Mechanism And Effect Of Water Mitigation On Confined Explosion Loading

By placing water around the explosives as water barriers,the damage to people and buildings caused by accidental explosions can be mitigated,and the dangerous area of explosive storage also can be reduced.In a confined space,water heated or vaporized can absorb a large amount of energy generated by the explosion,reducing the temperature of explosion products and the gas pressure.The experimental results showed that proper amount of water placed around the explosives can reduce the quasi-static pressure by more than 90%,which has a good application prospect.In order to evaluate the feasibility of water placed in the enclosed space to mitigate the explosion loading,the mechanism and the effect of water mitigation on the explosive loading has been studied.The studies on water attenuation of explosion load have been reported in domestic and foreign literatures.Most of them are conducted in numerical simulations while a few are carried out in experiments.Moreover,the majority of them are conducted in application sites such as underground tunnels,semi-closed spaces and free spaces,and almost all of them have achieved good mitigation effect on shock waves of explosions.However,it is found that pressure and impulse desities are somewhat higher with water in the full scale experiment performed in the Sweden-Club tunnel,which is different from the results of small scaled experiments.It is showed that the water mitigation phenomenon is very complex and more work is needed to understand the mechanisms of water mitigation in the confined space,including the water breakup process and the ways of explosion energy dissipated.In this paper,the movement of water driven by explosion is studied by means of experiment and numerical simulation.The influence of water on explosion load mitigation in different conditions is also investigated.As the result of these,the mitigation mechanism and energy dissipation mechanism of water on explosion loading in enclosed space are analyzed.Finally,preliminary verification of water as a mitigator in the explosion containment vessel are carried out.The results obtained are as follows:(1)The spherical explosion configuration that a spherical explosive is surrounded by spherical water was designed and applied to investigate the diffusion process of water subjected to the explosion loading.The high-speed camera was used to capture the dispersion process with the ratio of water to explosive weight less than 10 in order to demonstrate the evolution process of water such as injection,acceleration,breakup,atomization and so on.The diffusion speed of water and the evolution of air shock wave in front of water are investigated to analyze the concept of water mitigation on explosion load.A confined structure to generate water mist was designed as the experimental platform,which was used to carry out experiments to characterize the movement of water mist.The processes of water sprays breakup,atomization and oscillation in the closed space were explored.(2)Based on the method of using water to surround the explosive to reduce shock wave and energy of confined explosions,experiments of explosion with and without water were carried out to study the effect of water on confined explosion load in different conditions.Spherical explosion configurations were applied in these experimental test systems to measure the explosion loadings at the scaled distances of 0.128m/kg1/3,0.255m/kg1/3,0.468m/kg1/3 and 1.21m/kg1/3.The influences of different conditions including water-to-explosive ratio,the structure used to retain the water,scaled distance,air gap between water and explosive were studied according to the measured results of parameters such as reflected overpressure,quasi-static pressure,temperature,impulse and structural response.Firstly,the results show that the most important factor of water mitigation effect is the scaled distance.That larger scaled distance is more conducive to water mitigation.When the scaled distance is too small,water will not mitigate but enhance the explosion damage.Secondly,the important factor of water mitigation is the ratio of water mass to equivalent TNT mass of the explosive,which is usually 2?4 when the best mitigation effect is yielded at a certain scaled distance.When the scaled distance is large,the water-to-explosive ratio can be appropriately increased to improve the effect of water mitigation.Thirdly,the air gap between the water and the explosive is beneficial to the effect on the explosion loading.Besides,it is favorable to reduce the adverse effect of the water-containing structure on water mitigation that the material density or the wave impedance of the structure is decreased.(3)The mitigation effect of water on explosion loadings of spherical explosion configuration was investigated using the finite element program.The smoothed particle hydrodynamics(SPH)method was used to model the evolution process of water in the initial stage of explosion,such as jetting and breakup caused by shock wave.The calculation model can reproduce the movement process of water around the explosive,which gives an index to the mechanism of water mitigation on shock wave of the explosion.The LS-DYNA software used to simulate in the initial stage of explosion with water and the FLUENT software applied to calculate the following development of the explosion were combined to investigate the whole process of water movements and breakups etc,in order to study the water mitigation mechanics.The series of numerical simulations of explosions with and without water in different conditions such as water-to-explosive weight ratio were carried out in confined spaces using AUTODYN.The evolution of the flow field in the confined space was compared to the measured reflected overpressure waveform in order to study the formation mechanism of the pressure waveform of explosion with water.(4)Based on the results of numerical simulations,the high-speed photography and explosion tests with and without water,the mechanism of the influence of water around explosive on explosion loading is discussed.It is concluded that water absorbs the energy of explosion and converts it into the kinetic energy of water in the initial stage of explosion,thus reducing the air shock wave and prolonging its arrival time.The water impact on the structural wall close to the explosion can not be neglected,which means that both the measured reflected overpressure and the structural deformation of explosion with water are larger than that of explosion without water.The impact of water in form of sprays,mist and so on will be decreased rapidly when water is atomized and decelerated by the air resistance with the increase of propagation distance in the medium term of the explosion.As a result,water has a better mitigation effect on attenuating the shock wave reflected in the confined space and the oscillations of the reflected overpressure turn to be less than that without water.The heat exchange between water and high temperature gas of explosion product turns to be prominent in the later period of explosion.Accordingly,water heating and vaporization has a significant effect on the quasi-static pressure and the temperature rise in the confined space.Up to more than 40%reduction of the quasi-static pressure and the temperature rise is achieved by using water in our experiments.(5)The influence of water vaporization on mitigating quasi-static pressure is studied through theoretical analysis.When the load density(the ratio of the explosive equivalent and the effective volume of closed space)is constant,the more water vaporizes,the lower the pressure will be.When the ratio of the amount of water vaporized to the explosive equivalent is constant,the higher the load density,the lower the pressure will be.Compared to all sand walls,an explosion replacing some sand walls by water walls was performed in a containment vessel,to evaluate the mitigation effect of water walls used in confined space on blast wave.The feasibility of water walls as a mitigator applied in explosion vessel is verified preliminary,which can provide a reference for engineering application of water mitigation technology.The experimental results show that 1/5 of the sand walls replaced by water walls can effectively reduce the quasi-static pressure in the closed space,up to 50%of the quasi-static pressure reduction of the whole sand wall is achieved.Briefly,the mitigation effect has been improved by replacing some all sand walls with water walls.