Effect of exhaust gas recirculation and combustible oxygen mass fraction on emission index of NOx for counterflow methane/air diffusion flames
Exhaust gas recirculation has been proven to be an effective method of reducing emissions of oxides of nitrogen (NOx) from internal combustion engines. The present study validates this effect through flame simulations using detailed chemistry. Counterflow methane/air diffusion flames were simulated in one dimension in physical space using GRI-Mech 3.0 detailed chemistry mechanism, thermal, and transport data. The influence of Exhaust Gas Recirculation (EGR) on the flame structure and NOx emissions was studied by selecting higher than ambient inlet temperature and by varying the composition of the oxidizer stream. The Combustible Oxygen Mass Fraction (COMF), which represents the fraction of total inlet oxygen available for combustion, was varied by selecting EGR fractions between 5% and 40%. The results showed that the emission index of NOx (EINOx) is directly proportional to the COMF in agreement with recent experimental observations from engine studies. The EINOx computations also captured experimentally observed decrease in the pressure exponent reported in recent experimental studies. In addition to pressure and preheat, the effect of variation in strain rate on the flame structure and NOx emissions is also studied. Similar to low strain rate flames without EGR, low strain rate flames with EGR are also observed to have higher NOx emissions. A linear relation between COMF and stoichiometric fuel/air ratio has been observed. The application of detailed chemistry calculations to develop EINOx-COMF control algorithms is promising.
Naik, Purdue University.
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