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Diese Forschungen befassen sich mit den Flammbeständigkeitsmerkmalen von gewellten Flammenfängern bei Wasserstoffexplosionen und konzentrieren sich auf die Auswirkungen struktureller Parameter und der Wasserstoffkonzentration. Die Studie untersucht den Zusammenhang zwischen Flammausbreitung und Druckanstieg und zeigt, wie die Dicke flammhemmender Einheiten, die Porosität und die Wasserstoffkonzentration das Flammenverhalten und die Explosionsdynamik beeinflussen. Durch detaillierte experimentelle Verfahren und visuelle Daten werden die Auswirkungen dieser Parameter auf Flammenausbreitungsgeschwindigkeit, Druck und Temperatur geklärt. Die Ergebnisse zeigen, dass eine Erhöhung der Dicke von Flammschutzmitteln und eine Verringerung der Wasserstoffkonzentration den Erfolg der Flammbeständigkeit verbessern können. Darüber hinaus untersucht die Studie die zugrunde liegenden Mechanismen der Flammbeständigkeit, einschließlich physikalischer Effekte wie Stoßwellendämpfung und chemischer Effekte wie radikale Wechselwirkungen. Die Schlussfolgerungen betonen die Bedeutung der Optimierung der strukturellen Parameter von Flammschutzmitteln, um eine effektive Explosionssicherheit zu gewährleisten und katastrophale Schäden zu verhindern. Diese umfassende Analyse bietet wertvolle Einsichten in die Mechanismen der Flammhemmung und die Faktoren, die die Explosionsdynamik in Wasserstoffumgebungen beeinflussen.
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Abstract
The visualization experimental apparatus used to study the flame resistance characteristics of hydrogen explosions with a corrugated flame arrester is illustrated in Fig. 8.1. The main framework of this apparatus has been detailed in previous work (Cao et al. in Int J Hydrogen Energy 88:228–241, 2024). This section focuses specifically on the corrugated flame-retardant system, which comprises the flame-retardant unit and its shell, upper and lower pressure plates, and four threaded support rods. The multilayer flame-retardant units are tightly stacked using the upper and lower pressure plates and are housed within the flame-resistant unit shell. The system is secured by the threaded support rods, with height adjustments made via nuts. By varying the ripple height and the number of flame-retardant units, the structural parameters can be altered. A self-processed flame-retardant unit with a thickness of 20 mm and the diameter of 110 mm was adopted.
8.1 Experimental Apparatus and Procedures
The visualization experimental apparatus used to study the flame resistance characteristics of hydrogen explosions with a corrugated flame arrester is illustrated in Fig. 8.1. The main framework of this apparatus has been detailed in previous work [1]. This section focuses specifically on the corrugated flame-retardant system, which comprises the flame-retardant unit and its shell, upper and lower pressure plates, and four threaded support rods. The multilayer flame-retardant units are tightly stacked using the upper and lower pressure plates and are housed within the flame-resistant unit shell. The system is secured by the threaded support rods, with height adjustments made via nuts. By varying the ripple height and the number of flame-retardant units, the structural parameters can be altered. A self-processed flame-retardant unit with a thickness of 20 mm and the diameter of 110 mm was adopted.
The experimental conditions and parameters of the flame arrester are presented in Table 8.1. In order to prevent flame passing through the gap between the flame-retardant system and the inner wall surface, the gap was sealed by the high temperature explosion-proof sealing mud. The premixed gas inside the vessel was prepared according to Dalton’s law of partial pressures. During the experiment, a high-speed camera (f = 4000 fps) recorded the transient transformation processes of flames at both ends of the flame arrester. Two high-frequency pressure transmitters [Range (A) = 0–2.5 MPa; Response frequency (f) = 200 kHz] and two S-type platinum–rhodium thermocouples were employed to collect pressure and temperature data at both ends of the flame arrester. The micro-thermocouple with S-type was welded from Pt and Pt/Rh-10% with a diameter of 50 μm and its measurement range in a short period of time reached 2000 ℃. Additionally, ignition and data acquisition were controlled by a program and control acquisition system (f = 200 kHz/channel) developed using a custom programming language. In order to ensure the accuracy of data, the experiment under same flame resistance condition was repeated three times at least. And deviations of ΔPmax and Tmax were less 5.60% and 7.90%, respectively.
Table 8.1
Experimental condition parameters
Hydrogen concentration (c)/%
Porosity (ε)
Flame arrester unit thickness (H)/mm
1
20
0.30
20
2
40
0.50
40
3
60
0.65
60
4
80
0.70
80
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8.2 Results and Discussions
8.2.1 Analysis of Flame Resistance Characteristic
Figure 8.2 illustrates the relationship between flame propagation and pressure rise during a hydrogen explosion as flame resistance fails (c = 18%, ε = 0.30, H = 140 mm). There is a clear correlation between the flame passing through the corrugated flame arrester and the pressure rise under failure conditions. Initially, the flame developed a “spherical” structure. At 4.75 ms, the pressure at the lower end began to rise, but the slower flame propagation velocity resulted in a smaller pressure increase. As the flame accelerated, it transitioned from an “ellipsoidal” to a “finger” structure, leading to a noticeable increase in the rate of pressure rise. Under the influence of a reverse shock wave, the velocity history exhibited significant oscillations. As the flame approached the lower end of the flame arrester, an axial acceleration process occurred. At 10.25 ms, the velocity history peaked. Subsequently, the flame entered the narrow channel and stagnated. At 11.75 ms, the flame passed through the narrow channel, and the pressure rise rate reached its peak at 14.50 ms. After this point, the pressure rise rate at the lower end began to decrease, with a pressure peak occurring at 17.50 ms (ΔPmax = 0.32 MPa). In comparison to the lower end, the moment of pressure rise at the upper end was slightly delayed (t = 6.75 ms). Following passage through the flame arrester, the pressure rise at the upper end was not significant; however, it was noticeably higher than that at the lower end in relation to flame propagation velocity. With continued acceleration of flame propagation, the rate of pressure rise increased markedly. At t = 14.00 ms, the velocity history at the upper end reached its peak; however, the corresponding pressure rise rate did not reach its maximum until later. As explosive reactions progressed at the upper end, heat accumulation increased continuously. At 18.25 ms, the pressure rise rate peaked and was significantly higher than that at the lower end. Subsequently, as explosive reactions weakened, the pressure rise rate decreased, with a pressure peak appearing at 19.50 ms (ΔPmax = 0.36 MPa). Compared to the lower end, ΔPmax increased by 11.11%. Following this peak, pressure decreased significantly as the flame continued to extinguish.
Fig. 8.2
Analysis of flame resistance characteristic under the failure condition (c = 18%, ε = 0.30, H = 140 mm)
Figure 8.3 illustrates the effect of flame-retardant unit thickness on the flame propagation process (c = 15%, ε = 0.30). By calculating the derivative of flame front height over time, the velocity history can be obtained (see in Fig. 8.4). It is evident that the initial flame evolves from a “spherical” to an “ellipsoidal” structures. As the thickness of the flame-retardant unit increases, the velocity history is significantly reduced, particularly during the acceleration phase of the “finger” flame. This reduction occurs because the reverse action of the high-speed shock wave is intensified due to the obstruction caused by the flame arrester, which continuously accumulates pressure at the front end and slows down flame propagation.
Fig. 8.3
Effect of flame-retardant unit thickness on flame propagation process (c = 15%, ε = 0.30)
However, an evident acceleration phenomenon occurs as the flame approaches the end face of the flame-retardant arrester. At 16.00 ms, the flame first enters the narrow channel with a unit thickness of 80 mm. Subsequently, as the flame continues to enter the narrow channel, stagnation occurs, and the stagnation time gradually increases with greater flame-retardant unit thickness. At 22.00 ms, the flame passes through the narrow channel with H = 80 mm and accelerates, indicating a failure in flame resistance. In comparison to the lower end, the acceleration of flame propagation is more pronounced. As the thickness of the flame-retardant unit increases, the flame does not pass through the arrester to ignite the premixed gas at the upper end, demonstrating successful flame resistance. This indicates that variations in flame-retardant unit thickness can alter the velocity of flame propagation into the narrow channel, thereby affecting both shock wave attenuation and flame behavior within it.
Effect of Porosity
Figure 8.5 illustrates the effect of porosity on the flame propagation process (c = 15%, H = 140 mm). As shown in the velocity history (in Fig. 8.6), the flame development rate progressively slows down as porosity increases, particularly during the development stages of “spherical” and “ellipsoidal” flames. This occurs because lower porosity enhances the reverse action of the shock wave, leading to a greater disturbance at the flame front and a higher flame propagation velocity. As porosity increases, the propagation velocity of the flame decreases sequentially as it enters the flame arrester, dropping from 73.3 to 54.5 m/s.
Fig. 8.5
Effect of porosity on flame propagation process (c = 15%, H = 140 mm)
However, the stagnation time of the flame also decreases correspondingly. Notably, the flame first passes through the narrow channel and ignites the premixed gas at the upper end when ε = 0.70. Under a porosity condition of 0.30, the flame fails to pass through the narrow channel, resulting in successful flame resistance. Although a decrease in porosity can increase the flame propagation velocity at its front end, it also enhances the attenuation effects of the narrow channel on both the flame and shock wave. Furthermore, the increase in flame propagation velocity after passing through the flame arrester is more pronounced with higher porosity and decreases with lower porosity.
Effect of Hydrogen Concentration
Figure 8.7 illustrates the effect of hydrogen concentration on the flame propagation process (ε = 0.30, H = 140 mm). As shown in the velocity history in Fig. 8.8, the flame development rate gradually increases with higher hydrogen concentration, particularly during the development stages of “spherical” and “ellipsoidal” flames. This increase is attributed to the significant impact of hydrogen concentration on flame propagation velocity. Under lean-burn conditions, the velocity of flame propagation increases sequentially as hydrogen concentration increases, leading to reductions in both the moment of entry into the flame arrester and stagnation time within the narrow channel.
Fig. 8.7
Effect of hydrogen concentration on flame propagation process (ε = 0.30, H = 140 mm)
As c = 21%, the flame first passes through the flame arrester and ignites the premixed gas at the upper end, indicating a failure in flame resistance. Meanwhile, the velocity after passing through the arrester gradually increases with higher hydrogen concentration. However, at c = 12% and 15%, the flame stagnates within the narrow channel, indicating successful flame resistance. This suggests that the propagation velocity of the flame entering the corrugated flame arrester is a key factor affecting flame resistance and significantly influences acceleration of flame propagation at the rear end under failure conditions.
8.2.3 Analysis of Explosion Pressure
Effect of Flame-Retardant Unit Thickness
Figure 8.9 illustrates the effect of flame-retardant unit thickness on explosion pressure (ε = 0.30, c = 15%). It is evident that the influence of flame-retardant unit thickness on pressures at both ends aligns with its impact on velocity history. When H = 80 mm, flame resistance fails, resulting in the upper pressure being significantly higher than the lower pressure, with an increase of 25.71% at the upper end compared to the lower end. As the thickness of the flame-retardant unit increases, the flame does not pass through the flame arrester; consequently, the lower pressure becomes noticeably higher than the upper pressure, exhibiting a slight decreasing trend. Compared to the failure condition (H = 80 mm), pressure peaks at both ends decrease by 60.0% and 7.70%, respectively, under successful flame resistance conditions (H = 100 mm). This indicates that the pressure drop at the upper end is more pronounced during successful flame resistance; however, the extent of reduction becomes less significant as flame-retardant unit thickness continues to increase. As H = 140 mm, the pressure peaks at both ends decrease by 20.83% and 14.29%, respectively. The history of pressure rise rate can be obtained by calculating the derivative of pressure history over time. As the thickness of the flame-retardant unit increases, changes in the pressure rise rate exhibit a pattern similar to that of the pressure.
Fig. 8.9
Effect of flame-retardant unit thickness on pressure history (ε = 0.30, c = 15%)
Figure 8.10 illustrates the effect of porosity on explosion pressure (H = 140 mm, c = 15%). It is evident that both the pressure history and its rising rate exhibit similar change patterns with increasing porosity. At ε = 0.30, flame resistance is successful, with lower pressure exceeding upper pressure. However, as porosity increases to 0.50, the flame resistance fails, resulting in a significant increase in upper pressure and its rising rate. The pressure peaks at both ends increase by 57.14% and 9.52%, respectively. Notably, the change in the pressure rising rate is more pronounced than that of the pressure peak. As porosity increases, upper pressure rises, and the corresponding pressure rising rate history follows a similar variation pattern. ΔPmax at upper and lower ends are increased by 66.67% and 17.39%, respectively. And (dP/dt)max are increased by 90.10% and 20.99%, respectively. This indicates that an increase in porosity during flame resistance failure generates greater pressure at the rear end of the flame arrester. Therefore, the structural parameters of corrugated flame arresters selected for engineering applications must ensure effective flame resistance; otherwise, they may enhance pressure effects and cause serious damage. Considering allowable pressure drops, further optimization of flame-retardant system structure parameters is necessary.
Fig. 8.10
Effect of porosity on pressure history (H = 140 mm, c = 15%)
Figure 8.11 illustrates the effect of hydrogen concentration on explosion pressure (H = 140 mm, ε = 0.30). Hydrogen concentration significantly impacts explosion pressure and its rising rate at both ends, influencing the explosion reaction rate, which in turn affects the propagation velocity and intensity of the shock wave entering the narrow channel. The relationship between hydrogen concentration and explosion pressures, as well as their rising rates at both ends, follows a similar variation pattern; however, pressure changes at the two ends are not significant under opposing flame resistance effects. At c = 12% and 15%, flame resistance is successful, with lower pressure exceeding upper pressure. Conversely, at c = 18% and 21%, the flame resistance fails, resulting in upper pressure being significantly higher than lower pressure. As hydrogen concentration increases, pressures at both ends exhibit a slight upward trend. This indicates that hydrogen concentration has a notable impact on the rates of pressure rise at both the front and rear ends of the flame-retardant system, with the likelihood of failure increasing as concentration rises.
Fig. 8.11
Effect of hydrogen concentration on pressure history (H = 140 mm, ε = 0.30)
Figure 8.12 shows the effect of flame-retardant unit thickness on the flame temperature peak (ε = 0.30, c = 15%). It clear that the change in flame temperature peak (Tmax) presented the same pattern as the change in pressure peak. When H = 80 mm, the flame resistance fails, resulting in the temperature at the upper end being higher than that at the lower end. As the thickness of the flame-retardant unit increases, the flame resistance is successful, and Tmax decreases significantly, particularly at the upper end where the drop is more pronounced. Compared to a unit thickness of 80 mm, Tmax at the upper and lower ends corresponding to a unit thickness of 100 mm decreases by 79.48% and 15.63%, respectively. Meanwhile, Tmax at the lower end remains noticeably higher than at the upper end and does not reach the ignition temperature of hydrogen. Although the flame does not pass through the flame-retardant system, it still exhibits a significant temperature increase due to high-temperature unburned gas. This indicates that as unit thickness increases, the contact time between the narrow channel and the flame also increases, further enhancing heat absorption by the flame and consequently reducing the temperature at the flame front.
Fig. 8.12
Effect of flame-retardant unit thickness on temperature history (ε = 0.30, c = 15%)
Figure 8.13 presents the effect of porosity on the flame temperature peak (c = 15%, H = 140 mm). It is evident that Tmax at both ends under successful flame resistance is significantly lower than the failure, with the change in Tmax at the upper end being more pronounced. When flame resistance is successful, Tmax at the upper end is lower than at the lower end. Compared to a porosity of 0.30, Tmax at the upper and lower ends corresponding to a porosity of 0.50 increases by 9.11% and 9.68%, respectively. However, when flame resistance fails, Tmax at the upper end exceeds that at the lower end. Compared to a porosity of 0.45, Tmax at the upper and lower ends corresponding to a porosity of 0.65 increases by 9.29% and 10.35%, respectively. Additionally, Tmax at both ends corresponding to a porosity of 0.45 increases by 77.63% and 4.46%, respectively, compared to a porosity of 0.25. This indicates that reducing porosity increases the surface area of the flame in contact with the inner wall of the narrow channel, thereby enhancing the heat transfer rate and wall effect.
Fig. 8.13
Effect of porosity on temperature history (c = 15%, H = 140 mm)
Figure 8.14 shows the effect of hydrogen concentration on the flame temperature peak (ε = 0.30, H = 140 mm). It is evident that hydrogen concentration significantly affects the flame temperatures at both the upper and lower ends, thereby influencing the flame resistance effectiveness of the corrugated flame arrester in hydrogen explosions. At c = 12% and 15%, flame resistance is successful, with Tmax at the lower end exceeding that at the upper end. Additionally, Tmax increases noticeably with rising hydrogen concentration. Compared to a hydrogen concentration of 12%, Tmax at the upper and lower ends corresponding to a concentration of 15% increases by 21.34% and 14.29%, respectively. However, at c = 18% and 21%, the flame resistance fails, resulting in Tmax at the upper end being higher than at the lower end. As hydrogen concentration increases, Tmax sequentially rises. Compared to a hydrogen concentration of 18%, Tmax at the upper and lower ends corresponding to a concentration of 21% increases by 12.33% and 10.28%, respectively. This indicates that an increase in hydrogen concentration enhances both flame propagation velocity and temperature as they enter the narrow channel of the flame-retardant system, thus affecting the attenuation effect of the narrow channel on the flame. Furthermore, an increase in hydrogen concentration is detrimental to the explosion resistance of the corrugated flame arrester.
Fig. 8.14
Effect of hydrogen concentration on temperature history (ε = 0.30, H = 140 mm)
Figure 8.15 illustrates the schematic diagram of the hydrogen explosion flame quenching process within the narrow channel of a corrugated flame arrester. The flame resistance mechanism can be attributed to a combination of physical and chemical effects [2]. In terms of physical effects, the high-speed shock wave acted on the end face of flame-retardant unit before the high-temperature flame. Firstly, part of shock waves emerged the reverse propagation due to the obstruction of end face, thereby weakening the intensity of shock wave [3]. The reverse shock wave also exerts a blocking effect on flame propagation, reducing its velocity as it enters the narrow channel [4]. Meanwhile, discrete shock waves enter the narrow channel and continuously collide with the wall surface during propagation, further diminishing their intensity. As the flame came into contact with the end face of flame-retardant system, the continuous and smooth flame was also dispersed by the narrow channel and propagated inside it. The flame continuously exchanged heat with the wall face during propagation, which greatly reduced the temperature of flame front. As the temperature dropped below the ignition temperature of hydrogen, the flame resistance was successful [5]. Apart from the physical effect, the number and energy of free radicals activated for the main chain reactions (H2 + O2 = HO2 + H, H + O2 = O + OH, OH + H2 = H2O + H) of high-temperature flame caused by the hydrogen explosion were greatly reduced during the interaction with the wall surface. As the production rate of chain reaction was less than the destruction rate, the flame was quenched. The thickness and porosity of the flame-retardant unit, along with hydrogen concentration, influence flame resistance by altering shock wave intensity and flame behavior as they enter the narrow channel, as well as affecting physical and chemical interactions during internal propagation.
This experiment primarily investigated the flame resistance characteristics and influence mechanisms of the corrugated flame-retardant system on hydrogen explosions. Through visual flame resistance experiments, we clarified the effects of the flame-retardant system structure parameters and hydrogen concentration on flame propagation characteristics and explosion parameters at both ends. Additionally, the influences of flame propagation and shock wave attenuation within the narrow channel were summarized, further exploring the underlying explosion resistance mechanisms in depth. The conclusions are as follows:
First, a clear correlation exists between flame propagation and pressure rise at both ends of the flame-retardant system during explosion resistance. An evident oscillation occurs in the initial stage of flame propagation due to obstruction by the end face of the flame-retardant system. Additionally, the flame-retardant system significantly influences the flame propagation process at its front end, thereby affecting its velocity within the narrow channel. As flame resistance fails, the flame at the rear end experiences a noticeable acceleration. The structural parameters of the flame-retardant system and hydrogen concentration influence the velocity of flame propagation entering the narrow channel, as well as the physical and chemical effects during internal propagation, ultimately impacting the effectiveness of flame resistance.
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Second, increasing the thickness of the flame-retardant unit and decreasing hydrogen concentration can effectively reduce the flame propagation velocity at the front end, thereby influencing its velocity within the narrow channel and enhancing the probability of flame resistance success. However, while a reduction in porosity can increase the flame propagation velocity at the front end, the narrow channel exerts a stronger weakening effect on both the flame and shock wave. Additionally, the increase in flame propagation velocity after passing through the narrow channel is more pronounced with greater porosity, while it decreases with reduced porosity.
Third, the explosion pressure and flame temperature displayed a similar variation pattern to the velocity history, influenced by the structural parameters of the flame-retardant system and hydrogen concentration. The structural parameters of the corrugated flame arrester selected for engineering applications must ensure effective explosion resistance; otherwise, they may cause explosion enhancement and lead to catastrophic damage. Considering the pressure drop parameters in a normal operating environment, further optimization of the structural parameters of the corrugated flame-retardant system is necessary. Besides, the expansion chamber structure at the front end of the flame arrester should be optimized to ensure that the flame propagation velocity of entering its front-end is minimized and the concentration of transported gas inside the pipeline is reduced as much as possible.
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