several of these piloted flames to better understand the flame stabilization and mixed-mode combustion phenomena. Figure 2 shows the blowoff behavior vs recess distance, Lr, for flames with one part CH4 (volumetric) delivered through the central tube and two parts air delivered through the annulus, while using a premixed pilot flame burning C2H2, H2, air, CO2, and N2 in proportions to match the adiabatic equilibrium composition and temperature of stoichiometric CH4/air. (This mixture achieves the flame speed necessary for a robust pilot without adding complexity to the modeling problem.) The methane flame is most stable when the central tube is recessed by 75 mm, ten times the jet exit diameter, and the blowoff velocity asymptotes to the homogeneously mixed value as Lr approaches 300 mm.