Insights into the scalar structures in turbulent diffusion flames with progressive H2 addition using 1D spontaneous Raman scattering and simultaneous PIV-OH PLIF

This paper examined the effect of H 2 enrichment in a primary fuel (CH 4 ) on turbulent flame features using 1D spontaneous Raman scattering (SRS) and simultaneous particle Image Velocimetry (PIV), OH-Planar Laser Induced Fluorescence (PLIF) measurements. The experiments are conducted in a canonical non-premixed bluff body burner operating under typical central fuel jet-dominated flow mode. Downstream of the bluff body, the flow exhibits complex patterns, and it can be globally categorized into three successive zones: recirculation, neck, and jet-like zones. The flame undergoes local extinction in the neck zone, where the local flow-induced hydrodynamic strain rate (κ hyd ) is much higher than the flame extinction strain rate (κ ext ). It is well known that H 2 enrichment increases κ ext and thus modifies the probability of localized flame extinctions in the neck zone. Additionally, recent studies have shown the significance of preferential diffusion effects of H 2 in H 2 + CH 4 bluff body stabilized premixed and turbulent jet diffusion flames. Although 1D SRS measurements in canonical jet and bluff body stabilized non-premixed flames were studied extensively, to the best of our knowledge, differential diffusion has not been reported earlier in the non-premixed bluff body burner fueled with progressive H 2 addition. To better understand this phenomenon, five H 2 enrichment levels are considered: 0 %, 10 %, 30 %, 50 % and 80 % (in vol.). The simultaneous PIV and OH-PLIF measurements revealed the presence of local extinctions in the cases of H 2 enrichment ≤ 30 %, while local extinctions are not witnessed for H 2 = 50, 80 %. The conditional PDFs of the temperature in mixture fraction space obtained from the 1D SRS further confirmed this observation. Furthermore, the local instantaneous hydrogen/methane mass fraction ratio has been estimated to evaluate the differential diffusion effects. The results showed the dominance of the differential diffusion in the burner's near field, while the strong turbulence mixing effect weakens the differential diffusion in the far field.