This paper presents an experiment system in the open field, which comprises a charge structure (approximately 166.2 kg), a high-speed camera subsystem, and a pressure measurement subsystem. Through a series of experiments under the cylindrical clouds with different diameters, heights, and diameter-to-height ratios (
Some fuels, such as hydrocarbons and hydrocarbon oxides, cannot blast when they are stored separately. However, when they are mixed with the ambient air under the dynamic load, they form a certain shape vapor cloud of fuel-air mixtures (fuel-air cloud) and obtain explosive properties. The fuel-air clouds detonate if a powerful igniter source provides the proper energy for the initiation and the concentration of fuel in the cloud reaches the explosive limits. The detonation of fuel-air cloud produces the high overpressure, temperature, and impulse. Such conditional detonation of fuel-air cloud results in a serious explosion accident when the leaked fuel in storage, process, sale, and transport satisfies the formation condition of fuel-air cloud. However, it is an important step in the reformation for routine weapons [
Shock waves are the major hazard in explosion accidents. The overpressure is one of the main parameters to evaluate the risky level of shock waves. The characteristics of the overpressure of fuel-air mixtures have been extensively studied in recent years to recognize the cloud detonation process through accurate modeling and estimation conditions. Therefore, it is important to study the parameters affecting the detonation of the fuel-air cloud resulting in the overpressure field. Up to now, influencing factors of the overpressure of fuel-air cloud have been widely studied. Some works on the enclosed space have paid attention to the influencing factors of the overpressure production process of gas-liquid two-phase, gas-solid two-phase, and gas-liquid-solid three-phase within the vessel in laboratory [
The cloud shapes effectively affect the damage scope of the explosion near the ground [
The experimental system consisted of a charge structure (approximately 166.2 kg) coupled with a high-speed camera subsystem and a pressure measurement subsystem. The experiments were carried out in the open field with the charge structure. The detonator was placed at the center of the bottom cover with the radius of 1.5 m.
The process of change in the shape of cloud was monitored by means of the high-speed camera subsystem and the overpressure data were captured by the pressure measurement subsystem.
Solid-liquid mixtures were chosen as the fuels in the study due to their wide range of explosive limits in air, high damage potential, and high sensitivity to detonation [
Changing rate of various diameters at the fixed distances.
Distance/m | Increase rate% | |
---|---|---|
|
|
|
5–10 | — | — |
15 | 54.76 | 13.33 |
20 | 54.84 | 21.88 |
30 | 48.39 | 21.74 |
40 | 29.17 | 16.13 |
50 | 10.53 | 14.29 |
TNT was shaped into the cylindrical shape by press-fitting method to achieve the density of 1.63 kg/m3 and the total mass of TNT in the central charge is 1.08 kg. TNT (8 kg) in the initiator with the size of
As shown in Figure
Sketch of charge structure (not to scale).
The experimental setup shown in Figure
Experimental layout.
According to different delay time, the fuel-air mixture was used to form the certain cloud shape and the cloud was detonated by a separate initiator charge. The test parameters are as given below: canister stand-off (1.5 m), initiator stand-off (2.8 m), the interval between canister and booster (1.5 m), and delay of initiator charge [TNT] (240~280 ms).
The detonations were monitored by means of the pressure measurement subsystem composed of the pressure testing device and synchronization control device. The pressure testing device was composed of Kistler pressure sensors, sensor adapters, and data collection unit for storing the voltage values. Synchronization control device sent the order to the data collection systems to record the electricity signal as soon as it detected the detonation signal and the recorded data were stored in the hard disk. The pressure sensors were mounted on the steel plate with the diameter of 0.3 m and the plate surface was parallel to the ground. The main mounting lines were arranged from the charge center in four directions and the distances between the charge center and the sensors were, respectively, 5 m, 8 m, 10 m, 15 m, 20 m, 30 m, 40 m, and 50 m (Figure
For the detonation with a given cloud shape, the measured pressure varies with time and reaches the peak values in the front of pressure wave. The peak values are defined as the peak overpressure. With the measurement subsystem, the overpressure can be obtained on the basis of the given sensor’s sensitivity and magnification through calculation. The relationship of voltage and overpressure is expressed as
In order to measure the overpressure accurately, it is important to check and arrange all the devices before the experiments (Figure
After checking the devices, the fuse was triggered via an external signal firstly and the center charge then produced a powerful blast. The blast caused the shell damage, thus dispersing the fuel to form the certain fuel-air cloud in the air. After the delay of 240~280 ms, the fuel-air cloud was initiated by the detonator. Meanwhile, the pressure measurement subsystem collected the data and the experimental photos were taken by the high-speed cameras.
The data of cloud dispersal process were acquired by a V12 high-speed color photography system. The photographing frequency was 1000 frames per second. Recording time was no less than 2 s and the resolution ratio of each photo was 1280 × 720.
The fuel was dispersed by central charge. After a certain period, fuel-air mixture formed cylindrical fuel-air clouds. According to the characteristics of cylindrical clouds, the diameter (
Figure
Photos of cylinder clouds taken by a high-speed camera.
Diameter-to-height ratios
The experiments of fuel-air mixture explosions were carried out under the same ignition conditions: relative humidity of 90%, initial temperature of fuel-air mixture at 26°C, and the wind speed less than 5 m·s−1.
The cylinder shapes of the cloud with different diameters at the same cloud height of 4 m were chosen and compared with each other. The peak overpressure of each experiment was calculated as the average of the measured values at four points with the identical distance from the explosion center. In the typical results of experiments in Table
Figure
Comparison of overpressure under various diameters versus different distances.
The explosive cloud ignited by the powerful source immediately generated the detonation waves. When the detonation waves traveled to the ground along the propagation direction, it caused a Mach reflection which accelerated chemical reactions and raised the temperature. As a result, the detonable cloud produced the higher explosive overpressure and obtained the maximum overpressure. Then the detonation waves stably propagated. When it spread to the cloud border, detonation waves were attenuated to shock waves. According to the distribution law of peak overpressures, the overpressure field is divided into two zones: detonation wave zone and shock wave zone. In the experiment, in the detonation wave zone, the peak overpressure of fuel-air cloud was slightly changed slightly within the testing height range from 0 m to 10 m. In shock wave zone, the peak overpressures decrease significantly in the testing height range from 10 m to 50 m.
For the purpose of comparison, peak overpressures obtained within the same distance in different diameters are also shown in Figure
Table
The higher overpressure was produced in the larger diameter at the same distance and the decrease rate was smaller than that in the near field of shock waves. However, the decrease rates were consistent with each other in the far field of shock waves. For example, the overpressures within the diameter range from 22.7 m to 23.9 m were close to that at the distance of 50 m, 0.02 MPa. As a result, the overpressures in the far field of shock wave showed no obvious difference among different diameters.
The experimental results on the overpressure at different heights (3.8 m, 3.9 m, and 4.1 m) were obtained, respectively. The overpressures determined at the same distance are shown in Figure
Comparison of overpressure under various heights versus different distances.
As shown in Figure
Comparison of the peak overpressure under various heights with the same diameter of fuel-air cloud showed that the peak overpressure in the shock waves varied with the height. Although the peak overpressure varied with the height, the gaps among different groups decreased with the propagation of shock waves. In the shock wave zone, the peak overpressures gradually became close to each other. Therefore, compared with the diameter, the height within the studied range had little impact on the peak overpressure in the shock wave zone. According to the experimental result, the peak overpressure reached the highest value under the height of 3.9 m.
In order to expand the explosion scope, during the selection of the cloud shape, the following two points can be referenced. Firstly, under the pancake-shaped cloud, the range of overpressure field can be increased. Secondly, the larger diameter can realize the purpose within the certain height range.
According to the experiment results of cylindrical cloud with different diameters and heights, the pancake-shaped cloud with the larger diameter can boost the impact range of overpressure. Thus, the main characteristics of the overpressure field are largely dependent on the
The pancake-shaped clouds with the same volume and fuel mass were ignited at various
Peak overpressure with
Distance/m |
|
Rate | |||
---|---|---|---|---|---|
5.6 | 5.7 | 5.8 | Increase% |
Decrease% |
|
5–10 | 0.716 | 0.943 | 0.815 | — | — |
15 | 0.126 | 0.132 | 0.125 | 4.76 | 5.30 |
20 | 0.062 | 0.075 | 0.073 | 20.97 | 2.67 |
30 | 0.031 | 0.047 | 0.046 | 51.61 | 2.13 |
40 | 0.025 | 0.032 | 0.028 | 28.00 | 12.50 |
50 | 0.019 | 0.024 | 0.021 | 26.32 | 12.50 |
According to the above result, the
The Netherlands Organization Multi-Energy Model (TNO MEM) [
The TNO MEM implies that
Assuming that the behaviors of the fuel mixture are similar to those of diethyl ether, the peak overpressure within a certain distance can be estimated by TNO MEM method. The changes of overpressure with distance were computed according to (
Comparative experimental and theoretical overpressure of cloud detonation for different sizes cloud.
Distance/m | Experimental overpressure/MPa | TNO MEM overpressure/MPa | |||||
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Cloud shape | |||||||
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| ||
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0.126 | 0.195 | 0.221 | 0.126 | 0.12 | 0.124 | 0.31 |
|
0.062 | 0.096 | 0.117 | 0.062 | 0.064 | 0.066 | 0.068 |
|
0.031 | 0.046 | 0.056 | 0.034 | 0.038 | 0.039 | 0.042 |
|
0.024 | 0.031 | 0.036 | 0.025 | 0.026 | 0.028 | 0.031 |
|
0.019 | 0.021 | 0.024 | 0.019 | 0.019 | 0.021 | 0.020 |
In the paper, we studied the influence of the shape of cloud on detonation parameters. The main conclusions can be summarized as follows: According to the changing trend of the overpressure of fuel-air cloud, the range of overpressure was divided into the detonation wave zone and the shock wave zone. In the detonation wave zone, the maximum overpressure was acquired and the peak overpressures decreased significantly in the shock wave zone. The decrement of peak overpressure was between 82.40% and 20.83%. The overpressure of cylindrical fuel-air cloud at the same distance increases with the increase in the diameter. However, the height of cloud shows little impact on the overpressure in the shock wave zone. Therefore, increasing the diameter of cylindrical cloud can increase the overpressure range under the same conditions. Based on the influences of the diameter and height of cylindrical cloud on the overpressure, the explosion of pancake-shaped cloud of fuel-air mixtures realizes the larger overpressure range. Though the pancake-shaped cloud produces the larger range of overpressure, the influencing range of the overpressure is affected by diameter-to-height ratio
The study results can be applied in the industry and military. In the military, the tendency for fuel-air clouds to form pancake shapes could significantly enhance the influencing range of the overpressure effect. In the industry, the factors of cloud shape should be included in hazard assessments and hazard analysis when the large quantities of fuel were handled during the storage, processing, sale, and transport.
The authors declare that they have no competing interests.