APPLICATION OF THE REAXFF MOLECULAR SIMULATION METHOD FOR INVESTIGATING HYPERGOLICITY OF ENERGETIC IONIC LIQUIDS
Open Access
- Author:
- Singhai, Shashank
- Graduate Program:
- Mechanical Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- November 17, 2010
- Committee Members:
- Prof Adri Van Duin, Thesis Advisor/Co-Advisor
Adrianus C Van Duin, Thesis Advisor/Co-Advisor - Keywords:
- ionic liquid
energetic
hypergolic
simulation
Reaxff
molecular dynamics
combustion - Abstract:
- The conventional fossil fuels that are currently used for energy generation in different areas are non-renewable and harmful for the environment. This problem has made researchers and people around the world work towards finding alternatives to these fuels. One special area of application for certain type of fuels is their use as hypergolic space propellants. Hypergolic in the context of propellants refers to the process of spontaneous ignition upon contact with an external species such as an oxidizer. The most common hypergolic propellants in use today are a mixture of fuels like Hydrazine, Monomethyl Hydrazine (MMH) and Unsymmetrical Dimethyl Hydrazine (UDMH) along with oxidizers like White Fuming Nitric Acid (WFNA), Red Fuming Nitric Acid (RFNA), Inhibited Red Fuming Nitric Acid (IRFNA) and Nitrogen Tetroxide (NTO). Since hydrazines are carcinogenic, have a high vapor pressure, are low density fuels, evaporate easily and can be highly toxic if inhaled or internally ingested, there is a significant need for the development of new fuels with matching performance properties and lower toxicity. The high chemical reactivity of hypergolic fuels makes it extremely difficult to measure their reaction chemistry and reaction kinetics with certainty. Computer simulations like molecular dynamics (MD) methods can help us identify the intermediates and predict the reaction chemistry at an atomistic level. Results obtained using MD in analyzing combustion and thermal decomposition problems coupled with convincing results have prompted this study at the atomistic level to be performed in probing hypergolic behavior among ionic liquids. In this thesis, the ReaxFF reactive force field is used for performing MD simulations on a number of ionic liquid and oxidizer species combinations. We predict the reaction chemistry of these energetic IL’s and correlate it with earlier results published using QM calculations. Primary cation is 1-butyl-3-methyl imidazolium and anion is dicyanamide. Oxidizer species used are nitric acid (HNO3) and nitrogen tetroxide (N2O4). We have first trained the force field for nitric acid by training it against a set of available QM and experimental data. Then we use this force field to perform simulations and extract the data obtained from the output to retrain the force field. Drop-test simulations on a fuel and oxidizer combination and analysis of the chemistry are performed. We analyze the trajectory of the different cation and anion to find out the crucial intermediates from the reaction. In our simulations, we observe that when the fuel droplet comes in contact with the oxidizer at a velocity of 9-12 km/s, it undergoes vigorous reactions leading to the formation of key reaction intermediates HC2N3 and HC2N4O3. Cookoff simulation involving a steep temperature ramp-up of 2000K carried out with HC2N4O3 in a nitric acid environment eventually led to the formation of dinitrobiuret (H3C2N5O6). This reaction mechanism observed through ReaxFF is in good agreement with that proposed in QM and experimental studies [15, 32] indicating that ReaxFF provides a feasible computational approach for designing and analyzing hypergolic ionic liquid formulations.