Experimental study of N2 and CO2 dilution in CH4 fuel stream with buoyancy-induced low-stretch diffusion flames

Abstract

This study experimentally investigated the structures, dynamics and extinction behaviors of buoyancy-induced ultra-low-stretch diffusion flames with N2/CO2 dilution in a CH4 stream, using porous spherical burners with large radii. In quasi-steady states associated with low dilution and comparatively small fuel mixture injection speeds (uF), as stretch rate dropped and uF enlarged, the flame standoff distance increased, while the burner surface temperature decreased due to surface heat losses. The flame temperature, Tf, dropped at lower uF and smaller stretch rates, primarily because of reduced heat generation and increased flame radiative loss, respectively. The dilution effect of N2/CO2 reduced Tf. Moreover, the chemical and thermal effects of CO2 further decreased Tf and soot formation. With increased dilution, two instability patterns occurred before extinction. One was a stripes/holes pattern, evolving from waves/bumps and cellular flames, at higher uF. The other was a periodic holes pattern appearing at lower uF. These patterns were associated with Rayleigh-Taylor instability, thermal-diffusive instability and instability related to heat loss. The dilution extinction limits at varied stretch rates increased with increases in uF until they reached constant levels, corresponding to the intrinsic dilution limits of N2 and CO2 for CH4, and the suppression effect of CO2 was greater than that of N2. At the high dilution extinction limit, extinction was independent of uF and stretch rate, but dominated by lower heat release compared with robust flames, and also by substantial flame heat losses. At the low dilution extinction limit, both gas-phase and burner surface heat losses contributed to extinction, furthermore, lower stretched flames had obvious higher dilution extinction limits, primarily due to smaller gas-phase heat losses requiring less heat generation to balance. This work enriches the knowledge of ultra-low-stretch gaseous diffusion flames, especially the flame structure, instability and extinction, and also provides information regarding the dilution gas effects.