CO2-laser-induced deflagration of fuel/oxygen mixtures

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Abstract
Weakly focused pulsed‐CO2‐laser radiation has been examined as an ignition source for low‐pressure (85–100 Torr), near‐stoichiometric fuel/oxygen mixtures containing the following fuels: ethylene, methyl fluoride, methanol, ethanol, dimethyl ether, p‐dioxane, n‐propyl nitrate, and iso‐propyl nitrate. The data analysis includes characterization of the spatially dependent ir absorption in the test cell, calculation of appropriate medium temperatures from the absorbed laser energy, and correlation of observed ignition delay times tig with the calculated temperatures. Effects of hydrodynamic motion on the pressure, density, and temperature profiles of the irradiated samples were modeled using a computer code for two‐dimensional wave propagation. Code predictions are in accordance with experimental pressure‐time histories obtained using a piezoelectric transducer. Minimum ignition temperatures ranged from 590 K for the iso‐propyl nitrate/O2 system to 1645 K for CH3F/O2. The observed functional relationship between tig and temperature T was generally of the form ln tig =A/T+B, where A and B are constants. This relationship follows the form predicted by thermal and degenerate chain ignition theories, viz., ln tig =Eact/ RT+constant, where Eact corresponds to an overall activation energy. Using this relation, derived activation energies are 86, 57, 42, 47, ∼40, 13, and 12 kcal/mole for the CH3F/O2, CH3OCH3/O2, CH3OH/O2, C2H5OH/O2, C4H8O2/O2, n‐propyl nitrate/O2, and iso‐propyl nitrate/O2 mixtures, respectively. These results are in reasonable agreement with available data from shock‐tube and hot air stream injection techniques. In contrast, an anomalous, threshold‐like effect was observed for laser ignition of C2H4/O2. The applicability of ignition schemes of this type to time‐resolved kinetic spectroscopic studies is briefly discussed.

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