Radiation and Nitric Oxide Formation in Turbulent Non-premixed Jet Flames

Radiative heat transfer has a significant effect on nitric oxide (NO) formation in turbulent non-premixed flames. Consequently, predictive models of turbulent non-premixed flames must include an accurate radiation submodel. To investigate the importance of radiation submodels in modeling NO formation, multiscalar measurements of temperature and species were coupled with radiation measurements in a series of turbulent non-premixed jet flames. A range of fuel mixtures were considered including H2, H2/He, CO/H2/N2, CH4/H2/N2, and partially premixed CH4/air. This group of flames represents a range of complexity with regard to NO formation and is currently the subject of multiple modeling efforts. Measurements of radiant fraction, temperature, and NO mass fraction have been compared with previously obtained modeling results for the H2, H2/He, and CH4/air flames. The results show that an emission-only radiation submodel is adequate for modeling the hydrogen flames but not the CH4/air flames. In one CH4/air flame, the emission-only computations overpredict the radiant heat loss by a factor of 2.5. A comparison of adiabatic and radiative computations shows that the inclusion of radiative losses can reduce the predicted peak NO levels by as much as 57%. An accurate radiation submodel for hydrocarbon flames must account for radiative absorption. Spectrally resolved radiation calculations show that absorption by CO2 near 4.3 lm is primarily responsible for the increased optical density of the hydrocarbon flames. The series of turbulent jet flames considered here contains a range of CO2 levels and provide a basis for developing a realistic radiation model that incorporates absorption by CO2.