Fundamental Studies in Flameless Combustion and Implications to Reaction Zone Structure and Pollutants Formation

Abstract

The objective of this study is to investigate numerically the effects of strain rate, methane dilution by nitrogen, and air-side preheat on pollutant and molecular hydrogen production and the reaction zone structure during flameless and flame-based combustion in a non-premixed counter-flow geometry of diluted methane with preheat air. Fundamental flameless and lean flame-based combustion studies are useful in shedding light on their possible uses in fuel reforming, especially as it pertains to the production of cleaner fuels for fuel cell operation, or partial oxidation. The study centers on fuel concentrations from 5 to 15% (by volume) in intervals of 2% along with both varying inlet velocities (the relation to strain rate is described below) of 10 to 60 cm/s in intervals of 10 cm/s, and oxidizer preheat temperatures from 1100 to 2100 K in intervals of 200 K. The geometry is based on axisymmetric counterflow of fuel and oxidizer jets, which are separated by a distance of 5 cm at their inlets. Parametric studies of different fuel dilution rates show that NO[subscript x] mass fractions decrease with an increasing strain rate as well as decreasing fuel concentration in both flameless and flame-based combustion, while NO[subscript x] mass fractions increased with the air-side preheat temperature. The intermediate species, H₂, is found to follow a similar trend to NO[subscript x] as preheat temperatures and fuel concentrations were increased; however, varying strain rate has little or no effect on H2 concentrations. The distinct differences in behavior between NO[subscript x] and H₂ may be attributed primarily to the conflicting roles of strain in both increasing reactants? delivery to the reaction zone and its reduction of chemical residence time. Reaction flux analysis, which elucidates the chemical mechanisms governing the flameless and flame-based modes of burning concerning NO[subscript x], shows an important shift in the dominant mechanisms producing NO, from the thermal NO[subscript x] mechanism at high preheat temperatures, and to prompt NO[subscript x] at lower preheat temperatures.