A Numerical Study of the Effect of Flow Dynamics and Oxygen Concentration on Methane-Hydrogen Non-Premixed Flame in a Mild Combustion Regime

Mild is an acronym for moderate or intense low-oxygen dilution and refers to oxidizer high preheating and dilution. The Mild combustion has features which set it apart from other combustion systems, such as higher reaction zone volume, more uniform temperature distribution, lower reaction rate, lower heat release rate and lower Damkohler number in comparison with the ordinary combustion regime. In this paper, characteristics of the Mild regime were studied numerically. The experimental conditions of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154] were used for modelling in the present study. The EDC model was used to describe the turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI 2.11 full mechanism were used to represent the chemical reactions of H2/methane jet flame. The distribution of temperature, some species, reaction rates and also the importance of molecular diffusion for various O2 levels and jet Reynolds numbers were investigated. The results show that the molecular diffusion in Mild combustion cannot be ignored in comparison with the turbulent transport. Also, the method of inclusion of molecular diffusion in combustion modelling has a considerable effect on the accuracy of numerical modelling of Mild combustion. By decreasing the jet Reynolds number and reducing the oxygen concentration in the airflow, the influence of molecular diffusion on Mild combustion increases. Moreover, the effect of the molecular diffusion reduces on reaction zone when moving away from the nozzle. Although the agreement between numerical and experimental results are very satisfactory, by distancing from the nozzle, the rate of reactions increases while deviation from Mild condition and numerical errors grows.