A three-axis regime diagram for quantitative analyses of the mixing field structure in laminar and turbulent combustion

The combustion characteristics and stability are affected primarily by the mixing field structure. The goal of many practical systems is to achieve higher stability by creating Inhomogeneous, Partially Premixed, and Stratified (IPPS) environments. A three-axis regime diagram is proposed in this work to describe the mixing field structure of the IPPS, the non-premixed, and the premixed environments. The proposed axes of the diagram are the mean, the fluctuations, and the 2-D gradients of the conserved scalar mixture fraction.

Highly resolved two-dimensional measurements of the mixture fraction in highly stabilized burners with well-controlled mixture inhomogeneity using advanced Rayleigh scattering measurements are used in this study to investigate the abilities of the proposed diagram. The effects of the mixing level, equivalence ratio, and Reynolds number on the mixing field structure were investigated using this diagram adequately. The three-axis diagram provided quantitative detailed information on the mixing field structure at different operating conditions. In addition, the local mixing layer thickness data are extracted from the diagram based on the mixture fraction profiles and the corresponding mixture fraction gradients profiles in the mixture fraction space.

The level of mixture inhomogeneity and equivalence ratio significantly affect the mixing field structure, while the effect of the Reynolds number in turbulent conditions is weak. The correlations between the local mixing layer thickness and the main operating parameters are observed using its PDFs. Reducing the level of mixture inhomogeneity reduces the maximum mixture fraction gradients at zero fluctuations of the mean mixture fraction. The correlations are clear for further analytical investigation. This study shows that the proposed three-axis diagram is a useful tool to investigate and analyze the mixing field structure of the IPPS regimes.