Influence of Inlet Dilution of Reactants on Premixed Combustion in a Recuperative Furnace

This paper presents a numerical study by RANS modeling that investigates the effect of external dilution on the premixed combustion occurring in a recuperative furnace. Calculations are performed using the detailed GRI-Mech 3.0 mechanism to ensure the accuracy of the modeling. Results of the in-furnace flow, temperature, and concentrations of OH, O₂, CO₂ and NO _x are provided. It is found that the external dilution with the inert gas CO₂ plays a significant role in establishing the premixed MILD (Moderate or Intense Low-oxygen Dilution) combustion. Externally diluting the reactant mixture not only reduces the initial concentration of O₂ but also ensures a stronger internal dilution by recirculation of more hot combustion products. Importantly, the latter effect is more significant for achieving the MILD regime. There is a critical mass fraction of the diluent CO₂ present, below which MILD combustion cannot occur. While the traditional premixed flame produces much more NO _x than the MILD combustion, the emission of NO _x appears to result most from the thermal-NO route and least from the N₂O route no matter which mode occurs. Moreover, the present simulation demonstrates that the MILD mode occurs over a wider range of initial reactant conditions for premixed combustion than for diffusion combustion.