Superequilibrium and thermal nitric oxide formation in turbulent diffusion flames

Measurements and modeling of the formation of superequilibrium radicals and nitric oxide in atmospheric pressure turbulent jet diffusion flames are presented which quantify the influence of superequilibrium on thermal NOx formation. Variation of fuel gas compositions (CO/H2/N2, CO/H2/CO2, and CO/H2/Ar) permits partial separation of chemical and fluid mechanical effects. Superequilibrium OH radical concentrations are measured by single-pulse laser saturated fluorescence and NO and NO2 concentrations by probe sampling and chemiluminescent detection. Four different types of probes were used to quantify probe sampling effects. In turbulent reaction zones, virtually all of the NOx in the flame occurred in the form of NO but far downstream of the flame nearly half of the NOx occurred as NO2. Thermal NOx maximized near stoichiometric flame zones; the rich shift observed by others may be a probe sampling artifact. In turbulent CO/H2/N2 jet diffusion flames, both measurements and a nonequilibrium turbulent combustion model show that superequilibrium decreases average temperatures by 250K, increases average OH concentrations by a factor of 4-6, and increases thermal NOx formation principally by broadening the range of mixture fraction (both rich and lean) where thermal NOx is formed. Calculated increases in thermal NOx due to superequilibrium in turbulent CO/H2/N2 jet diffusion flames are factors of 2.5 at 1 atm and 1.4 at 10 atm. The two-scalar pdf model predicts that thermal NOx yield is independent of Reynolds number in disagreement with previous experimental reports. © 1987