DOI: 10.1002/prep.201100150 Study of the Effect of Covolumes in BKW Equation of State on Detonation Properties of CHNO Explosives Muhamed Suceska,* [a] How-Ghee Ang, [a] and Hay Yee Serene Chan [a] 1 Introduction Thermochemical codes use thermodynamics to simultane- ously solve state variables and the chemical composition of detonation products, and hence obtain a prediction of det- onation parameters. This problem is relatively straightfor- ward, given that the equations of state of the gaseous and solid products are known [1]. One of the most difficult parts of this problem is to accu- rately describe the equations of state of the product gases. Various gaseous equations of state (EOS) have been pro- posed until now to describe the behavior of the detonation products gases at extremely high pressures and tempera- tures. Some of them are: Becker-Kistiakowsky-Wilson (BKW) EOS [2], Johnes-Cowperthwaite-Zwisler (JCZ) EOS [3], modi- fied van der Waals EOS [4], modified virial EOS (VLW EOS) [5], etc. The semi-empirical Becker-Kistiakowsky-Wilson (BKW) EOS is one of the most extensively used and is given by: pV g RT ¼ 1 þ xe bx ¼ FðxÞ ð1Þ Where x ¼ kk V g ðT þ qÞ a ; k ¼ S g x i k i ; x i ¼ n i =Sn i ð2Þ p is the pressure, V g is the molar gas volume and R is the gas constant. a, b, k, and q are adjustable parameters and shall be referred to as BKW constants henceforth. x i and k i represent the mole fraction and covolume of each product species i. As noted by Fried et al. [1], care should be taken when considering the covolumes in BKW EOS. The term covo- lume is somewhat a misnomer. In the high density limit, the covolume acts as an effective molecular volume multi- plied by a stiffness factor. However, in the low density limit it can be related to the virial coefficients. It is worth noting at this point that the BKW EOS is based upon a repulsive potential applied to the virial equation of state. Since there is no general relation between molecular volume and the virial coefficient, covolumes cannot be uniquely identified with a physical molecular property. Hence, Fried suggested [a] M. Suceska, H.-G. Ang, H. Y. Serene Chan Energetics Research Institute Nanyang Technological University 50 Nanyang Avenue, Block N1-B4a-02 Singapore 639798, Singapore *e-mail: msuceska@ntu.edu.sg Abstract : Due to its simplicity, the Becker-Kistiakowsky- Wilson (BKW) equation of state has been used in many thermochemical codes in the calculation of detonation properties. Much work has been done in the calibration of the BKW EOS parameters to achieve agreement with exper- imental detonation velocities and pressures thus resulting in many different sets of BKW constants (a, b, k and q) and covolumes of detonation products, with varying levels of accuracy over broad density limits, i.e. broad pressure limits. The covolumes of the product gases in BKW EOS may be regarded as measures of intermolecular interac- tions, and their values should affect the predicted detona- tion properties, particularly at higher explosives densities. This work aims to study the effect of covolumes on calcu- lated values of detonation parameters. Several sets of covo- lumes available from literature and derived by different methods (matching experimental Hugoniots of individual products, by stochastic optimization, and calculated from van der Waals radii), were studied. In addition, the covo- lumes of the product gases were also calculated by ab initio methods. The effect of covolumes is studied compar- ing detonation properties calculated using different sets of covolumes, and experimental data for a series of standard CHNO explosives. It was found that it is possible to repro- duce experimental detonation velocities and pressures within reasonable accuracy (root mean square error of less than 5 % for all tested sets) using different set of covo- lumes, and simultaneously optimizing constants in BKW EOS. However, different values of covolumes strongly affect the composition of detonation products at the Chapman- Jouguet state. It particularly applies to oxygen-deficient ex- plosives and at higher densities, where formic acid appears to be an important detonation product. Keywords: BKW equation of state · Covolume · Detonation · Detonation properties Propellants Explos. Pyrotech. 2013, 38, 103 – 112  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 103 Full Paper