Study On Slow Cook-off Response Characteristics Of Melt Cast Explosives Under Constraints

The response characteristics of ammunition under the condition of slow cook-off are a hot topic in the research of ammunition thermal safety,but current research mainly focuses on the pre-ignition stage,the research on the response characteristics of ammunition in the post-ignition stage is rarely discussed.In order to explore the influence of different constraint conditions on the slow cook-off response characteristics of ammunition,takes the ammunition filled with melt cast explosive Comp-B as the research object,four kinds of cook-off ammunitions with different venting areas were designed,which were 0%(fully constrained),2.0%,2.5% and 3.5% of the total surface area of the charge,respectively.Using a disposable electric heating device,a cook-off test was conducted on cook-off ammunition with a size of ? 35 mm ? 116 mm under different constraint conditions at a temperature rise rate of 3.3℃/h.The test results show that the venting structure can be smoothly activated near ignition moment and form a venting channel;compared with the fully constrained condition,the response time of the cook-off ammunition with the venting area ratio of 2.0%,2.5% and 3.5% was extended by 2.5h,3.6h and 4.5h respectively;when the venting area ratio is small,there is no significant change in the response violence of cook-off ammunition,but when the venting area ratio is greater than 2.5%,the response violence of cook-off ammunition can be reduced to “combustion reaction”.On the basis of experimental research,a simulation analysis of the pre-ignition stage of explosives in the slow cook-off process was conducted using Fluent software,exploring the influence of the venting structure on heat transfer in the cook-off process and the mechanism of the venting structure.For the fully constrained cook-off ammunition,the fluidity of the melted explosives is weak,and the UCM model can be used for numerical simulation.For the cook-off ammunition containing a venting structure,before the venting structure is activated,the entire structure can be considered as a closed,fully constrained structure,and can also be simulated using the UCM model;after the venting structure is activated,the fluidity of explosive increases,and the simulation using a multi-phase flow model is more realistic.The results of numerical simulation show that the existence of venting structures will have an impact on the internal heat transfer characteristics of explosives under the condition of slow cook-off;due to the stronger thermal conductivity of the metal shell,the cook-off ammunition designed with a venting structure receives less heat from the outside in the same time than the fully constrained ammunition;the activation of the venting structure near the ignition moment is related to the pressure accumulation in the ammunition caused by the acceleration of the spontaneous thermal reaction rate of the explosive.Based on the law of mass conservation and the equation of gas state,a model of gas pressure growth in the post-ignition period of cook-off ammunition is established considering the effects of the state of the explosive at the ignition time and the venting of the pressure relief hole,and a simulation calculation model is established using the Simulink platform to study the influence of venting structure characteristics on the pressure growth in the post-ignition period.The calculation results show that the peak pressure of the cook-off ammunition with the venting area ratio of 2.0%,2.5% and 3.5% is 151.11 MPa,99.71 MPa and 31.53 MPa respectively,while the maximum internal pressure that the shell can withstand is not higher than 120.77 MPa.Therefore,under the condition of slow cook-off,when the designed venting area ratio is greater than 2.5%,the shell will not be damaged for the cook-off ammunition filled with melt cast explosive Comp-B,and the larger the proportion of the venting area,the safer the ammunition.The above research conclusions have certain reference value for the analysis of the cook-off mechanism of melt cast explosives and the design of the venting structure for new safety munitions.