NOx Scaling and Stability Characteristics of Turbulent Non-premixed Jet Flames of H2/CO Syngas

Abstract

NOx emission and stability characteristics of turbulent non-premixed jet flames of syngas hydrogen/carbon monoxide blends were investgated experimentally. The non-premixed jets in this study were formed by central fuel jet and surrounding coaxial air jet, the type widely used in practical combustors. Correlations among flame residence time, global strain rate of the jet and NOx emission were studied for single jet flames and stability limits and local flow characteristics were examined quantitatively for coaxial flames. For this studies, various measurement techniques, including NOx concentration measurement, OH* Chemiluminescence, particle image velocimetry and planar laser-induced fluorescence of OH, were used and measurements were focused on extreme near field of the flames. The measured flame lengths of single jet flame of syngas increase with Ref and the flame lengths also increase with the Froude number. Thus, it was confirmed that the jet flames investigated in this study are in the region of buoyancy-momentum transition. The NOx emission decreases with increase of Ref since the decreasing effect on the flame residence time by the increased fuel jet velocity is more significant than the increasing effects by lengthened flame. As the CO content increases in syngas fuel, the NOx emission decreases since the flame length decreases due to higher stoichiometric mixture fraction. The modified NOx scaling, EINOx/(L/Uf) ~ (Uf/df*)-n, which adopted simplified residence time, L/Uf, satisfies n = -1/2, same as in hydrogen flames, for each fuel composition of syngas in buoyancy-momentum transition region. Futher modified NOx scaling, EINOx/fst/(L/Uf) ~ (Uf/df*)1/2, which considered NOx production per unit air comsumption, finally collapsed all datasets of syngas into a single line. The detachment stability characteristics of attached H2 and syngas jet flames with coaxial air were investigated by observing flame behaviors with varying fuel jet and air jet velocities. The critical fuel jet velocity of the single jet flame of syngas decreases with increase of CO content in syngas fuel. Trends of stability limit have correspondence with the maximum burning velocity in high CO content region and with the stoichiometric mass ratio in high H2 content region. Hydrogen jet flame show that the stability limits are almost independent of the coaxial nozzle size in the fuel/air stream velocity domain while blowout of the lifted flames is related to the jet turbulent characteristics. Stability limit curve in fuel/air velocity domain shift upward with increase of rim thickness. Non-monotonic (ascending/descending: regime I/regime II) trends were observed for all syngas compositions in the fuel/coaxial air velocity domain. Local extinction on the rim was observed in regime I and local extinction at the starting point of shear layer near nozzle was observed by Chemiluminescence imaging. Stability limit in regime I could be scaled by fuel/air mass flux. For the syngas compositions, the stability curve extrema are found to be identical for identical nozzle lip thickness cases when the fuel nozzle velocity is normalized by the single jet critical velocity. For understanding the detachment phenomenon in descending region (Regime II), strain rate and OH layer characteristics were studied experimentally with simultaneous PIV and OH PLIF measurements. The mean principal strain rate S1 on OH layer decrease with the axial distance and the maximum values of them are located near the nozzle exit. In some cases, the maximum S1 on OH layer near nozzle exit appears higher in the stable flames than in the flames near detachment. For various fuel jet Reynolds number, the flames near detachment condition have almost identical minimum mean OH layer thicknesses δ<OH> near the nozzle exit. The S1 on OH layer and the OH thickness δOH present inverse relationships, however, δOH is broadly scattered within a small S1 range. The pdfs of δOH show that the distribution is skewed toward lower values within a higher S1 range while broad distribution around higher values appears within a lower S1 range. Moreover, the most probable value of δOH shifts toward lower value near the minimum mean δ<OH> as the flame condition is varied from stable to near detachment.