On flammability limits of dry CO/O-2 opposed-jet diffusion flames

The response of dry CO/O-2 opposed-jet diffusion flames as a function of stretch rate was computationally studied with detailed chemistry, transport properties, and radiation representation. While CO oxidation was greatly facilitated with the presence of hydrogen-contained species, the simplicity of the dry CO/O-2 system allowed a comprehensive understanding of the interactions between kinetics and radiation. Adiabatic and non-adiabatic flames were compared under various conditions. For the non-adiabatic flame calculations, a statistical narrowband radiation model and its optically thin limit were employed and compared. Results show that aim accurate radiation model is required for the quantitative and qualitative predictive capability. The effect of varying the reactant inlet temperatures was also systematically investigated. With increasing inlet temperature, the non-adiabatic flame isola enlarged and hence the system became more flammable. This trend continued with increasing ambient temperature until the unstable branch finally met time frozen branch. The merging point is known as a double-point bifurcation. Further increasing the ambient temperature led to an Omega -shaped flame response, with one blowoff extinction turning point and one spontaneous ignition state. Furthermore, the effect of suppressant addition on the flammability limits was examined. Two suppressants-CO2 and Ar-were compared, with the former as a radiatively participating species. A reversal in the trend of suppressant effectiveness was found at low stretch rates as compared with higher stretch rates. Implications of the present findings for microgravity fire safety are also discussed.