Onset Temperatures in Hot Wire Ignition of AN‐Based Emulsions

Abstract

Hot wire ignition experiments were carried out recently at the Canadian Explosives Research Laboratory on a few emulsion formulations. The data indicate that there is a pressure-dependent onset temperature beyond which the wire temperature increases at an accelerated rate. In order to explain this observation and to detect this temperature more consistently, particularly at low pressures, the data are reanalysed by comparing the experimental wire temperature with that predicted from theory for the heating of an inert material. For this purpose, an analytical theory from the literature is reviewed and the numerical solution developed in this report is described. The latter can deal with more general solutions with variable thermal properties and chemical reactions in the condensed medium surrounding the wire. A review of available data on the vapour pressure of ammonium nitrate (AN)/water solutions is carried out and one set of data is correlated analytically to allow the general prediction for any concentration of water. A model is proposed to predict the phase transition temperature based on the energy balance of the endothermic effects of water vapourization and AN dissociation/ decomposition and the heat input from the hot wire. The predicted onset temperatures are shown to be in reasonable to excellent agreement with experiment depending on the compositions, which include emulsions with 9%, 18% water and additional ingredients of sodium nitrate, sodium perchlorate and urea. It is postulated that the minimum burning pressure (MBP) would lie on the transition temperature/pressure curve. However, locating this point requires an accurate prediction of the heat feedback from the exothermic reaction zone in the gas phase.

Analysis of combustion waves arising in the presence of a competitive endothermic reaction

“…Such a configuration has, for example, been used in laboratory experiments on the burning of ammonium nitrate (AN), in the context of emulsion explosives [Turcotte et al .2008;Chan and Turcotte 2009]. The extensive literature on the dissociation of AN reveals a surprising variety of representations of the chemistry involved.…”

“…Importantly, some are exothermic and some endothermic, and it has been common to assume a lumped representation of just two competing processes, one endothermic, for example [Sinditskii et al, 2005] In numerical simulations of the ignition of AN [Hughes et al 2011], self propagating fronts were not found when values of the kinetic parameters appropriate for 'pure' AN were used; this is in accord with the extensive literature on AN combustion. However, for values modified to take a crude account of the effect of additives [Chan and Turcotte 2009], such travelling combustion fronts were observed to develop for wide ranges of initial conditions and parameter values.…”

Numerical Study on Thermal Safety of Emulsion Matrix in Emulsifier

“…S. K. Chan and R. Turcotte et al. established a testing system to study the thermal safety of emulsion matrix under different ambient pressure and proposed the minimum burning pressure concept and analyzed the key influencing factors. In addition, some scholars have studied the thermal safety of emulsion matrix in the screw pump.…”

Influence of Physical Characteristics and Ingredients on the Minimum Burning Pressure of Ammonium Nitrate Emulsions

“…Various MBP measurement methodologies for AWEs have been developed over the years [2 -4] and recent research has demonstrated that ignition source power and geometry play an important role in order to ensure reproducible localized thermal ignition in these explosives [5,6]. Consequently, in some of the MBP data collected previously, lack of propagation may have been confused with improper ignition.…”