Different Ignition Modes and Temperature Evolution of Typical Hypergolic Ionic Liquids

Abstract

The hypergolic ignition behaviors of three ionic liquids [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] reacting with white fuming nitric acid (WFNA) were investigated using the drop test approach. Two high-speed cameras with/without a long-distance microscope and a time-resolved infrared camera were used to simultaneously record the hypergolic ignition process and the reaction region temperature evolution. Results showed two distinct hypergolic ignition modes. Specifically, for [BMIm][DCA] and [EMIm][DCA], some “vapor smoke” is generated after the fuel droplet contacts the WFNA pool, followed by sudden ejection of liquids induced by microexplosion. The measured temperature in the observation window is found to first increase, then decrease, and rise again. For [EMIm][CDB], no microexplosion is observed and the temperature monotonically increases before hypergolic ignition. It is inferred that microexplosion is caused by sufficient accumulation of gaseous intermediates or products underneath the liquid surface, and the monotonic or nonmonotonic increasing temperature is a characteristic of ignition modes. A conceptual model is proposed to illustrate the two ignition modes. In addition, the droplet impact velocity (U0) was varied from 1.1 to 2.1 m/s in the experiment. It is found that the vapor delay time (VDT), explosion delay time (EDT), and ignition delay time (IDT) are all reduced with U0 by at most 50%. For a given U0, the three time scales decrease in the order [BMIm][DCA], [EMIm][DCA], and [EMIm][CDB] (IDT as low as 14.4 ms).