Shock-initiated ignition of natural gas-air mixtures

Ignition delay times in air for hydrocarbon mixtures that simulate natural gas were measured following the shock heating of such mixtures in reflected shock waves. The onset of ignition was determined by time-resolved absorption of the 3.39 μm He−Ne laser line. Measured induction times ranged from 15–1200 μs over the temperature interval 1200–1850 K at total densities near 3×10 −5 mol/cm 3 , corresponding to a pressure of about 4 atm at 1600 K. It was found that the induction time of a mixture of 1% CH 4 in air is shortened by factors of 3, 9 and 13 for the addition of 0.2% C 3 H 8 , 0.2% C 2 H 6 +0.1% C 3 H 8 and 0.2% C 2 H 6 +0.1% C 3 H 8 +0.1% n -C 4 H 10 , respectively. The temperature variation of the induction times for these fuel mixtures is essentially constant and given by: exp (26000±600 K/T), in agreement with ignition delay studies of CH 4 in O 2 /Ar mixtures. The activation energies for ignition of mixed fuels are thus controlled by the oxidation mechanism of CH 4 . A qualitative interpretation based on changes of initiation and branching rates introduced by the additives is provided to account for the shorter induction times.

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