A Study of Flame Dynamics Induced by A Dual-Pulse Laser Ignition Technique

This study describes the ignition and flame dynamics generated using a dual-pulse laser pre-ionization technique. The new technique involves the use of two nanosecond pulses for inducing gas heating and initiating combustion. Initially, a UV pulse $(\lambda =266$ nm) from the fourth harmonic of a Nd:YAG laser is used to pre-ionize a small volume of gas inside of the combustion chamber forming a weakly ionized plasma channel $(\mathrm {n}_{e} \sim 3 \mathrm {x}10 ^{16}$ cm-3). The cold plasma produced by the UV pulse (T~ 600 -1000K) is subsequently heated by an NIR $(\lambda =1064$ nm) pulse that follows ~ 10ns after the preionization pulse. The NIR beam adds energy into the gas through inverse bremsstrahlung absorption of radiation and increases the temperature of the plasma to T~ 2000-3000 K. Ignition of propane-air mixtures at various equivalence ratios was successfully achieved using the technique presented above and the results are contrasted with the more common laser breakdown/spark ignition technique that uses a single NIR pulse. Preliminary results show that the dual-pulse technique allows for ignition of leaner mixtures ($\phi =0.6)$ as compared to conventional laser breakdown ignition ($\phi =0.7)$. In addition, analysis of the pressure data collected during the combustion events suggests that the combustion efficiency (defined here as the fraction of the chemical energy of the fuel converted into heat) is also higher using the new technique. Measurements of the plasma energy absorption show that both techniques require similar (absorbed) energy for ignition $(\mathrm {E}_{abs} \sim 15$ mJ); however, the dual-pulse achieves this with less incident pulse energy, i.e., total combined pulse energy of 50 mJ $(\mathrm {E}_{UV}=20$ mJ, $\mathrm {E}_{NIR}=30$ mJ), as compared to needing incident 75 mJ for single pulse NIR. Moreover, studying the chemiluminescence emitted by the OH* radical $(\lambda _{OH\ast }=308$ nm) that is naturally produced during the combustion event using an ICCD camera revealed that the flame dynamics can be very different for the two techniques. The NIR initiated flames propagate as a toroidal structure owing to the vorticity induced by the shock wave that follows the spark, a situation that generates excessive flame stretching which can lead to quenching for lean mixtures. In contrast, the flames generated using the dual-pulse technique propagate as a roughly spherical front (depending on the offset of the two beam waists) with less pronounced stretching.