Detailed Soot Modelling in Laminar and Turbulent Reacting Flows

Abstract: In the present work a detailed kinetic soot model is applied to study the formation and oxidation of soot particles in laminar premixed and turbulent non-premixed flames. The soot model used in this study is based on a detailed description of the chemical and physical processes involved in the formation of soot. In the first part of the study the soot model is extended to include the formation of charged soot particles in laminar premixed flames. In the second part the base soot model is applied to non-premixed flames under laminar and turbulent conditions. Laboratory flames and also practical combustion devices such as Diesel engines, gas turbines and a furnace black reactor, which is used to produce carbon black, are simulated. It is well known from experiments that a part of the soot particles in flames are charged due to their low ionisation potential. To investigate the effect of charged soot particles on soot growth the base soot model is extended to include the thermal ionisation of soot particles. Besides the conservation equations for the moments for neutral particles, the conservation equations for the moments of positively and negatively charged soot particles are solved. The enhancement of charged-charged and charged-neutral particle coagulation due to image and coulomb forces is rigorously accounted for. A low-pressure acetylene/oxygen premixed laminar flame is simulated and the results are in good agreement to measurements of charged particle properties. In comparison to other uncertainties in the model, when including charged particles the results do not vary significantly from the model with only neutral particles. Two different approaches are used to couple models for detailed chemistry, soot formation and turbulent flow. In the first approach, based on the laminar flamelet model, a transport equation for the soot mass fraction is solved in the flow field while the rates are obtained from pre-calculated flamelet libraries. It is assumed that the rates of soot formation are decoupled from the flow field depending on the mixture fraction and the scalar dissipation rate only. The mean rates are obtained using prescribed probability density functions (PDF) for mixture fraction and scalar dissipation rate. In addition the flamelet library model is extended to account for radiation effects. Good agreement to measurements is obtained in the laminar as well as in the turbulent flame. In order to reduce computational costs the flamelet library approach is further simplified and applied to simulate soot formation in Diesel engines and gas-turbine combustors. In the second approach, a partially stirred plug flow reactor model (PaSPFR) based on the PDF method is used to simulate the production of carbon black in the furnace black process. It is shown that the IEM and the Curl mixing models are consistent with the method of moment while the Binomial-Langevin model is not. Measured data of carbon black particle properties from a carbon black reactor are simulated with reasonable accuracy. Furthermore it is found that non-homogeneities in the reactor effect the formation of soot significantly.