Turbulent and Molecular Diffusion in He + CO2/Air Axisymmetric Coaxial Jets

Many attractive alternative fuels, originated from biomass gasification or as crude oil refinery by-products, contain a mixture of gasses mainly based on hydrogen, methane, CO and some impurities. In spite of the reasonably good knowledge and understanding of combustion behavior of “pure” fuel, the combustion of fuel mixtures is still scarcely understood. Moreover during the combustion process of fuels mixture the various components inside the mixture can behave in different ways due to their various chemical and physical characteristics. In particular, it has been suggested that the higher molecular diffusivity of hydrogen compared to those of other gases could lead to major changes in the flame structure as well as in pollutants formation mechanisms. The aim of the present study is to investigate the effects of molecular diffusivity in a turbulent field and in particular to compare molecular and turbulent diffusivities in axisymmetric coaxial jets, at ambient temperature, in which the central jet consists of a He and CO2 mixture, issuing into ambient air. The mixture is selected due to the fact that He and CO2 have highly disparate binary molecular diffusivities. The molecular diffusivity is evaluated by applying the Rayleigh scattering technique, while the turbulent diffusivity is evaluated with the LDV technique