Probing the local radiative quenching during the transition from a non-smoking to a smoking laminar coflow ethylene/air non-premixed flame

This paper experimentally documents the transition from the “closed-tip” flame configuration to the “open-tip” one that a laminar axisymmetric coflow ethylene/air non-premixed flame experiences with increasing the fuel flow rate at the smoke point in terms of soot temperature and volume fraction distributions. To this end, the two-dimensional soot temperature and volume fraction fields are measured by the two-color Modulated Absorption/Emission (2C-MAE) technique. The MAE setup has been specifically extended to a third spectral range centered at a lower wavelength (405 nm). With this new 3C-MAE technique, information on the level of scattering attributed to soot particles can be obtained. The experimental investigations are combined with radiative heat transfer computations to gain a comprehensive understanding of the radiative quenching of soot oxidation that happens at the flame tip responsible for soot release from the flame at the smoke point. In addition, the field of spectral scattering coefficient at 405 nm can be estimated, confirming that the non-scattering approximation is valid within the context of radiative models in this kind of flame. As a result, numerical simulations implementing radiative models without scattering should be able to decently capture the transition at the smoke point. Since the simulation of such a transition is crucial to the control of carbonaceous nanoparticles release process from flames, an original contribution of the present paper to this challenge is the supplemental material that contains the measured two-dimensional fields of both soot temperature and volume fraction.