Research On The Thermal Hazard And Temperature Gradient Effect In The Adiabatic Calorimetry Of Two Energetic Materials

Energetic materials are widely used because of their high energy density,fast heat and gas production rate and strong work ability.However,due to the metastable structure of energetic materials and their sensitivity to thermal and other external excitation conditions,it is very easy to cause accidents in the industrial process and bring harm to people and the environment.Among these excitation conditions,heat is the most common form,so the thermal hazard of energetic materials has always been an important focus of safety research.However,the reliability of thermal hazard assessment results of energetic materials largely depends on the accuracy of kinetic model and calorimetric parameters.As one of the most important means to obtain thermal decomposition parameters of energetic materials,adiabatic calorimetry can give the thermal effect and pressure effect under adiabatic condition,so as to better simulate the thermal runaway behavior of reaction in plant scale.However,the temperature gradient effect often exists in adiabatic heat,which seriously affects the accuracy of calorimetric parameters and kinetic models.Therefore,it is necessary to study the temperature gradient effect of the adiabatic decomposition system of energetic materials,so as to evaluate its thermal hazard more accurately.In this thesis,two typical energetic materials(2,4-Dinitrotoluene(2,4-DNT)and N-Nitrodihydroxyethyl dinitrate(DINA))were studied.Firstly,the thermal behavior of 2,4-DNT and DINA under dynamic,isothermal and adiabatic conditions were tested by differential scanning calorimeter(DSC)and accelerating rate calorimeter(ARC).These parameters such as the onset temperature(T₀),peak temperature(T_p),specific heat release(Q),specific gas production(V_g),temperature rise rate(d T/dt)and pressure rise rate(d P/dt)were obtained.The decomposition kinetic model and kinetic parameters of two substance were evaluated and verified based on the calorimetric data under different modes combining with isoconversional method and kinetic model fitting method.The results show that the severity of thermal runaway reaction of 2,4-DNT and DINA is“catastrophic”,and both of them are highly explosive.The thermal decomposition of 2,4-DNT conforms to theA?^{r 1}?B?^{r 2}?C reaction model,the first step is an autocatalytic reaction,and the second step is an N-order reaction.The thermal decomposition of DINA conforms to the reaction model ofA?^{r 1}?B?^{r 2}?C?^{r 3}?D?^{r 4}?E,which is a four-step consecutive autocatalytic reaction,and both substances exhibit strong autocatalytic characteristics.Then,using the Therm Ex module of TSS software based on the kinetic models of 2,4-DNT and DINA to simulate its thermal explosion and thermal runaway behavior under different working conditions,and predict the critical thermal explosion temperature(T_cr),critical ambient temperature(T_?),self-accelerating decomposition temperature(SADT)and thermal explosion induction period under adiabatic condition(TMR_ad).The influence of packaging conditions,initial temperature and loading of sample on these parameters were analyzed qualitatively and quantitatively.The results show that:(1)Under the same loading capacity,the T_cr and T_?of the former are lower than those of the latter in comparison with the standard 1 m³ container and multiple stacked small packages;(2)Under the same initial temperature,the thermal explosion induction period under non-adiabatic condition(?)of the former is shorter,which all indicate that the former is more dangerous;(3)Under the same package material and external conditions,the smaller the sample loading,the higher the SADT,which is more conducive to the safety of its storage and transportation process.Then,the mathematical model of thermal decomposition process of substance in closed adiabatic condition was established based on the reaction rate equation,combined with the mass conservation,momentum conservation,energy conservation,gas state equation and natural convection and laminar flow model;ARC results of 45%glucose solution were used to compare and verify.Combined with ARC experimental research and Computational Fluid Dynamics(CFD)numerical simulation method,the unsteady numerical simulation of thermal decomposition process of 2,4-DNT and DINA inside ARC vessel was carried out to obtain the temperature field and heat change in the system,and then the dynamic variation of temperature gradient and thermal inertia in the adiabatic system was analyzed.The results show that:(1)The maximum temperature gradient of 2,4-DNT and DINA is 182.4?·cm^-1 and 320?·cm^-1,respectively;(2)In the process of adiabatic decomposition,the temperature gradient effect first increases and then decreases,when the heat release is the fastest,it is most obvious,and then decreases until it reaches zero;(3)The effective thermal inertia?_effalso presents a similar pattern,that is,?_eff is basically consistent with the theoretical value?when the reaction rate is low,with the increase of heat release rate,?_eff gradually decreases,?_eff is the lowest when the heat release is the fastest,and then returns to the initial state;(4)With the enhancement of the exothermic characteristics,the more obvious the temperature gradient effect,the greater the?_eff deviation from the theoretical value,and the greater the deviation of the calorimetric data correction in the high self-heating section.Finally,the established CFD mathematical model combined with the specific reaction rate equation was used to further study the causes,the influencing factors and the calculation deviation of the kinetic and thermal hazard parameters for the temperature gradient,so as to seek the optimization model of the temperature gradient and the accurate method of obtaining the adiabatic thermal decomposition parameters.The results show that:(1)There are positive correlation,positive correlation and negative correlation between heat production rate,heat production and thermal conductivity of samples and temperature gradient effect,respectively.That is to say,for strong exothermic materials,a small number of materials can be mixed with inert materials with high thermal conductivity to maintain better temperature uniformity and balance;(2)Among the three adiabatic temperature control models B1,B2 and B3,the order of temperature gradient optimization is as follows:B3>B2>B1,and B3 model can provide approximately ideal adiabatic test conditions;(3)The order of accuracy of adiabatic decomposition parameters is:B3(?_eff=1.0)>B2(VSP,T_c)>B1(ARC,T_c)>B1(ARC,T_b).Through the application of the model and method,its reliability is well proved.In summary,this thesis is of practical significance and application value for the construction of complex kinetic model of energetic materials,accurate acquisition of adiabatic parameters,thermal hazard assessment,prevention and control of industrial thermal hazard,also provides a useful reference for the application of CFD method in the study of thermal decomposition of energetic materials.