Experimental investigation on the stability of turbulent swirling methane/air-O2 flames

Abstract

This work investigates the combustion characteristics of CH4-air/O2 turbulent swirling flames in a co-axial burner with radial fuel injection. The burner configuration consists of two coaxial tubes with a swirler positioned within the annular space for the oxidizer flow. The central tube delivers the fuel through eight holes placed that are uniformly distributed along the circumference of the inner tube nearby the burner exit. The experiments were carried out in a 25-kW combustion chamber of dimensions 50x50x120 cm3. Stereo Particle Image Velocimetry (SPIV) was used to characterize the different velocity fields, whereas the OH* Chemiluminescence is employed to examine the flame structure and stability. The exhaust gas compositions were measured using multi-gas analyzers. The structure and stability of flames, temperature evolutions, CO2, and pollutant emissions (NOx, CO) were examined under different parameters, such as: oxygen enrichment (21–50 %), global equivalence ratio (0.4–1), swirl number (0.5 and 1.4), injection type of fuel (axial or radial). Experiments were conducted at a constant power of 9.41 kW and variable powers in the range from 9.41 to 22.42 kW. The present results show that the oxygen enrichment decreases the lift-off height and enhances the flame stability using constant flame power. Increasing the swirl number improves the flame stability and produces a better attached flame. At constant flame power and variable oxidizer flow rate, a high oxygen concentration yields lower NOx emissions. For lean combustion with excessive oxygen enrichment over 35 %, retaining the same flame power leads to a significant decrease in NOx emissions. The radial injection of CH4 does not affects the NOx emissions, however, the axial injection is shown to be advantageous in terms of CO emissions in the present studied configurations.