No abstract

Emulsions based on ammonium nitrate (AN) and water locally ignited by a heat source do not undergo sustained combustion when the pressure is lower than some threshold value usually called the Minimum Burning Pressure (MBP). This concept is now being used by some manufacturers as a basis of safety. However, before a technique to reliably measure MBP values can be designed, one must have a better understanding of the ignition mechanism. Clearly, this is required to avoid under ignitions which could lead to the erroneous interpretation of failures to ignite as failures to propagate. In the present work, facilities to prepare and characterize emulsions were implemented at the Canadian Explosives Research Laboratory. A calibrated hot-wire ignition system operated in a high-pressure vessel was also built. The system was used to study the ignition characteristics of five emulsion formulations as a function of pressure and ignition source current. It was found that these mixtures exhibit complicated pre-ignition stages and that the appearance of endotherms when the pressure is lowered below some threshold value correlates with the MBP. Thermal conductivity measurements using this hot-wire system are also reported.

A hot-spot initiated grain burning model is presented which simulates the reaction of an aerated liquid explosive. This model assumes hot spot formation by bubble collapse. The explosive is assumed to react via bulk thermal reaction and laminar burning. The burning fraction is obtained by numerical simulation which accounts for the random distribution of the size and location of the initial air bubbles which are collapsed before burning. Eyring's bum-rate is used to predict the burning velocity of the flame envelopes.This reaction model is applied with a 2-D non-ideal detonation theory to predict the velocity-diameter relationship of the explosive at various initial densities. There is good agreement between theory and experiment for densities down to 0.7 liquid density.

It has been well established that sustained combustion in ammonium nitrate water-based emulsions (AWEs) can only occur if the ambient pressure is held above some threshold value, usually referred to as the minimum burning pressure (MBP). For the commercial explosives industry, a good knowledge of the MBP for particular AWE formulations is essential to estimate safe operating pressures for the associated manufacturing and handling processes. In these processes, AWE products are most often pumped in closed systems and at elevated temperature. While previous studies have established that the MBP can depend critically on major ingredients, its dependence on physical characteristics such as temperature and viscosity had never been investigated. Moreover, the consequences of alterations in measurement methodologies on the resulting measured MBP values had not been studied.In the present work, MBP measurements were performed for a number of AWE systems representative of those found in the commercial explosives industry. The effect of the presence or absence of certain ingredients on the MBP will be discussed. In particular, the influence of glass or polymeric microspheres and testing geometry on the combustion behaviour of some AWEs will be evidenced. Experimental results showing the effect of other physical factors such as initial product viscosity and temperature on the measured MBP values are also presented.

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.