Comparison of gas-phase mechanisms applied to RDX combustion model

Two detailed gas-phase chemical mechanisms for RDX – Yetter and coworkers, herein ‘Y2’ [K. Prasad, R.A. Yetter, M.D. Smooke, Combust. Sci. Technol. 124 (1997) p. 35.]; Cal. Tech. group, herein ‘CTM’ [(a) A.D. Chakraborty, R.P. Muller, S. Dasgupta, W.A. Goddard, III, J. Phys. Chem. A 104 (2000) 2261. (b) D. Chakraborty, R.P. Muller, S. Dasgupta, W.A. Goddard, III, J. Comput. Aided Mater. Des. 8 (2001) 203. (c) D. Chakraborty, R.P. Muller, S. Dasgupta, W.A. Goddard, III, Available from: http://www.wag.caltech.edu/home/rpm/projects/hedm/] – have been tested using a recently developed combustion model. The results are compared with each other and experimental data. Burning rates predicted using CTM are about 15% higher than Y2, but both compare well with experimental data across a wide pressure range. Also, majority species profiles are in reasonable agreement with data from a 0.5 atm pressure experiment. However, comparison of predicted trace species profiles to experiments indicates neither mechanism reproduces all measured trace species well; furthermore, most of these trace species occur along main reaction pathways. Detailed chemical analysis indicates the main initial RDX reaction is surprisingly very different for the two mechanisms. NO₂ scission dominates using Y2, but HONO elimination dominates using CTM, in spite of the NO₂ scission reaction having by far the largest RDX decomposition rate coefficient in each mechanism. Analysis shows the unexpected result using CTM is due to a curious global kinetics phenomenon arising in the product pathway: the ring-opening reaction, RDXR → RDXRO, where RDXR is the cyclic radical formed upon NO₂ scission, has a much smaller rate coefficient in CTM compared to Y2. This causes the reaction to be a bottleneck, and so the NO₂ scission reaction goes into partial equilibrium rather than being forwards. Tests were performed to see how the predicted burning rates would be affected by changes in some of the most sensitive rate parameters. Some of the key parameters leading to the differing predictions have been identified. These results will help guide future efforts to understand and develop an accurate representation of the actual RDX combustion chemistry.