Gas Dynamic Features of Self Ignition of Non Diluted Fuel/Air Mixtures at High Pressure

Abstract

The self ignition of several non diluted fuel/air mixtures at high pressure is studied. Hydrogen, iso-octane and n-heptane have been used as fuels. Experiments have been performed using the shock tube technique. Various observation methods, such as recording of pressure and of light band emission and shadow cinematography have been applied. The type of self ignition as well as the ignition delay times can be determined using these techniques. The self ignition of all fuels takes place in many different modes. At high temperatures ignition at the end wall is achieved. All three fuels under investigation show mild and strong ignition as described by Oppenheim (1985). Mild ignition - characterized by establishing a deflagrative combustion mode at seperate independent reaction centers - also occurs at low temperatures, when the ignition process is within some limits-randomly distributed in time and space (hot spot ignition). Hydrogen shows this hot spot ignition with and without transition to detonation (DDT). In the case of n-heptane the hot spot ignition always ends up in a transition to detonation. The iso-octane/air mixture exhibits very clearly both ignition modes with and without DDT. The fuels n-heptane and iso-octane both show in the regime of mild ignition flame zones with spherical or planar structure, depending on the temperature. The planar structure, so far, has not been observed for hydrogen. It seems that this behavior can be explained by the sensitivity of the chemical system to disturbances in temperature. This interpretation can be regarded as an extension to the coherence theory explaining the strong ignition limit (Meyer and Oppenheim, 1971b).

Key words:

• Self ignition
• secondary explosion
• hydrogen
• n-heptane
• iso-octane
• shock tube