Characterisation of oxy-fuel flames using laser based diagnostics techniques

Abstract

Oxygen enhanced oxy-fuel laminar and turbulent flame structure in a co-flow non-premixed jet burner are investigated. The measurement of intermediate species such as hydroxyl (OH) and formaldehyde (CH2O) and temperature are the focus of this work. The species concentrations were measured using planar laser induced fluorescence (PLIF) and the temperature using Rayleigh scattering. ‘Traditional Rayleigh’ requires a constant Rayleigh cross-section throughout the combustion process. This is impossible in high temperature oxy-fuel flames due to thermal decomposition. Derived temperature from Rayleigh signals is hence prone to inaccuracy. A direct comparison of measured and numerically-calculated Rayleigh signals can eliminate this error. Numerical Rayleigh signals are relatively easily calculated with knowledge of temperature and species concentration. The feasibility of adopting this procedure to validate the numerical model was investigated in laminar and turbulent flames. Sensitivity studies including radiation models, chemical kinetics mechanisms and the Soret effect were performed in laminar flames. Another Rayleigh technique, polarised/ depolarised Rayleigh was employed in a joint temperature, OH and CH2O measurement. The effect of varying O2 and jet Reynolds number on the flame structure was investigated. The applicability of determining heat release rate (HRR) using the product of [OH]x[CH2O] was also determined. [OH]x[CH2O] and HRR showed good spatial correlation in the main oxidation zone, but underestimated HRR in the secondary oxidation zone. Finally, analysis of thermal diffusion structures using high resolution polarised/ depolarised Rayleigh was performed. The analysis revealed the thickness of the diffusion layer is proportional to the temperature, axial location and O2 concentration. Increase of Reynolds number, however, reduces layer thickness. In summary, this work has used a suite of optical diagnostics to make the first structural survey of high temperature oxy-fuel flames, starting with overall flame shape through macroscopic localised extinction to microscopic thermal diffusion.