Laser-induced fluorescence measurements in venturi cascaded propane gas jet flames

A propane jet diffusion flame at a burner exit Reynolds number of 5100 was examined with a set of venturis of specific sizes and spacing arrangement. The venturi cascading technique resulted in a decrease of 33% in NO emission index along with a 24% decrease in soot emission from the flame, compared to the baseline condition (same flame without venturis). The temperature measurements showed that, in the near-burner region, the venturi cascaded flame has an average temperature lower by 5% than the baseline flame. However, in the mid-flame and far-burner regions, it has a temperature 13 and 12% higher, respectively. The NO composition measurements revealed that in the near-burner, mid-flame and far-burner regions, the venturi cascaded flame has lower NO concentrations by 16, 5 and 3%, in average values respectively, compared to the baseline case. In order to understand the above results and observations, Laser induced fluorescence (LIF) spectroscopy was employed to detect the minor species (OH, CH and CN) in the baseline and venturi cascaded flames. The LIF measurements, in the near-burner region of the venturi cascaded flame, indicated an average decrease of 18, 24 and 12% in OH, CH and CN radical species respectively, from their baseline values. However, in the mid-flame region, a 40% average increase in OH, from its baseline value, was observed. In this particular region, the CH or CN radicals were not detected. The LIF results were used to understand the thermochemical processes involved including soot and NO formation. It showed that OH radical, in the downstream locations, was mostly affected by soot rather than by temperature. In addition, prompt NO mechanism appeared to play a significant role besides the conventional thermal NO mechanism.