Oxides of nitrogen emissions from turbulent jet flames: Part I—Fuel effects and flame radiation

Measurements of oxides of nitrogen emission indices, flame radiant fractions, and visible flame dimensions were made for turbulent jet diffusion flames covering a wide range of flow conditions. Objectives of the study were to explain the observed scaling of NOx emissions with flow variables and to understand the interrelationships among NOx, flow conditions, and flame radiation. The flames were vertical and stabilized with hydrogen pilot flames on straight tube burners. Flow conditions were varied by changing the initial jet velocity and/or the burner tube diameter. Four burner sizes were used, with diameters ranging from 2.18 to 6.17 mm; and four fuel types, having a wide range of sooting tendencies, were employed: methane, ethylene, propane, and a 57%CO43%H2 (by volume) mixture. The ranges of Reynolds numbers and Froude numbers explored were 3,130–88,500 and 218 to 2.8 × 106, respectively. The effects of flow parameters and fuel type on radiant losses are shown to be important in determining the NOx emissions from simple jet flames. For high-temperature flames (T > 2050 K), overall NOx production rates for all four fuels were found to scale with characteristic flame temperatures deduced from the measured radiant fractions in a manner consistent with Zeldovich kinetics. This successful scaling of NOx production rates with global flame temperatures and residence times is consistent with, but does not prove, the view that much of the NOx emitted by jet flames is formed in large-scale eddies at the flame tip. NOx production rates higher than expected from the thermal mechanism alone are observed for the hydrocarbon fuels at lower flame temperatures (<2050 K), with the NOx production rates ranking in the same order as sooting tendencies. This suggests that gas-molecular radiation is more relevant than broadband radiation from soot for determining temperatures in NO formation zones. Prompt NO and/or other soot-NO interactions may also be important for the hydrocarbon fuels in this temperature regime. Previously reported Reynolds and Froude number dependencies for NOx production rates are examined and found to be consistent with flame heat loss characteristics.