The ignition process measurements and performance evaluations for hypergolic ionic liquid fuels: [EMIm][DCA] and [BMIm][DCA

Abstract

Bipropellant liquid rocket engines are typically fueled by hydrazines that exhibit extreme toxicity and carcinogenity. Recently hypergolic ionic liquids (HILs) have been emerging as environmentally friendly propellant that holds the potential for hydrazine replacement due to their low volatility, easy and safe storage and handling. The HIL is a novel concept both for chemistry community and for combustion community, and plenty of research waits to be initiated to promote practical propulsion application, especially for comprehensive understanding on their fundamental combustion. In this work, hypergolic ignition of two HILs (1-ethyl-3-methylimidazolium dicyanamide([EMIm][DCA]) and 1-butyl-3-methyl-imidazolium dicyanamide ([BMIm][DCA]) were tested with three oxidizers (white fuming nitric acid (WFNA), red fuming nitric acid(RFNA), and nitrogen tetroxide). A drop test system was designed to control the collision event of a single droplet of fuel with an oxidizer pool. Two independent high speed cameras were spontaneously employed to capture the self-luminous images of the whole hypergolic ignition process as well as the initial impact process. The results showed that hypergolic ignition process of [EMIm][DCA]-WFNA is quite similar to that of monomethyl hydrazine-WFNA. Combined with thermo physical properties of HILs, hypergolic ignition process of [EMIm][DCA]-WFNA is proposed. To qualitatively demonstrate the overall hypergolic ignition and reactivity, the explosion delay time and the ignition delay times of various combinations of the fuel-oxidizer pairs were determined. The effects of the Weber number on the explosion delay time and the ignition delay time of [EMIm][DCA]-WFNA are quantitatively investigated. Hypergolicity of different propellant combinations is tested to investigate the influence of oxidizer’s oxidative capacity and HILs’ cation structure. Additionally, the propulsive performance of the two dicyanamide-based IL fuels was evaluated and compared with traditional hydrazine-based fuels, based on a steady rocket model under the same initial conditions. Results show that using WFNA as the oxidizer, BMIM DCA and EMIM DCA presented a comparable propulsive performance to that of UDMH.