Experimental And Numerical Researches On The Afterburning Of Thermobaric Explosives

Thermobaric bombs,as new type of weapons,can effectively target the hidden targets in the confined spaces,such as caves,tunnels,underground facilities and so on.As the main charges of thermobaric bombs,thermobaric explosives have different energy release process from traditional high explosives.After the detonation of thermobaric explosives,afterburning reaction takes place,which utilizes the surrounding air to increase the energy output and form a high-temperature,high-pressure,and anoxic environment in a confined space to damage the targets.Therefore,deeply understanding the energy releasing of afterburning reaction and the evolution process of post-combustion flow field causing by the explosion of thermobaric explosives are important for evaluating the damage effect accurately and effectively.This paper focuses on the afterburning energy releasing and the post-combustion flow field evolution,experimental and numerical researches are carried out.An experimental device for evaluating the afterburning rate of thermobaric explosives is designed and a coupled finite difference–material point method for solving the gas-solid two-phase reaction flow is established.The numerical program of the post-combustion flow field of thermobaric explosives is developed.Based on this program,the process of the afterburning and the explosion characteristics in a confined space are investigated.The main work and innovations of this paper include the following aspects:1.A sleeve-slider experimental device was designed,and the relationship between the state parameters of detonation products and the movement parameters of the slider was established.By measuring the displacement history of the slider driven by the detonation products of the aluminized explosive,the histories of the pressure and specific volume of the detonation products with time are obtained.And the afterburning energy releasing curve of the aluminized explosive is determined based on the principle of energy conservation.This experimental method makes up for the shortcoming of the current experimental methods that can only obtain the static energy output of thermobaric explosives.2.A gas-solid two-phase reaction flow model of the post-combustion flow field of a thermobaric explosive was established,which includes the two-phase conservation equations,the mass,momentum,energy interactions between the two phases,and the chemical reaction model.A quick method for calculating the multi-component combustion reaction suited to fluid dynamic calculating and a calculation method for different types of energy released by thermobaric explosives were developed.A gas equation of state that can uniformly describe the detonation products,air and combustion products in the post-combustion flow field was established.3.A coupled finite difference–material point method for solving gas-solid two-phase reaction flow was established.The gas phase was calculated by the wave propagation method,which has the good ability to track the material interface.Considering the calculation problem caused by the strong rarefaction waves happening at the interface between the detonation products and air,a "five-wave" Riemann solver was proposed to improve the wave propagation method.The solid phase was calculated by the material point method.The discrete format of the mass equation was derived to consider the mass change induced by the particle combustion.A collision algorithm was developed to handle the elastic and viscous collisions between the solid particles.The coupling method was realized by using a uniform grid as the discrete grid for finite difference method and the background grid for material point method.The gas-solid interactions were directly calculated at the unified grid node using the fourth-order Runge-Kutta format.The time stability conditions depending on the gas-solid interactions were proposed.The coupled finite difference–material point method fully utilizes the respective advantages where the finite difference method captures the discontinuity of the flow field and the material point method tracks the particle movement,which overcomes the numerical difficulties encountered by the pure Eulerian method in solving the motion equation of solid phase.4.The numerical simulation program of the post-combustion flow field of thermobaric explosive was developed in C/C++ language.The accuracy of the gas phase numerical calculating was verified by the shock tube problem and the air explosion problem.The validity for simulating the post-combustion flow field of thermobaric explosives and the predictive ability for the quasi-static parameters of confined space were verified by the explosion tests in closed containers.The influences of grid size,turbulence,and the generation order of combustion products on the calculation results were analyzed.The results show that the combustion calculation of gas phase has grid convergence,and the influences of turbulence on the movement process of high-pressure gas and the evolution of the interface are confined.The influence of the generation order on the quasi-static pressure can be ignored.5.Basing on the simulation program,the post-combustion reaction process and the explosion characteristics were studied.In the part of the reaction process,the critical ignition diameters of the particles drived by the center detonation products of RDX range from 45μm to 50μm for the active-shell-type thermobaric explosives.The disturbance of the interface between the detonation product and the air by the particles can promote the mixing and increase the rate of gas phase combustion.Compared with the active-shell-type thermobaric explosives,the heterogeneous ones have a higher proportion of anaerobic combustion and increase the early shock wave overpressure,but have little influence on the impulse.In the part of the explosion characteristics,the explosion phenomenon in the finite space has the size effect,the shock wave overpressure and the quasi-static pressre are different for the different sizes,and the pressure waveform and the reaction rate of aluminum powder are different after normalization.The secondary peak value and impulse of the shock wave of the temperature-pressure explosive increase with the increase of the aluminum powder content and the time interval between the primary and secondary wave peaks decreases.For the same content of aluminum powder,the shock wave overpressure of the thermobaric explosive with larger particle diameter is smaller in the near field and larger in the far field.Moreover,the effects of aluminum powder content and particle diameter on the quasi-static parameters after the explosion of thermobaric explosive in a confined space were investigated.The results show that the quasi-static pressure,quasi-static temperature,and oxygen consumption capacity increase with the increase of aluminum powder content.The particle diameter has effect on the energy releasing proportion of anaerobic combustion and the overpressure on the wall,but has little effect on the quasi-static parameters.The numerical simulation’s results of the quasi-static parameters are generally similar to the theoretical model’s results,but the values are smaller.This is because the numerical simulation takes into account the mixing process of the fuel and the oxidant in the post-combustion reaction of the thermobaric explosive,and the energy release efficiency cannot reach 100%.