Modified Vitiation in a Moderate or Intense Low-Oxygen Dilution (MILD) Combustion Furnace

Abstract

This paper reports on the effects of various operational and geometric parameters on mixing and vitiation in a laboratory-scale furnace operating with natural gas and under the moderate or intense low-oxygen dilution (MILD) regime. The study is carried out through numerical modeling. Seven independent parameters are considered: i.e., mass fraction (f) of fuel diluents (CO2 and N2), air preheat temperature (Ta), global equivalence ratio (ϕ), air nozzle exit diameter (Da), fuel nozzle exit diameter (Df), fuel–air nozzles separation distance (S), and fuel injection angle from the furnace floor (α). The modeling is initially verified through a comparison to measurements by Szegö et al. (Szegö, G. G., Dally, B. B., and Nathan, G. J.Combust. Flame 2008, 154, 281−295) in the same furnace geometry. It is shown that varying each of the parameters f, ϕ, Da, Df, S, and α can considerably influence the fuel-jet penetration distance and the recirculation rate of the exhaust gas, two important quantities for establishing the MILD combustion. Relatively, the geometric parameters Da, Df, S and α play more effective roles in controlling the vitiation rate and, hence, “flame” characteristics. Also, it is revealed that influences of all of the parameters, except S and α, can be represented by that of the ratio of the fuel injection momentum to the air injection momentum.