Visualization of lean methane-air ignition behind a reflected shock wave.

• Simultaneous schlieren and direct photography was used to visualize the ignition of methane-air. • Ignition starts with ignition kernels randomly distributed throughout the mixture. • Above a critical temperature strong ignition occurs randomly away from the reflecting wall. • The strong ignition delay time correlates with an Arrhenius model. • The experimental global activation energy does not match the predicted value for a constant volume. Methane is the primary fuel typically used in dual-fuel compression ignition engines. The ignition of methane-air, equivalence ratios of 0.25, 0.5 and 1.0, was studied using the reflected shock technique. High-speed schlieren and direct photography (taken obliquely to the shock tube axis) in an optically accessible 76 mm square cross-section test section were used to identify ignition sites in three-dimensional space and to obtain the ignition delay time, corroborated with pressure measurements. Tests were carried out at a nominal reflected pressure of 10 bar and temperatures in the range of 880–1500 K. The ignition process started with multiple flame kernels, randomly distributed throughout the mixture, which resulted in a slow rise in pressure. Above a critical reflected shock temperature, this relatively slow deflagration process abruptly terminated with strong ignition, believed to correspond to the onset of a detonation. The location of the detonation initiation was random, and typically occurred away from the end-wall. The minimum temperature for strong ignition was found to be 1015 K, 1054 K, and 1189 K for equivalence ratios of 0.25, 0.5, and 1.0, respectively. The strong-ignition delay time data yielded a global activation energy of 12.9 kcal/mol and 17.2 kcal/mol for equivalence ratios of 0.25 and 0.5, respectively. These experimental values are significantly smaller than constant volume model predictions of 35–36 kcal/mol. [ABSTRACT FROM AUTHOR]