Experimental and chemical kinetic study of laminar flame characteristics and ammonia-alcohol synergistic effect for C1∼C5 alcohol-ammonia composite combustion

Currently, studies on using carbon-neutral alcohol-based oxygenated fuels to improve the speed and stability of ammonia laminar flame have attracted extensive attention from researchers. In this study, the laminar flame speed, flame cell structure and flame self-accelerating as well as the Markstein length of C1∼C5 alcohol-ammonia composite combustion have been explored by high-speed schlieren system in constant-volume combustion chamber together with chemical kinetics, and it was found that there is a synergistic effect on both laminar flame speed and flame stability of C1∼C5 alcohol-ammonia composite combustion, finally the ammonia-alcohol synergistic effect was investigated based on Taguchi method. The results show that, except for methanol addition which is particularly effective, the addition of C1∼C5 alcohols can significantly improve the laminar flame speed of ammonia, and the enhancement effect decreases first and then increases with the extension of the molecular chain in added alcohols. The chemical kinetics are attributed to the fact that during ammonia-alcohol composite combustion, the intensity of NH₃-NH₂-NH dehydrogenation and CH₂O-CO exothermic reactions reduces first and then increases with the extension of alcohol molecular chain, and methanol addition is relatively easier to generate CH₂O and HNO which can significantly increase the strength of ammonia-methanol synergistic effect. Besides, the addition of C1∼C5 alcohols can keep the ammonia flame in a self-accelerating status and effectively increase the Markstein length. However, the onset of cellular flame is delayed, the intensity of self-accelerating is weakened and the enhancement effect of flame stability is reduced with the extension of molecular chain in added alcohols. Combining the ammonia-alcohol laminar flame characteristics and Taguchi method, it can be found that the strength of ammonia-alcohol synergistic effect is strongest when methanol or n-pentanol are added, and the laminar flame speed of NH₃/CH₃OH and NH₃/n-C₅H₁₁OH are greater than those of pure methanol and pure n-pentanol flame in alcohol addition ratio of 80 %-initial pressure of 7 bar/5 bar-φ = 1.2, respectively. Meanwhile, ammonia-alcohol composite combustion can alleviate the limitations imposed by increasing pressure or equivalent ratio on laminar flame speed or Markstein length, respectively. In C1∼C5 alcohol-ammonia combustion laminar flame, the optimal conditions for rapid and stable combustion as well as the strongest ammonia-alcohol synergistic effect are alcohols addition ratio of 80 %.