Laser thermal ignition using a dual-pulse approach
2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All Rights Reserved. Laser induced heating using a dual-pulse approach with application to laser ignition is investigated. The method proposed herein uses an ultraviolet (UV, =266 nm) laser pulse to achieve initial gas pre-ionization followed by an overlapped near-infrared (NIR, =1064 nm) pulse for energy addition into the pre-ionized gas. The plasma energy absorption for each individual beam is measured as well as for the two combined pulses. The results show that the energy absorption for the IR pulse is significantly increased if it is preceded by the UV pulse. For the IR pulse alone, absorption is observed at energies of 65 mJ and higher (at a pressure p=1 bar). However, if the IR beam follows a 10 mJ UV pulse, significant absorption is noticed for laser energies as low as E1064nm=20 mJ. This represents an appreciable decrease in energy requirements for laser ignition (~3 times less energy). The gas heating is also determined using a Rayleigh scattering technique. We note that initial gas heating is achieved by the UV pulse. We have measured temperatures in excess of T=600 K at 50 s after the laser pulse (E266nm=25 mJ). Adding the overlapped IR pulse can raise the gas temperature even further to T=1268 K (E266nm=10 mJ, E1064nm=20 mJ). Finally, laser ignition using the dual-pulse technique is demonstrated for a propane-air mixture at =1.0 (p0=1 bar, T0=300 K). This represents, to our knowledge, the first demonstration of laser ignition using a non-resonant dual-pulse approach.