Flame front visualization in turbulent premixed ethylene/air flames by laser-induced photofragmentation fluorescence

The elucidation of premixed ethylene/air combustion mechanisms holds theoretical significance for optimizing engines such as pulse detonation engines and rotating detonation engines. The flame front structure constitutes a vital factor of premixed ethylene/air combustion mechanisms, while the prevailing planar laser-induced fluorescence techniques currently in use fall short of directly visualizing the reaction zone of ethylene flames. Here, we, for the first time, employ the planar laser-induced photofragmentation fluorescence technique to achieve direct visualization of the reaction zone in premixed ethylene/air jet flames over a broad range of equivalence ratios (phi phi = 0.4-1.8). The application of a 212.8 nm laser for the photofragmentation of ethylene combustion intermediates results in the generation of abundant C2 2 Swan bands fluorescence. Through a comparative analysis of experimental and simulation outcomes, it is established that this fluorescence primarily originates from C2* 2 * produced after the three-photon photofragmentation of C2H2, 2 H 2 , which is present in relatively high concentrations in ethylene/air flames, enabling direct visualization of the flame front structure. To enhance the signal-to-noise ratio (SNR), we further adopt a pump-probe approach by introducing a 516.5 nm probe laser to excite the low- energy state C2 2 produced from the 212.8 nm laser-induced photofragmentation of C2H2. 2 H 2 . This methodology results in a doubling of the overall imaging SNR.