Thermochemical Code, Cheetah

Exp-6 potential models can be validated through several independent means. Fried and Howard33 have considered the shock Hugoniots of liquids and solids in the decomposition regime where thermochemical equilibrium is established. As an example of a typical thermochemical implementation, consider the Cheetah thermochemical code.32 Cheetah is used to predict detonation performance for solid and liquid explosives. Cheetah solves thermodynamic equations between product species to find chemical equilibrium for a given pressure and temperature. From these properties and elementary detonation theory, the detonation velocity and other performance indicators are computed. 

Because of the lack of high-pressure experimental reaction rate data for HMX and other explosives with which to compare, we produce in Figure 15 a comparison of dominant species formation for decomposing HMX that have been obtained from entirely different theoretical approaches. The concentration of species at chemical equilibrium can be estimated through thermodynamic calculations with the Cheetah thermochemical code.32,109... 

It may however, be noted that these values for different parameters are influenced by purity, morphology and particle size of the sample. The calculated values of VOD for FOX-7 and RDX are 9090ms 1 and 8940 ms 1 respectively (Cheetah Thermochemical Code). Based on these results, it was concluded that FOX-7 is better than RDX which is used as a benchmark explosive for comparison with other explosives. Consequently, it is an attractive ingredient for application in high performance IM compliant explosive formulations. FOX-7 also increases the burning-rate in propellants and as a natural consequence, is of interest for high performance propellants. 

One of the attractive features of thermodynamic modeling is that it requires very little information regarding the unreacted energetic material under elevated conditions. The elemental composition, density, and heat of formation of the material are the only information needed. Since elemental composition is known once the material is specified, only density and heat of formation needs to be predicted. The Cheetah thermochemical code offers a general-purpose, easy to use, thermodynamic model for a wide range of materials. 

The Cheetah thermochemical code uses assumptions about the interactions of unlike molecules to determine the equation of state of a mixture. The accuracy of these assumptions is a crucial issue in the further development of the Cheetah code. We have tested the equation of state of a mixture of methanol and ethanol in order to determine the accuracy of Cheetah s mixture model. Cheetah uses an extended Lorenz-Berthelot mixture approximation [138] to determine the interaction potential between unlike species from that of like molecules ... 

The thermodynamic theory for exp-6 mixtures of polar materials is now implemented in the thermochemical code Cheetah.32 We considered first the major polar detonation products H20, NH3, CO, and HF. The optimal exp-6 parameters and dipole moment values for these species were determined by fitting to a variety of available experimental data. We find, for example, that a dipole moment of 2.2 Debye for water reproduces very well all available experiments. Incidentally, this value is in very good agreement with values typically used to model supercritical water.50... 

Lu, J. P., Evaluation of the Thermochemical Code-CHEETAH 2.0 for Modelling Explosives Performance, DSTO-DR-1199, Defence Science and Technology Organization (Australian Government), Edinburgh, 2001. 

Non-ideal explosives have Chapman-Jouguet (C-J) detonation parameters (pressure/velocity) significantly different from that expected from a thermochemical / hydrodynamic code for equilibrium and steady state calculations. Therefore, common computer codes such as Cheetah, EXPL05 and others often do not correctly predict the detonation parameters of non-ideal explosives such as ANFO or me-talized explosives. 

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