Laser-induced spark ignition of DME-air mixtures in microgravity: Comparison of ignition characteristics between normal gravity and microgravity

This work addressed laser-induced spark ignition (LSI) of dimethyl ether (DME)-air mixtures in a microgravity environment which was simulated in a laboratory-scale drop tower, to study fundamental ignition characteristics in microgravity. A laser-induced spark was adopted as a pilot source because LSI has potential benefits as compared to conventional electrical spark ignition: no wall effects, no heat-loss to electrodes, etc. Ignition characteristics, therefore, can be investigated without effects of ignition source via LSI. DME-air mixtures filled in a combustion chamber on a framework package were ignited during free-fall. The ignition energy and flame kernel development were investigated with varying the equivalence ratio and the laser pulse energy. The minimum ignition energy (MIE) was calculated statistically via the logistic regression method. Comparing MIE and flame kernel development between normal gravity and microgravity, MIE in microgravity was lower than that in normal gravity over tested equivalence ratios ranging from 0.575 to 0.675. The difference of MIE became significant with decreasing the equivalence ratio, the maximum value of which was 7.0 mJ at the lowest equivalence ratio. Flame kernel was deformed in normal gravity due to buoyant force. The flame kernel in the bottom region was often not able to develop against buoyant force. Ignition succeeded consequently when only the top of flame kernel developed into a self-sustaining flame to propagate throughout the entire chamber. This flame deformation was noticeable as the equivalence ratio decreased because the flame propagation speed also decreased. On the other hand, in microgravity, reduced buoyant force allowed the flame kernel to stably develop into a self-sustaining flame. This is the first time that both MIE and flame kernel development via LSI were found experimentally to be well affected by gravity.