Laser Ignition of Iso-Octane Air Aerosols

Abstract

Iso-octane aerosols in air have been ignited with a focused Nd:YAG laser at pressures and temperatures of 100 kPa and 270 K and imaged using schlieren photography. The aerosol was generated using the Wilson cloud chamber technique. The droplet diameter, gas phase equivalence ratio and droplet number density were determined. The input laser energy and overall equivalence ratio were varied. For 270 mJ pulse energies initial breakdown occurred at a number of sites along the laser beam axis. From measurements of the shock wave velocity it was found that energy was not deposited into the sites evenly. At pulse energies of 32 mJ a single ignition site was observed. Overall fuel lean flames were observed to locally extinguish, however both stoichiometric and fuel rich flames were ignited. The minimum ignition energy was found to depend on the likelihood of a droplet existing at the focus of the laser beam.3

Complex chemistry simulations of spark ignition in turbulent sprays

“…This behaviour is due to the limited number of droplets present inside, and therefore ignited by, the small kernel. This is consistent with the observation that the likelihood of ignition depends on the number of particles in the hot volume [7]. The hot kernel needs to reach a minimum size to overcome molecular and turbulent diffusion and to ignite enough droplets for heat production.…”

“…The positive effect of rapid evaporation and the detrimental effect of higher turbulence intensity on ignition performance were demonstrated in several studies [2,[4][5][6][7][8]. Those effects were included in the general model for minimum ignition energy for overall lean sprays [9].…”

“…Following successful spark ignition, a flame propagates in a uniform cloud of droplets. The concept of effective equivalence ratio, i.e., the gas phase equivalence ratio at the flame front, has been successful in describing the flame speed in a spray over a range of overall equivalence ratio and droplet sizes (i.e., overall rich sprays or lean sprays and large droplets) [7,[10][11][12][13][14]. However, this one-dimensional model does not account for numerous observations that the flame speed is affected by the wrinkling of the flame by droplets [10] and by spatial mixture fraction inhomogeneities in the interdroplet spacing which lead to diffusion flames surrounding droplets where chemical reactions are expected to occur at stoichiometry [15][16][17][18].…”

Ignition of turbulent non-premixed flames

“…The minimum ignition energy with laser-induced sparks for droplet-air mixtures is discussed in Refs. [255,296].…”

An Experimental Investigation of Kerosene Droplet Breakup by Laser-Induced Blast Waves

“…A review of theoretical and experimental investigations on spray ignition was given in 1998 by Aggarwal [14], revealing that the great majority of experimental work has focused on providing the MIE in dependency of the SMD, global equivalence ratio, fuel volatility, mixture velocity, and pressure. Investigations in recent years continued this approach, mainly focusing on MIE estimation and detailed characterization of phase 2 in academic [8,12,13,15,16] and industrial test rigs [17][18][19]. However, the authors of this paper are not aware of any study explaining the mechanisms that actually occur in a spray when an ignition spark turns into a flame kernel.…”