Effect of CO₂ concentration in biogas on the mixing field structure and stability of turbulent planar partially premixed flames

Biogas combustion and stability are affected by the CO₂ concentration. The aim of this study is to investigate this effect in biogas combustion on the mixing field structure, mixing layer thickness, and hence the combustion stability. The Rayleigh scattering technique was used for highly resolved two-dimensional mixture fraction measurements in a concentric flow slot burner that produces highly turbulent planar flames. Biogas was simulated by mixtures of methane and CO₂ with a range of concentrations from 5% to 30% of CO₂ by volume. A recent three-axis regime diagram was used for a detailed quantitative illustration of the mixing field structure, supporting detailed analysis and understanding of the effects of CO₂. A quantitative measure of the level of mixture inhomogeneity was developed and calculated from the three-axis regime diagram. This shows a linear increase in the degree of inhomogeneity by increasing the CO₂ concentration, attributed to the physical properties of Biogas, and by increasing the equivalence ratio. On the other hand, more mixing and a lower level of mixture inhomogeneity were achieved by increasing the mixing length and the Reynolds number. The local mixing layer thickness was significantly decreased by increasing the mixing length, and was slightly decreased by increasing the CO₂ concentration. The maximum mixture fraction gradient is significantly increased by increasing the CO₂ concentration and the equivalence ratio. On the other hand, a variable trend of the gradients was observed for the mixing length with a peak at L/D = 7. This explained the highest stability of Biogas at this mixing length in our previous study, with an expected increase in the production of the main radicals.