Time-resolved imaging of flame kernels: Laser spark ignition of H2/O2/Ar mixtures

Laser Ignition of Iso-Octane Air Aerosols

“…6 for a number of shock fronts. Although laser ignition does not provide instantaneous energy input, and the initial plasma volume is finite, blast wave analysis is often applied to gas phase laser ignition with reasonable agreement (Spiglanin et al 1995 andBradley et al, 2004). In the present work the estimated energy deposited in each ignition site, E, was found from measurements of the shock wave radius, r s ,.…”

“…In the case of aerosols the energy at the focus was less than this because the pulse energy was attenuated by the droplets prior to the focal point. The kernel developed as a classical ignition kernel with movement back towards the laser after 100 µs generating a 'mushroom' shape (or third lobe) (Spiglanin, 1995). The centre of the ignition kernel at 100 µs was used to define the focal point of the ignition system.…”

“…Laser ignition has been proposed as a potential ignition source that can offer some advantages over conventional systems such as the ability to position the ignition source optimally for ignition, rather than for mounting or durability reasons (Ronney, 1994 andEl-Rabii et al, 2004). A variety of laser ignition strategies are available and these include the use of: a high flux density from a focused pulsed laser to breakdown the fuel-air premixture (Spiglanin, 1995); a wavelength tuned laser to excite a specific species within a mixture (Lucas, 1987;Forch and Miziolek, 1991) or a laser beam that is focused onto a target thereby reducing the energy flux density required (Bach et al, 1969). To date, the first option appears to be the most practical, employing relatively simple and comparatively cheap laser systems and permitting the movement of the ignition position by steering the beam.…”

“…A large amount of work has been undertaken on the laser ignition of gas phase fuel-air premixtures for a variety of fuels that include hydrogen and methane (Spiglanin, 1995;Phuoc and White, 1999). Laser intensities of the order of 200 GW/cm 2 for air at 100 kPa and 300 K are required to generate a plasma in a gas (Bradley et al, 2004).…”

“…However, after approximately 50 µs the kernel begins to propagate along the axis and towards the incoming laser igniter. This results in laser ignition kernels having a distinctive 'mushroom' shape (Spiglanin et al, 1995).…”