IGNITION OF H2/O2/NH3, H2/AIR/NH3 AND CH4/O2/NH3 MIXTURES BY EXCIMER-LASER PHOTOLYSIS OF NH3

Volumetric ignition of H-2/O2, H-2/air, and CH4/O2 mixtures (in an open flow system, initially at 1 atm and room temperature) is achieved via photolysis of a small amount of NH3 present in the flow mixtures. The photolysis is accomplished by an ArF excimer laser operated at 193 nm. The ignition appears to be homogenous, since the ignition-delay times measured at several locations are approximately identical. The minimum ignition-energy density is measured to be 137 +/- 8, 190 +/- 20, and 380 +/- 30 mJ/cm3 for stoichiometric mixtures of H-2/O2/NH3, H-2/air/NH3, and CH4/O2/NH3, respectively, and appears to be insensitive to fuel-equivalence-ratio values between 0.35 and 3.0. The ignition-delay time depends strongly on both the initial NH3 concentration and the laser-energy-deposition density. However, the ignition-delay time is not sensitive to the fuel equivalence ratio. The measured minimum ignition-energy density appears to be substantially lower than that predicted by a kinetic modeling calculation. This result implies that the hot H atoms produced by the photolysis of NH3 may play an important role in facilitating the ignition.