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

This study experimentally investigated the structures, dynamics and extinction behaviors of buoyancyinduced ultra-low-stretch diffusion flames with N (2)/CO (2) dilution in a CH (4) stream, using porous spherical burners with large radii. In quasi-steady states associated with low dilution and comparatively small fuel mixture injection speeds (u F), as stretch rate dropped and u F enlarged, the flame standoffdistance increased, while the burner surface temperature decreased due to surface heat losses. The flame temperature, T f, dropped at lower u F and smaller stretch rates, primarily because of reduced heat generation and increased flame radiative loss, respectively. The dilution effect of N (2)/CO (2) reduced T f. Moreover, the chemical and thermal effects of CO (2) further decreased T f 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 u F. The other was a periodic holes pattern appearing at lower u F. 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 u F until they reached constant levels, corresponding to the intrinsic dilution limits of N (2) and CO (2) for CH (4), and the suppression effect of CO (2) was greater than that of N (2). At the high dilution extinction limit, extinction was independent of u F 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. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

This study experimentally investigated the structures, dynamics, and extinction behaviors of buoyancy-induced ultra-low-stretch diffusion flames with N(2)/CO(2) dilution in a CH(4) stream. The results showed that the characteristics and extinction process of the flame varied under different dilution and fuel mixture injection speed conditions. The dilution effect of N(2)/CO(2) reduced the flame temperature and soot formation. The extinction of the flame was influenced by stability instability and heat losses.