Chemiluminescent footprint of premixed ammonia-methane-air swirling flames

This work reports on the chemiluminescence signature of premixed ammonia-methane-air swirling flames. Wide ranges of equivalence ratios (0.6 ≤ Φ ≤ 1.3), ammonia fractions in the fuel blend (0 ≤ XNH3 ≤ 0.70), and Reynolds numbers (10,000 ≤ Re ≤ 40,000) were investigated to understand effects of these parameters on the light emitted by these flames. Excited radicals contributing to chemiluminescence in the UV and visible ranges were confirmed, namely NO*, OH*, NH*, CN*, CO2*, CH*, and NH2*. With non-intrusive flame monitoring in mind, various ratios of chemiluminescence intensities were carefully studied because these allow removing effects of time-varying flame surface area that is inherent in turbulent flames. Consistent with previous findings in laminar flames, ratios CN*/OH*, CN*/NO*, and NH*/CH* were found to be promising candidates. Ratios CN*/OH* and CN*/NO* were identified as potential surrogates for equivalence ratio if XNH3 ≥ 0.20 and 0.05 ≤ XNH3 ≤ 0.50, respectively. Ratio NH*/CH* was identified as a potential surrogate for the ammonia fraction in the fuel blend provided that equivalence ratio is roughly known. Ratio Blue/NH2*, obtained exclusively from measurements in the visible range, is another interesting surrogate for the ammonia fraction but its sensitivity to Reynolds number may limits its range of applications. Trends of measured exhaust NO concentration with equivalence ratio and ammonia fraction were found to qualitatively match that of NO*, NH*, and CN*, implying that emissions from these excited radicals could be used to monitor the NO performance of practical ammonia-methane-air flames.