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. 2024 Mar 14;9(12):14539–14550. doi: 10.1021/acsomega.4c00555

Study on the Synergistic Suppression Effect and Mechanism of N2/Ultrafine Water Mist on Liquefied Petroleum Gas Explosion

Bei Pei 1,*, Yuliang Han 1, Liwei Chen 1, Ziwei Hu 1, Zhiqi Wu 1, Hang Lv 1, Wentao Ji 1
PMCID: PMC10976399  PMID: 38559942

Abstract

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Liquefied petroleum gas (LPG) is widely used for its cleanliness and high efficiency in industry and city life. In order to improve the suppression effect on LPG explosion, a constant volume combustion bomb was used to investigate the synergistic influence of N2/ultrafine water mist on the explosion and combustion characteristics of 6% premixed LPG/air gas. The results showed that (1) the effect of a single ultrafine water mist on suppressing LPG explosion is unstable. When the concentration of ultrafine water mist is low, the flame acceleration in the initial stage of explosion is promoted, and when the ultrafine water mist with a mass fraction of 420 g/m3 is introduced, the maximum pressure rise rate increases. (2) The combination of N2/ultrafine water mist has a synergistic effect on LPG explosion. Compared to the individual suppression effects, the combination of N2/ultrafine water mist showed more effective suppression on the explosion pressure, flame propagation, and flame instability of LPG explosion. (3) Through the mechanism analysis, it is found that the combined action of N2/ ultrafine water mist can better reduce the mole fraction and ROP peak of active free radicals such as H, O, and OH by inhibiting the main reaction of generating H, O, and OH radicals during the explosion of LPG, resulting in the reduction of flame free radicals in the explosion system, thus effectively inhibiting the chain reaction of ignition and explosion of LPG. This research can provide guidance for a better understanding and implementation of gas–liquid two-phase suppression technology for LPG explosion.

1. Introduction

Liquefied petroleum gas (LPG) is a basic chemical raw material and clean fuel that is widely used in industrial and civil applications.1,2 In recent years, under the guidance of the “dual-carbon goal”,3,4 LPG has been further used in production and city life due to its cleanliness and low cost. However, because of the high specific gravity and high chemical activity of LPG, it can easily accumulate in low-lying areas after leakage. Once ignited, it poses a high risk of explosion and combustion accidents. In recent years, both domestic and international incidents involving LPG explosions and fires have occurred with increasing frequency. For instance, the truck tanker accident in India in 2012 resulted in substantial property damage and casualties.5 In 2020, the “6.13” tanker explosion accident in Wenling caused 20 deaths, and 172 people were hospitalized.6 Therefore, conducting research on LPG explosion characteristics and explosion suppression techniques holds great significance for industrial and urban safety.

Currently, research on the explosion characteristics of LPG primarily concentrates on the explosion pressure, flame propagation characteristics, and influencing factors. In terms of explosion characteristics, Wang et al.7 conducted tests on the explosive characteristics of gasoline–air mixtures in a 907.5 L oil tank. They observed that with an increase in gasoline concentration the maximum explosion pressure and maximum pressure rise rate initially increased and then decreased within a certain range. Furthermore, it was noted that the maximum explosion pressure exhibited a linear increasing trend, while the maximum pressure rise rate showed a logarithmic increasing trend with increasing heat energy. Huo and Chow8 carried out a flame propagation study of an in-tube premixed LPG explosion. Their findings indicated that the flame propagation velocity at the explosion point depends on both the turbulent combustion velocity and the expansion ratio. Razus et al.9 revealed that the addition of dilution inert gas can mitigate the severity of LPG explosions and decrease the flame front speed by extending the duration of heat loss during combustion. Wang and Liang and Wang et al.10,11 observed that the addition of hydrogen increased the sensitivity coefficient of reactants C3H8 and C4H10, the maximum ROP of free radicals H, O, and OH, and the explosive strength of the reaction. For the LPG/H2/air mixture, adding N2 and CO2 can inhibit its explosion strength, and the explosion suppression effect of CO2 is better than that of N2. It is found by simulation software that the addition of N2 and CO2 reduces the maximum ROP of free radicals H, O, and OH. Wang et al.12 obtained through experiments that repeatedly placing rectangular obstacles increased the maximum explosion pressure and the revised deflagration index, shortened the time to reach the maximum pressure, and significantly elevated the explosion hazard level of LPG–air mixtures.

At present, inert gases such as N2 and CO2 are commonly employed in the research on LPG explosion suppression technology. Kai et al.13 pointed out that both N2 and CO2 provide good inhibition of syngas/air explosions. Shang et al.14 concluded from their experiments that the inhibition effect of CO2 and N2 is primarily due to a thermal effect, meaning that the inhibition effect is achieved by reducing the temperature. Li et al.15 found that CO2 is obviously superior to N2 in suppressing the explosion of CH4- air mixtures in a closed space. Kumar and Mishra et al.16 observed that the introduction of N2 resulted in an increase in flame length across all lip thickness, with flame length decreasing as lip thickness increased. Du et al.17 conducted experimental research on the suppression of nonpremixed gasoline–air mixture explosions in a confined tunnel using N2. Their results indicated that under nonpremixed suppression conditions, both the maximum pressure value and the rate of pressure rise during the explosion were lower compared to conditions without nonpremixed suppression. Luo et al.18 discovered that the inclusion of N2 and CO2 led to a reduction in the maximum pressure, maximum pressure rise rate, and average flame propagation speed during LPG explosions. Furthermore, they observed that the inerting effect became more pronounced with higher concentrations of these gases. Tu et al.19 researched the inhibitory effect of N2 on free radicals in the reaction process by numerical simulation. They discovered that the addition of N2 significantly reduced the concentration of free radicals, thereby inhibiting the explosion.

Water mist dilution is a common method for suppressing LPG explosions. Water mist is cost-effective and readily accessible.20 It has no environmental pollution, possesses robust heat absorption and cooling capabilities, and can promptly block and reduce thermal radiation. Additionally, it offers advantages in isolating oxygen and suppressing explosions.21 As a result, this method has garnered increasing attention from scholars both domestically and internationally. Li et al.22 found that water mist can significantly reduce the deflagration of propane/air, and the characteristics of the spray and the nozzle’s installation position have a significant impact on the explosion-proof effect. Xu et al. and You et al.23,24 found that the temperature, delay time, flame speed, and overpressure of methane explosions are significantly reduced in the presence of ultrafine water mist. Ananth et al.25 discovered that the inhibitory effect of fine water mist is primarily due to latent heat absorption, followed by sensible heat absorption by water vapor (and droplets) and momentum absorption by subdroplets into high-velocity gases.

Pei et al.26−28 found that the combination of inert gas and fine water mist can prevent the explosive phenomenon that may occur with fine water mist alone. The two together exhibit a synergistic inhibitory effect. N2/ultrafine water mist has a synergistic inhibitory effect on gas explosions, and this inhibitory effect tends to stabilize as the concentration of the N2/ultrafine water mist increases. Cao et al.29 found that the addition of NaCl to ultrafine water mist can further enhance the explosion suppression effect. Holborn et al.30,31 conducted research indicating that the combination of high-density water mist and nitrogen is an effective approach to reduce the explosion intensity of hydrogen, and this combined method outperforms a single water mist.

In summary, it has been observed that the combination of inert gas and ultrafine water mist can significantly enhance the explosion suppression effectiveness for combustible gases, such as methane. However, the literature primarily focuses on single-component combustible gases such as methane and hydrogen. Since LPG consists of propane, butane, and other combustible components and its specific gravity exceeds that of air, it is imperative to investigate the explosion suppression characteristics of gas–liquid two-phase LPG detonation for the safety of the petrochemical industry. This study takes N2/ultrafine water mist as an example to comparatively examine the suppression characteristics of gas–liquid two-phase explosion suppression on LPG explosion using a constant volume combustion bomb. Additionally, the reaction mechanism of the N2/ultrafine water mist was analyzed using CHEMKIN-Pro software. The research findings will offer valuable technical insights for LPG explosion suppression in both petrochemical production and urban domestic gas utilization.

2. Experimental Section

2.1. Experimental System

As shown in Figure 1, the experimental setup comprises key components, including a constant volume combustion bomb, a high-speed schlieren system, an ignition system, a gas distribution system, an ultrafine water mist generation and distribution device, a data acquisition system, and a synchronized control system.

Figure 1.

Figure 1

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Diagram of experimental equipment.

The constant volume combustion bomb consists of a double-layer stainless steel sphere with a volume of 20 L and a pressure measurement range of 0–2.0 MPa. The model of the high-speed camera is a Speed Sense VEO 710, the pixel is set to 1280 × 700 resolution, and the shooting frequency is 5000 fps. The schlieren model is CQW300, which is mainly composed of a light source, a slit, a small reflector, a main reflector, and a knife edge. The high-speed camera and schlieren instrument can capture and photograph the explosion flame and analyze the flame propagation characteristics. The ignition system consists of a spark generator and an ignition electrode. In this experiment, the electrode spacing is 3 mm, and the ignition delay time is 100 ms. The gas distribution system consists of an air compressor, gas valve, gas pipe, gas cylinder, GM-240200 mass flow controller (MFC), etc., and the intake is controlled by a computer. The ultrafine water mist generating and conveying system consists of an atomizing device, a sealed square water storage box, a pipeline, etc. The ultrasonic atomizing device adopts a three-head all-copper atomizer, and the working frequency of the atomizing blade is 1700 kHz. After many measurements, the average generation rate of the ultrafine water mist generating system is 11.2 g/min, and the ultrafine water mist is introduced into a constant volume combustion bomb through air. However, gaseous H2O was used in the CHEMKIN-Pro calculation because liquid H2O cannot be calculated with CHEMKIN-Pro.32 The average particle size of ultrafine water mist used in the experiment is 6 μm, which evaporates into a gaseous state during the reaction, so this paper sets ultrafine water mist as gaseous water. The volume fraction of H2O (CO2), φ, is defined as

2.1. 1

Here, X refers to the volume of the specific species in the mixtures. The volume of the ultrafine water mist is 10%, and the N2 volume is 8%.

2.2. Experimental Methods

To analyze the effects of different N2 and ultrafine water mist concentrations on the explosion as well as the flame propagation characteristics of LPG, the experimental gas was prepared by the China Jiyuan LPG Company. LPG consists of 30% propane and 70% butane by volume. The volume fraction of LPG was set at 6%, setting the N2 flow rate at 8, 16, and 24% (volume fraction), and the ultrafine water mist flow time at 15, 30, and 45 s. It can be calculated according to the output rate of ultrafine water mist of 11.2 g/min, and the corresponding mass concentrations are 140, 280, and 420 g/m3. The purity of N2 reached 99.9%.

During the experiment, the constant volume combustion bomb was first evacuated to negative pressure by using a vacuum pump. Subsequently, LPG and N2 were introduced sequentially following the Dalton partial pressure method. Ultrafine water mist was introduced into the constant volume combustion bomb along with air, and the intake rate was regulated by a mass flow controller. The introduction time was controlled using the switch on the ultrafine water mist generator, after which the generator was turned off. Finally, air was added to equalize the internal pressure of the constant volume combustion bomb with the atmospheric pressure. Once the gas inlet was completed, the mixture of sample gas and water mist was allowed to stand for 30 s to ensure even mixing. Ignition was computer-controlled, and the pressure acquisition system and picture acquisition system were simultaneously triggered to record the explosion pressure and images. After the experiment, the constant volume combustion bomb was purged with four times the volume of air to remove waste gas and ensure that the container was dry. To guarantee the accuracy of the experimental data, each set of working conditions was repeated five times. All the tests were carried out in the environment of P = 0.1 MPa and T = 298.3 K.

2.3. Data Processing

In this paper, the radius of a spherical flame is calculated by the equal area method.32,33 The stretched flame propagation velocity, Sn, can be calculated by

2.3. 2

The stretching rate α and the length of Markstein are obtained. The stretch ratio is the derivative of the logarithmic value of an infinitesimal area on a flame with respect to time, and the calculation method is as follows

2.3. 3

At the initial stage of flame propagation, there is a time when the pressure changes little. At this time, the propagation speed Sn of the stretching flame and the flame stretching rate α can be approximately linear

2.3. 4

Sl represents the nonstretched flame propagation velocity, and Lb denotes the Markstein length. The Markstein length reflects the flame’s sensitivity to stretching, with values less than zero indicating flame instability and values greater than zero indicating flame stability.

3. Analysis and Discussion

3.1. Influence on LPG Explosion Pressure

3.1.1. Influence of a Single Inhibitor

The explosion pressure is a crucial parameter that reflects explosion characteristics, and the rate of pressure rise effectively demonstrates the explosion strength. Figure 2 shows the maximum explosion pressure (Pmax), time to reach the maximum explosion pressure (ta), maximum pressure rise rate [(dP/dt)max], and time to reach the maximum pressure rise rate (tb) of 6% LPG exploded by N2 with different volume fractions. It was observed that as the volume fraction of N2 increased, both Pmax and (dP/dt)max gradually decreased, while ta and tb gradually increased. This is consistent with previous research results.18

Figure 2.

Figure 2

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Effects of N2 on Pmax, ta, (dP/dt)max and tb of 6% LPG explosion.

Figure 3 depicts the effects of varying concentrations of ultrafine water mist on the Pmax, ta, (dP/dt)max, and tb of a 6% LPG explosion. As shown in Figure 3, within the test concentration range, the decrease in Pmax remains within 20%, and the explosion suppression effect of the ultrafine water mist is inferior to that of N2. Upon the introduction of 140 and 280 g/m3 ultrafine water mist, ta increases by 38.2 and 28.1%, (dP/dt)max decreases by 28.7 and 32.5%, and tb extends by 32.4 and 6.4%, respectively. However, when the concentration of ultrafine water mist is further raised to 420 g/m3, ta is 12.5% earlier, (dP/dt)max increases by 17.3%, and tb is 24.5% earlier. Although ultrafine water mist can reduce Pmax, there is no clear pattern for (dP/dt)max, ta, and tb, suggesting that the suppression effect of ultrafine water mist on LPG explosion does not improve with increasing mass concentration because the use of a single ultrafine water mist can lead to vapor-pressure-induced explosion suppression instability. Cao et al.34 demonstrates that steam pressure, as part of the explosion pressure in a closed container, influences the overall explosion pressure.

Figure 3.

Figure 3

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Effects of ultrafine water mist on Pmax, ta, (dP/dt)max, and tb of a 6% LPG explosion.

3.1.2. Influence of N2/Ultrafine Water Mist Gas–Liquid Two-Phase Inhibitor

To elucidate the impact of N2/ultrafine water mist on the pressure of a 6% LPG explosion, this study selected the combination of N2/ultrafine water mist with a mass concentration of 280 g/m3 for analysis Figure 4 illustrates the effect of N2/ultrafine water mist on Pmax, ta, (dP/dt)max, and tb of 6% LPG. According to Figure 4, when the N2/ultrafine water mist gas–liquid two-phase explosion suppressant is applied, the explosion suppression effect is significantly enhanced. When 280 g/m3 of ultrafine water mist is combined with 24% N2, Pmax decreases to 0.427 MPa, and ta extends to 690.8 ms. Compared to the 6% LPG explosion alone, Pmax and (dP/dt)max decrease by 44.5 and 92.8%, respectively, while ta and tb increase by 310.7 and 57.2%, respectively. It is evident that when both are combined, the reduction in Pmax and (dP/dt)max is more pronounced, and ta and tb experience significant delays. This indicates a synergistic enhancement in the pressure suppression effect of N2/ultrafine water mist on LPG explosion when used together, surpassing the effect of each as a single inhibitor and avoiding the situation that (dP/dt)max increases and the unstable explosion suppression effect under the condition of a single ultrafine water mist, and its suppression effect on LPG explosion pressure is significantly better than that of a single inhibitor. This improvement is attributed to the prolonged residence time of ultrafine water mist in the flame zone due to N2 inertization, leading to enhanced evaporation heat absorption, dilution, and a further reduction in combustion rate, resulting in superior LPG explosion suppression.28

Figure 4.

Figure 4

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Effects of N2/ultrafine water mist on Pmax, ta, (dP/dt)max, and tb of a 6% LPG explosion.

3.2. Influence on Flame Propagation Characteristics during the Initial Stage of LPG Explosion

When combustible gases burn in enclosed containers, an interaction occurs between pressure wave reflections and the combustion wave, which can alter the flame propagation characteristics. Therefore, it is essential to investigate the impact of detonation suppressants on flame instability during LPG explosions. To thoroughly study the effect of N2/ultrafine water mist on flame propagation in the early stage of LPG explosion, the change in the flame propagation characteristics in the early stage of flame propagation from the moment of ignition to the moment when the flame develops to the size of the window as well as the moment when the flame is self-accelerating (i.e., the moment when the cellular structure is generated) is analyzed.

3.2.1. Influence of a Single Inhibitor

3.2.1.1. Flame Propagation Speed

Figures 5 and 6 depict the effects of N2 and ultrafine water mist on the flame propagation speed of a 6% LPG explosion, respectively. The changes in flame propagation speed during LPG explosions can be divided into two periods: I is the ignition phase, during which the flame accelerates significantly in the initial ignition stage; II is the flame development phase, where the flame expands from the center, forming a spherical flame that spreads outward and gradually fills the entire window. During this phase, the flame speed gradually stabilizes. Concurrently, the flame propagation speed decreases progressively with the increase of the volume fraction of N2. The flame speed of the 6% LPG explosion eventually stabilizes at approximately 1.8 m/s. However, when 24% N2 is introduced, the flame propagation speed significantly decreases and stabilizes at approximately 0.7 m/s, which is 61.1% lower than that of the 6% LPG explosion. This demonstrates that N2 can reduce the flame propagation speed and inhibit LPG explosions. In contrast, under the influence of ultrafine water mist alone, the flame propagation speed during the LPG explosion does not decrease with increasing concentration. When the concentration of ultrafine water mist is 280 and 420 g/m3, the flame propagation speed stabilizes at approximately 1.7 and 1.5 m/s, representing only an 5.6 and 16.7% decrease compared to the 6% LPG explosion. However, at concentrations of 140 g/m3 ultrafine water mist, the flame propagation speed increases and the flame velocity fluctuates significantly. This indicates that the inhibitory effect of a single ultrafine water mist on the flame propagation velocity of LPG is not evident, and its inhibition effect on LPG explosions is unstable.

Figure 5.

Figure 5

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Effects of N2 on the flame propagation speed of 6% LPG.

Figure 6.

Figure 6

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Effects of ultrafine water mist on the flame propagation speed of 6% LPG.

3.2.1.2. Flame Structure

Figure 7 depicts the spherical flame propagation images of 6% LPG at various moments under the influence of N2. These images show the ignition time (1 ms), 6 ms, midpoint, time of reaching the edge of the window, formation time of the cell flames, and flame images in the later stages of the explosion. Taking the example of a 6% LPG explosion, after ignition, the flame expanded outward in a spherical shape, spreading in a laminar state, with cracks and folds appearing on the flame’s surface. At 13 ms, the flame reached the midpoint, with a radius half that of the spherical radius, exhibiting a cellular structure. By 26 ms, the flame reached the edge of the window. Compared to the earlier flame structure, more wrinkles appeared on the flame’s surface, and the cellular structure became more pronounced. Large cells gradually differentiated into smaller cells. At 52 ms, a complete cellular structure emerged, and the flame exhibited cellular instability. The surface area of the flame increased, enhancing convection and heat transfer between the flame surface and unburned gas. This led to an increased combustion rate and self-acceleration, ultimately intensifying the explosion. Figure 7 also illustrates the impact of different volume fractions of N2 on the flame structure of LPG explosions. With the addition of 8% N2, the time for the flame to reach half of the window extended to 15 ms, and reaching the edge of the window took 31 ms. Complete cells appeared at 85 ms, which shows that 8% N2 can weaken the flame acceleration in the initial stage of ignition and the initial explosion intensity of LPG.35 Similarly, when introducing 16 or 24% N2, the flame arrival time was further delayed, the number of cellular structures decreased, surface wrinkles were reduced, and the flame exhibited slight floating. The suppression effect improved with the increase in the volume fraction of N2. This indicates that N2 can inhibit the flame propagation of 6% LPG and reduce its explosion intensity.

Figure 7.

Figure 7

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Schlieren image of the effect of N2 on the 6% LPG explosion flame at different times.

Figure 8 displays schlieren images of the spherical flame propagation of a 6% LPG explosion at various times under the influence of an ultrafine water mist. In comparison to a single N2, the impact of ultrafine water mist on the cellular structures is less pronounced. When 280 and 420 g/m3 ultrafine water mist was introduced, the flame exhibited a delay, and the number of cell structures decreased. However, with the introduction of 140 g/m3 ultrafine water mist, the flame reached each stage earlier, indicating acceleration of the flame and intensified flame propagation. This further suggests that the influence of a single ultrafine water mist on the explosion flame structure is unstable, and when the amount of ultrafine water mist is insufficient, it promotes flame propagation.

Figure 8.

Figure 8

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Schlieren image of the effect of ultrafine water mist on the 6% LPG explosion flame at different times.

3.2.1.3. Flame Acceleration Characteristics

According to Figure 9, when the volume fraction of N2 is 8, 16, and 24%, the unstretched flame propagation speed of LPG at the initial stage of the explosion decreases to 1.55, 1.34, and 0.75 m/s, respectively. As the volume fraction of N2 increases, the range of flame elongation gradually narrows and the Markstein length increases. The Markstein length of the 6% LPG explosion flame was 0.11 mm, and after adding 8, 16, and 24% N2, the Markstein length increased to 0.20, 0.35, and 0.59 mm, representing increases of 81.8, 218.2, and 436.4%, respectively. This indicates that N2 has a significant inhibitory effect on the flame propagation of LPG explosion, and the inhibitory effect increases with the increase of the volume fraction of N2.

Figure 9.

Figure 9

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Relationship between flame propagation speed and elongation under different volume fraction of N2.

3.2.2. Impact of N2/Ultrafine Water Mist Gas–Liquid Two-Phase Inhibitor

3.2.2.1. Flame Propagation Speed

Figure 10 illustrates the impact of N2/ultrafine water mist on the flame propagation speed of a 6% LPG explosion. It is evident that when they are used in conjunction, the flame propagation speed decreases more significantly in comparison to using N2 or ultrafine water mist as standalone inhibitors. When 24% N2 and 280 g/m3 ultrafine water mist are introduced, the flame propagation speed stabilizes at approximately 0.5 m/s, which is 69% lower than that of a pure 6% LPG explosion. A comprehensive comparison reveals that the N2/ultrafine water mist exhibits a synergistic effect in reducing the flame propagation speed of a 6% LPG explosion.

Figure 10.

Figure 10

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Effects of the N2/ultrafine water mist on the flame propagation speed of a 6% LPG explosion.

3.2.2.2. Flame Structure

N2 and 280 g/m3 ultrafine water mist were selected to act together to further analyze the effect of two-phase inhibitors on the flame structure in the early stage of LPG explosion. The flame propagation images shown in Figure 11 reveal that when used in combination, N2/ultrafine water mist results in a further reduction in flame propagation speed and delayed arrival times at various stages compared to using N2 or 280 g/m3 ultrafine water mist alone. In the case of 24% N2 and 280 g/m3 ultrafine water mist used together, for instance, when 24% N2 or 280 g/m3 ultrafine water mist is applied alone, the flame reaches the edge of the window in 60 and 28 ms, respectively. The fullest cellular structure appeared at 166 and 61 ms, respectively. When both are used together, the flame arrival at the edge of the window is extended to 90 ms, the flame cellular structure essentially disappears, and the flame exhibits a floating phenomenon. The self-acceleration phenomenon of the flame in the initial stage of ignition is further weakened with the increase of volume fraction of N2, thus reducing the initial explosion intensity of LPG explosion, enabling the ultrafine water mist to better play its physical heat absorption and chemical explosion suppression functions,35 and avoiding the influence of a single ultrafine water mist on the flame structure.

Figure 11.

Figure 11

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Schlieren image and the effect of N2/ultrafine water mist on the 6% LPG explosion flame at different times.

3.2.2.3. Flame Acceleration Characteristics

According to Figure 12, under the combined effect of N2/ultrafine water mist, the flame propagation speed gradually decreases, the flame stretch rate gradually decreases, and the corresponding stretch rate range gradually narrows. Additionally, the Markstein length shows an increasing trend. Compared to a single inhibitor, two-phase inhibitors have the most significant impact on increasing the Markstein length. When 8, 16, 24% N2, and 280 g/m3 of ultrafine water mist are combined, the unstretched flame propagation velocity decreases to 1.47, 1.19, and 0.60 m/s, respectively. Furthermore, the Markstein length increases to 0.40 0.69, and 1.13 mm, respectively. Compared to 6% LPG, the Markstein length increases by 263.6, 527.3, and 927.3%, respectively, which is a greater increase than that achieved with a single inhibitor. This indicates that the combined application of N2/ultrafine water mist is more effective in suppressing flame instability during the initial stages of LPG explosion compared to using a single inhibitor. This observation aligns with the results analyzed from the flame structure images above.

Figure 12.

Figure 12

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Relationship between flame propagation velocity and elongation under different N2/ultrafine water mist concentrations.

3.3. Analysis of Synergistic Effects

Table 1 provides a comprehensive comparison of the synergistic suppression effect of N2/ultrafine water mist on 6% LPG explosions. From Table 1, it is evident that compared to the effect of a single inhibitor, the gas–liquid two-phase inhibitor demonstrates a more significant suppression effect on explosion pressure, with larger reductions in Pmax and (dP/dt)max. Simultaneously, there is a greater decrease in flame propagation speed and a more substantial increase in Markstein length, indicating a more pronounced effect on enhancing flame stability. In summary, N2/ultrafine water mist exhibits a synergistic effect on suppressing LPG explosions, and their combined action results in superior explosion suppression. Taking 16% N2 and 280 g/m3 ultrafine water mist as an example. Pmax decreased by 10.3 and 10.5% when N2 or ultrafine water mist acted alone but decreased by 34.8% when they were employed together, which was higher than the sum of the two reduction ranges. ta was extended by 72.7%, while the extension ranges of single N2 and ultrafine water mist were 26.6 and 28.1%, respectively. (dP/dt)max also shows the same law; under the combined application of N2/ultrafine water mist, (dP/dt)max decreases by 77.3%, while the decreasing ranges of single N2 or ultrafine water mist are 18.6 and 32.5%, respectively, which are greater than the sum of the decreasing ranges of N2/ultrafine water mist. N2 and ultrafine water mist have obvious synergistic effects on Pmax and (dP/dt)max. Similarly, the combined application of a N2/ultrafine water mist has a synergistic effect on reducing the flame propagation speed and increasing the Markstein length. When N2 ultrafine water mist is employed together, the flame propagation speed decreases by 44.4%, while when N2 or ultrafine water mist is used alone, the decreases are 27.8 and 5.6%, respectively, which is greater than the sum of the decreases under the action of a single inhibitor, and the Markstein length increases by 527.3%, while when N2 or ultrafine water mist is used alone, the decreases are 218.2 and 200%, respectively, which is greater than the sum of the increases under the action of a single inhibitor.

Table 1. Changes in Explosion Parameters under the Action of a N2/Ultrafine Water Mist.

inhibitory composition variation range of Pmax/% variation range of ta/% variation range of (dP/dt)max/% variation range of tb/% variation range of flame propagation speed/% variation range of Markstein length/%
0            
8% N2 –5.9 +13.2 –11.2 +9.6 –16.7 +81.8
16% N2 –10.3 +26.6 –18.6 +24.1 –27.8 +218.2
24% N2 –18.8 +139.4 –71.5 +90.8 –61.1 +436.4
140 g/m3 H2O –8.2 +38.2 –28.7 +32.4 +38.9  
280 g/m3 H2O –10.5 +28.1 –32.5 +6.4 –5.6 +200.0
420 g/m3 H2O –14.3 –12.5 +17.3 –24.5 –16.7  
8% N2 + 280 g/m3 H2O –18.1 +31.9 –46.9 +17.9 –22.2 +263.6
16% N2 + 280 g/m3 H2O –34.8 +72.7 –77.3 +27.6 –44.4 +527.3
24% N2 + 280 g/m3 H2O –44.5 +310.7 –92.8 +57.2 –72.2 +927.3
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4. Analysis of Inhibition Mechanisms

In order to explore the principle, N2/ultrafine water mist combined suppression effect on LPG explosion. Utilizing CHEMKIN-Pro simulation software, a simulation study was conducted using the laminar combustion calculation module to simulate the explosion of C3H8 and C4H10 (the main components of LPG, the ratio is 3:7) gas mixtures under the impact of N2/ultrafine water mist. The reaction kinetic mechanism used in this study is GRI Mech3.0, which encompasses 53 components and 325 elementary reactions. Table 2 lists the primitive reactions corresponding to the reaction steps shown in Figure 13. Previous studies have shown that this reaction kinetics mechanism is superior to other reaction models and can elucidate the influence of elementary reactions on the reactants during the reaction process.32 However, due to the complexity of analyzing all elementary reactions, this paper focuses on analyzing the key elementary reactions.

Table 2. Main Reaction Steps and Basic Reactions of Free Radical Concentration.

reaction step basic reaction reaction step basic reaction
R1 O2 + H ⇔ O + OH R2 O+ H2 ⇔ H+ OH
R3 OH + H2 ⇔ H + H2O R4 2OH ⇔ O + H2O
R16 HO2+H ⇔ 2OH R31 OH + CO2 ⇔ CO2 + H
R40 HCO + H2O ⇔ CO + H + H2O R59 O2 + CH2 ⇔ HCO + OH
R88 CH3 + H(+M) ⇔ CH4(+M) R89 CH3 + O ⇔ H + CH2O
R123 CH4 + H ⇔ CH3 + H2 R124 CH4 + O ⇔ CH3 + OH
R125 OH + CH4 ⇔ CH3 + H2O R149 O2 + HCCO ⇔ OH + 2CO
R155 C2H3(+M) ⇔ C2H2 + H(+M) R157 C2H2 + O ⇔ CH2 + CO
R158 C2H2 + O ⇔ H + HCCO R249 C2H4 + H(+M) ⇔ C2H5(+M)
R284 C2H4 + H ⇔ C2H3 + H2 R251 C2H4 + O ⇔ C2H3 + OH
R252 C2H4 + O ⇔ CH3 + HCO R254 C2H4 + OH ⇔ C2H3 + H2O
R631 C4H10 + H ⇔ sC4H9 + H2    
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Figure 13.

Figure 13

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ROP of H, O, and OH radicals in each reaction step: (a) ROP of H radicals; (b) ROP of O radicals; and (c) ROP of OH radicals.

Gas combustion and explosion are essentially complex chain reactions, in which the free radicals H, O, and OH play an important role, especially in the induction period of gas explosion.36Figure 13 illustrates the ROP of H, O, and OH radicals at each reaction step during LPG explosions. In Figure 13a, the most critical basic reactions for H radicals are R3: OH + H2 ≤=> H + H2O and R1: H + O2 ≤=> O + OH. R3 is the primary reaction for H radical generation, while R1 is the main reaction for H radical consumption. Conversely, Figure 13c shows that OH radicals exhibit the opposite behavior; R3 is the main reaction for OH radical consumption, and R1 is the primary reaction for OH radical generation. In Figure 13b, for O radicals, R1 is the main reaction for the generation of O radicals, while R158: C2H2+O ≤=> HCCO + H becomes the primary reaction for O radical depletion.

Figures 14 and 15 depict the changes in the molar fractions of H, O, and OH radicals as well as the ROP following the introduction of N2/ultrafine water mist, respectively. When N2 or ultrafine water mist is added, fundamental reactions involving the generation and consumption of H, O, and OH radicals remain unchanged. However, the molar fractions of H, O, and OH radicals decrease, indicating that the addition of N2 or ultrafine water mist can impede the generation of H, O, and OH radicals. This is due to a decrease in the ROP peak of the main reactions R3 and R1, which produce H, O, and OH. Therefore, in this paper, the influence of N2/ ultrafine water mist on explosive chain reactions of C3H8 and C4H10 was analyzed by comparing the molar fraction of H, O, and OH radicals and the change of ROP peak value in the basic reaction. When N2/ultrafine water mist is introduced, a significant decrease in the molar fractions of H, O, and OH radicals is observed. Furthermore, the effect of adding N2/ultrafine water mist in reducing the molar fractions of H, O, and OH radicals is notably superior to that of adding a single N2 or ultrafine water mist. This demonstrates that N2/ultrafine water mist exhibits a synergistic inhibitory effect, better suppressing the generation of H, O, and OH radicals, better reducing the ignition sensitivity of LPG and preventing the explosive chain reaction. This also explains why the explosion parameters are significantly reduced when N2/ultrafine water mist works synergistically.

Figure 14.

Figure 14

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Molar fraction of H/O/OH radicals under different volume fractions of N2 and H2O: (a) ROP of H radicals; (b) ROP of O radicals; and (c) ROP of OH radicals.

Figure 15.

Figure 15

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ROP of H, O, and OH radicals after N2 and H2O: (a) ROP of H radicals; (b) ROP of O radicals; and (c) ROP of OH radicals.

5. Conclusions

In this study, the synergistic inhibition of N2/ultrafine water mist on a 6% LPG explosion was studied in a constant volume combustion bomb. The inhibition mechanism was analyzed with CHEMKIN-Pro software, and the main conclusions were as follows:

  • (1)

    The inhibitory effect of a single ultrafine water mist on LPG explosion was unstable. When 140 g/m3 ultrafine water mist is introduced, compared with pure LPG explosion, the flame propagation speed is accelerated, the flame stability is reduced, the flame cell structure is more obvious, and the time to reach the end of the window is advanced. When 420 g/m3 ultrafine water mist is introduced, the maximum pressure rise rate is increased by 17.3% compared with 6% LPG explosion. This is because the single ultrafine water mist is greatly influenced by the vapor pressure formed by the evaporation of water mist, so it cannot play a stable inhibitory effect on LPG explosion.

  • (2)

    The combined application of N2/ultrafine water mist demonstrates a remarkable synergistic effect, the decrease of explosion pressure of LPG by both inhibitors is greater than the sum of the decrease of single inhibitor. In addition, with the increase of volume fraction of N2, both the maximum explosion pressure and the maximum pressure rise rate decrease significantly, and the arrival time is prolonged.

  • (3)

    When N2/ultrafine water mist is combined, the reduction in flame propagation speed and the increase in the Markstein length are significantly more pronounced compared to the effect of a single inhibitor, and the flame exhibits a noticeable upward movement. This is because the addition of N2 weakens the acceleration phenomenon of flame in the initial stage of ignition and weakens the initial explosion intensity of LPG, thus prolonging the action time of ultrafine water mist, improving its explosion suppression effect and avoiding the explosion promotion phenomenon caused by steam pressure when a single ultrafine water mist acts.

  • (4)

    Through mechanism analysis, it can be concluded that when N2/ ultrafine water mist is used in combination, it shows more obvious inhibition on the generation of H, O, and OH free radicals. This is because N2/ ultrafine water mist can better inhibit the main reactions of generating H, O, and OH radicals by LPG explosion, thus inhibiting the molar fraction and ROP peak of H, O, and OH radicals. Therefore, the total amount of flame free radicals in the explosion system is significantly reduced, which leads to a significant improvement in explosion mitigation efficiency.

Acknowledgments

This paper was financially supported by the research fund provided by the Science & Technology Innovation Talents in Universities of Henan Province (no. 22HASTIT027) and Scientific and Technological Key Project of Henan Province (no. 222102320142).

The authors declare no competing financial interest.

References

  1. Troncoso K.; Segurado P.; Aguilar M.; Soares Da Silva A. Adoption of LPG for cooking in two rural communities of Chiapas, Mexico. Energy Policy 2019, 133, 110925. 10.1016/j.enpol.2019.110925. [DOI] [Google Scholar]
  2. Sharma A.; Parikh J.; Singh C. Transition to LPG for cooking: A case study from two states of India. Energy Sustainable Dev. 2019, 51, 63–72. 10.1016/j.esd.2019.06.001. [DOI] [Google Scholar]
  3. Xu Y.; Zhao H. Study on the Environmental Costs of Petrochemical Enterprises Based on Double Carbon Targets. Environ. Resour. Ecol. J. 2022, 6 (5), 90. 10.23977/erej.2022.060510. [DOI] [Google Scholar]
  4. Zhou Z.; Liu Y.; Du J. Analysis on the constraint mechanism of transportation carbon emissions in the Pearl River Delta based on ‘Dual carbon’ goals. Syst. Sci. Control Eng. 2022, 10 (1), 854–864. 10.1080/21642583.2022.2107116. [DOI] [Google Scholar]
  5. Bariha N.; Mishra I. M.; Srivastava V. C. Fire and explosion hazard analysis during surface transport of liquefied petroleum gas (LPG): A case study of LPG truck tanker accident in Kannur, Kerala, India. J. Loss Prev. Process Ind. 2016, 40, 449–460. 10.1016/j.jlp.2016.01.020. [DOI] [Google Scholar]
  6. Lyu S.; Zhang S.; Huang X.; Peng S.; Li J. Investigation and modeling of the LPG tank truck accident in Wenling, China. Process Saf. Environ. Prot. 2022, 157, 493–508. 10.1016/j.psep.2021.10.022. [DOI] [Google Scholar]
  7. Wang S.; Wu D.; Guo H.; Li X.; Pu X.; Yan Z.; Zhang P. Effects of concentration, temperature, ignition energy and relative humidity on the overpressure transients of fuel-air explosion in a medium-scale fuel tank. Fuel 2020, 259, 116265. 10.1016/j.fuel.2019.116265. [DOI] [Google Scholar]
  8. Huo Y.; Chow W. K. Flame propagation of premixed liquefied petroleum gas explosion in a tube. Appl. Therm. Eng. 2017, 113, 891–901. 10.1016/j.applthermaleng.2016.11.040. [DOI] [Google Scholar]
  9. Razus D.; Brinzea V.; Mitu M.; Oancea D. Burning Velocity of Liquefied Petroleum Gas (LPG)-Air Mixtures in the Presence of Exhaust Gas. Energy Fuels 2010, 24 (3), 1487–1494. 10.1021/ef901209q. [DOI] [Google Scholar]
  10. Wang J.; Liang Y. Effect of hydrogen on explosion characteristics of liquefied petroleum gas-air mixtures. Int. J. Hydrogen Energy 2022, 47 (6), 4255–4263. 10.1016/j.ijhydene.2021.11.024. [DOI] [Google Scholar]
  11. Wang J.; Liang Y.; Zhao Z. Effect of N2 and CO2 on explosion behavior of H2-Liquefied petroleum gas-air mixtures in a confined space. Int. J. Hydrogen Energy 2022, 47 (56), 23887–23897. 10.1016/j.ijhydene.2022.05.152. [DOI] [Google Scholar]
  12. Wang T.; Yi W.; Liang H.; Yang P.; Luo Z.; Sun L.; Cheng F.; Kang X.; Feng Z.; Deng J. Experimental research on the pressure and flame propagation behaviors of LPG-air mixtures in a double obstructed tube. J. Loss Prev. Process Ind. 2023, 82, 104979. 10.1016/j.jlp.2023.104979. [DOI] [Google Scholar]
  13. Kai Z.; Xufeng Y.; Minggao Y.; Rongjun S.; Lei W., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct. 2019,. [Google Scholar]
  14. Shang R.; Zhang Y.; Zhu M.; Zhang Z.; Zhang D.; Li G. Laminar flame speed of CO2 and N2 diluted H2/CO/air flames. Int. J. Hydrogen Energy 2016, 41 (33), 15056–15067. 10.1016/j.ijhydene.2016.05.064. [DOI] [Google Scholar]
  15. Li M.; Xu J.; Wang C.; Wang B. Thermal and kinetics mechanism of explosion mitigation of methane-air mixture by N2/CO2 in a closed compartment. Fuel 2019, 255, 115747. 10.1016/j.fuel.2019.115747. [DOI] [Google Scholar]
  16. Kumar P.; Mishra D. P. Characterization of bluff-body stabilized LPG jet diffusion flame with N2 dilution. Energy Convers. Manage. 2008, 49 (10), 2698–2703. 10.1016/j.enconman.2008.04.007. [DOI] [Google Scholar]
  17. Du Y.; Zhang P.; Zhou Y.; Wu S.; Xu J.; Li G. Suppressions of gasoline-air mixture explosion by non-premixed nitrogen in a closed tunnel. J. Loss Prev. Process Ind. 2014, 31, 113–120. 10.1016/j.jlp.2014.07.012. [DOI] [Google Scholar]
  18. Luo Z.; Wei C.; Wang T.; Su B.; Cheng F.; Liu C.; Wang Y. Effects of N2 and CO2 dilution on the explosion behavior of liquefied petroleum gas (LPG)-air mixtures. J. Hazard. Mater. 2021, 403, 123843. 10.1016/j.jhazmat.2020.123843. [DOI] [PubMed] [Google Scholar]
  19. Tu Y.; Xu M.; Zhou D.; Wang Q.; Yang W.; Liu H. CFD and kinetic modelling study of methane MILD combustion in O2/N2, O2/CO2 and O2/H2O atmospheres. Appl. Energy 2019, 240, 1003–1013. 10.1016/j.apenergy.2019.02.046. [DOI] [Google Scholar]
  20. Grant G.; Brenton J.; Drysdale D. Fire suppression by water sprays. Prog. Energy Combust. Sci. 2000, 26 (2), 79–130. 10.1016/S0360-1285(99)00012-X. [DOI] [Google Scholar]
  21. Yang K.; Zhang P.; Yue C.; Chen K.; Ji H.; Xing Z.; Hao Y.; Jiang J. Experimental research on methane/air explosion inhibition using ultrafine water mist containing methane oxidizing bacteria. J. Loss Prev. Process Ind. 2020, 67, 104256. 10.1016/j.jlp.2020.104256. [DOI] [Google Scholar]
  22. Li G.; Pan C.; Liu Y.; Zhu X.; Ni X.; Zhao X.; Chen G.; Wang X. Evaluation of the effect of water mist on propane/air mixture deflagration: Large-scale test. Process Saf. Environ. Prot. 2021, 147, 1101–1109. 10.1016/j.psep.2021.01.034. [DOI] [Google Scholar]
  23. Xu H.; Li Y.; Zhu P.; Wang X.; Zhang H. Experimental study on the mitigation via an ultra-fine water mist of methane/coal dust mixture explosions in the presence of obstacles. J. Loss Prev. Process Ind. 2013, 26 (4), 815–820. 10.1016/j.jlp.2013.02.014. [DOI] [Google Scholar]
  24. You H.; Yu M.; Zheng L.; An A. Study on Suppression of the Coal Dust/Methane/Air Mixture Explosion in Experimental Tube by Water Mist. Procedia Eng. 2011, 26 (C), 803–810. 10.1016/j.proeng.2011.11.2240. [DOI] [Google Scholar]
  25. Ananth R.; Willauer H. D.; Farley J. P.; Williams F. W. Effects of Fine Water Mist on a Confined Blast. Fire Technol. 2012, 48 (3), 641–675. 10.1007/s10694-010-0156-y. [DOI] [Google Scholar]
  26. Pei B.; Yang Y.; Li J.; Yu M. Experimental Study on Suppression Effect of Inert Gas Two Fluid Water Mist System on Methane Explosion. Procedia Eng. 2018, 211, 565–574. 10.1016/j.proeng.2017.12.049. [DOI] [Google Scholar]
  27. Pei B.; Yu M.; Chen L.; Zhu X.; Yang Y. Experimental study on the synergistic inhibition effect of nitrogen and ultrafine water mist on gas explosion in a vented duct. J. Loss Prev. Process Ind. 2016, 40, 546–553. 10.1016/j.jlp.2016.02.005. [DOI] [Google Scholar]
  28. Pei B.; Li J.; Wang Y.; Wen X.; Yu M.; Jing G. Synergistic inhibition effect on methane/air explosions by N2-twin-fluid water mist containing sodium chloride additive. Fuel 2019, 253, 361–368. 10.1016/j.fuel.2019.05.035. [DOI] [Google Scholar]
  29. Cao X.; Ren J.; Zhou Y.; Wang Q.; Gao X.; Bi M. Suppression of methane/air explosion by ultrafine water mist containing sodium chloride additive. J. Hazard. Mater. 2015, 285, 311–318. 10.1016/j.jhazmat.2014.11.016. [DOI] [PubMed] [Google Scholar]
  30. Holborn P. G.; Battersby P.; Ingram J. M.; Averill A. F.; Nolan P. F. Estimating the effect of water fog and nitrogen dilution upon the burning velocity of hydrogen deflagrations from experimental test data. Int. J. Hydrogen Energy 2013, 38 (16), 6882–6895. 10.1016/j.ijhydene.2013.03.063. [DOI] [Google Scholar]
  31. Holborn P. G.; Battersby P.; Ingram J. M.; Averill A. F.; Nolan P. F. Modelling the mitigation of hydrogen deflagrations in a vented cylindrical rig with water fog and nitrogen dilution. Int. J. Hydrogen Energy 2013, 38 (8), 3471–3487. 10.1016/j.ijhydene.2012.12.134. [DOI] [Google Scholar]
  32. Wei S.; Yu M.; Pei B.; Zhu Z.; Zhang Z. Suppression of CO2 and H2O on the cellular instability of premixed methane/air flame. Fuel 2020, 264, 116862. 10.1016/j.fuel.2019.116862. [DOI] [Google Scholar]
  33. Xu C.; Zhong A.; Wang H.; Jiang C.; Sahu A.; Zhou W.; Wang C. Laminar burning velocity of 2-methylfuran-air mixtures at elevated pressures and temperatures: Experimental and modeling studies. Fuel 2018, 231, 215–223. 10.1016/j.fuel.2018.05.082. [DOI] [Google Scholar]
  34. Cao X.; Wang Z.; Lu Y.; Wang Y. Numerical simulation of methane explosion suppression by ultrafine water mist in a confined space. Tunn. Undergr. Space Technol. 2021, 109, 103777. 10.1016/j.tust.2020.103777. [DOI] [Google Scholar]
  35. Pei B.; Yu M.; Chen L.; Wang F.; Yang Y.; Zhu X. Experimental study on the synergistic inhibition effect of gas-liquid two phase medium on gas explosion. J. Loss Prev. Process Ind. 2017, 49, 797–804. 10.1016/j.jlp.2016.12.008. [DOI] [Google Scholar]
  36. Zhou S.; Gao J.; Luo Z.; Hu S.; Wang L.; Wang T. Role of ferromagnetic metal velvet and DC magnetic field on the explosion of a C3H8/air mixture-effect on reaction mechanism. Energy 2022, 239, 122218. 10.1016/j.energy.2021.122218. [DOI] [Google Scholar]

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