On the structure of the near field of oxy-fuel jet flames using Raman/Rayleigh laser diagnostics

An experimental study on turbulent non-premixed jet flames is presented with focus on CO₂-diluted oxy-fuel combustion using a coflow burner. Measurements of local temperatures and concentrations of the main species CO₂, O₂, CO, N₂, CH₄, H₂O and H₂ were achieved using the simultaneous line-imaged Raman/Rayleigh laser diagnostics setup at Sandia National Laboratories. Two series of flames burning mixtures of methane and hydrogen were investigated. In the first series, the hydrogen molar fraction in the fuel was varied from 37% to 55%, with a constant jet exit Reynolds number ReFᵤₑₗ of 15,000. In the second series the jet exit Reynolds number was varied from 12,000 to 18,000, while keeping 55% H₂ molar fraction in the fuel. Besides local temperatures and concentrations, the results revealed insights on the behaviour of localized extinction in the near-field. It was observed that the degree of extinction increased as the hydrogen content in fuel was decreased and as the jet Reynolds number was increased. Based on the distribution of the temperature, a fully burning probability index able to quantify the degree of extinction along the streamwise coordinate was defined and applied to the present flame measurements. A comparison of measured conditional mean of mass fractions and laminar flame calculations underlined the significant level of differential diffusion in the near-field that tended to decrease farther downstream. The results also showed high local CO levels induced by the high content of CO₂ in the oxidizer and flame products. A shift of maximum flame temperature was observed toward the rich side of the mixture fraction space, most likely as a consequence of reduced heat release in the presence of product dissociation. Main characteristics of laser Raman scattering measurements in CO₂-diluted oxy-fuel conditions compared to air-diluted conditions are also highlighted. Most data, including scalar fluctuations and conditional statistics are available upon request.