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An experimental investigation on explosion behaviors of syngas-air mixtures in a vessel with a large blockage ratio perforated plate
Fuel 264 (2020) 116842
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An experimental investigation on explosion behaviors of syngas-air mixtures in a vessel with a large blockage ratio perforated plate
T
Lu-Qing Wanga, , Hong-Hao Maa,b, Zhao-Wu Shena ⁎
a
CAS Key Laboratory of Mechanical Behavior and Design of Materials (LMBD), Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui 230026, PR China b State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China
ARTICLE INFO
ABSTRACT
Keywords: Syngas Explosion Perforated plate Maximum explosion pressure Deflagration index
Explosions may occur in the presence of obstacles, which was not given enough considerations. In this study, an experimental study was conducted to investigate the influence of a perforated plate on the explosion characteristics of syngas-air mixtures in a constant volume vessel. The syngas, containing 50% H2 and 50% CO, was selected as the test fuel. The maximum explosion pressure, heat loss to the vessel wall as well as the perforated plate, explosion time and deflagration index were examined. The turbulence enhancement factor, which was defined as the amplification of the burning velocity when a perforated plate was introduced, was also evaluated. The maximum explosion pressure was always decreased with a perforated plate due to the heat transferred to the obstacle. In addition, the maximum explosion pressure decreased as the distance between the obstacle and ignitor decreased. The heat loss to unit area of the obstacle was larger than that of the vessel walls. When a perforated plate existed, the explosion time was decreased. The maximum pressure rise rate could be affected by the obstacle in both positive and negative ways, depending on the blockage ratio and location of the perforated plate. Lastly, the turbulence enhancement factor was found to be up to 3 with a 0.99 blockage ratio.
1. Introduction Nowadays, carbon-based fossil fuels are still the major energy sources for the power output worldwide. The fossil fuels are nonrenewable, and due to the continuous utilization, the reserves are becoming depleted. Besides, the emissions of fossil energy during the industrial processes are environmentally unfriendly, which intensifies the pollution and greenhouse effect. Therefore, the development of clean renewable has been advocated in recent years. Syngas, mainly made up of hydrogen and carbon monoxide, is very promising in the 21st century [1–4]. Generally, syngas can be produced via various processes, such as steam reforming, carbon dioxide reforming, auto-thermal reforming (ATR), gasification of biomass and wastes, etc. [5–8]. It plays an important part in the synthesis of ammonia and methanol. Meanwhile, syngas combustion is promising in the integrated gasification combined cycle (IGCC) [9,10]. However, the safety problem related with syngas explosion cannot be neglected in view of the low ignition energy and wide flammability range. Once an accidental explosion occurs in a congest confined space, the high explosion pressure and temperature will cause huge damage to the infrastructure. For the ha-
⁎
zard assessment, explosion indices must be considered quantitatively [11]. The maximum explosion pressure (Pmax ) and explosion time ( , the time interval between an ignition and Pmax ) are two fundamental parameters for the evaluation of explosion process, which can be read directly in the pressure-time history. The maximum pressure rise rate ((dp / dt ) max ) is of importance for hazardous level assessment and the design of explosion-proof equipment [12,13]. The deflagration index (K G ) can be correlated with the volume of an enclosure (V ) [14,15]:
K G = (dp / dt )max · V 1/3
(1)
Up to now, numerous studies have been conducted to give insight into the explosion characteristics of syngas. Sun [16] studied the explosion behaviors of syngas with various hydrogen contents and found that the fuel composition has great influence on the explosion characteristics. Xie et al. [9,10] systematically investigated the explosion indices of syngas diluted with steam or carbon dioxide at elevated temperatures and pressures. Salzano et al. [1] analyzed the effect of CO2 on the explosion pressure and burning velocity of syngas in oxygen-enriched air environment experimentally and numerically. Di
Corresponding author. E-mail address: [email protected] (L.-Q. Wang).
https://doi.org/10.1016/j.fuel.2019.116842 Received 15 October 2019; Received in revised form 4 December 2019; Accepted 6 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 264 (2020) 116842
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Nomenclature
U
Latin
A BR C D d dp / dt H KG N n P Q q R Rf S SL T
flame propagation speed (m/s) vessel volume (L)
Greek
constant blockage ratio surface area (m2) vessel diameter (mm) hole diameter (mm) explosion pressure rise rate (MPa/s) vessel height (mm) deflagration index (MPs·m/s) hole number molecular number pressure (MPa) heat loss (kJ) heat loss per area (kJ/m2) vessel radius (mm) flame radius (mm) distance between perforated plate and ignitor (mm) laminar burning velocity (m/s) temperature (K)
constant adiabatic coefficient explosion time (ms) equivalence ratio turbulence enhancement factor Superscript
0 1
without perforated plate with perforated plate
Subscript
0 a max tr
Sarli et al. [17] performed investigations on the explosion pressure and the pressure rise rate of wood chip-derived syngas at different temperatures. Tran et al. [18] discussed the influence of hydrocarbons on the explosion behaviors of syngas. More specially, the explosion parameters of syngas in turbulent environments were studied by Sun [13,19]. Recently, Xie et al. [20] studied the intrinsic flame instability parameter (e.g., Markstein length and critical Peclet number) of H2/ CO/air mixtures diluted with CO2 and steam. In practical, there may exist obstructions in confined spaces where explosion hazards occur [21]. It is well known that the explosion process is affected significantly by obstacles. In long obstacle-laden tubes, the flame may undergo deflagration to detonation transition (DDT) rapidly, which has been given much consideration [22–28]. The explosion parameters in vessels with low aspect ratio can be affected by the vessel geometry, the obstacle type and location, the mixture property, etc. Phylaktou et al. [21] studied the effect of spherical-grid obstacles (203 mm in diameter) on explosions in methane-, propane-, ethylene- and hydrogen-air mixtures in a 0.5 m-long, 0.5 m-diameter
initial condition end of combustion maximum value transferred to external
vessel. The flame speed was found to be significantly increased outside the spherical grid; meanwhile, the explosion time was shortened. The explosion behaviors of CH4-air mixtures in a cylindrical chamber (340 mm diameter and 490 mm length) filled with a spherical wire were reported by Kindracki et al. [29]. They found that Pmax and (dp / dt ) max were not influenced by the introduction of the obstacle. Kumar et al. [30] studied the explosion characteristics of H2-steam-air in a 2.3 m diameter spherical vessel with gratings. They indicated that Pmax and (dp / dt ) max were decreased due to the heat transferred to the gratings. Recently, Wang et al. [31–33] studied the effect of a single annular obstacle on the explosion parameters of hydrogen-methane-air mixtures. Both the blockage ratio and location of the obstacle had dual effects on (dp / dt ) max . Depending on the producing process technologies, the mole fraction of H2 (or CO) in syngas can be various [19]. In this study, only the syngas containing 50% H2 and 50% CO was considered as the test fuel, which also attracted widespread attentions in previous work [3,9,10,13,34–36]. The explosion characteristics of H2/CO in air were
Fig. 1. Sketch of experiment setup.
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Fig. 2. Details of perforated plates used in this study, including hole number (N ) and blockage ratio (BR ).
achieved by the spark induced by two electrodes at the center. A stainless steel perforated plate (5 mm thickness) was fixed in the vessel above the ignitor. Three kinds of perforated plates were adopted, depending on the hole number (N ) or the blockage ratio (BR ). The diameter of the hole was d = 21 mm, and the distance between two adjacent holes was also 21 mm (see Fig. 2). Therefore, the blockage ratios (BR = 1 N ·d 2/ D 2 ) were 0.99, 0.97 and 0.95, respectively. The distance between the lower surface of the perforated plate and the electrodes were 35 mm (S/H = 1/6) and 70 mm (S/H = 1/3). In this study, syngas containing 50% H2 and 50% CO was used as the test fuel. The equivalence ratios ( ) of syngas-air mixtures were ranged from 0.6 to 3.0. Firstly, the vessel was vacuumized to an initial pressure less than 100 Pa. A digital pressure gauge was connected to the vessel to monitor the pressure. Secondly, the components (i.e., H2, CO and air) of the mixtures were guided into the vessel based on Dolton’s law. Note that all the experiments were performed at the initial pressure of 0.1 MPa and the ambient temperature (305 K). The mixtures were allowed to be mixed for five minutes to ensure the quiescent condition. Lastly, the mixture was ignited at the center. The explosion pressure histories were captured by a pressure transducer (PCB CA102B06) on the top wall, combined with a signal conditioner (482C05) and an oscilloscope. Before the next shot, the vessel was evacuated and flushed by dry air, which could eliminate the effect of the gas products. Every experiment was repeated for at least three times to test the repeatability, and the standard deviation was represented by the error bar. Dahoe claimed that there exist intrinsic fluctuations of the pressuretime curves [37]. The maximum explosion pressure (Pmax ) cannot be distinguished accurately. Therefore, the pressure-time curves should be smoothed before data processing. In this study, the sample rate was set as 100 kHz, which was the same with previous studies [9–11,13,16,19]. Therefore, the second-order Savitzky-Golay method [38] was adopted.
Fig. 3. Smoothed pressure and pressure rise rate curve at stoichiometric condition without perforated plate.
investigated experimentally in a cylindrical vessel with various perforated plates. The objectives of the present study are as follows: (a) the effect of blockage ratio (equivalently, the number of holes) on the explosion indices; (b) the effect of location of the perforated plate on the explosion indices. Meanwhile, the heat loss during an explosion and turbulence enhancement factor were evaluated. 2. Experimental Fig. 1 shows the schematic of the experimental equipment. Both the height and diameter of the cylinder vessel are 210 mm (H / D =1), in which the explosion experiments were carried out. The ignition was
Fig. 4. Explosion pressure evolution (a) at different equivalence ratios with no perforated plate and (b) stoichiometric syngas-air with various perforated plates.
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Fig. 6. Adiabatic flame temperature and explosion pressure of 50% H2 + 50%CO + air mixture.
Fig. 7. Laminar burning velocity of 50%H2 + 50%CO + air mixtures.
by differentiating the p (t ) curve, from which the maximum pressure rise rate could be attained. 3. Results and discussion Fig. 4(a) exhibits the pressure evolutions of syngas-air mixtures at different equivalence ratios without the perforated plate. Each curve exhibits the same growing trend, i.e., the explosion pressure increases due to the continuous heat release, and then decreases due to the complete consumption of the reactants. From = 0.6 to = 1.6, the pressure rise rate increases monotonously and subsequently, the rise rate decreases with the equivalence ratio. The influence of the perforated plate in the pressure development for the mixture at = 1.0 is shown in Fig. 4(b). Obviously, the maximum explosion pressure is decreased when a perforated plate is introduced; besides, the time reaching Pmax is advanced. Interestingly, there exists a sudden inflexion in the pressure-time curve for N = 1 (BR = 0.99), which causes the more rapid pressure rise. Similar scenario was found in hydrogen-methane-air mixtures with an orifice plate (BR = 0.94 ) [33]. As indicated in Fig. 4(b), the maximum rate of pressure rise (the maximum slope of the tangent) for N = 1 is attained right after the inflexion. This
Fig. 5. Maximum explosion pressure under various distances between ignitor and perforated plate.
Considering other factors (e.g., electric noise and mechanical vibration noise) which could affect the oscillation of the pressure curve, the window width of 125 points was adopted. Fig. 3 shows the smoothed curves of p (t ) and dp / dt of stoichiometric syngas-air mixture without a perforated plate. Obviously, Pmax and can be read directly from the smoothed p (t ) curve. The pressure rise rate (dp / dt ) curve was obtained
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Fig. 8. Maximum explosion pressure under various blockage ratios, (a) S = 35 mm and (b) S = 70 mm.
indicates that this perforated plate accelerates the pressure rise significantly. However, in the cases of N = 3 and N = 5, no obvious inflexion can be observed. Fig. 5 exhibits the effect of the location of the perforated plate on Pmax at different equivalence ratios. As the equivalence ratio increases, Pmax increases firstly and decreases after the peak value of Pmax is reached. The maximum values of Pmax are obtained at = 1.2 , with or without a perforated plate. Nishimura et al. [39] indicated that the adiabatic explosion pressure (Pa ) can be given by
Pa = P0·
Ta na · T0 n 0
Pmax is lower. In addition, the difference between the maximum explosion pressures of S = 70 mm and S = 35 mm is increased with the decrease of N (equivalently, the increase of BR ). This means that Pmax is affected by the location of the perforated plate more significantly for increased blockage ratio. When the equivalence ratio is larger than 1.4, however, there is no obvious difference between the maximum explosion pressures to a specific mixture. This may be ascribed to the higher flame propagation speed at larger equivalence ratios. Fig. 7 shows the laminar burning velocity (SL ) reported in previous studies [45–49]. One 3.0 are can see the laminar burning velocities in the range of 1.6 1.6 since higher than those of < 1.6. The heat release is faster at the laminar burning velocity is closely related to the exothermicity and reactivity [50]. This manifests that the location of the obstacle has no effect on the maximum explosion pressure (or heat loss) when the flame speed exceeds a certain value. The effect of BR on the maximum explosion pressure is shown in Fig. 8. For S = 35 mm, Pmax decreases with the increase of blockage ratio. This is because the contacting surface is decreased for the perforated plate with less holes. However, the difference between the va1.6 is not obvious, which also can be ascribed to the lues of Pmax at higher flame speed at the fuel-rich side. In the case of S = 70 mm, again, there exists no difference between the measured maximum explosion pressures if the uncertainties are taken into consideration. The largest divergence is about 1.6% (obtained at = 0.6). As claimed by Leyer [51], the total heat loss during an explosion, Qtr , or the heat loss to unit area, qtr , could be calculated by
(2)
Herein, T and n are temperature and molecular number, respectively, and the subscripts ‘0 ’ and ‘a ’ represent the initial and the end conditions. In this study, Ta (the adiabatic flame temperature) and Pa were calculated by the chemical equilibrium software GASEQ [40], assuming no heat loss during an explosion process. Ten compounds were considered as the products: N2, H2O, CO2, CO, O2, OH, H, O, N2 and NO (hydrocarbon/O2/N2 products). As shown in Fig. 6, both the peak values of Ta and Pa are attained at = 1.2. Besides, Pmax is always less than the correspond Pa due to the heat loss in the practical situation. Once a perforated plate is introduced into the vessel, one can see that the maximum explosion pressure is decreased to a specific mixture. In a confined vessel, the adiabatic explosion pressure is determined by the mixture properties [13]. Thus, Pa would not be changed in the presence of a perforated plate because the total heat release remains constant. Mitu and Brandes [41] demonstrated that the heat release could be separated into two parts, i.e., the heat accumulated in the vessel (reflected by Pmax ) and the heat loss to the external. Therefore, the maximum explosion pressure is decreased due to the additional heat transferred to the perforated plate. For a given blockage ratio, Pmax is higher for the larger S for < 1.6. After an ignition, a flame kernel would be formed, which then propagates outward spherically. The flame propagation speed (U ) can be calculated by the following expression [42–44]
U = A· Rf
V
Qtr = a
qtr =
V · C0
1
·(Pa
1 1
a
0 Pmax )
·(Pa
(4)
0 Pmax )
(5)
in which and C are the adiabatic coefficient and inner surface, and the superscript ‘0’ represents ‘without obstacle’. As shown in Figs. 5 and 8, Pmax (or heat loss) is almost independent of S and BR for higher flame speed. Therefore, the heat transferred to the vessel wall can be regarded as a constant even when a perforated plate is introduced in to the vessel. The heat loss to unit area of the perforated plate is
(3)
where A and are two positive constants and Rf is the flame radius. Eq. (3) indicates that the flame propagation speed increases with the flame radius. The flame-obstacle contacting time is longer when the obstacle is closer to the ignitor (S = 35 mm ); hence, more heat loss occurs and
qtr =
V · C1
1 a
1
0 ·(Pmax
1 Pmax )
(6)
The superscript ‘1’ represents ‘with obstacle’. In this study, the
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Fig. 9. Heat loss per unit of wall and perforated plate. Fig. 10. Explosion time under various distances between ignitor and plate.
thickness of the perforated plate is 5 mm, and the explosion duration is relatively short; hence, the heat transfers to both the upper and lower surface of the obstacle. Thus, the surface in Eq. (6) can be calculated by the following equation:
C1 = 2 ·BR·R2
plate, calculated by Eqs. (5) and (6), is shown in Fig. 9. Generally, the heat transferred to unit surface of the obstacles is higher than that of the vessel wall. When the perforated plate is close to the ignitor (S = 35 mm), the heat loss is more significant due to the lower flame propagation speed and hence longer flame-obstacle contacting time. For the mixtures with low reactivity ( < 1.6 ), the differences between
(7)
The heat loss to unit surface to the vessel wall and the perforated 6
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Fig. 11. Explosion time under various blockage ratios.
the heat loss are more significant, compared with the high sensitive mixtures. This is also because the flame speed is lower. Fig. 10 shows the explosion time, , at different equivalence ratios and locations. For all scenarios, the shortest occurs at = 1.6. As suggested by Xie et al. [9], there exists a positive feedback between the explosion time and the laminar burning velocity. The premixed mixtures can be depleted more rapidly when the corresponding laminar burning velocity is higher, resulting shorter explosion time. In addition, the explosion time is also affected by the maximum adiabatic flame temperature [18], i.e., higher Ta results in shorter . As plotted in Figs. 6 and 7, the maximum SL and Ta occurs at = 2.0 and = 1.2 , respectively. Therefore, the shortest explosion time is obtained between = 2.0 and = 1.2 , i.e., = 1.6. This is ascribed to the combined effects of the two factors. Compared with the scenario without a perforated plate, the explosion time is always decreased. When a flame interacts with the perforated plate, the flame front is distorted. The increased surface of the flame front accelerates the chemical reaction rate and hence shortens the explosion. The explosion time is also influenced by the location of the perforated plate. For the largest blockage ratio (BR = 0.99), decreases in the order of no plate, S = 70 mm and S = 35 mm (see Fig. 10(a)). However, as the blockage ratio decreases, the difference between S = 70 mm and S = 35 mm is not obvious. As reported in previous studies [30–32], the obstacle in a closed vessel could affect the explosion time in an opposite way, i.e., the explosion time can be increased due to the heat-sink effect of the obstacle (hence decreased chemical reaction rate). The dual effects of the perforated plates (BR = 0.97 and BR = 0.95) make the explosion time of S = 70 mm and S = 35 mm very close. Fig. 11 presents the effect of the blockage ratio on the explosion under the same location. The explosion time obtained with BR = 0.99 is always the minimum, indicating that the increased BR has more turbulence-generating effect. For S = 35 mm (Fig. 11(a)), the flame speed is lower when flame-obstacle contacting occurs. The explosion time is increased in the order of BR = 0.99, BR = 0.97 and BR = 0.95. In the case of S = 70 mm , the explosion time obtained with BR = 0.97 and BR = 0.95 is very close. This indicates that both the heat-sink and turbulence-generating effects influence the explosion time when the perforated plate is far away from the ignitor. The effect of obstacle location on the maximum explosion pressure rise rate, (dp / dt ) max , is shown in Fig. 12. Meanwhile, the deflagration
index, K G , is also presented. Both the maximum (dp / dt ) max and K G obtained without a perforated plate is obtained at = 1.6, which is consistent with that reported by Sun [13]. The deflagration index obtained without a perforated plate is smaller than 30 MPa·m/s, indicating that the mixtures is within a safe level [52]. As shown in Fig. 12(b, c), for the perforated plates with BR = 0.95 and BR = 0.97 , the deflagration indexes are still smaller than 30 MPa·m/s. However, the K G is increased significantly with the introduction of the BR= perforated plate (Fig. 12(a)). From = 0.8 to = 2.0 , the deflagration indexes are larger than 30 MPa·m/s, i.e., the most hazardous level. This confirms again the significant turbulence-generating role played by the BR=perforated plate. For BR = 0.99, (dp / dt ) max increases with the increase of S . However, the peak value of Pmax occurs at = 1.2 , which is different from that of the ‘no-plate’ scenario. This is because that the 1.6. The turbulence induced flame propagation speed is lower than by the perforated plate is more intense for low flame speed; meanwhile, Pmax occurs at = 1.2 . The two positive effects make (dp / dt ) max to be the largest at = 1.2 . In the cases of BR = 0.97 and BR = 0.95, the turbulence-generating effect is weakened due to the decreased blockage ratio. Thus, the inflection equivalence ratio is observed at = 1.6 , indicating that the laminar burning velocity has more dominant influence on (dp / dt ) max for decreased blockage ratio. As shown in Fig. 12(c), (dp / dt ) max decreases with the increase of S . Therefore, the heat-sink effect of the perforated plate, instead of the turbulence-generating effect, dominates the (dp / dt ) max behavior for BR = 0.95. The maximum pressure rise rate is dominated by the combination of the two competitive effects for the medium blockage ratio (see Fig. 12(b)); thus, the relationship between (dp / dt ) max and S is not monotonous. Fig. 13 shows the effect of blockage ratio on the maximum pressure rise rate. Uniformly, (dp / dt ) max increases with the increase of the blockage ratio. Bradley and Mitcheson [53] suggested that the pressure rise rate should be correlated with SL and the explosion pressure, Fig. 14
3SL 0 dp = (Pa dt Ra
P0) 1
P0 P
1/ a
Pa Pa
P P0
2/3
(8)
in which is the density. Later, Faghih et al. [54] claimed the maximum pressure rise rate could be expressed as
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SL, a =
1 R P · · max 3 Pmax P0 P0
0
·(dp / dt )max
(10)
For a closed vessel without a perforated plate, there exists no turbulence generated from the fluctuation of the flow field [21]. Therefore, we define a parameter, the turbulence enhancement factor:
= SL1, a/SL0, a
(11)
As indicated by Wang et al. [55,56], the flame front could be wrinkled after passing through a perforated plate, resulting higher turbulent flame burning velocity. As the initial pressure and turbulence intensity increase, the turbulent flame burning velocity increases significantly. However, one can see that for BR = 0.95 and BR = 0.97 , ranges from 0.71 to 1.25. At < 1.0 , the fuel-lean side, the turbulence enhancement factor obtained with BR = 0.99 are significantly increased. The maximum value of is found to be ~3.13 with S = 75 mm. This indicates that the turbulence induced by the perforated plate is strongly dependent on the blockage ratio. Compared with Refs. [55,56], there exists no turbulence before an ignition, and the initial pressure is 0.1 MPa in this study. In the case of BR = 0.95 and BR = 0.97 , the combination of the heat-sink and turbulence-generation effects result in low turbulent burning velocity. 4. Conclusion In the present study, the explosion characteristics of syngas-air mixtures in a vessel with a perforated plate were examined experimentally. The effects of the distance between the obstacle and the ignitor (S ) and the blockage ratio (BR ) were considered. Main conclusions are as follows: (1) The maximum explosion pressure (Pmax ) is decreased in the presence of a perforated plate due to the additional heat loss. The heat loss is larger when the perforated plate is close to the ignitor, thus, Pmax is decreased with the decrease of S . Compared with the vessel wall, the heat transferred to unit surface of the perforated plate is more significant. (2) The explosion time ( ) is decreased due to the turbulence induced by the perforated plate. For the perforated plate with BR = 0.99, decreases as the distance between the obstacle and the ignitor increase. As the blockage ratio decreases (BR = 0.97 and BR = 0.95), no obvious difference between the explosion time can be detected for various obstacle locations. (3) The effects of the perforated plate on the maximum rate of pressure rise ((dp / dt ) max ) strongly depends on S and BR . Generally, (dp / dt ) max is increased with the increase of the blockage ratio. For BR = 0.99, the turbulence-generating of the perforated plate has more dominant effect on (dp / dt ) max . Thus, the maximum rate of pressure rise is increased significantly in existence of a perforated. However, as the blockage ratio decreases, the role of heat-sink of the obstacle participates in to affect (dp / dt ) max . Therefore, the maximum pressure rise rate can be decreased with a perforated plate. (4) When the blockage ratios are 0.97 and 0.95, the turbulence enhancement factors ( ) are close to unit, indicating that the induced turbulence is weak. For BR = 0.99, can be as large as about 3 at the fuel-lean side.
Fig. 12. Maximum rate of pressure rise under various distances between ignitor and plate.
(dp / dt ) max =
3SL, a P ·P0· a R P0
1 ·
Pa P0
1/ 0
(9)
Declaration of Competing Interest
Experimentally, the adiabatic explosion pressure is reflected by the maximum explosion pressure. Thus, the laminar burning speed, SL, a (note again, the subscript ‘a ’ represents the end of the combustion), can be calculated by
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Fig. 14. Turbulence enhancement factor under different obstacle conditions.
Acknowledgements This research was financially supported by the National Natural Science Foundation of China under Project NOs. 51674229 and 51874267, and China Postdoctoral Science Foundation Grant under Project Nos. 2019TQ0310 and 2019M660154. References [1] Salzano E, Basco A, Cammarota F, et al. Explosions of syngas/CO2 mixtures in oxygen-enriched air. Ind Eng Chem Res 2011;51(22):7671–8. [2] Liu J, Wang J, Zhang N, et al. On the explosion limit of syngas with CO2 and H2O additions. Int J Hydrogen Energy 2018;43(6):3317–29. [3] Wang LQ, Ma HH, Shen ZW, et al. Detonation behaviors of syngas-oxygen in round and square tubes. Int J Hydrogen Energy 2018;43(31):14775–86. [4] Wang WQ, Sun ZY. Experimental studies on explosive limits and minimum ignition energy of syngas: a comparative review. Int J Hydrogen Energy 2019;44(11):5640–9. [5] Rostrup-Nielsen JR. New aspects of syngas production and use. Catal Today 2000;63(2–4):159–64. [6] Rostrup-Nielsen JR. Syngas in perspective. Catal Today 2002;71(3–4):243–7. [7] Yung MM, Jablonski WS, Magrini-Bair KA. Review of catalytic conditioning of biomass-derived syngas. Energy Fuels 2009;23(4):1874–87. [8] Göransson K, Söderlind U, He J, et al. Review of syngas production via biomass DFBGs. Renew Sustain Energy Rev 2011;15(1):482–92. [9] Xie Y, Wang J, Cai X, et al. Pressure history in the explosion of moist syngas/air mixtures. Fuel 2016;185:18–25. [10] Xie Y, Wang X, Bi H, et al. A comprehensive review on laminar spherically premixed flame propagation of syngas. Fuel Process Technol 2018;181:97–114. [11] Sun ZY, Li GX. Turbulence influence on explosion characteristics of stoichiometric and rich hydrogen/air mixtures in a spherical closed vessel. Energy Convers
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L.-Q. Wang, et al. gas explosions. J Loss Prev Process Ind 2005;18(3):152–66. [38] Savitzky A, Golay MJE. Smoothing and differentiation of data by simplified least squares procedures. Anal Chem 1964;36(8):1627–39. [39] Nishimura I, Mogi T, Dobashi R. Simple method for predicting pressure behavior during gas explosions in confined spaces considering flame instabilities. J Loss Prev Process Ind 2013;26(2):351–4. [40] Morley C, Gaseq A. A chemical equilibrium program for windows. Version 0.79 b, URL: http://www.gaseq.co.uk [accessed November 10, 2010]; 2005. [41] Mitu M, Brandes E. Influence of pressure, temperature and vessel volume on explosion characteristics of ethanol/air mixtures in closed spherical vessels. Fuel 2017;203:460–8. [42] Bychkov VV, Liberman MA. Stability and the fractal structure of a spherical flame in a self-similar regime. Phys Rev Lett 1996;76(15):2814. [43] Blinnikov SI, Sasorov PV. Landau-Darrieus instability and the fractal dimension of flame fronts. Phys Rev E 1996;53(5):4827. [44] Wu F, Jomaas G, Law CK. An experimental investigation on self-acceleration of cellular spherical flames. Proc Combust Inst 2013;34(1):937–45. [45] Prathap C, Ray A, Ravi MR. Investigation of nitrogen dilution effects on the laminar burning velocity and flame stability of syngas fuel at atmospheric condition. Combust Flame 2008;155(1–2):145–60. [46] Hassan MI, Aung KT, Faeth GM. Properties of laminar premixed CO/H/air flames at various pressures. J Propul Power 1997;13(2):239–45. [47] Sun H, Yang SI, Jomaas G, et al. High-pressure laminar flame speeds and kinetic modeling of carbon monoxide/hydrogen combustion. Proc Combust Inst
2007;31(1):439–46. [48] Bouvet N, Chauveau C, Gökalp I, et al. Experimental studies of the fundamental flame speeds of syngas (H2/CO)/air mixtures. Proc Combust Inst 2011;33(1):913–20. [49] Xie Y, Wang J, Xu N, et al. Thermal and chemical effects of water addition on laminar burning velocity of syngas. Energy Fuels 2014;28(5):3391–8. [50] Ranzi E, Frassoldati A, Grana R, et al. Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels. Prog Energy Combust Sci 2012;38(4):468–501. [51] Leyer JC. Contributions a l’étude des instabilités de la combustion et des phenomènes de transfer de chaleur aux parois dans le cas des combustions à volume constant. Rev Gén Therm 1970;98:121–38. [52] National Fire Protection Association. Guide for venting of deflagrations. NFPA68; 2002. [53] Bradley D, Mitcheson A. Mathematical solutions for explosions in spherical vessels. Combust Flame 1976;26:201–17. [54] Faghih M, Gou X, Chen Z. The explosion characteristics of methane, hydrogen and their mixtures: a computational study. J Loss Prev Process Ind 2016;40:131–8. [55] Wang J, Zhang M, Huang Z, et al. Measurement of the instantaneous flame front structure of syngas turbulent premixed flames at high pressure. Combust Flame 2013;160(11):2434–41. [56] Wang J, Yu S, Zhang M, et al. Burning velocity and statistical flame front structure of turbulent premixed flames at high pressure up to 1.0 MPa. Exp Therm Fluid Sci 2015;68:196–204.
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Full Length Article
An experimental investigation on explosion behaviors of syngas-air mixtures in a vessel with a large blockage ratio perforated plate
T
Lu-Qing Wanga, , Hong-Hao Maa,b, Zhao-Wu Shena ⁎
a
CAS Key Laboratory of Mechanical Behavior and Design of Materials (LMBD), Department of Modern Mechanics, University of Science and Technology of China, Hefei, Anhui 230026, PR China b State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China
ARTICLE INFO
ABSTRACT
Keywords: Syngas Explosion Perforated plate Maximum explosion pressure Deflagration index
Explosions may occur in the presence of obstacles, which was not given enough considerations. In this study, an experimental study was conducted to investigate the influence of a perforated plate on the explosion characteristics of syngas-air mixtures in a constant volume vessel. The syngas, containing 50% H2 and 50% CO, was selected as the test fuel. The maximum explosion pressure, heat loss to the vessel wall as well as the perforated plate, explosion time and deflagration index were examined. The turbulence enhancement factor, which was defined as the amplification of the burning velocity when a perforated plate was introduced, was also evaluated. The maximum explosion pressure was always decreased with a perforated plate due to the heat transferred to the obstacle. In addition, the maximum explosion pressure decreased as the distance between the obstacle and ignitor decreased. The heat loss to unit area of the obstacle was larger than that of the vessel walls. When a perforated plate existed, the explosion time was decreased. The maximum pressure rise rate could be affected by the obstacle in both positive and negative ways, depending on the blockage ratio and location of the perforated plate. Lastly, the turbulence enhancement factor was found to be up to 3 with a 0.99 blockage ratio.
1. Introduction Nowadays, carbon-based fossil fuels are still the major energy sources for the power output worldwide. The fossil fuels are nonrenewable, and due to the continuous utilization, the reserves are becoming depleted. Besides, the emissions of fossil energy during the industrial processes are environmentally unfriendly, which intensifies the pollution and greenhouse effect. Therefore, the development of clean renewable has been advocated in recent years. Syngas, mainly made up of hydrogen and carbon monoxide, is very promising in the 21st century [1–4]. Generally, syngas can be produced via various processes, such as steam reforming, carbon dioxide reforming, auto-thermal reforming (ATR), gasification of biomass and wastes, etc. [5–8]. It plays an important part in the synthesis of ammonia and methanol. Meanwhile, syngas combustion is promising in the integrated gasification combined cycle (IGCC) [9,10]. However, the safety problem related with syngas explosion cannot be neglected in view of the low ignition energy and wide flammability range. Once an accidental explosion occurs in a congest confined space, the high explosion pressure and temperature will cause huge damage to the infrastructure. For the ha-
⁎
zard assessment, explosion indices must be considered quantitatively [11]. The maximum explosion pressure (Pmax ) and explosion time ( , the time interval between an ignition and Pmax ) are two fundamental parameters for the evaluation of explosion process, which can be read directly in the pressure-time history. The maximum pressure rise rate ((dp / dt ) max ) is of importance for hazardous level assessment and the design of explosion-proof equipment [12,13]. The deflagration index (K G ) can be correlated with the volume of an enclosure (V ) [14,15]:
K G = (dp / dt )max · V 1/3
(1)
Up to now, numerous studies have been conducted to give insight into the explosion characteristics of syngas. Sun [16] studied the explosion behaviors of syngas with various hydrogen contents and found that the fuel composition has great influence on the explosion characteristics. Xie et al. [9,10] systematically investigated the explosion indices of syngas diluted with steam or carbon dioxide at elevated temperatures and pressures. Salzano et al. [1] analyzed the effect of CO2 on the explosion pressure and burning velocity of syngas in oxygen-enriched air environment experimentally and numerically. Di
Corresponding author. E-mail address: [email protected] (L.-Q. Wang).
https://doi.org/10.1016/j.fuel.2019.116842 Received 15 October 2019; Received in revised form 4 December 2019; Accepted 6 December 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature
U
Latin
A BR C D d dp / dt H KG N n P Q q R Rf S SL T
flame propagation speed (m/s) vessel volume (L)
Greek
constant blockage ratio surface area (m2) vessel diameter (mm) hole diameter (mm) explosion pressure rise rate (MPa/s) vessel height (mm) deflagration index (MPs·m/s) hole number molecular number pressure (MPa) heat loss (kJ) heat loss per area (kJ/m2) vessel radius (mm) flame radius (mm) distance between perforated plate and ignitor (mm) laminar burning velocity (m/s) temperature (K)
constant adiabatic coefficient explosion time (ms) equivalence ratio turbulence enhancement factor Superscript
0 1
without perforated plate with perforated plate
Subscript
0 a max tr
Sarli et al. [17] performed investigations on the explosion pressure and the pressure rise rate of wood chip-derived syngas at different temperatures. Tran et al. [18] discussed the influence of hydrocarbons on the explosion behaviors of syngas. More specially, the explosion parameters of syngas in turbulent environments were studied by Sun [13,19]. Recently, Xie et al. [20] studied the intrinsic flame instability parameter (e.g., Markstein length and critical Peclet number) of H2/ CO/air mixtures diluted with CO2 and steam. In practical, there may exist obstructions in confined spaces where explosion hazards occur [21]. It is well known that the explosion process is affected significantly by obstacles. In long obstacle-laden tubes, the flame may undergo deflagration to detonation transition (DDT) rapidly, which has been given much consideration [22–28]. The explosion parameters in vessels with low aspect ratio can be affected by the vessel geometry, the obstacle type and location, the mixture property, etc. Phylaktou et al. [21] studied the effect of spherical-grid obstacles (203 mm in diameter) on explosions in methane-, propane-, ethylene- and hydrogen-air mixtures in a 0.5 m-long, 0.5 m-diameter
initial condition end of combustion maximum value transferred to external
vessel. The flame speed was found to be significantly increased outside the spherical grid; meanwhile, the explosion time was shortened. The explosion behaviors of CH4-air mixtures in a cylindrical chamber (340 mm diameter and 490 mm length) filled with a spherical wire were reported by Kindracki et al. [29]. They found that Pmax and (dp / dt ) max were not influenced by the introduction of the obstacle. Kumar et al. [30] studied the explosion characteristics of H2-steam-air in a 2.3 m diameter spherical vessel with gratings. They indicated that Pmax and (dp / dt ) max were decreased due to the heat transferred to the gratings. Recently, Wang et al. [31–33] studied the effect of a single annular obstacle on the explosion parameters of hydrogen-methane-air mixtures. Both the blockage ratio and location of the obstacle had dual effects on (dp / dt ) max . Depending on the producing process technologies, the mole fraction of H2 (or CO) in syngas can be various [19]. In this study, only the syngas containing 50% H2 and 50% CO was considered as the test fuel, which also attracted widespread attentions in previous work [3,9,10,13,34–36]. The explosion characteristics of H2/CO in air were
Fig. 1. Sketch of experiment setup.
2
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Fig. 2. Details of perforated plates used in this study, including hole number (N ) and blockage ratio (BR ).
achieved by the spark induced by two electrodes at the center. A stainless steel perforated plate (5 mm thickness) was fixed in the vessel above the ignitor. Three kinds of perforated plates were adopted, depending on the hole number (N ) or the blockage ratio (BR ). The diameter of the hole was d = 21 mm, and the distance between two adjacent holes was also 21 mm (see Fig. 2). Therefore, the blockage ratios (BR = 1 N ·d 2/ D 2 ) were 0.99, 0.97 and 0.95, respectively. The distance between the lower surface of the perforated plate and the electrodes were 35 mm (S/H = 1/6) and 70 mm (S/H = 1/3). In this study, syngas containing 50% H2 and 50% CO was used as the test fuel. The equivalence ratios ( ) of syngas-air mixtures were ranged from 0.6 to 3.0. Firstly, the vessel was vacuumized to an initial pressure less than 100 Pa. A digital pressure gauge was connected to the vessel to monitor the pressure. Secondly, the components (i.e., H2, CO and air) of the mixtures were guided into the vessel based on Dolton’s law. Note that all the experiments were performed at the initial pressure of 0.1 MPa and the ambient temperature (305 K). The mixtures were allowed to be mixed for five minutes to ensure the quiescent condition. Lastly, the mixture was ignited at the center. The explosion pressure histories were captured by a pressure transducer (PCB CA102B06) on the top wall, combined with a signal conditioner (482C05) and an oscilloscope. Before the next shot, the vessel was evacuated and flushed by dry air, which could eliminate the effect of the gas products. Every experiment was repeated for at least three times to test the repeatability, and the standard deviation was represented by the error bar. Dahoe claimed that there exist intrinsic fluctuations of the pressuretime curves [37]. The maximum explosion pressure (Pmax ) cannot be distinguished accurately. Therefore, the pressure-time curves should be smoothed before data processing. In this study, the sample rate was set as 100 kHz, which was the same with previous studies [9–11,13,16,19]. Therefore, the second-order Savitzky-Golay method [38] was adopted.
Fig. 3. Smoothed pressure and pressure rise rate curve at stoichiometric condition without perforated plate.
investigated experimentally in a cylindrical vessel with various perforated plates. The objectives of the present study are as follows: (a) the effect of blockage ratio (equivalently, the number of holes) on the explosion indices; (b) the effect of location of the perforated plate on the explosion indices. Meanwhile, the heat loss during an explosion and turbulence enhancement factor were evaluated. 2. Experimental Fig. 1 shows the schematic of the experimental equipment. Both the height and diameter of the cylinder vessel are 210 mm (H / D =1), in which the explosion experiments were carried out. The ignition was
Fig. 4. Explosion pressure evolution (a) at different equivalence ratios with no perforated plate and (b) stoichiometric syngas-air with various perforated plates.
3
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Fig. 6. Adiabatic flame temperature and explosion pressure of 50% H2 + 50%CO + air mixture.
Fig. 7. Laminar burning velocity of 50%H2 + 50%CO + air mixtures.
by differentiating the p (t ) curve, from which the maximum pressure rise rate could be attained. 3. Results and discussion Fig. 4(a) exhibits the pressure evolutions of syngas-air mixtures at different equivalence ratios without the perforated plate. Each curve exhibits the same growing trend, i.e., the explosion pressure increases due to the continuous heat release, and then decreases due to the complete consumption of the reactants. From = 0.6 to = 1.6, the pressure rise rate increases monotonously and subsequently, the rise rate decreases with the equivalence ratio. The influence of the perforated plate in the pressure development for the mixture at = 1.0 is shown in Fig. 4(b). Obviously, the maximum explosion pressure is decreased when a perforated plate is introduced; besides, the time reaching Pmax is advanced. Interestingly, there exists a sudden inflexion in the pressure-time curve for N = 1 (BR = 0.99), which causes the more rapid pressure rise. Similar scenario was found in hydrogen-methane-air mixtures with an orifice plate (BR = 0.94 ) [33]. As indicated in Fig. 4(b), the maximum rate of pressure rise (the maximum slope of the tangent) for N = 1 is attained right after the inflexion. This
Fig. 5. Maximum explosion pressure under various distances between ignitor and perforated plate.
Considering other factors (e.g., electric noise and mechanical vibration noise) which could affect the oscillation of the pressure curve, the window width of 125 points was adopted. Fig. 3 shows the smoothed curves of p (t ) and dp / dt of stoichiometric syngas-air mixture without a perforated plate. Obviously, Pmax and can be read directly from the smoothed p (t ) curve. The pressure rise rate (dp / dt ) curve was obtained
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Fig. 8. Maximum explosion pressure under various blockage ratios, (a) S = 35 mm and (b) S = 70 mm.
indicates that this perforated plate accelerates the pressure rise significantly. However, in the cases of N = 3 and N = 5, no obvious inflexion can be observed. Fig. 5 exhibits the effect of the location of the perforated plate on Pmax at different equivalence ratios. As the equivalence ratio increases, Pmax increases firstly and decreases after the peak value of Pmax is reached. The maximum values of Pmax are obtained at = 1.2 , with or without a perforated plate. Nishimura et al. [39] indicated that the adiabatic explosion pressure (Pa ) can be given by
Pa = P0·
Ta na · T0 n 0
Pmax is lower. In addition, the difference between the maximum explosion pressures of S = 70 mm and S = 35 mm is increased with the decrease of N (equivalently, the increase of BR ). This means that Pmax is affected by the location of the perforated plate more significantly for increased blockage ratio. When the equivalence ratio is larger than 1.4, however, there is no obvious difference between the maximum explosion pressures to a specific mixture. This may be ascribed to the higher flame propagation speed at larger equivalence ratios. Fig. 7 shows the laminar burning velocity (SL ) reported in previous studies [45–49]. One 3.0 are can see the laminar burning velocities in the range of 1.6 1.6 since higher than those of < 1.6. The heat release is faster at the laminar burning velocity is closely related to the exothermicity and reactivity [50]. This manifests that the location of the obstacle has no effect on the maximum explosion pressure (or heat loss) when the flame speed exceeds a certain value. The effect of BR on the maximum explosion pressure is shown in Fig. 8. For S = 35 mm, Pmax decreases with the increase of blockage ratio. This is because the contacting surface is decreased for the perforated plate with less holes. However, the difference between the va1.6 is not obvious, which also can be ascribed to the lues of Pmax at higher flame speed at the fuel-rich side. In the case of S = 70 mm, again, there exists no difference between the measured maximum explosion pressures if the uncertainties are taken into consideration. The largest divergence is about 1.6% (obtained at = 0.6). As claimed by Leyer [51], the total heat loss during an explosion, Qtr , or the heat loss to unit area, qtr , could be calculated by
(2)
Herein, T and n are temperature and molecular number, respectively, and the subscripts ‘0 ’ and ‘a ’ represent the initial and the end conditions. In this study, Ta (the adiabatic flame temperature) and Pa were calculated by the chemical equilibrium software GASEQ [40], assuming no heat loss during an explosion process. Ten compounds were considered as the products: N2, H2O, CO2, CO, O2, OH, H, O, N2 and NO (hydrocarbon/O2/N2 products). As shown in Fig. 6, both the peak values of Ta and Pa are attained at = 1.2. Besides, Pmax is always less than the correspond Pa due to the heat loss in the practical situation. Once a perforated plate is introduced into the vessel, one can see that the maximum explosion pressure is decreased to a specific mixture. In a confined vessel, the adiabatic explosion pressure is determined by the mixture properties [13]. Thus, Pa would not be changed in the presence of a perforated plate because the total heat release remains constant. Mitu and Brandes [41] demonstrated that the heat release could be separated into two parts, i.e., the heat accumulated in the vessel (reflected by Pmax ) and the heat loss to the external. Therefore, the maximum explosion pressure is decreased due to the additional heat transferred to the perforated plate. For a given blockage ratio, Pmax is higher for the larger S for < 1.6. After an ignition, a flame kernel would be formed, which then propagates outward spherically. The flame propagation speed (U ) can be calculated by the following expression [42–44]
U = A· Rf
V
Qtr = a
qtr =
V · C0
1
·(Pa
1 1
a
0 Pmax )
·(Pa
(4)
0 Pmax )
(5)
in which and C are the adiabatic coefficient and inner surface, and the superscript ‘0’ represents ‘without obstacle’. As shown in Figs. 5 and 8, Pmax (or heat loss) is almost independent of S and BR for higher flame speed. Therefore, the heat transferred to the vessel wall can be regarded as a constant even when a perforated plate is introduced in to the vessel. The heat loss to unit area of the perforated plate is
(3)
where A and are two positive constants and Rf is the flame radius. Eq. (3) indicates that the flame propagation speed increases with the flame radius. The flame-obstacle contacting time is longer when the obstacle is closer to the ignitor (S = 35 mm ); hence, more heat loss occurs and
qtr =
V · C1
1 a
1
0 ·(Pmax
1 Pmax )
(6)
The superscript ‘1’ represents ‘with obstacle’. In this study, the
5
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Fig. 9. Heat loss per unit of wall and perforated plate. Fig. 10. Explosion time under various distances between ignitor and plate.
thickness of the perforated plate is 5 mm, and the explosion duration is relatively short; hence, the heat transfers to both the upper and lower surface of the obstacle. Thus, the surface in Eq. (6) can be calculated by the following equation:
C1 = 2 ·BR·R2
plate, calculated by Eqs. (5) and (6), is shown in Fig. 9. Generally, the heat transferred to unit surface of the obstacles is higher than that of the vessel wall. When the perforated plate is close to the ignitor (S = 35 mm), the heat loss is more significant due to the lower flame propagation speed and hence longer flame-obstacle contacting time. For the mixtures with low reactivity ( < 1.6 ), the differences between
(7)
The heat loss to unit surface to the vessel wall and the perforated 6
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Fig. 11. Explosion time under various blockage ratios.
the heat loss are more significant, compared with the high sensitive mixtures. This is also because the flame speed is lower. Fig. 10 shows the explosion time, , at different equivalence ratios and locations. For all scenarios, the shortest occurs at = 1.6. As suggested by Xie et al. [9], there exists a positive feedback between the explosion time and the laminar burning velocity. The premixed mixtures can be depleted more rapidly when the corresponding laminar burning velocity is higher, resulting shorter explosion time. In addition, the explosion time is also affected by the maximum adiabatic flame temperature [18], i.e., higher Ta results in shorter . As plotted in Figs. 6 and 7, the maximum SL and Ta occurs at = 2.0 and = 1.2 , respectively. Therefore, the shortest explosion time is obtained between = 2.0 and = 1.2 , i.e., = 1.6. This is ascribed to the combined effects of the two factors. Compared with the scenario without a perforated plate, the explosion time is always decreased. When a flame interacts with the perforated plate, the flame front is distorted. The increased surface of the flame front accelerates the chemical reaction rate and hence shortens the explosion. The explosion time is also influenced by the location of the perforated plate. For the largest blockage ratio (BR = 0.99), decreases in the order of no plate, S = 70 mm and S = 35 mm (see Fig. 10(a)). However, as the blockage ratio decreases, the difference between S = 70 mm and S = 35 mm is not obvious. As reported in previous studies [30–32], the obstacle in a closed vessel could affect the explosion time in an opposite way, i.e., the explosion time can be increased due to the heat-sink effect of the obstacle (hence decreased chemical reaction rate). The dual effects of the perforated plates (BR = 0.97 and BR = 0.95) make the explosion time of S = 70 mm and S = 35 mm very close. Fig. 11 presents the effect of the blockage ratio on the explosion under the same location. The explosion time obtained with BR = 0.99 is always the minimum, indicating that the increased BR has more turbulence-generating effect. For S = 35 mm (Fig. 11(a)), the flame speed is lower when flame-obstacle contacting occurs. The explosion time is increased in the order of BR = 0.99, BR = 0.97 and BR = 0.95. In the case of S = 70 mm , the explosion time obtained with BR = 0.97 and BR = 0.95 is very close. This indicates that both the heat-sink and turbulence-generating effects influence the explosion time when the perforated plate is far away from the ignitor. The effect of obstacle location on the maximum explosion pressure rise rate, (dp / dt ) max , is shown in Fig. 12. Meanwhile, the deflagration
index, K G , is also presented. Both the maximum (dp / dt ) max and K G obtained without a perforated plate is obtained at = 1.6, which is consistent with that reported by Sun [13]. The deflagration index obtained without a perforated plate is smaller than 30 MPa·m/s, indicating that the mixtures is within a safe level [52]. As shown in Fig. 12(b, c), for the perforated plates with BR = 0.95 and BR = 0.97 , the deflagration indexes are still smaller than 30 MPa·m/s. However, the K G is increased significantly with the introduction of the BR= perforated plate (Fig. 12(a)). From = 0.8 to = 2.0 , the deflagration indexes are larger than 30 MPa·m/s, i.e., the most hazardous level. This confirms again the significant turbulence-generating role played by the BR=perforated plate. For BR = 0.99, (dp / dt ) max increases with the increase of S . However, the peak value of Pmax occurs at = 1.2 , which is different from that of the ‘no-plate’ scenario. This is because that the 1.6. The turbulence induced flame propagation speed is lower than by the perforated plate is more intense for low flame speed; meanwhile, Pmax occurs at = 1.2 . The two positive effects make (dp / dt ) max to be the largest at = 1.2 . In the cases of BR = 0.97 and BR = 0.95, the turbulence-generating effect is weakened due to the decreased blockage ratio. Thus, the inflection equivalence ratio is observed at = 1.6 , indicating that the laminar burning velocity has more dominant influence on (dp / dt ) max for decreased blockage ratio. As shown in Fig. 12(c), (dp / dt ) max decreases with the increase of S . Therefore, the heat-sink effect of the perforated plate, instead of the turbulence-generating effect, dominates the (dp / dt ) max behavior for BR = 0.95. The maximum pressure rise rate is dominated by the combination of the two competitive effects for the medium blockage ratio (see Fig. 12(b)); thus, the relationship between (dp / dt ) max and S is not monotonous. Fig. 13 shows the effect of blockage ratio on the maximum pressure rise rate. Uniformly, (dp / dt ) max increases with the increase of the blockage ratio. Bradley and Mitcheson [53] suggested that the pressure rise rate should be correlated with SL and the explosion pressure, Fig. 14
3SL 0 dp = (Pa dt Ra
P0) 1
P0 P
1/ a
Pa Pa
P P0
2/3
(8)
in which is the density. Later, Faghih et al. [54] claimed the maximum pressure rise rate could be expressed as
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SL, a =
1 R P · · max 3 Pmax P0 P0
0
·(dp / dt )max
(10)
For a closed vessel without a perforated plate, there exists no turbulence generated from the fluctuation of the flow field [21]. Therefore, we define a parameter, the turbulence enhancement factor:
= SL1, a/SL0, a
(11)
As indicated by Wang et al. [55,56], the flame front could be wrinkled after passing through a perforated plate, resulting higher turbulent flame burning velocity. As the initial pressure and turbulence intensity increase, the turbulent flame burning velocity increases significantly. However, one can see that for BR = 0.95 and BR = 0.97 , ranges from 0.71 to 1.25. At < 1.0 , the fuel-lean side, the turbulence enhancement factor obtained with BR = 0.99 are significantly increased. The maximum value of is found to be ~3.13 with S = 75 mm. This indicates that the turbulence induced by the perforated plate is strongly dependent on the blockage ratio. Compared with Refs. [55,56], there exists no turbulence before an ignition, and the initial pressure is 0.1 MPa in this study. In the case of BR = 0.95 and BR = 0.97 , the combination of the heat-sink and turbulence-generation effects result in low turbulent burning velocity. 4. Conclusion In the present study, the explosion characteristics of syngas-air mixtures in a vessel with a perforated plate were examined experimentally. The effects of the distance between the obstacle and the ignitor (S ) and the blockage ratio (BR ) were considered. Main conclusions are as follows: (1) The maximum explosion pressure (Pmax ) is decreased in the presence of a perforated plate due to the additional heat loss. The heat loss is larger when the perforated plate is close to the ignitor, thus, Pmax is decreased with the decrease of S . Compared with the vessel wall, the heat transferred to unit surface of the perforated plate is more significant. (2) The explosion time ( ) is decreased due to the turbulence induced by the perforated plate. For the perforated plate with BR = 0.99, decreases as the distance between the obstacle and the ignitor increase. As the blockage ratio decreases (BR = 0.97 and BR = 0.95), no obvious difference between the explosion time can be detected for various obstacle locations. (3) The effects of the perforated plate on the maximum rate of pressure rise ((dp / dt ) max ) strongly depends on S and BR . Generally, (dp / dt ) max is increased with the increase of the blockage ratio. For BR = 0.99, the turbulence-generating of the perforated plate has more dominant effect on (dp / dt ) max . Thus, the maximum rate of pressure rise is increased significantly in existence of a perforated. However, as the blockage ratio decreases, the role of heat-sink of the obstacle participates in to affect (dp / dt ) max . Therefore, the maximum pressure rise rate can be decreased with a perforated plate. (4) When the blockage ratios are 0.97 and 0.95, the turbulence enhancement factors ( ) are close to unit, indicating that the induced turbulence is weak. For BR = 0.99, can be as large as about 3 at the fuel-lean side.
Fig. 12. Maximum rate of pressure rise under various distances between ignitor and plate.
(dp / dt ) max =
3SL, a P ·P0· a R P0
1 ·
Pa P0
1/ 0
(9)
Declaration of Competing Interest
Experimentally, the adiabatic explosion pressure is reflected by the maximum explosion pressure. Thus, the laminar burning speed, SL, a (note again, the subscript ‘a ’ represents the end of the combustion), can be calculated by
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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