The problem of soot formation in combustion devices is gaining rising importance due to its negative effects on human health; for this reason more stringent limitations concerning the emissions of pollutants have been introduced in order to improve the air quality. Detailed kinetic models for soot formation can be successfully used to help identify the conditions that reduce soot formation, but unfortunately the direct coupling of detailed kinetic schemes and CFD is a very difficult task, especially when considering the typical dimensions of the computational grids used for complex geometries and industrial applications. Moreover, turbulent flows are characterized by strong interactions between fluid mixing and chemical reactions, which cannot be neglected in most cases. Therefore, despite the continuous increase in the speed of computational tools, simplified approaches must be taken into account.

While the thermal field and most chemical species in turbulent flames can be successfully modeled using non-equilibrium chemistry via flamelet libraries and presumed probability distribution functions (PDF), the same approach is not able to properly describe the soot formation, due to its comparatively slow chemistry. In order to partially overcome these difficulties, an individual balance equation must be introduced and solved for soot particles: this allows to take into account the interactions between turbulence and chemistry with a higher level of detail and to accurately describe the evolution of soot particles.

In the present work the formation and oxidation of soot particles is predicted in two well-documented turbulent non-premixed flames at atmospheric pressure, fed with ethylene and methane respectively, using a coupled radiation/flamelet combustion model. The soot population balance equation is solved using the Direct Quadrature Method of Moments (DQMOM), a recent and efficient approach based on a quadrature approximation of the size distribution of soot particles. A simple semi-empirical model for the description of soot nucleation, growth, oxidation and aggregation is adopted. In order to take into account the effects of turbulent fluctuations, two different approaches for the closure of source terms in the soot population balance equation are implemented and compared.

The numerical predictions show a reasonable agreement with the experimental measurements; in particular the so-called “uncorrelated closure” seems to warrant the best and most reliable results. Moreover it is evident that turbulent fluctuations must be carefully taken into account to obtain a satisfactory prediction of soot volume fraction. The accurate coupling between the soot production rates and the radiative heat transfer is also necessary to correctly predict the soot volume fraction field. Finally the predicted soot amount in the two turbulent flames investigated is relatively insensitive to the semi-empirical model adopted for describing the nucleation process.