Effect of metal powders on explosion of fuel-air explosives with delayed secondary igniters

Yong-xu Wng ,Yi Liu ,Qi-ming Xu ,Bin Li ,Li-feng Xie ,*

a School of Chemical Engineering,Nanjing University of Science and Technology,Nanjing,Jiangsu,210094,PR China

b Xian Modern Chemistry Research Institute,Xian,Shaanxi,710065,PR China

c China Research and Development Academy of Machinery Equipment,Beijing,100089,PR China

Keywords:Fuel air explosive Aluminum powder Boron Magnesium hydride Explosion performance

ABSTRACT In order to improve the energy level of fuel air explosive(FAE)with delayed secondary igniters,high energetic metal powders were added to liquid fuels mainly composed of ether and isopropyl nitrate.Metal powders’explosive properties and reaction mechanisms in FAE were studied by high-speed video,pressure test system,and infrared thermal imager.The results show that compared with pure liquid fuels,the shock wave overpressure,maximum surface fireball temperature and high temperature duration of the mixture were significantly increased after adding high energetic metal powder.The overpressure values of the liquid-solid mixture at all measuring points were higher than that of the pure liquid fuels.And the maximum temperature of the fireball was up to 1700 °C,which was higher than that of the pure liquid fuels.After replacing 30%of aluminum powder with boron or magnesium hydride,the shock wave pressure of the mixture was further increased.The high heat of combustion of boron and the hydrogen released by magnesium hydride could effectively increase the blast effect of the mixture.The improvement of the explosion performance of boron was better than magnesium hydride.It shows that adding high energetic metal powder to liquid fuels can effectively improve the explosion performance of FAE.

1.Introduction

Fuel air explosive(FAE)has attracted wide attention owing to its completely different from traditional condensed-phase explosives in terms of concept,principle,composition,performance and operational efficiency[1-4].Compared with condensed-phase explosives,the FAE is loaded with fuel rather than explosives,the oxidant is provided by the air at the explosion site,thus its charging efficiency is higher.The fuel firstly forms an explosive mixture under the action of the booster explosive and then explodes during the usage.Therefore,it has a larger explosion pressure range and duration.

In order to improve the explosion performance of FAE,the component of FAE should try to select components with high heat of combustion[5].Aluminum powder is the most commonly used metal powder in various types of mixed explosives and propellants.Researchers have explored the impact of aluminum powder size and content on explosive performance and the reaction mechanism of aluminum powder in explosives[6-9].Duan et al.[10]studied the effects of Al/O on pressure properties of confined explosion from aluminized explosives.Peuker et al.[11]studied Particle size effects on blast and overpressure enhancement in aluminized explosives,the experimental results show that particle size effects are surprisingly negligible over the range of 3-40μm.As a metal with higher heat of combustion than aluminum powder,boron has been used in propellants[12].However,in condensed-phase explosives,the application of boron is still in the experimental stage due to the high ignition temperature and slow reaction kinetics[13].Chen et al.[14]studied the explosion of Aluminized explosive compositions(RDX/Al/AP and RDX/Al/B/AP)and found that the shock wave overpressure did not increase significantly when boron was added.Researchers[15]pointed out that the initial high temperature environment and appropriate oxygen concentration were conducive to the release of boron energy.Magnesium hydride is a new type of hydrogen storage material with the hydrogen storage capacity of 7.6%.The released hydrogen can effectively increase the blast effect of explosives.Adding magnesium hydride to energetic materials can effectively increase the energy level[16,17].

In order to increase the energy level of FAE,high energetic metal powders,including aluminum powder,aluminum/boron mixture,and aluminum/magnesium hydride mixture,were added to liquid fuels mainly composed of diethyl ether and isopropyl nitrate.Diethyl ether is very volatile in the air with a low boiling point,and the explosion limit of isopropyl nitrate is between 2 and 100%,which can play a sensitizing role making the mixture more prone to explosion.The pressure test system and infrared thermal imager were used to compare the parameters of shock wave overpressure,fireball surface temperature and thermal energy,and the reaction mechanism of high energetic metal powder in FAE was analysed.The present study provided us with a way of enhancing the blast effect of FAE.

2.Experimental

2.1.Experimental sample

The fuel mixtures are filled in the cylindrical device which consists of detonator,booster tube,booster explosive and shell.The diameter and length of cylindrical device is 90 mm and 200 mm.The thickness of shell composed of unplasticized polyvinyl chloride(UPVC)is 3 mm.The inner diameter of the booster tube is 24 mm.The bottom of the booster tube is located at 1/3 of the length of the shell.TNT was used as the booster explosive.Fuel is made up of mixtures of the liquid fuels(Ethyl ether and Isopropyl nitrate)and high energetic metal powder in different proportions as seen in Table 1.The liquid fuels used in the experiments were of analytical purity,and the purity was greater than 99%.The SEM pictures of the high energetic metal powder are shown in Fig.1.The aluminum is flake aluminum powder,the water cover area is 0.75 m2/g,and the appearance is silver-gray,petal-shaped.Boron is amorphous boron and has a median diameter of 0.1μm.Magnesium hydride has a wide particle size distribution with a median diameter of 3μm.The ratio of aluminum powder and other two metal powders is 7:3.In this study,four different types of FAE were prepared,and two tests were performed for each type of FAE.The schematic diagram of the cylindrical device was shown in Fig.2.

2.2.Experimental setup

The schematic diagram of experimental setup was shown in Fig.3.The experimental device was placed on a plastic stool with a height of 0.6 m.The second ignition explosive was two 160 g TNT explosive charges with a distance of 1.35 m from the explosion site.Six pressure sensors were mounted in the pressure measuring instrument to measure the pressure histories.The horizontal distances of pressure sensors from explosion site are 1 m,2 m,3 m,4 m,5 m,and 6 m,respectively.The dispersion process and explosion process were captured by a high-speed camera with the frame rate of 2000 fps and an infrared thermal imager with the frame rate of 30 fps.Since it is difficult for the fuel-solid mixture to explode in the accumulation state,it is necessary to disperse the fuel into the air firstly to form an explosive mixture,and then make the cloud explode by introducing a certain intensity of external energy.The primary booster explosive and the secondary ignition explosive are controlled by a delayed ignition device,and the time interval set for this experiment is 40 ms.

3.Results and discussion

3.1.Fuel dispersion process

After the ignition switch is pressed,the booster explosive first explodes.The mixtures were pushed forward by the force of the explosion that formed a fuel cloud,and the fuel mixtures were broken up into droplets and granules.The fuel dispersion process was shown in Fig.4.It can be seen from Fig.4 that the mixtures mainly move outward in the form of a jet in the first 5 ms,and the cloud diameter grows faster.After that,the growth rate of the cloud diameter slows down,and the fuel diffuses freely under the action of shock waves and aerodynamic drag.Throughout the experiment,we kept the position of the high-speed video and FAE device unchanged,and measured the cloud diameter through the scale.The fuel cloud diameter as the function of time was depicted in Fig.5.It can be seen from Fig.5 that the cloud diameter increases rapidly in the first 5 ms,and then the growth rate slows down.Zabelka[18]divided the fuel dispersion process into two section:the movement of fuel was mainly determined by the force of the explosion,which was called the near-field section;the movement of fuel was mainly determined by the aerodynamic resistance,which was called the far-field section.Combined Figs.4 and 5,it can be considered that the first 5 ms was the near-field section,and the far-field section was followed.Before the second ignition,the cloud diameter was rankedasether/aluminum＞ether/aluminum/boron＞ether＞ether/aluminum/magnesium hydride.The loading volume of the four groups of experiments was the same,and the diameter of the cloud formed by the ether/aluminum and the ether/aluminum/boron mixture were basically the same.Due to the low charge weight of pure liquid fuel,the cloud diameter formed was small.However,the ether/aluminum/magnesium hydride mixture had the same charge but formed the lowest cloud diameter,indicated that the cloud dispersion process of this mixture was not ideal.The typical FAE and TNT overpressure variation trend with distance was shown in Fig.6.Since the fuel first forms an explosive mixture in the air,during the propagation of the shock wave in the cloud zone,the fuel reaction would provide energy support,and the pressure decay rate in the cloud zone was slower than outside area or even remain unchanged[19].The larger the cloud area leads to the larger the scope of damage and destruction to the target.In order to obtain the best explosion effect,the cloud was generally required to be a flat cylinder,which can increase the direct range of cloud explosion.Fig.7 shows the cloud shape formed by 4 types FAE at 40 ms.It can be seen that the cloud shapes of the ether/aluminum mixture and the ether/aluminum powder/boron mixture were flatter than the ether/aluminum/magnesium hydride mixture.

Table 1Experimental sample components and weight.

Fig.1.SEM image of high energetic metal powders.

Fig.2.Schematic diagram of the experimental device.

Fig.3.Schematic diagram of experimental setup.

3.2.Cloud explosion process

After 40 ms,the cloud was ignited by the second ignition explosives.The liquid fuel and liquid-solid fuel cloud explosion process were shown in Fig.8.As can be seen from Fig.8,under the effect of the second ignition explosives,the entire cloud of pure liquid fuel exploded in a short time(＜10 ms),as for the liquid-solid mixture,it can be clearly seen that liquid-solid mixture did not explode immediately at the edge of the cloud.It shows that the reaction speed of liquid fuel was faster than that of high energy metal powder.After 90 ms,the explosive fireball formed by pure liquid fuel began to become smaller and the flame gradually disappeared.But for liquid-solid mixtures,the explosive fireball started to become smaller after 140 ms.Combined the pictures of cloud explosion process and the explosion process of non-ideal explosive,the cloud explosion process of liquid-solid mixture can be divided into three stages:First,the vapor and some droplets in the cloud were directly explosion under the excitation of ignition energy;then the remained liquid droplets and some metal powders react under high temperature and pressure,at this stage,a large amount of energy will be released to enhance the blast effect.Finally,the remained metal powders deflagrate or combustion and increase the high temperature duration of the cloud fireball.

3.3.Shock wave overpressure

The shock wave overpressure curve of the experimental test was basically similar.Taking liquid fuel as an example,the overpressure-time curves at 1 m,2 m,3 m,4 m,5 m,and 6 m was shown in Fig.9.As can be seen from Fig.9,there were two peaks on the curve.The pressure values of the two peaks were shown in Tables 2 and 3.It can be seen from Table 2 that the first peak measured values of the four types were not much different.Taking the average value,Fig.10 shows the comparison between the measured value and the calculated value of 160 g TNT and 320 g TNT.In Fig.10,the measured value was between the two calculated values.It indicated that the first peak was caused by the two 160 g TNT charges,the second peak was caused by the cloud explosion.

Because of the volume of the FAE device is fixed,so it can only hold 700 g of liquid fuel.But a large number of experimental data show that the distribution of FAE overpressure still conforms to the explosion similarity law outside the cloud zone[20].In order to analyze the pressure data under the same charge,takingas the abscissa and the average shock wave overpressure as the ordinate,the overpressure generated by the liquid fuel can be fitted by Eq.(1).whereΔpis the shock wave overpressure,kPa;Ris the distance from the explosion site,m;wis the fuel mass,kg.

The overpressure value of liquid fuel at different distances under 1000 g charge can be calculated by Eq.(1).The peak overpressure with different distances at the same charge was shown in Fig.11.

Fig.4.Pictures of fuel dispersion process.

Fig.5.Cloud diameter with change of tim.

Fig.6.FAE and TNT peak overpressure variation with distance.

It can be seen from Fig.11 that the peak overpressure of pure liquid was lowest.After the metal powder was added,the overpressure had been greatly increased,which indicated that the addition of metal powder can effectively increase the shock wave overpressure for FAE.The shock wave overpressure was ranked as ether/aluminum/boron＞ether/aluminum/magnesium hydride ether/aluminum＞ether.For aluminum powder,the combustion flame temperature is 3727°C,which is much higher than the boiling point of 2518°C,so the combustion reaction of aluminum powder conforms to the Glassman criterion.That is,the flame temperature of metal burning in gaseous state(equal to the oxide’s boiling point)must be much higher than the boiling point of the metal[21].Therefore,aluminum powder can be burned in gaseous form and the reaction is more complete under higher temperature condition.Besides,the aluminum powder can react with the explosion products like carbon dioxide and water that effectively promote the explosion effect[22].When the boron and magnesium hydride were added to the aluminum powder,the shock wave overpressure further increased.The heat of combustion of boron is about twice than aluminum powder,but its higher ignition temperature and slower reaction kinetics make its application in condensed-phase explosives not ideal.However,the special reaction mode of Fuel Air Explosive was suitable for the energy release of boron.FAE itself does not contain oxygen.The reaction oxygen comes from the air,and the amount of oxygen is sufficient.The high temperature and high pressure environment formed by the explosion of liquid fuel and aluminum powder can also effectively trigger boron reacts.At the same time,FAE belongs to volume explosion,and the explosion reaction time is longer than condensed phase explosive.The above conditions make it possible to significantly increase the blast effect of FAE by adding boron to aluminum.As a hydrogen storage material,Magnesium hydride releases its stored hydrogen at high temperatures,which is beneficial to the explosion reaction and enhances the blast effect.At the same time,magnesium is also a metal of high heat of combustion,which can increase the explosion heat of the system.Therefore,after adding magnesium hydride to the aluminum powder,the shock wave overpressure increased,but the amplitude was not as good as boron.

3.4.Surface temperature and thermal effect of fireball

Fig.7.Pictures of cloud at 40 ms.

Fig.8.Comparison of cloud explosion process with different components.

Fig.9.Typical pressure history of cloud explosion.

Table 2Overpressure data of first peak.

The infrared thermal imager was used to capture the flame surface temperature after second ignition,the maximum surface flame temperature change curve over time was shown in Fig.12.Taking the average value,the maximum surface flame temperature and duration of high temperature were shown in Table 4.After the FAE explodes,an explosive fireball will form and it lasts for a period of time.It can be seen from Table 4 that the temperature of the fireball reaches 103°C order and the high temperature duration reaches 102ms order.It can meet the ignition and detonation conditions of some flammable and explosive materials.Therefore,FAE also has the characteristics of burning weapons,which is one of the differences between FAE and other condensed phaseexplosives.By with the thermal effects of the explosion,the FAE can effectively attack targets in the jungle,bunkers and trenches.

Table 3Overpressure data of second peak.

Fig.10.The measured value and calculated value with different distances.

Fig.11.The peak overpressure with different distances at the same charge.

Fig.12.The maximum surface temperature of the fireball with different time.

The fireball of pure liquid fuel had the minimum surface flame temperature and high temperature duration(temperature high than 1000°Cand 500°C)caused by low calorific value.When the aluminum powder was added to the liquid fuels,because the aluminum powder was ignited under heating conditions,it will be a lot of energy from this reaction between aluminum and oxygen,increasing the surface temperature of the fireball,and the high temperature will continue for a period of time.The aluminum/boron mixture has the highest fireball surface temperature and the longest duration of high temperature,indicated that when boron participates in the reaction,the high calorific value and postcombustion reaction of boron can effectively improve the thermal effect of FAE.The thermal effect of the ether/aluminum/magnesium hydride mixture was not as good as the ether/aluminum powder.It may be because the hydrogen in magnesium hydride mainly enhances the shock wave effect,and the magnesium had a lower combustion calorific value than aluminum.

The formula proposed by former researcher[23]was used to analyze the thermal effect in this work.The heat energy(Q)and distance(R)from the explosion site as follows:

whereQis the heat radiation energy per unit area,J/m2;wis fuel mass,kg;θis fireball temperature,K;Dis fireball diameter,m;BGis 2.04×104,Fis the constant is 161.7.According to Eq.(2),the picture of heat energy with different distances from explosion site was present in Fig.13.It can be found that the heat energy of the aluminum/boron mixture had the highest thermal effect at each measuring location,the heat energy of the aluminum is second and the aluminum/magnesium hydride mixture is third,the pure liquid fuels has the lowest thermal effect.

4.Conclusion

The dispersion and explosion process of liquid-soild mixture was studied in open field with the influence of different high energetic metal powders.The explosion characteristic and thermal effect were analysed by the high-speed video,pressure data and infrared thermal imager.Experimental results show that in the same volume,after adding aluminum powder to the liquid fuel,the shock wave overpressure and thermal effect of the mixture were greatly improved.The addition of aluminum powder can effectively increase the explosion performance of FAE.After replacing 30% of aluminum powder with boron or magnesium hydride in the ether/aluminum mixture,the shock wave overpressure further increased.The cloud diameter formed by the ether/aluminum/boron mixture was basically the same as the ether/aluminum mixture,but the cloud diameter of the ether/aluminum/magnesium hydridemixture was reduced by 24%.The temperature of the fireball formed by the ether/aluminum/boron mixture was the highest,and the heat energy at different distances was the largest.In the FAE,the addition of boron improves the explosion effect better than magnesium hydride.

Table 4The maximum surface flame temperature and high temperature duration of fireball.

Fig.13.Heat energy with different distances from explosion site.

Declaration of competing interest

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements

This study was supported by the Young Scientists Fund of the National Natural Science Foundation of China(No.11802136).