Flame structure and radiation characteristics of CO/H2/CO2/air turbulent premixed flames at high pressure

Abstract

Experimental study on turbulent premixed flames for a CO/H2/CO2/air mixture as a model of coal gasification syngas in a high pressure environment was performed up to 1.0MPa. CH4/air flames with laminar burning velocity and adiabatic flame temperature identical to those of CO/H2/CO2/air mixture were also employed, and then the flame structure and turbulent burning velocity analyzed using OH-PLIF as well as flame radiation characteristics were compared. Results showed that in the case of the CO/H2/CO2/air flame, very fine cusps were seen, these small cusps being generated on a large scale wrinkled flame front even for low u′/SL. Flame surface density of the CO/H2/CO2/air flame was higher than that of the CH4/air flame, and the mean volume of the turbulent flame region of CO/H2/CO2/air flame was smaller than that of the CH4/air flames at high pressure. Bending of ST/SL with u′/SL for the CO/H2/CO2/air flame was not observed in this experiment, while such bending was clearly seen for the CH4/air flame. The difference in the bending characteristics can be explained based on the scale relation between the smallest scale of flame wrinkles represented by the fractal inner cutoff and characteristic flame instability scale. The spectrum of flame radiation for the CO/H2/CO2/air flame at 0.5MPa showed continuous emission in the visible range of the wavelength, while that for CH4/air flames for the equivalence ratio of unity showed very intense H2O emission in the IR range. When the flames with identical adiabatic flame temperature for the CO/H2/CO2/air flame and the CH4/air flame were compared, the total radiation intensity of the former mixture was observed to be strong. The intense emission from the CO/H2/CO2/air flame is thought to be one of the mechanisms which generate very fine flame cusps as enhanced cellar instability for smaller Lewis number.