Experimental Study on Duct-Vented Explosion of Hydrogen–Air

Article pubs.acs.org/IECR

Experimental Study on Duct-Vented Explosion of Hydrogen−Air Mixtures in a Wide Range of Equivalence Ratio Jin Guo,† Changjian Wang,*,‡ and Xuanya Liu*,§ †

College of Environment and Resources, Fuzhou University, Fuzhou 350116, People’s Republic of China School of Civil Engineering, Hefei University of Technology, Hefei 230009, People’s Republic of China § Tianjin Fire Research Institute, Tianjin 300381, People’s Republic of China ‡

ABSTRACT: A relief duct is often used for indoor vented enclosures to discharge unburnt and burnt gas mixtures to outdoors, but its use can significantly affect pressure relief and flame behavior. In the current paper, duct-vented explosions of hydrogen−air mixtures with equivalence ratios ranging from 0.8 to 4.0 were experimentally investigated in a small cylindrical vessel with a 50-cmlong relief duct. Different from “coherent deflagration” as previously reported, the maximum pressure rise rate in the duct is always larger than that in the vessel. Three pressure peaks can be identified on the external pressure profile, which result from the burst of the vent cover, the burn-up in the duct, and the external explosion, respectively. Whether the second or the third pressure peak is the dominant one depends on the hydrogen equivalence ratio. Compared with simple vented explosions, the use of a relief duct decreases the pressure relief efficiency and the external flame size for all equivalence ratios tested.

1. INTRODUCTION Explosion venting is an effective method to protect equipment against accidental internal gas or dust explosions. For those indoor vented enclosures, a relief duct is necessary to discharge the unburnt and burnt gas mixtures to outdoors so as to avoid the damage to other indoor equipment or personnel.1 Explosion venting of dusts2,3 and hydrocarbon−air mixtures4−9 through a duct has been investigated extensively, and it has been found that the presence of a duct can significantly increase the maximum internal overpressure compared with the cases without a relief duct (simple vented explosion).10−14 The factors, which are expected to be responsible for the above phenomenon, include frictional loss of the outflow and inertia of the gas column in the duct,6,15 acoustic and Helmholtz oscillations in the vessel,5 and turbulent combustion (secondary explosion or burn-up) in the duct.5,6 Through a numerical method, Ferrara et al.12 found that the severity of the ductvented explosion is mainly attributed to the turbulent combustion in the duct rather than the other factors. The experiments by Molkov10 also showed that the turbulent combustion in the duct plays the leading role because the internal overpressure was much reduced to a level in a simply vented explosion when the combustion was suppressed by sprinkling water into the duct. Ferrara et al.14 investigated the interaction between the explosion in the vessel and duct for methane and propane and found that the maximum pressure rise rates in the vessel and duct are nearly equal and named it “coherent deflagration”. Moreover, the use of a relief duct may cause the transition from deflagration to detonation (DDT) under some suitable conditions,7,16,17 and in this case venting cannot be used to reduce the maximum overpressure in © 2016 American Chemical Society

enclosures. Therefore, vents on ducts are also necessary to prevent a deflagration from transitioning into a detonation.1 Compared with the extensive studies on duct-vented explosions of dusts and hydrocarbons, little work has been done about hydrogen, which is widely used in refining, chemical synthesis of materials, nuclear plants, fuel cell cars and hydrogen fueling stations, etc. The experimental results by Kumar et al.18 show that the peak pressure in the vessel increases with hydrogen concentration increasing from 8.5 to 20% and venting is effective in reducing the internal peak pressure for low hydrogen concentrations due to long combustion times (slow burning rate). Kasmani19 found that the characteristics of duct-venting of hydrogen were different from those of methane and propane because hydrogen has a higher burning rate and is prone to undergo DDT. Besides the works about lean hydrogen−air mixtures,18,19 duct-vented explosion of hydrogen−air mixtures is worthy of more detailed investigations. One important issue is how the pressure profile and flame behave in a wider range of hydrogen concentration, because any concentration between the lower and upper flammable limits may be reached in real life. For this purpose, experiments with hydrogen−air mixtures with equivalence ratio ranging from 0.8 to 4.0 were carried out in a small vented vessel with a relief duct, which may be representative of such systems as instrumentation boxes, etc. Received: Revised: Accepted: Published: 9518

May 26, 2016 August 3, 2016 August 17, 2016 August 17, 2016 DOI: 10.1021/acs.iecr.6b02029 Ind. Eng. Chem. Res. 2016, 55, 9518−9523

Article

Industrial & Engineering Chemistry Research

2. EXPERIMENTAL SECTION Figure 1 shows the schematic of the venting configuration. It consists of a cylindrical vessel with two symmetrical 10-cm-long

in the vessel (P1) increases monotonically to its peak value and then decreases. However, the pressure profiles are much more complicated at the neck exit (P2) and in the duct (P3). Taking ϕ = 3.0 as an example, at the time of vent failure (t1), Figure 3

Figure 1. Schematic of venting configuration. P1−P4: piezoelectric pressure transducers.

Figure 3. Schlieren images of the spreading flame. ϕ = 3.0. t1−t3 correspond to the times in Figure 2.

necks at its waist and a 50-cm-long relief duct. Both the internal diameter and the length of the cylindrical vessel are 25 cm. The square cross section of the necks and the duct is 7 cm × 7 cm. At both ends of the cylindrical vessel, two quartz windows were mounted for allowing optical access necessary to the schlieren system. A thin aluminum foil fitted between the relief duct and one of the vessel necks was used as a vent cover, and the exit of the other vessel neck was sealed with a blind flange. Ignition was performed by a spark plug mounted in the blind flange, and the ignition energy was kept about 500 mJ in every test. Seven hydrogen−air equivalence ratios (ϕ) of 0.8, 1.0, 1.2, 1.6, 2.0, 3.0, and 4.0 were taken into account. All tests were repeated three times at the initial pressure and temperature of 1 atm and 300 K, respectively. The measuring systems and experimental procedures are quite similar to our earlier work.20 The flame images were captured by a high-speed schlieren system and a high-speed camera. Four piezoelectric pressure transducers, P1−P4, were employed to record pressure histories, as shown in Figure 1. The four pressure transducers were all coated with a thin layer of silicon grease to avoid thermal effects on the pressure measurements. P1, P2 (2 cm away from vent cover), and P3 were mounted flush with the inner surface of the wall, and P4 was on the axis of the apparatus. Experiments without the 50cm-long relief duct were also conducted with the locations of P1 and P2 unchanged and P3 moved to the axis to verify the effect of the short relief duct on explosion venting.

shows that the flame is still far from the vent cover, so unburnt hydrogen−air mixtures are first vented through the duct. Under the effect of the vented flow, P3 first increases slowly and then reaches a “plateau” before the internal flame enters the neck at the time of t2, as shown in Figure 2. During the time interval between t1 and t2, P2 is higher than P3 because of the frictional loss of vented unburnt gases. Shortly after t2, the vented unburnt gas mixtures in the duct, which become turbulent,4 are ignited, and consequently vigorous burn-up in duct occurs.6,7 For all equivalence ratios tested in current experiments, P2 and P3 increase steeply to reach their peaks with amplitude higher than P1, which makes the mixtures in the duct flow reversely into the vessel and a reverse flow forms as a result, as shown in Figure 3. Under the effect of blockage in the duct,6,15 the turbulence of the internal flame due to the reverse flow,6,8,13 and the interaction of the internal flame with the back-propagating pressure wave,5,21,22 the efficiency of the internal pressure relief was impaired. As a result, the slope of P1 becomes steeper after t3, as shown in Figure 2. Figure 4 presents the pressure rise rate in the vessel and duct for ϕ = 0.8. For the sake of clarity, the data of pressure rise rates have been smoothed by 20-point averaging. Ferrara et al.14 found that the magnitudes of the maximum pressure rise rates in the vessel and duct are very similar for propane−air mixtures and named it “coherent deflagration”. However, the present experiments do not seem to support this. As shown in Figure 4, the pressure rise rate in the vessel dP1/dt is much lower than that in the duct dP3/dt, which can be observed for all equivalence ratios tested in current experiments as summarized in Figure 5. The fact that the maximum dP3/dt is much larger than dP1/dt may be due to the more intense turbulence4 and much higher ignition energy of the combustible gas mixtures formed in the duct.

3. RESULTS AND DISCUSSION 3.1. Pressure Profiles in Vessel and Duct. It is found that the pressure profiles in the vessel and duct vary in quite a similar way for ϕ ranging from 0.8 to 4.0, and typical pressure− time histories are shown in Figure 2. After ignition, the pressure

Figure 2. Pressure time histories in vessel and duct. 9519

DOI: 10.1021/acs.iecr.6b02029 Ind. Eng. Chem. Res. 2016, 55, 9518−9523

Article

Industrial & Engineering Chemistry Research

with a further increase of ϕ, the combustible gas mixtures in the duct become richer which results in the decrease of dP3/dt. 3.2. External Pressure Profile. Three pressure peaks can be observed in the external pressure profile, as shown in Figure 6. The first pressure peak p1 with quite low amplitude appears at about 2 ms after vent failure. The second pressure peak p2 with much higher amplitude forms shortly after the appearance of the maximum pressure in the duct, which was followed by the third pressure peak p3. It is found that p1 is resulted from the weak shock wave due to the rupture of the vent cover as shown in Figure 7. p2 is determined by the shock wave due to Figure 4. Pressure rise rate vs time. ϕ = 0.8.

Figure 7. Schlieren images of the external flow field. ϕ = 1.2.

the vigorous burn-up in duct. Also, p3 has a close relation with the explosion that occurred in the outer space. After vent failure, Figure 7 shows that a part of the unburnt hydrogen−air mixtures is vented through the relief duct to form a cloud of the combustible mixtures in the outer open space. The external explosion is triggered when the cloud of the combustible mixtures is ignited by the flame rushing out from the duct,17,23−25 so a shock wave forms and consequently p3 appears. In current experiments, there is not much difference in the vent burst pressure for various ϕ values, which yields the nearly identical speed of a weak shock wave due to vent failure, so p1, with an amplitude in a range of several kilopascals, appears nearly at the same time (2 ms) after vent failure. However, p2 and p3 are found to depend on ϕ, as shown in Figure 8. As discussed above, p2 and p3 depend on the intensity of the vigorous burn-up in duct and the external explosion, respectively. When the lean unburnt hydrogen−air mixtures are vented through the duct to the outer open space, they become leaner and leaner from the duct to the outer space, which results in a low p2 and a lower p3. For example, p3 can hardly be observed for ϕ = 0.8. When ϕ increases from lean to rich, the combustible mixtures in the duct and in the outer space probably approach stoichiometric, so the intensity of the burn-up in the duct and the external explosion increases.

Figure 5. Maximum pressure rise rate vs ϕ.

Figure 5 also shows the relation between the maximum pressure rise rate in the duct and vessel with ϕ. The maximum dP3/dt first increases and then decreases sharply with ϕ increasing from 0.8 to 3.0, and decreases slowly with a further increase of ϕ. However, no such sensitive dependence of the maximum pressure rise rate in the vessel dP1/dt was found. One important factor which leads to the evident change of dP3/dt with ϕ is the concentration of the combustible gas mixtures formed in the duct after vent failure. As discussed above, the unburnt hydrogen−air mixtures in the vessel are first vented after vent failure, which are diluted by the fresh air in the duct. It can be deduced that the combustible gas mixtures in the duct approach stoichiometric with ϕ increasing from lean to slightly rich, and as a result the maximum dP3/dt increases. But

Figure 6. Pressure time histories for ϕ = 1.2. 9520

DOI: 10.1021/acs.iecr.6b02029 Ind. Eng. Chem. Res. 2016, 55, 9518−9523

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Industrial & Engineering Chemistry Research

Figure 8. External pressure time histories for different ϕ values.

Figure 10. Typical pressure time histories.

Correspondingly, the amplitudes of p2 and p3 also increase. But with a further increase of ϕ, p2 and p3 decrease because the combustible mixtures get richer and richer, especially in the relief duct; therefore, p2 is nearly absent for ϕ = 4.0, as shown in Figure 8. Experimental results also show that the external pressure peak varies with ϕ. Figure 8 and Figure 9 show that p2 is much higher than p3 for ϕ < 2.0. However, the two pressure peaks have nearly identical amplitude for ϕ = 2.0, and p3 exceeds p2 for richer hydrogen−air mixtures.

Figure 11. Maximum internal overpressure vs ϕ.

duct. First, the maximum internal overpressures in both cases first increase and then decrease with ϕ increasing from 0.8 to 4.0, and the maximum values appear on the rich side. Second, the use of the 50-cm-long duct increases the maximum internal overpressure compared with the simple vented explosions for all equivalence ratios tested, and the difference also depends on ϕ. Figure 11 shows that the difference between the maximum internal overpressures for the two cases is large for ϕ ranging from 0.8 to about 3.0, which is resulted from the vigorous burnup in the duct. However, it is relatively small for ϕ = 4.0 due to the weak burn-up. The above phenomenon proved that the burn-up in the duct plays the leading role in reducing the pressure relief efficiency in duct-vented explosions.10,12 The use of the relief duct also has a significant effect on the maximum external overpressure. As shown in Figure 10, the maximum external overpressure in duct-vented explosion is much higher than that in simple vented explosion for ϕ = 1.0, which is also true for ϕ ranging from 0.8 to 2.0. But for ϕ = 3.0 and 4.0, the latter equals or slightly exceeds the former. The reason is that the burn-up inducted pressure peak in ductvented explosions, which is dominant in the external overpressure time history for ϕ < 2.0, is absent in a simple vented explosion. But for richer mixtures, the external explosion induced pressure peak becomes the dominant one, so the use of a 50-cm-long duct has no significant effect on it. Besides the internal and external overpressures, the use of the relief duct affects the evolution of the external flames including the flame speed and size, as shown in Figure 12 and Figure 13. Compared with the simple vented explosions, the flame was

Figure 9. p2 − p3 vs ϕ.

3.3. Comparison with Simple Vented Explosions. Figure 10 presents the comparison of the overpressures between the cases with and without the 50-cm-long relief duct for ϕ = 1.0. As reported in the previous investigations,10−14 the use of a short relief duct affects significantly the internal overpressure compared with the simple vented explosions. Figure 10 shows that P1 in both cases still has the same amplitude at the early stage after vent failure. Then P1 in the experiment with a duct becomes slightly higher before the internal flame rushes out of the vessel due to the frictional loss of the outflow and inertia of the gas column in the duct.6,15 But P1 in the duct-vented experiments increases more quickly under the effect of the back-propagating pressure wave and the reverse flow due to the vigorous burn-up in the duct,6 and the difference of P1 between the two cases becomes larger until the maximum pressures in the vessel are reached nearly at the same time. Figure 11 compares the maximum internal overpressures as a function of ϕ between the cases with and without the relief 9521

DOI: 10.1021/acs.iecr.6b02029 Ind. Eng. Chem. Res. 2016, 55, 9518−9523

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Industrial & Engineering Chemistry Research

Figure 12. Flame speed vs location. ϕ = 1.6.

2. Three peaks can be identified in the external pressure profile, which are resulted from the rupture of the vent cover, the burn-up in the duct, and the external explosion, respectively. The second pressure peak is the dominant one for ϕ < 2.0, and the third peak becomes dominant for ϕ = 3.0 and 4.0. 3. Compared with the simple vented explosions, the use of the 50-cm-long relief duct always increases the maximum internal overpressure but decreases the flame size.

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AUTHOR INFORMATION

Corresponding Authors

Figure 13. Typical external flame. ϕ = 1.6. t is the time when external flame appears.

*E-mail: [email protected] (C.W.). *E-mail: [email protected] (X.L.). Notes

The authors declare no competing financial interest.

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significantly accelerated in the relief duct because of the small vent area and the vigorous burn-up. Taking ϕ = 1.6 as an example, Figure 12 shows that the flame speed increases from about 99 m/s when it enters the neck to its maximum value of about 900 m/s when it rushes out of the relief duct. However, the maximum flame speed is about 575 m/s in the simple vented explosion. It has been found that a jet-structured flame forms in the outer space after the occurrence of the external explosion,26 and there is not much difference in the speeds of the jet-structured flames between the experiments with and without a relief duct, as shown in Figure 12. Figure 13 presents a typical external flame with and without a duct for ϕ = 1.6. The external flame looks thinner and shorter in the experiments with a duct than those without a duct especially. The reason may be that, in duct-vented explosions, a part of the discharged unburnt gas mixtures was combusted in the duct and the remaining part was vented into the outer space at a higher speed, so the cloud of the external combustible mixtures is thinner.

ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (No. 51276177) and the Key Laboratory of Building Fire Protection Engineering and Technology of MPS (No. KFKT2015MS05).

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REFERENCES

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4. CONCLUSIONS Experimental studies on duct-vented explosions of hydrogen− air mixtures with values of the equivalence ratio ϕ ranging from 0.8 to 4.0 were conducted in a small cylindrical vessel with a 50cm-long duct. The effect of the relief duct on the internal and external overpressures and flame behaviors were investigated. The major conclusions are summarized below: 1. The burn-up in duct causes the local pressure to temporarily exceed the internal one, and as a result a reverse flow forms. The maximum pressure rise rate in the duct, which is always larger than that in the vessel, first increases and then decreases with the increase of ϕ. 9522

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DOI: 10.1021/acs.iecr.6b02029 Ind. Eng. Chem. Res. 2016, 55, 9518−9523