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

Abstract

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 NO x emissions with flow variables and to understand the interrelationships among NO x , 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% CO 43% H 2 (by volume) mixture. The ranges of Reynolds numbers and Froude numbers explored were 3,130–88,500 and 218 to 2.8 × 10 6, respectively. The effects of flow parameters and fuel type on radiant losses are shown to be important in determining the NO x emissions from simple jet flames. For high-temperature flames (T > 2050 K), overall NO x 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 NO x production rates with global flame temperatures and residence times is consistent with, but does not prove, the view that much of the NO x emitted by jet flames is formed in large-scale eddies at the flame tip. NO x production rates higher than expected from the thermal mechanism alone are observed for the hydrocarbon fuels at lower flame temperatures (<2050 K), with the NO x 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 NO x production rates are examined and found to be consistent with flame heat loss characteristics.