A turbulent combustion model for soot formation at the LES subgrid-scale using virtual chemistry approach

Abstract

Designing efficient and clean combustion devices to reduce soot formation in practical applications is a major concern nowadays. This could be achieved by performing predictive simulations in turbulent sooting combustors. Large Eddy Simulation (LES) is an attractive approach to predict flame/turbulence interactions at small scales including soot phenomena. However, because of the peculiarly complex nature of soot production phenomena, turbulent sooting flames remain very difficult to simulate and are computationally expensive. Thus, reduced gas-phase kinetics and soot models should be included in numerical simulations. The reduced virtual chemistry approach is a great alternative to address this problem since it introduces an optimized and flexible strategy to tackle multi-mode turbulent combustion and pollutants prediction. Also, the virtual chemistry approach has recently demonstrated its capability in reproducing soot phenomena in laminar flames including radiative heat transfer. In the present work, the virtual chemistry methodology is extended to resolve turbulent sooting flames using LES. Based on analysis of flame and soot chemical time scales along with turbulent time scale, an original hybrid turbulent combustion model formulation is proposed, which degenerates to the flamelet regime in the flame front region and to a PSR formalism in the post-flame zone, characterized by slow soot chemistry. The subgrid-scale model for turbulence-chemistry-soot interactions is then implemented in an LES framework. The model is challenged in the turbulent non-premixed sooting flame from Sandia Laboratory. Results indicate that the LES subgrid-scale model has a significant impact on soot formation and confirms previous observations from the literature in LES of non-premixed turbulent sooting flames. Additionally, results show that the virtual chemistry can reproduce flame-soot characteristics of a turbulent sooting jet flame and are comparable to other state-of-the-art works.