Study On The Mechanism Of Mixed Melt-cast Explosive In Slow Cook-off

The melt-cast explosive is one of the most widely used main charges in the modern armament industry.In order to explore the properties of mixed melt-cast explosive in the slow cook-off test,such as the flow characteristics,temperature variation regularities,self-heating reaction,and the distribution of ignition location,et.al,the typical mixed melt-cast explosive—Comp-B,was selected as the research object.The cook-off experiments were tested under different heating rates for 3 kinds of bomb size by a self-designed one-time heating device.In the experiments,the temperature changing curves of the whole experimental process were obtained by the preset miniature armored thermocouples.The result shows that,for every kind of cook-off bomb,despite the difference in heating rate,the final ignition position is always close to the top cap of the bomb.Under the heating rate of 1℃/min,it’s possible that the explosive responded before all of it melt-down when the bomb size is large.Moreover,the temperature curves of the measuring points revealed that the upper part of the explosive melted quickly than the lower part in the melting process.But when the heating rate reduced into 3.3/h,the temperature difference between the mid and center of the bomb is increasing with the bomb size increased.For the larger cook-off bomb,the variation characteristics of internal temperature are different to some extent,and the temperature curves of each size of the bomb also show that the liquid explosive did not seem to flow from the beginning of melting.To understand more and further about the flow behavior of the melt-castable Comp-B in the cook-off process,we did a numerical simulation of Comp-B based on the viscosity model of Bingham fluid.The result of the simulation shows that the flow and heat transfer characteristics of Comp-B in the cook-off process are directly affected by the yield flow characteristic of Bingham fluid.But in essence,the influence of bomb size and heating rate to the flow characteristic of explosive,it can be summed up to the influence about internal temperature difference to the yielding process of the explosive,and the changing of viscosity is very complicated in the whole process of cook-off.In the condition of larger bomb size or quicker heating rate,the temperature difference in the bomb is large when the melting occurred,this will generate the buoyancy force in the liquid explosive,and the buoyancy force will cause the shear stress subsequently.When the shear stress exceeded the Yield stress threshold of the explosive,the yielding occurred,and the liquid explosive started to flow,the natural convection appeared.Whereafter,the heat convection will influence the heat transfer mode in the explosive,change the melting process and the temperature field distribution.Thus,the upper part of the explosive in the bomb melted quickly than the lower part.After all of the explosives melted down,the temperature field slowly presented like a typical liquid phase temperature field,the temperature of the upper part was higher than the lower part,and this will finally lead that the explosive responded at the upper position.When the heating rate was slow or the bomb size was small,the internal temperature difference was still tiny after the explosive completely melted down,the buoyancy lift is too little to force the explosive yielding,it was still almost static in the liquid explosive,and the effect of convective heat transfer on the overall heat transfer mode of explosive was very weak.Until the self-heating of the explosive occurred,and the internal temperature difference raised and exceed a certain value,the explosive started to flow,and the internal temperature distribution and ignition location changed with that.Moreover,a scientific explanation was proposed for the controversial topics in this research field about whether the natural convection was existing in the slow cooking-off process of Comp-BThe viscosity model of explosives that used in this research can be applied to most of the mixed melt-cast explosives in theory,and the conclusions can provide technical guidance in further research about the ignition characteristic of melt-cast explosives when under the thermal stimulation.