Raman-rayleigh measurements in bluff-body stabilised flames of hydrocarbon fuels

Bluff-body stabilised turbulent nonpremixed flames of methane and methanol are investigated. In the flame region downstream of the recirculation zone, turbulent mixing rates become very intense at high enough velocities and the interactions between turbulence and chemistry becomes significant. Simultaneous, space- and time-resolved, Raman-Rayleigh measurements of temperature and the mass fractions of fuel, CO, CO₂, H₂, H₂O, O₂ and N₂ are presented for flames with low turbulent mixing rates as well as ones close to blowoff. The flames also range from being fuel-jet dominant, with just hot gases stabilising the flame to the nozzle, to coflow-air dominant with a fully visible recirculating zone. In methanol flames, the recirculation region is visibly soot-free, while in methane flames it is sooting and hence causes significant interferences on the laser Raman scattered light from all species especially CO₂. The results are presented in the form of scatter plots as well as probability density functions conditioned with respect to mixture fraction. It is found that bluff-body stabilised flames close to extinction show similar characteristics to pilot-stabilised free jet-flames of the same fuel. Namely, a bimodal approach to blowoff with an increasing percentage of locally extinguished samples as the turbulent mixing rates incrase. In methane flames, the peak levels of CO and H₂ are higher than those calculated for laminar flamelets and this is consistent with measurements in pilot-stabilised flames. In methanol flames, the bimodality is more distinct and the peak levels of CO are lower than those measured in pilot-stabilised flames.