Thermal decomposition and combustion modeling of RDX monopropellant and RDX-TAGzT pseudo-propellant
Open Access
- Author:
- Khichar, Mayank
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 20, 2020
- 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
Robert Graham Melton, Outside Member
Daniel Connell Haworth, Program Head/Chair - Keywords:
- Thermal decomposition
Combustion
modeling
RDX
TAGzT
Quantum chemical calculation
TGA
DSC
FTIR
Mechanism
Liquid Phase
Diffusion
Deflagration - Abstract:
- The present study aims to advance the current understanding of the complex thermal decomposition and combustion events of nitramine-based monopropellants and pseudo- propellants. In particular, the focus is on the elucidation of chemistry and physics involved in the thermal decomposition and combustion of cyclotrimethylenetrinitramine (RDX) and a pseudo- propellant containing RDX along with a well-known burn rate modifier, bis(triaminoguanidinium) 5,5'-azotetrazolate (TAGzT). The key challenges of thermal decomposition and combustion modeling that the current study addresses are (1) the unavailability of detailed reaction mechanisms for RDX and TAGzT with accurate rate kinetics data in the condensed phase, (2) limited understanding of the complex physical processes occurring during propellant combustion, and (3) insufficient information about the chemical interactions among RDX, TAGzT and their decomposition products. A systematic approach is followed to accomplish the research objective. The overall advancement in the understanding of thermal decomposition and combustion is divided into six different sections. The first three sections focus on RDX whereas the later sections focus on TAGzT and its interactions with RDX. In section one, a recently developed reaction mechanism focused on the initiation reactions of RDX decomposition in the liquid phase is expanded and validated. The reaction mechanism was expanded by investigating the reactions of the intermediate species leading to the formation of experimentally observed final decomposition products. The validation involves a comparison of experimental results obtained from confined rapid thermolysis at various set temperatures. In the experiments, the decomposition occurs in the liquid phase, which results in the desorption of species into the gas phase. The temporal behavior of the mole fractions is obtained using FTIR spectroscopy data. A species conservation model was developed to simulate the confined rapid thermolysis experiments. The model incorporates the detailed liquid-phase reaction mechanism. The rate parameters in the reaction mechanism were optimized by comparing the experimental and computational results. With the optimized parameters, the computational model reproduces the experimentally observed trends with reasonable accuracy. In section two, the liquid-phase reaction mechanism of RDX is used to study its slow decomposition. Considering the need for accurate vaporization rates in the propellant combustion models, sublimation and vaporization rates of RDX were estimated over a wide range of temperatures. Simultaneous thermal analysis was carried at various slow heating rates – 5, 10, and 15 oC/min using a coupled TGA/DSC-FTIR system. In the solid phase, the mass loss occurs mainly due to the sublimation of RDX, whereas, in the liquid phase, both vaporization and thermal decomposition play a significant role. The extent of thermal decomposition was estimated using a computational model based on a recently developed detailed liquid-phase decomposition mechanism for RDX. For each of the heating rates, a suitable match between computational and experimental mass loss and species evolution profiles was achieved. As evident from the FTIR data, a major part of the mass loss occurs because of the evolution of decomposition products, such as N2O, CH2O, NO2, NO, HCN, H2O, CO, and CO2. Results show that vaporization accounts for 29.6, 34, and 35.9 % of the total mass loss for the 5, 10, and 15 oC/min heating rates, respectively. Relatively more RDX vaporizes at higher heating rates because of the initiation of the boiling phenomenon at higher sample temperatures. In section three, the significance of liquid phase decomposition during constant pressure combustion of RDX is investigated. A melt-layer (liquid) model was developed which considers detailed chemical kinetics in the liquid phase. The extent of decomposition in the melt layer was calculated and the results show that RDX mole fraction at the propellant burning surface is 90% at atmospheric pressure. Whereas, for higher pressures the extent of RDX decomposition is found to be significantly higher. This suggests that the significance of liquid phase decomposition increases with pressure and is important for rocket motor working conditions. Hence, a combustion model was developed which considers detailed chemical kinetics in both liquid and gas phases. RDX combustion wave-structure obtained from the model was analyzed and validated against the experimental data. The combustion model was further improved by incorporating the experimentally observed bubble formation phenomenon inside the two-phase melt layer. The gas- phase reaction mechanism of RDX was also updated by adding new RDX decomposition pathways. In section four, the liquid-phase decomposition process of TAGzT is investigated using a combined experimental and computational approach. Sub-milligram samples of TAGzT were heated at rates of about 2000 K/s to a set temperature (230 to 260 °C) where liquid-phase decomposition occurred under isothermal conditions. Fourier transform infrared (FTIR) spectroscopy and time-of-flight mass spectrometry (ToFMS) were used to acquire transmittance spectra and mass spectra of the evolved gas-phase species from the rapid thermolysis, respectively. FTIR spectroscopy was also used to acquire the transmittance spectra of the condensate and residue formed from the decomposition. N2, NH3, HCN, N2H4, triaminoguanidine and 3-azido-1,2,4- triazol-4-ide anion were identified as products of liquid-phase decomposition. Quantum chemical calculations were used for confirming the identity of the species observed in experiments and for identifying elementary chemical reactions that formed these species. Based on the calculated free energy barriers of these elementary reactions, important reaction pathways were identified for the formation of each of the product species. In section five, quantum chemical calculations were performed to develop a detailed reaction mechanism for liquid-phase decomposition of TAGzT. The developed mechanism has 495 elementary reactions and 355 species. Simultaneous thermal analysis experiments were also performed at three different heating rates: 5, 10, and 15oC/min using a coupled TGA/DSC-FTIR system. The experimentally obtained mass loss and species evolution profiles were used to validate and optimize the developed mechanism. Due to a large number of elementary reactions and species in the detailed mechanism, it cannot be directly used in a combustion model without sacrificing computational efficiency. The detailed mechanism was reduced by identifying and removing elementary reactions whose rate-of-progress remained low throughout the decomposition process. The reduced mechanism has 90 elementary reactions and 70 species, and it was deduced using a detailed kinetic model. The analysis leads to the conclusion that TAGzT decomposition occurs in two steps – 1) anion decomposition and 2) cation decomposition. The anionic part primarily decomposes via a single pathway, whereas the cationic part decomposes via five competing pathways. The experimentally observed rapid decomposition behavior of TAGzT is due to its highly exothermic decomposition pathways and the autocatalytic effects of HCN, CN-, and cyanamide on the decomposition process. The major residual products of TAGzT decomposition are identified as formoguanamine and melamine. In section six, a combustion model for RDX-TAGzT mixture is presented. The model developed for RDX monopropellant was extended to handle a multi-component system. The detailed liquid-phase mechanisms for RDX and TAGzT were including in the combustion model. Efforts were also made to understand the interaction among RDX, TAGzT, and their decomposition products. The liquid-phase decomposition process of RDX-TAGzT mixture was investigated using a combined experimental and computational approach. Simultaneous thermal analysis experiments were also performed at a heating rate of 15oC/min using a coupled TGA/DSC-FTIR system. Two different mixture compositions were studied with RDX:TAGzT ratio of 90:10 and 80:20. Quantum chemical calculations were used for identifying elementary chemical reactions that form the experimentally observed species. These reactions explain the early decomposition of RDX in presence of TAGzT. The liquid-phase mechanism in the combustion model was updated by adding the interaction reactions. The combustion wave-structure obtained from the model was analyzed and validated against the data from literature.