| A common method to reduce the response intensity of a munition under thermal stimulation is to design a venting structure on the ammunition body.In order to investigate the role of the venting structure characteristics in the response process of the ammunition during the slow cookoff experiments,takes the typical mixed melt cast explosive Comp-B as the research object,based on the pressure balance method,three sizes of cookoff ammunitions with different venting structures were designed,in which the shell sizes of the ammunitions areФ35mm,Ф76mm and Ф130mm,and the venting hole area is 2%,2.5% and 3% of the surface area of the charge respectively.Then the multi-point temperature measurement cookoff experiments were carried out at the heating rate of 3.3℃/h.The experiment results show that the action of venting holes is the main factor leading to the delayed ignition time of ammunitions.With the same proportion area of the venting holes,the larger the size of the ammunition,the shorter the ignition delay time after the venting hole acts.With the increase of venting hole area,the slow cookoff response grade of different sizes of ammunitions will be reduced to combustion.On the basis of experimental research,software Fluent was used to numerically simulate the two stages of the slow cookoff process before and after the ignition of the explosive respectively.Based on the Universal Cookoff Model（UCM）,a model was established for the calculation of explosive Comp-B cookoff after the melting of the explosive with buoyancy driven flow,and the reaction rate varied with the pressure and reaction process,etc.The changes of the internal temperature field and internal pressure of the explosive during the slow cookoff heating process were studied.The results show that the pressure rise rate inside explosive Comp-B during the cookoff process is slow to fast,and the venting structure makes the cookoff ammunition less sealed,and the internal pressure is lower than the sealed ammunition at the same time before the venting hole is flushed open;after the venting hole is flushed open,the reaction gas is released,which reduces the gas phase reaction rate and leads to the decrease of the internal temperature of the explosive,and the release of the reaction gas and the bubbledriven convection together lead to the delay of the ignition time.The decreasing degree of selfheating reaction rate of explosive caused by ventilation increases with the increase of ammunition size.The different ignition time of different sizes of ammunitions with venting structure is due to the different effect of their venting structures on the delay of ignition time.The internal pressure of the confined ammunition at the moment of ignition decreases with the increase of the ammunition size.The effect of convective heat transfer makes the ignition point appear in the top area of the explosive,while the effect of the venting hole has little effect on the location of the ignition point.Based on the source term and dynamic grid technology,the end-face retreat and gas flow phenomena of explosive combustion after ignition are simulated,and the influence of the venting structure characteristics on the pressure release inside the ammunition is studied.The simulation results show that the pressure peak in the ammunition will appear at the early stage after ignition and the peak decreases with increasing venting area,and the size of the peak affects the intensity evolution of the explosive after ignition.The temperature field in the free space of the ammunition with different venting areas remains uniform.Multiple venting holes uniformly distributed along the circumference of the end face of the ammunition with the same venting area lead to a higher pressure peak than a single venting hole in the geometric center of the end face,increasing the risk of violent response.The above findings have important reference value for the analysis of the cookoff mechanism of melt cast explosives and the design of the venting structure. |
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