Experimental investigation of the competitive relationship between soot formation and chemiluminescence in laminar ammonia-ethylene inverse diffusion flames

Ammonia (NH3) is a viable carbon-free alternative fuel, and its blending affects soot formation and chemiluminescence, which compete with each other in hydrocarbon fuel flames. However, this competitive relationship has not yet been investigated in NH3-blended flames. Given the high scalar dissipation rate, inverse diffusion flames (IDFs) are the ideal target for simultaneously studying soot formation and chemiluminescence. This fundamental experimental study focuses on soot formation and its competitive relationship with the chemiluminescence of laminar NH3-C2H4 IDFs. Seven IDF cases with NH3 blending ratios (XNH3) ranging from 0 to 0.30 are investigated. Two-dimensional distributions of soot volume fraction (SVF), temperature, and chemiluminescence are obtained. The results show that NH3 blending elevates the soot inception position and causes the color of chemiluminescence to transition from blue-green to blue, then purple. NH3 blending exhibits a stronger suppression of soot formation in IDFs than in normal diffusion flames. For every 0.05 increase in XNH3, the peak temperature decreases by 30 K, and the peak soot formation rate and soot loading decrease by 30 % and 23 %-52 %, respectively. Soot yield is negatively correlated with XNH3. NH3 blending increases the residence time required for soot inception and shortens it for soot formation. A competitive relationship between soot formation and chemiluminescence is also observed in NH3-blended flames, which is further enhanced and even dominated by nitrogen-containing excited radical formation. NH3 blending causes CN* and NH2* chemiluminescence to be observed on the fuel-rich side of the stoichiometric zone and broadens the region favorable for excited radical formation. The increased XNH3 decreases the peak CH* and C2* chemiluminescence intensities and enhances those of CN* and NH2*, which are responsible for the transition in the characteristic color. This study reports a fundamental database for laminar NH3-C2H4 IDFs and facilitates further investigation and practical applications of NH3 combustion.