| The equation of state (EOS) of detonation products is essential for describing detonation properties of explosives, it is not only the core parameter in numerical simulations of detonation process of explosives but also the primary characterization of the power of explosives. Due to some shortcomings of empirical EOS used in engineering, such as the simplicity of model, the narrow scope of application and the inefficiency of extrapolation, it is not precise enough to describe the complicated physical phenomenon of detonation products. For a more precise description, it’s necessary to develop the EOS theory based on the components of detonation products. The American CHEQ code using for calculation of the EOS of detonation products is just based on such theory, but it’s not publicly available for the cause of classification.In the paper, an improved EOS model of detonation products is proposed by using the public resources of CHEQ for reference. The major improvements are as follows:(1) The component of equilibrium state for detonation products is determined by solving chemical equilibrium equations of mixed system based on the minimum Gibbs free energy principle. Comparing with the linear method used in CHEQ, the steepest descent method used in this paper is more stable for solving equations and it can be used to solving equilibrium equations of any detonation products. (2) The EOS of gaseous detonation products is investigated using the improved one-fluid van der Waals mixture model and Ross’s modification of hard-sphere variation theory. Comparing with the CHEQ code, firstly, the potential function parameters of each molecular products are recalibrated, secondly, the atomic products are considered and the corresponding method for determining the potential function parameters are developed, and finally the interaction between different molecules is improved. (3) The condensed phase of carbon is taken as a mixture of graphite, diamond, graphite-like liquid and diamond-like liquid, and the phase under different state (temperature and pressure) is calculated by minimizing the Gibbs free energy. Comparing with the three-phase model (graphite, diamond, liquid carbon) used in CHEQ, the new model is a more realistic description of the aggregates and properties of carbon after detonation. Basing on the proposed physical model, a new CHEQ-like code is developed to calculate the equation of state of detonation products.The main physical mechanism in detonation process of explosive is studied. (1) The dissociation of molecular products under high temperature and pressure is investigated and the interaction potential parameters of atom-atom and dissociated molecule-atom are calculated by a new proposed method using the experimental Hugoniot data of molecular fluid. Comparing with the traditional corresponding states law method, the new parameters are in good agreement with the shock experiments data in a very wide pressure range. (2) The supercritical fluid phase separation phenomenon between different molecular products is investigated and the corresponding interaction potential parameters are obtained by using the detonation parameters of specific explosives. The non-ideal mixing correction coefficients between the major detonation products H2O, CO2 and N2 are:kN2-H2o=1.03, kN2-CO2=1.035, kH2O-CO2=0.96. (3) The non-equilibrium effect of carbon is studied. The possible existing form of carbon after detonation is analyzed. The EOS of clusters of carbon is obtained by using the four-phase equilibrium state EOS of carbon and the detonation parameters of explosives.In this article, the EOS of detonation products of several explosives is investigated. (1) The detonation velocity and pressure of PETN under different initial density is calculated and they are in good agreement with the experimental values in a wide range of density and temperature. The equilibrium components at the CJ point are computed and corresponding analysis and comparison is made with the results of BKW and LJD equation of state. The isentropic of PETN is obtained, the area under the isentrope in PV diagram is close to the value of JWL EOS which is calibrated by experimental results. It indicates that powers obtained by the two methods are comparative, however, the isentrope of CHEQ-like code is smoother than that of JWL EOS during the late expansion process. The calculated isentrope γ by CHEQ-like begins to monotonously decrease from CJ point, and finally is close to the value of ideal gas, instead of the calculated two peaks phenomenon using JWL EOS. According to detailed analysis, it is found that the two peaks phenomenon results from the function form of JWL EOS, and does not correspond to the actual physical process. Therefore, the results of this paper are more reasonable.(2) The properties of detonation products of RDX under different initial density are investigated. It is found that all detonation products are in gaseous phase when the initial density is less than 1.20g/cm3, the solid phase will gradually precipitate when increasing the initial density. The detonation velocity and pressure obtained by using uniform mixing model is in good agreement with experimental results when the initial density is less than 1.60 g/cm3. The difference between calculated and experimental values enlarges when continuously increasing density and it’s due to the supercritical fluid phase separation phenomenon. The improved results can be obtained by taking account this phenomenon into the EOS model.(3) The detonation parameters of PBX9502 at CJ point is calculated, and they are in good agreement with experimental values. The calculated isotherm is close to that obtained by using Sesame data library. An inflection point exists in the isotherm at 5802K, it is caused by the phase transition of free carbon. The transition from diamond-like phase carbon to solid phase carbon makes abrupt change in the thermodynamical quantities and component of products. (4) The detonation parameters of several other explosives are also investigated and compared with the experimental values and the calculated results of CHEQ code.
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