Thermal decomposition and combustion of RDX and HMX: Thermolysis experiments and molecular modeling
Open Access
- Author:
- Patidar, Lalit
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 05, 2019
- Committee Members:
- Stefan Thynell, Dissertation Advisor/Co-Advisor
Stefan Thynell, Committee Chair/Co-Chair
Richard A Yetter, Committee Member
Adrianus C Van Duin, Committee Member
Michael Matthew Micci, Outside Member
Daniel Connell Haworth, Program Head/Chair - Keywords:
- Energetic Materials
Thermal Decomposition
Combustion
Quantum mechanics - Abstract:
- The cyclic compounds RDX and HMX are the most important nitramine energetic ingredients commonly used in many applications, including, among others, explosives and rocket propellants. In this work, a detailed reaction mechanism was developed for the thermal decomposition of nitramines RDX and HMX in the liquid phase using extensive quantum mechanics calculations and important reaction pathways were discovered. The reaction mechanism was further expanded by investigating the reactions of the intermediate species leading to the formation of experimentally observed final decomposition products. The comprehensive mechanism was then validated for the case of HMX using synergetic application of thermal decomposition experiments and kinetic modeling. CH2O and N2O were detected as the major decomposition products at all heating rates considered in the TGA (thermogravimetric analysis) experiments and all set temperatures considered in the CRT (confined rapid thermolysis) experiments. Other decomposition products that were detected and quantified include H2O, NO2, NO, HCN, CO and CO2. A homogeneous liquid-phase reactor model was developed to simulate the TGA and CRT experiments. Computational mass loss and species evolution profiles were in reasonable agreement with the corresponding experimental results, thus validating the comprehensive reaction mechanism. Based on a sensitivity analysis, important reactions were identified that lead to the simultaneous formation of CH2O and N2O. Autocatalytic prompt oxidation pathway via addition of HONO molecules due to the cage effect and hydrogen abstraction via NO2 were found to be the dominant pathways for the decomposition of HMX and various intermediates species. In addition to the liquid-phase mechanism, the gas phase decomposition mechanism was also updated based on a comparative ab-initio study by adding the early ring-opening and hydrogen abstraction reactions along with the reactions of species evolving from the liquid-phase decomposition. G4(MP2) method was found to provide most accurate results for enthalpies of formation of nitramine species when benchmarked against experimental data as well W1BD method. For the calculations of reaction barrier heights using DFT, M06-2X functional was found to provide accurate results when benchmarked against G4(MP2) method. Variational effects in the transition state were found to be negligible and thermodynamic formulation of the conventional transition state theory was used to calculate rate constants with improved tunneling corrections using Eckart method. Molecular parameters required for the calculations of transport properties during combustion modeling were obtained using intermolecular potentials for various organic energetic materials. Quantum mechanics calculations were used to parametrize analytical Buckingham potentials which were then used to obtain Lennard-Jones collision parameters. Instead of choosing a particular bath gas and a combining rule, a novel approach was proposed to obtain the selfinteraction Lennard-Jones parameters using all bath gases and different combining rules via leastsquare optimization. A comparative sensitivity analysis was also performed and the temperature gradient controlling the burn-rate of RDX was found to be more sensitive to the transport parameters than to the reaction rate parameters. The detailed liquid- and gas-phase mechanisms along with the thermodynamic and transport properties were used in a multiphase combustion model for HMX monopropellant. The pressure dependent burn-rate, and temperature and species profiles in liquid and gas phase were obtained. The importance of including the liquid phase decomposition in combustion modeling was also analyzed. The predicted burn-rate and melt layer thickness are in excellent agreement with the experimental values.