Investigating the effects of natural gas mixture inhomogeneity imposed at the nozzle exit of a concentric flow conical nozzle burner

The interaction between natural gas and air in the inhomogeneous, partially premixed, stratified (IPPS) regime governs the reaction, flame structure, and combustion stability in practical systems. The mixing field in the IPPS regime exhibits local interaction between rich and lean pockets at different scales, so that the local mixing layer thickness and the mixture fraction gradient are two of the main characteristic parameters. The current investigation primarily addresses these parameters in laminar and turbulent natural gas flames within a concentric-flow conical nozzle (CFCN) burner, where mixing is well controlled across the extremes of fully premixed and non-premixed conditions. In addition, a quantitative evaluation of the level of mixture inhomogeneity has been conducted to address the details of the mixture fraction distribution for different operating parameters in laminar and turbulent combustion. Highly resolved measurements of the mixture fraction distribution at the nozzle exit of the CFCN burner were performed using the Rayleigh scattering technique. The data are illustrated in a recently developed three-axis regime diagram. A recently introduced factor representing the level of mixture fraction inhomogeneity was evaluated and used to provide a quantitative measure of the mixing field inhomogeneity for different operating conditions. The data exhibit useful correlations between the level of mixture inhomogeneity and the main operating parameters. In addition, the current study provided a strong correlation between the level of mixture inhomogeneity and the stability of IPPS turbulent natural gas flames in the CFCN burner for the first time. The stability of the IPPS flames within the CFCN burner is highest at δ MI = 0.05. The stability is then reduced at lower and higher values where the flame changes to either fully premixed or non-premixed conditions. The local mixing layer thickness and mixture fraction gradients extracted from the measurements affect the flame stability and can thus be used for quantitative analyses of the mixing field structure. The mixing layer thickness affects the interaction between the rich and lean pockets within the IPPS structure. The current study provided a useful tool for the design of a highly stabilized industrial gaseous fuel burner with a low level of pollutants.