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Experimental study on the detonation initiation using pipe bundle geometries in CH4–2H2–3O2 mixture
Energy 199 (2020) 117468
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Energy journal homepage: www.elsevier.com/locate/energy
Experimental study on the detonation initiation using pipe bundle geometries in CH4e2H2e3O2 mixture Xuxu Sun, Shouxiang Lu* State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, 230027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 July 2019 Received in revised form 12 February 2020 Accepted 23 March 2020 Available online 30 March 2020
Effects of the pipe bundles number and position on the deflagration to detonation transition (DDT) are investigated systematically for the first time in CH4e2H2e3O2 mixture. Four pressure transducers (PCB102B06) are used to obtain the average velocity by recording the time-of-arrival of the combustion wave. Meanwhile, the soot foil is employed to register the detonation cellular patterns. The results indicate that the critical pressure for DDT is reduced significantly after the pipe bundles are introduced into the tube. The optimum position for DDT occurs at position-II. At position-I, the flame acceleration is more significant for n ¼ 4 case, no difference can be observed among n ¼ 3, 4 and 5 cases at position-II, and the critical pressure is lower with the increases of pipe bundle number at position-III. The process of DDT closely depends on the flame jets. Finally the parameter of DH/l is introduced to quantify the critical condition of DDT. The critical value of DH/l nearly fluctuates around 1 at positions-I and II, and it is far greater than 1 at position-III. This indicates that in addition to the wave propagation mechanisms, the DDT regime is also closely related to the initiation process. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Pipe bundle structures DDT Flame jets Critical condition DH/l
1. Introduction The detonation mode of combustion characterized by a very high overpressure about 15e20 initial pressure [1] has been paid to lots of attention in the past. In general, a detonation wave can be produced by directly initiating with a large amount of energy or flame acceleration leading to deflagration to detonation transition (DDT). However, in most industrial facilities, no ignition source is sufficient to ignite a detonation wave directly. Almost all detonations originate from a weak ignition source. Then the combustion can accelerate and undergo transition to a detonation wave under appropriate boundary conditions. Deflagration to detonation transition (DDT) in a tube filled with premixed combustible mixture has become one of the hottest research topics in the field of gaseous detonation for practical applications and the determination of explosion hazards [2]. There are two stages in the process of DDT [3]: the flame acceleration and the detonation onset. In the flame acceleration phase, the slow subsonic flame has to be accelerated at least to the speed of sound in the combustion products prior to the onset of detonation otherwise
* Corresponding author. E-mail address: [email protected] (S. Lu). https://doi.org/10.1016/j.energy.2020.117468 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
the detonation cannot form [4e8]. Effect of obstacle size and spacing on the initial stage of flame acceleration was investigated by Ciccarelli et al. [9] in a rough tube, who pointed out that the flame velocity will be enhanced significantly with the increase of blockage ratio (BR) and mixture sensitivity. Johansen and Ciccarelli [10] systematically studied the flame acceleration in an obstructed channel. They found that the flame acceleration closely depends on the structure of vortex forming downstream of each obstacle. Afterwards, Ciccarelli et al. [11] found that the shock-flame interaction is the main reason of detonation formation. Masri et al. [12] experimentally investigated the effect of obstacle geometries on the flame acceleration in a tube. The results indicated that the optimum acceleration is observed in the case of square crosssection obstacle. Ciccarelli et al. [13] studied the flame acceleration in a narrow rectangular cross-section channel with three various boundary conditions, i.e., smooth wall and a rough surface with or without porosity. They concluded that the flame can be accelerated beyond the products speed of sound by the interaction of chemical-acoustic. Heretofore, the mechanisms of DDT in a tube or channel filled with obstacles have been investigated extensively [14e17]. In the past, the mode that the wave velocity lies between the theoretical CJ detonation speed and the products speed of sound is often called as the quasi-detonation regime [18]. The limit of this regime is also
2
X. Sun, S. Lu / Energy 199 (2020) 117468
referred to as DDT limit. Peraldi et al. [19] and Knystautas et al. [20] claimed that the DDT limit can be quantified as d/l > 1 (d represents the hole diameter and l is the detonation cell size). And then the criterion is confirmed by Cross and ciccarelli [17] and Ciccarelli et al. [21]. Depre et al. [22e24] systematically investigated the DDT limit for hydrogen-air mixtures in a tube with different diameters, and the criterion of pdc ¼ l was proposed. Afterwards, Lee [25] suggested that pdc is the maximum characteristic size of tube, and it should be related to the characteristic parameter of detonation sensitivity, i.e., detonation cell size l. Zhang et al. [26] further studied the detonation limit and determined the criteria of pdc ¼ l. Moreover, in some studies [27e30], they claimed that the DDT limit is also governed by the obstacle spacing. Therefore, Dorofeev et al. [29] proposed the criterion of L/l > 7 considering the effect of obstacle spacing. Herein, L is the characteristic length. In a round tube filled with continuous orifice plates, the L can be defined as: L¼(SþD)/2/(1-d/D)
(1)
where S represents the spacing between obstacles; D is the inner diameter of tube; d is the hole diameter of orifice plate. Chao et al. [31] experimentally investigated the onset of detonation in hydrogen-air mixtures. Two various channels and three different obstacle hole diameters were employed. They found that the critical condition for DDT closely depends on the obstacle orifice diameter, independent of the tube diameter. Grondin and Lee [32] further studied the onset of detonation downstream of a perforated plate in C3H8e5O2 and C2H2-2.5O2-70%Ar mixtures, i.e., irregular mixture with irregular transverse wave pattern and regular mixture with regular cellular structure. The results indicated that the onset of detonation regime is significantly related to the mixture types. The DDT and detonation propagation limits were studied by Cross and ciccarelli [17] in a rectangular cross-section channel. They found that the DDT limit is governed by the wave propagation regime, independent of DDT process. Furthermore, the pipe bundle geometries were also used to investigate the propagation mechanism of DDT and detonation wave. In the studies of Ishii and Tanaka [33] and Monwar et al. [34], the pipe bundle structures were employed to accelerate the detonation formation by the process of deflagration to detonation transition (DDT). Sun et al. [35] experimentally investigated the propagation regime of a detonation through three different pipe bundle geometries, and determined the optimum number of pipe bundle. On the basis of the work described above, it can be found that although the mechanism of DDT has been studied for many years, the roles of pipe bundle geometries on the DDT were given little attention in the past. Moreover, the influence of pipe bundle positions and numbers on DDT fails to be considered, and the underlying mechanism is still worth exploring. In practice, the pipe bundle geometries are very common in process industries. The explosion accidents propagating through the pipe bundles are likely to occur in the future, which is given less attention. In this study, the propagation behaviors of DDT for stoichiometric CH4e2H2e3O2 mixture in a round tube filled with three various pipe bundle geometries were investigated experimentally. Moreover, the roles of pipe bundle position and number on the DDT were also studied systematically for the first time. The critical condition was analyzed quantitatively based on the experimental measured average velocity. The detonation cellular patterns were recorded after the combustion wave propagation through the pipe bundle geometries. The effect of pipe bundle structures on the initiation mechanism of a detonation was analyzed in detail. The experimental results obtained in this study can provide some theoretical references for optimizing the design of detonation arrester and help reduce or even eliminate the
damage to people’s lives and property caused by the accident explosion in the future. Furthermore, in our previous study [36], the effect of the number of pipe bundles on the detonation transmission has been investigated. Combined with the results of this study, a complete research system from ignition to detonation formation can be obtained. 2. Experimental set-up In this study, a circular tube with an inner diameter of 90 mm and 2.5 m long was employed as the detonation tube, as shown in Fig. 1. Three pipe bundle geometries were used to investigate the mechanisms of deflagration to detonation transition (DDT). The details about the pipe bundles can refer our previous literature [36]. Just a brief description is given here. The pipe bundle geometries are created by inserting several smaller diameter pipes into the big tube. Three various pipe bundles can be obtained by changing the number of small pipes of n ¼ 3, 4 and 5, as shown in Fig. 1b. The specific size of the pipe bundles are tabulated in Table 1. Moreover, the effect of pipe bundle position on the DDT is also explored. Three different positions are considered: (1) position-I represents the pipe bundle downstream of the ignitor; (2) the distance between the pipe bundle and the ignitor is 500 mm in position-II; (3) the distance between the pipe bundle and the ignitor is further increased to 1000 mm in position-III. Herein, in order to the convenience of experiment operation, only three pipe numbers and positions are considered. In our future work, more experimental conditions will be employed to comprehensively explore the roles of pipe bundle geometries on DDT. The stoichiometric CH4e2H2e3O2 mixture was used as the test gas. The specific experimental procedures are as follows: (1) the CH4e2H2e3O2 mixture was introduced a 150 L mixing tank for at least 24 h to ensure the homogeneity prior to each shot; (2) the detonation tube was evacuated by a vacuum pump to lower than 100 Pa, and then the premixed mixture was guided into the tube to desired initial pressure by the way of partial pressure. A piezometer (SXT-4A, 0e150 kPa) with an accuracy of ±0.06% full scale was used to monitor the static pressure in the tube or the mixing tank; (3) the premixed mixtures were ignited at the left endplate by an electric spark from the discharge of a capacitor bank. The ignition energy is about 100 mJ; (4) four PCB pressure transducers (PCB102B06) were mounted on the wall of the tube to record the time-of-arrival of the combustion wave, from which the average velocity can be determined. Meanwhile, the smoked foil was also used to record the detonation cellular patterns by inserting a 0.1 mm thick stainless steel plate covered with uniform soot into the test section prior to each shot. Of note is that the cell size used in this study was obtained from our previous study [37]. 3. Results and discussion 3.1. Effect of pipe bundles on the DDT The average velocity measured in the smooth tube and the theoretical CJ detonation velocity (VCJ) calculated by the CHEMKIN package [38] both are presented in Fig. 2 for reference. It can be observed that the critical pressure (Pc) for DDT is 53 kPa in the smooth tube. The combustion wave can propagate at about the theoretical CJ value when the initial pressure (P0) is greater than Pc. Below the critical pressure, no steady velocity can be obtained, and the average velocity is decayed by gradually decreasing the initial pressure. Based on previous study [1], flame acceleration can be roughly divided into two stages in the smooth tube. Firstly, the initial laminar flame surface will be wrinkled because of the L-D instability after ignition by forming the cellular flame structure. The
X. Sun, S. Lu / Energy 199 (2020) 117468
3
Fig. 1. Sketch of experimental apparatus (a), pipe bundle geometries [36] (b).
Table 1 The detailed sizes of tube bundle geometries used in this study [36]. Number of small pipes (n)
Outer diameter (d/mm)
Wall thickness (d/mm)
Length (l/mm)
Interval (x/mm)
3 4 5
20
2
500
8
4
X. Sun, S. Lu / Energy 199 (2020) 117468
Fig. 2. The average velocity as a function of initial pressure in the smooth tube.
flame area will be increased progressively with the development of cellular flame, leading to the increases of flame velocity; secondly, the flame surface is further enhanced by the expansion of combustion products, causing the flow interaction with the tube wall. Fig. 3 presents the effects of pipe bundle positions on the DDT at the same small pipe number, n ¼ 3, 4 and 5. The critical pressure for DDT is reduced significantly after the pipe bundle geometries are guided into the tube. The specific values of the critical pressure are tabulated in Table 2 for the cases of different positions. The minimum of critical pressure occurs at position-II about 12 kPa. The maximum of critical pressure is observed at position-III, and the critical pressure at position-I is somewhere in between. This indicated that the propagation behaviors of DDT closely depends on the position of pipe bundles, and the optimum position for DDT at position-II. In addition, the influence of the number of small pipe on DDT at the same position is given in Fig. 4. It can be found that the flame acceleration is more significant in the case of n ¼ 4 at position-I, no difference can be observed among n ¼ 3, 4 and 5 cases at position-II, and the critical pressure is lower with the increases of small pipe number at position-III. The difference discussed above can be attributed to the different initiation process for DDT, and the specific details will be analyzed in the next section. Generally, it can be found that the run-up distance of DDT can be reduced clearly after the pipe bundle geometries are introduced into the tube. Based on previous studies [39,40], the process of flame acceleration in obstructed tube is easy to understand. Following ignition, the flame velocity will be enhanced due to the L-D instability by increasing the flame surface firstly. Then the flame surface is further increased obviously because of the existence of obstacle by producing turbulence, and the continuous flame acceleration can be observed. In addition, the KeH and R-M instabilities can also increase the flame surface significantly. In this present geometries, another factor that can achieve a fast flame acceleration cannot be ignored, i.e., flame jet initiation of detonation. In the study of Knystautas et al. [41], they reported that the jet initiation is produced by the intense mixing of combustion products. Afterwards, the large-scale eddies can be observed due to these fine-scale turbulence, which facilitates the detonation propagation for the onset of detonation. Moreover, Thomas and Jones Fig. 3. The average velocity as a function of initial pressure at different positions, (a) n ¼ 3, (b) n ¼ 4, (c) n ¼ 5.
X. Sun, S. Lu / Energy 199 (2020) 117468
5
Table 2 The critical pressure obtained from various positions. Number
n ¼ 3 (kPa)
n ¼ 4 (kPa)
n ¼ 5 (kPa)
Position-I Position-II Position-III
12 17 25
12 13 29
12 15 32
[42] found that a detonation can be formed by the jet initiation despite under conditions where a detonation cannot propagate into an unconfined space clearly. By comparing the average velocity obtained from various pipe bundle cases at position-II, it can be seen that the process of DDT is little dependent on the number of small pipes, and only related to the location of the pipe bundles. The critical pressures are all 12 kPa. To better understand the behaviors for DDT, the cellular patterns recorded at the critical condition are presented in Fig. 5, where the left edge corresponds to the exit of pipe bundle geometries. The detonation propagates from left to right, and the localized zone downstream of the pipe bundles are amplified for reference. It can be found that multiple “V” marks are recorded on the smoke foil induced by the flame jet. This is similar to that be observed by Ishii and Tanaka [33], indicating the DDT process is governed by the multiple flame jets. Moreover, the fine-cell can be observed immediately after the combustion wave propagates through the pipe bundles in the case of n ¼ 3, as shown in Fig. 5a. However, no-cell is recorded inside the “V” shape for the cases of n ¼ 4 and 5, and the fine-cell structures are seen after these “V” marks, as shown in Fig. 5b and 5c. This indicated that it is easier to ignite a detonation in the case of n ¼ 3 despite the critical pressures are same among n ¼ 3, 4 and 5. 3.2. Analysis of critical condition In the past, the parameter of d/l is often used to investigate the DDT and detonation propagation limits in previous studies [19e25]. However, in this present pipe bundle geometry, the term of the hydraulic diameter (DH) is more proper to be employed to study the critical condition of DDT. Because the hydraulic diameter (DH) is often used to investigate the fluid problems in noncircular pipes. In this study, the hydraulic diameter can be defined as [36]:
DH ¼ 4
pn 4
h o. D2 nd2 nðd 2dÞ2 p½D þ nd þ nðd 2dÞ (2)
where D is inner diameter of big tube; n is the number of small pipes; d represents the outer diameter of small pipe, d is the wall thickness of small pipes. Fig. 6 gives the average velocity as a function of DH/l at different positions for the case of n ¼ 3. The critical values of DH/l are 1.93, 1.12 and 7.04 at the positions of I, II and III, respectively. With the number of small pipes are further increased to 4 and 5 (see Figs. 7 and 8), the tendency of data is similar to that obtained from n ¼ 3 case. The experiment results are tabulated in Table 3, including the critical pressure, cell size at each Pc and the critical values of DH/l. The minimum of DH/l occurs at position II. This can be used to explain the reason of the optimum pipe bundle position discussed above. Moreover, of note is that the critical value of DH/l can fluctuate around 1 approximately at positions I and II. This is consistent the previous criterion of d/l > 1 [19,20], indicating the detonation can propagate only when the hole diameter can accommodate at least one cell size. But the critical value of DH/l is far greater than
Fig. 4. The average velocity as a function of initial pressure in the cases of different pipe bundles, (a) position-I, (b) position-II, (c) position-III.
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Fig. 5. Typical cellular patterns at P0 ¼ 12 kPa in the position-II, (a) n ¼ 3, (b) n ¼ 4, (c) n ¼ 5.
X. Sun, S. Lu / Energy 199 (2020) 117468
Fig. 6. The average velocity as a function of DH/l in the case of n ¼ 3.
7
Fig. 8. The average velocity as a function of DH/l in the case of n ¼ 5.
1 at position-III, as shown in Fig. 9. This indicates that in addition to the wave propagation mechanisms govern the DDT process, the initiation regime also has an important effect on the DDT.
4. Conclusion In this study, the DDT process is investigated experimentally by using three different pipe bundle geometries. Moreover, the effect of pipe bundle positions on DDT is also explored in detail. Some new and interesting results can be obtained as follows:
Fig. 7. The average velocity as a function of DH/l in the case of n ¼ 4.
(1) In the smooth tube, the critical pressure (Pc) is about 53 kPa. At the initial pressure greater than Pc, the combustion wave can propagate at about the theoretical CJ detonation velocity with small deficit; below the critical pressure, no steady velocity can be observed. (2) The critical pressure is reduced significantly after the pipe bundle geometries are guided into the tube. The minimum of critical pressure occurs at position-II, and the maximum of Pc is obtained at position-III. This indicates the optimum position for DDT at position-II. At position-I, the flame acceleration is more significant in the case of n ¼ 4, no difference can be observed among n ¼ 3, 4 and 5 cases at position-II, and the critical pressure is lower with the increases of small pipe number at position-III. Moreover, the process of DDT closely depends on the flame jets. (3) To quantify the critical condition of DDT, the term of DH/l is introduced. It can be found that the critical value of DH/l nearly fluctuates around 1 at positions I and II, and the critical value of DH/l is far greater than 1 at position-III. This indicates that in addition to the wave propagation mechanisms, the DDT regime closely depends on the initiation process.
8
X. Sun, S. Lu / Energy 199 (2020) 117468
Table 3 Experimental results of DDT at the critical condition. n
3 4 5
DH (mm)
38.73 32.15 27.33
Position-I
Position-II
Position-III
Pc (kPa)
l (mm)
DH/l
Pc (kPa)
l (mm)
DH/l
Pc (kPa)
l (mm)
DH/l
17 13 15
20.10 32.40 24.50
1.93 0.99 1.12
12 12 12
34.50 34.50 34.50
1.12 0.93 0.79
25 29 32
5.50 3.10 1.76
7.04 10.37 15.53
Fig. 9. DH/l as a function of pipe bundle position.
Declaration of competing interest No interest conflict. Acknowledgements This work was supported by the National Key R&D Program of China [No.2016YFC0802101] and the Fundamental Research Funds for the Central Universities [No. WK2320000040]. References [1] Ciccarelli G, Dorofeev S. Flame acceleration and transition to detonation in ducts. Prog Energy Combust Sci 2008;34(4):499e550. [2] Maeda S, Fujisawa M, Ienaga S, Hirahara K, Obara T. Effect of sandpaper-like small wall roughness on deflagration-to-detonation transition in a hydrogeneoxygen mixture. Proc Combust Inst 2019;37(3):3609e16. [3] Shepherd JE, Lee JHS. On the transition from deflagration to detonation. Major research topics in combustion. New York, NY: Springer; 1992. p. 439e87. [4] Chapman WR, Wheeler RN. The propagation of flame in mixtures of methane and air. J Chem Soc (London) 1926:2139. [5] Lee JHS, Knystautas R, Chan CK. Turbulent flame propagation in obstacle-filled tubes. Proc Combust Inst 1985;20:1663e72. [6] Moen IO. The influence of turbulence on flame propagation in obstacle environments. First international specialist meeting on fueleair explosions, vol. 16. Montreal: University of Waterloo Press SM study; 1981. p. 101e13. 1982. [7] Wagner HGg. Some experiments about flame acceleration, First international specialist meeting on fueleair explosions, vol. 16. Montreal: University of Waterloo Press SM study; 1981. 1982. pp. 101e35. [8] Peraldi O, Knystautas R, Lee JHS. Criteria for transition to detonation in tubes. Proc Combust Inst 1986;21:1629e37. [9] Ciccarelli G, Fowler CJ, Bardon M. Effect of obstacle size and spacing on the initial stage of flame acceleration in a rough tube. Shock Waves 2005;14(3): 161e6. [10] Johansen CT, Ciccarelli G. Visualization of the unburned gas flow field ahead of an accelerating flame in an obstructed square channel. Combust Flame 2009;156(2):405e16. [11] Ciccarelli G, Johansen CT, Parravani M. The role of shockeflame interactions on flame acceleration in an obstacle laden channel. Combust Flame 2010;157(11):2125e36. [12] Masri AR, Ibrahim SS, Nehzat N, Green AR. Experimental study of premixed flame propagation over various solid obstructions. Exp Therm Fluid Sci 2000;21(1e3):109e16.
[13] Ciccarelli G, Johansen C, Kellenberger M. High-speed flames and DDT in very rough-walled channels. Combust Flame 2013;160(1):204e11. [14] Zhang B, Liu H, Yan BJ. Effect of acoustically absorbing wall tubes on the nearlimit detonation propagation behaviors in a methane-oxygen mixture. Fuel 2019;236:975e83. [15] Zhang B, Liu H. The effects of large scale perturbation-generating obstacles on the propagation of detonation filled with methane-oxygen mixture. Combust Flame 2017;182:279e87. [16] Zhang B. The influence of wall roughness on detonation limits in hydrogenoxygen mixture. Combust Flame 2016;169:333e9. [17] Cross M, Ciccarelli G. DDT and detonation propagation limits in an obstacle filled tube. J Loss Prev Process Ind 2015;36:380e6. [18] Moen I, Lee JH. The mechanism of transition from deflagration to detonation in vapor cloud explosion. Prog Energy Combust Sci 1980;6:359e89. [19] Peraldi O, Knystautas R, Lee JHS. Criteria for transition to detonation in tubes, Symp. Int.) Combust. 1988;21(1):1629e37. [20] Knystautas R, Lee JHS, Peraldi O, Chan CK. Transmission of a flame from a rough to a smooth-walled tube. Prog Astronaut Aeronaut 1986;106:37e52. [21] Ciccarelli G, Wang Z, Lu J, Cross M. Effect of orifice plate spacing on detonation propagation. J Loss Prev Process Ind 2017;49:739e44. [22] Dupre G, Peraldi O, Joannon J, Knystautas R. Limit criterion of detonation in circular tubes. Prog Astronaut Aeronaut 1991;133:156e69. [23] Dupre G, Knystautas R, Lee JHS. Near-limit propagation of detonation in tubes. Prog Astronaut Aeronaut 1986;106:244e59. [24] Dupre G, Joannon J, Knystautas R. Unstable detonations in the near-limit regime in tubes. Proc Combust Inst 1990;23:1813e20. [25] Lee JHS. Dynamic parameters of gaseous detonations. Annu Rev Fluid Mech 1984;16:311e36. [26] Zhang B, Liu H, Yan BJ. Investigation on the detonation propagation limit criterion for methane-oxygen mixtures in tubes with different scales. Fuel 2019;239:617e22. [27] Teodorczyk A, Lee JHS, Knystautas R. Propagation mechanism of quasi-detonations. Symp. (Int.) Combust. Elsevier. 1989;22(1):1723e31. [28] Gu LS, Knystautas R, Lee JHS. Influence of obstacle spacing on the propagation of quasi-detonation. Dyn. Explos. 1988:232e47. [29] Dorofeev SB, Sidorov VP, Kuznetsov MS, Matsukov ID, Alekseev VI. Effect of scale on the onset of detonations. Shock Waves 2000;10(2):137e49. [30] Teodorczyk A, Lee JHS, Knystautas R. Photographic study of the structure and propagation mechanisms of quasi-detonations in rough tubes. Prog Astronaut Aeronaut 1990;133:223e40. [31] Chao J, Otsuka T, Lee JHS. An experimental investigation of the onset of detonation. Proc Combust Inst 2005;30(2):1889e97. [32] Grondin JS, Lee JHS. Experimental observation of the onset of detonation downstream of a perforated plate. Shock Waves 2010;20(5):381e6. [33] Ishii K, Tanaka T. A study on jet initiation of detonation using multiple tubes. Shock Waves 2005;14(4):273e81. [34] Monwar M, Yamamoto Y, Ishii K, Tsuboi T. Detonation propagation in narrow gaps with various configurations. J Therm Sci 2007;16(3):283e8. [35] Sun X, Li Q, Wang L, Guo J, Li C, Lu S. Experimental investigation of detonation propagation in hydrogen-air mixtures in a tube filled with bundles. Exp Therm Fluid Sci 2019;102:316e24. [36] Sun X, Li Q, Lu S. Effect of bundle geometries on the detonation velocity behaviors in stoichiometric hydrogen-air mixture. Int J Hydrogen Energy 2019;44(40):22519e26. [37] Sun X, Li Q, Li C, Lu S. Detonation propagation characteristics for CH4-2H23O2 mixtures in a tube filled with orifice plates. Int J Hydrogen Energy 2019;44(14):7616e27. [38] Kee RJ, Rupley FM, Miller JA. Sandia national laboratories report SAND898009. 1989. USA. [39] Matsukov I, Kuznetsov M, Alekseev V, Dorofeev S. Photographic study of the characteristic regimes of turbulent flame propagation, local and global quenching in obstructed areas. Report No. 315/20088504/INR. prepared for FZK-INR; 1998. [40] Matsukov I, Kuznetsov M, Alekseev V, Dorofeev S. Photographic study of unstable turbulent flames in obstructed channel. Proceedings of the 17th international colloquium on the dynamics of explosions and reactive systems. Heidelberg, Germany: Universitat Heidelberg; 1999, ISBN 3-932217-01-2. July 25e30, 1999. [41] Knystautas R, Lee JHS, Moen I, Wagner HGg. Direct initiation of spherical detonation by a hot turbulent gas jet. Proceedings of the 17th symposium (international) on combustion. 1979. p. 1235e45. [42] Thomas GO, Jones A. Some observation of the jet initiation of detonation. Combust Flame 2000;120:392e8.
Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Experimental study on the detonation initiation using pipe bundle geometries in CH4e2H2e3O2 mixture Xuxu Sun, Shouxiang Lu* State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, 230027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 July 2019 Received in revised form 12 February 2020 Accepted 23 March 2020 Available online 30 March 2020
Effects of the pipe bundles number and position on the deflagration to detonation transition (DDT) are investigated systematically for the first time in CH4e2H2e3O2 mixture. Four pressure transducers (PCB102B06) are used to obtain the average velocity by recording the time-of-arrival of the combustion wave. Meanwhile, the soot foil is employed to register the detonation cellular patterns. The results indicate that the critical pressure for DDT is reduced significantly after the pipe bundles are introduced into the tube. The optimum position for DDT occurs at position-II. At position-I, the flame acceleration is more significant for n ¼ 4 case, no difference can be observed among n ¼ 3, 4 and 5 cases at position-II, and the critical pressure is lower with the increases of pipe bundle number at position-III. The process of DDT closely depends on the flame jets. Finally the parameter of DH/l is introduced to quantify the critical condition of DDT. The critical value of DH/l nearly fluctuates around 1 at positions-I and II, and it is far greater than 1 at position-III. This indicates that in addition to the wave propagation mechanisms, the DDT regime is also closely related to the initiation process. © 2020 Elsevier Ltd. All rights reserved.
Keywords: Pipe bundle structures DDT Flame jets Critical condition DH/l
1. Introduction The detonation mode of combustion characterized by a very high overpressure about 15e20 initial pressure [1] has been paid to lots of attention in the past. In general, a detonation wave can be produced by directly initiating with a large amount of energy or flame acceleration leading to deflagration to detonation transition (DDT). However, in most industrial facilities, no ignition source is sufficient to ignite a detonation wave directly. Almost all detonations originate from a weak ignition source. Then the combustion can accelerate and undergo transition to a detonation wave under appropriate boundary conditions. Deflagration to detonation transition (DDT) in a tube filled with premixed combustible mixture has become one of the hottest research topics in the field of gaseous detonation for practical applications and the determination of explosion hazards [2]. There are two stages in the process of DDT [3]: the flame acceleration and the detonation onset. In the flame acceleration phase, the slow subsonic flame has to be accelerated at least to the speed of sound in the combustion products prior to the onset of detonation otherwise
* Corresponding author. E-mail address: [email protected] (S. Lu). https://doi.org/10.1016/j.energy.2020.117468 0360-5442/© 2020 Elsevier Ltd. All rights reserved.
the detonation cannot form [4e8]. Effect of obstacle size and spacing on the initial stage of flame acceleration was investigated by Ciccarelli et al. [9] in a rough tube, who pointed out that the flame velocity will be enhanced significantly with the increase of blockage ratio (BR) and mixture sensitivity. Johansen and Ciccarelli [10] systematically studied the flame acceleration in an obstructed channel. They found that the flame acceleration closely depends on the structure of vortex forming downstream of each obstacle. Afterwards, Ciccarelli et al. [11] found that the shock-flame interaction is the main reason of detonation formation. Masri et al. [12] experimentally investigated the effect of obstacle geometries on the flame acceleration in a tube. The results indicated that the optimum acceleration is observed in the case of square crosssection obstacle. Ciccarelli et al. [13] studied the flame acceleration in a narrow rectangular cross-section channel with three various boundary conditions, i.e., smooth wall and a rough surface with or without porosity. They concluded that the flame can be accelerated beyond the products speed of sound by the interaction of chemical-acoustic. Heretofore, the mechanisms of DDT in a tube or channel filled with obstacles have been investigated extensively [14e17]. In the past, the mode that the wave velocity lies between the theoretical CJ detonation speed and the products speed of sound is often called as the quasi-detonation regime [18]. The limit of this regime is also
2
X. Sun, S. Lu / Energy 199 (2020) 117468
referred to as DDT limit. Peraldi et al. [19] and Knystautas et al. [20] claimed that the DDT limit can be quantified as d/l > 1 (d represents the hole diameter and l is the detonation cell size). And then the criterion is confirmed by Cross and ciccarelli [17] and Ciccarelli et al. [21]. Depre et al. [22e24] systematically investigated the DDT limit for hydrogen-air mixtures in a tube with different diameters, and the criterion of pdc ¼ l was proposed. Afterwards, Lee [25] suggested that pdc is the maximum characteristic size of tube, and it should be related to the characteristic parameter of detonation sensitivity, i.e., detonation cell size l. Zhang et al. [26] further studied the detonation limit and determined the criteria of pdc ¼ l. Moreover, in some studies [27e30], they claimed that the DDT limit is also governed by the obstacle spacing. Therefore, Dorofeev et al. [29] proposed the criterion of L/l > 7 considering the effect of obstacle spacing. Herein, L is the characteristic length. In a round tube filled with continuous orifice plates, the L can be defined as: L¼(SþD)/2/(1-d/D)
(1)
where S represents the spacing between obstacles; D is the inner diameter of tube; d is the hole diameter of orifice plate. Chao et al. [31] experimentally investigated the onset of detonation in hydrogen-air mixtures. Two various channels and three different obstacle hole diameters were employed. They found that the critical condition for DDT closely depends on the obstacle orifice diameter, independent of the tube diameter. Grondin and Lee [32] further studied the onset of detonation downstream of a perforated plate in C3H8e5O2 and C2H2-2.5O2-70%Ar mixtures, i.e., irregular mixture with irregular transverse wave pattern and regular mixture with regular cellular structure. The results indicated that the onset of detonation regime is significantly related to the mixture types. The DDT and detonation propagation limits were studied by Cross and ciccarelli [17] in a rectangular cross-section channel. They found that the DDT limit is governed by the wave propagation regime, independent of DDT process. Furthermore, the pipe bundle geometries were also used to investigate the propagation mechanism of DDT and detonation wave. In the studies of Ishii and Tanaka [33] and Monwar et al. [34], the pipe bundle structures were employed to accelerate the detonation formation by the process of deflagration to detonation transition (DDT). Sun et al. [35] experimentally investigated the propagation regime of a detonation through three different pipe bundle geometries, and determined the optimum number of pipe bundle. On the basis of the work described above, it can be found that although the mechanism of DDT has been studied for many years, the roles of pipe bundle geometries on the DDT were given little attention in the past. Moreover, the influence of pipe bundle positions and numbers on DDT fails to be considered, and the underlying mechanism is still worth exploring. In practice, the pipe bundle geometries are very common in process industries. The explosion accidents propagating through the pipe bundles are likely to occur in the future, which is given less attention. In this study, the propagation behaviors of DDT for stoichiometric CH4e2H2e3O2 mixture in a round tube filled with three various pipe bundle geometries were investigated experimentally. Moreover, the roles of pipe bundle position and number on the DDT were also studied systematically for the first time. The critical condition was analyzed quantitatively based on the experimental measured average velocity. The detonation cellular patterns were recorded after the combustion wave propagation through the pipe bundle geometries. The effect of pipe bundle structures on the initiation mechanism of a detonation was analyzed in detail. The experimental results obtained in this study can provide some theoretical references for optimizing the design of detonation arrester and help reduce or even eliminate the
damage to people’s lives and property caused by the accident explosion in the future. Furthermore, in our previous study [36], the effect of the number of pipe bundles on the detonation transmission has been investigated. Combined with the results of this study, a complete research system from ignition to detonation formation can be obtained. 2. Experimental set-up In this study, a circular tube with an inner diameter of 90 mm and 2.5 m long was employed as the detonation tube, as shown in Fig. 1. Three pipe bundle geometries were used to investigate the mechanisms of deflagration to detonation transition (DDT). The details about the pipe bundles can refer our previous literature [36]. Just a brief description is given here. The pipe bundle geometries are created by inserting several smaller diameter pipes into the big tube. Three various pipe bundles can be obtained by changing the number of small pipes of n ¼ 3, 4 and 5, as shown in Fig. 1b. The specific size of the pipe bundles are tabulated in Table 1. Moreover, the effect of pipe bundle position on the DDT is also explored. Three different positions are considered: (1) position-I represents the pipe bundle downstream of the ignitor; (2) the distance between the pipe bundle and the ignitor is 500 mm in position-II; (3) the distance between the pipe bundle and the ignitor is further increased to 1000 mm in position-III. Herein, in order to the convenience of experiment operation, only three pipe numbers and positions are considered. In our future work, more experimental conditions will be employed to comprehensively explore the roles of pipe bundle geometries on DDT. The stoichiometric CH4e2H2e3O2 mixture was used as the test gas. The specific experimental procedures are as follows: (1) the CH4e2H2e3O2 mixture was introduced a 150 L mixing tank for at least 24 h to ensure the homogeneity prior to each shot; (2) the detonation tube was evacuated by a vacuum pump to lower than 100 Pa, and then the premixed mixture was guided into the tube to desired initial pressure by the way of partial pressure. A piezometer (SXT-4A, 0e150 kPa) with an accuracy of ±0.06% full scale was used to monitor the static pressure in the tube or the mixing tank; (3) the premixed mixtures were ignited at the left endplate by an electric spark from the discharge of a capacitor bank. The ignition energy is about 100 mJ; (4) four PCB pressure transducers (PCB102B06) were mounted on the wall of the tube to record the time-of-arrival of the combustion wave, from which the average velocity can be determined. Meanwhile, the smoked foil was also used to record the detonation cellular patterns by inserting a 0.1 mm thick stainless steel plate covered with uniform soot into the test section prior to each shot. Of note is that the cell size used in this study was obtained from our previous study [37]. 3. Results and discussion 3.1. Effect of pipe bundles on the DDT The average velocity measured in the smooth tube and the theoretical CJ detonation velocity (VCJ) calculated by the CHEMKIN package [38] both are presented in Fig. 2 for reference. It can be observed that the critical pressure (Pc) for DDT is 53 kPa in the smooth tube. The combustion wave can propagate at about the theoretical CJ value when the initial pressure (P0) is greater than Pc. Below the critical pressure, no steady velocity can be obtained, and the average velocity is decayed by gradually decreasing the initial pressure. Based on previous study [1], flame acceleration can be roughly divided into two stages in the smooth tube. Firstly, the initial laminar flame surface will be wrinkled because of the L-D instability after ignition by forming the cellular flame structure. The
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3
Fig. 1. Sketch of experimental apparatus (a), pipe bundle geometries [36] (b).
Table 1 The detailed sizes of tube bundle geometries used in this study [36]. Number of small pipes (n)
Outer diameter (d/mm)
Wall thickness (d/mm)
Length (l/mm)
Interval (x/mm)
3 4 5
20
2
500
8
4
X. Sun, S. Lu / Energy 199 (2020) 117468
Fig. 2. The average velocity as a function of initial pressure in the smooth tube.
flame area will be increased progressively with the development of cellular flame, leading to the increases of flame velocity; secondly, the flame surface is further enhanced by the expansion of combustion products, causing the flow interaction with the tube wall. Fig. 3 presents the effects of pipe bundle positions on the DDT at the same small pipe number, n ¼ 3, 4 and 5. The critical pressure for DDT is reduced significantly after the pipe bundle geometries are guided into the tube. The specific values of the critical pressure are tabulated in Table 2 for the cases of different positions. The minimum of critical pressure occurs at position-II about 12 kPa. The maximum of critical pressure is observed at position-III, and the critical pressure at position-I is somewhere in between. This indicated that the propagation behaviors of DDT closely depends on the position of pipe bundles, and the optimum position for DDT at position-II. In addition, the influence of the number of small pipe on DDT at the same position is given in Fig. 4. It can be found that the flame acceleration is more significant in the case of n ¼ 4 at position-I, no difference can be observed among n ¼ 3, 4 and 5 cases at position-II, and the critical pressure is lower with the increases of small pipe number at position-III. The difference discussed above can be attributed to the different initiation process for DDT, and the specific details will be analyzed in the next section. Generally, it can be found that the run-up distance of DDT can be reduced clearly after the pipe bundle geometries are introduced into the tube. Based on previous studies [39,40], the process of flame acceleration in obstructed tube is easy to understand. Following ignition, the flame velocity will be enhanced due to the L-D instability by increasing the flame surface firstly. Then the flame surface is further increased obviously because of the existence of obstacle by producing turbulence, and the continuous flame acceleration can be observed. In addition, the KeH and R-M instabilities can also increase the flame surface significantly. In this present geometries, another factor that can achieve a fast flame acceleration cannot be ignored, i.e., flame jet initiation of detonation. In the study of Knystautas et al. [41], they reported that the jet initiation is produced by the intense mixing of combustion products. Afterwards, the large-scale eddies can be observed due to these fine-scale turbulence, which facilitates the detonation propagation for the onset of detonation. Moreover, Thomas and Jones Fig. 3. The average velocity as a function of initial pressure at different positions, (a) n ¼ 3, (b) n ¼ 4, (c) n ¼ 5.
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Table 2 The critical pressure obtained from various positions. Number
n ¼ 3 (kPa)
n ¼ 4 (kPa)
n ¼ 5 (kPa)
Position-I Position-II Position-III
12 17 25
12 13 29
12 15 32
[42] found that a detonation can be formed by the jet initiation despite under conditions where a detonation cannot propagate into an unconfined space clearly. By comparing the average velocity obtained from various pipe bundle cases at position-II, it can be seen that the process of DDT is little dependent on the number of small pipes, and only related to the location of the pipe bundles. The critical pressures are all 12 kPa. To better understand the behaviors for DDT, the cellular patterns recorded at the critical condition are presented in Fig. 5, where the left edge corresponds to the exit of pipe bundle geometries. The detonation propagates from left to right, and the localized zone downstream of the pipe bundles are amplified for reference. It can be found that multiple “V” marks are recorded on the smoke foil induced by the flame jet. This is similar to that be observed by Ishii and Tanaka [33], indicating the DDT process is governed by the multiple flame jets. Moreover, the fine-cell can be observed immediately after the combustion wave propagates through the pipe bundles in the case of n ¼ 3, as shown in Fig. 5a. However, no-cell is recorded inside the “V” shape for the cases of n ¼ 4 and 5, and the fine-cell structures are seen after these “V” marks, as shown in Fig. 5b and 5c. This indicated that it is easier to ignite a detonation in the case of n ¼ 3 despite the critical pressures are same among n ¼ 3, 4 and 5. 3.2. Analysis of critical condition In the past, the parameter of d/l is often used to investigate the DDT and detonation propagation limits in previous studies [19e25]. However, in this present pipe bundle geometry, the term of the hydraulic diameter (DH) is more proper to be employed to study the critical condition of DDT. Because the hydraulic diameter (DH) is often used to investigate the fluid problems in noncircular pipes. In this study, the hydraulic diameter can be defined as [36]:
DH ¼ 4
pn 4
h o. D2 nd2 nðd 2dÞ2 p½D þ nd þ nðd 2dÞ (2)
where D is inner diameter of big tube; n is the number of small pipes; d represents the outer diameter of small pipe, d is the wall thickness of small pipes. Fig. 6 gives the average velocity as a function of DH/l at different positions for the case of n ¼ 3. The critical values of DH/l are 1.93, 1.12 and 7.04 at the positions of I, II and III, respectively. With the number of small pipes are further increased to 4 and 5 (see Figs. 7 and 8), the tendency of data is similar to that obtained from n ¼ 3 case. The experiment results are tabulated in Table 3, including the critical pressure, cell size at each Pc and the critical values of DH/l. The minimum of DH/l occurs at position II. This can be used to explain the reason of the optimum pipe bundle position discussed above. Moreover, of note is that the critical value of DH/l can fluctuate around 1 approximately at positions I and II. This is consistent the previous criterion of d/l > 1 [19,20], indicating the detonation can propagate only when the hole diameter can accommodate at least one cell size. But the critical value of DH/l is far greater than
Fig. 4. The average velocity as a function of initial pressure in the cases of different pipe bundles, (a) position-I, (b) position-II, (c) position-III.
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Fig. 5. Typical cellular patterns at P0 ¼ 12 kPa in the position-II, (a) n ¼ 3, (b) n ¼ 4, (c) n ¼ 5.
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Fig. 6. The average velocity as a function of DH/l in the case of n ¼ 3.
7
Fig. 8. The average velocity as a function of DH/l in the case of n ¼ 5.
1 at position-III, as shown in Fig. 9. This indicates that in addition to the wave propagation mechanisms govern the DDT process, the initiation regime also has an important effect on the DDT.
4. Conclusion In this study, the DDT process is investigated experimentally by using three different pipe bundle geometries. Moreover, the effect of pipe bundle positions on DDT is also explored in detail. Some new and interesting results can be obtained as follows:
Fig. 7. The average velocity as a function of DH/l in the case of n ¼ 4.
(1) In the smooth tube, the critical pressure (Pc) is about 53 kPa. At the initial pressure greater than Pc, the combustion wave can propagate at about the theoretical CJ detonation velocity with small deficit; below the critical pressure, no steady velocity can be observed. (2) The critical pressure is reduced significantly after the pipe bundle geometries are guided into the tube. The minimum of critical pressure occurs at position-II, and the maximum of Pc is obtained at position-III. This indicates the optimum position for DDT at position-II. At position-I, the flame acceleration is more significant in the case of n ¼ 4, no difference can be observed among n ¼ 3, 4 and 5 cases at position-II, and the critical pressure is lower with the increases of small pipe number at position-III. Moreover, the process of DDT closely depends on the flame jets. (3) To quantify the critical condition of DDT, the term of DH/l is introduced. It can be found that the critical value of DH/l nearly fluctuates around 1 at positions I and II, and the critical value of DH/l is far greater than 1 at position-III. This indicates that in addition to the wave propagation mechanisms, the DDT regime closely depends on the initiation process.
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Table 3 Experimental results of DDT at the critical condition. n
3 4 5
DH (mm)
38.73 32.15 27.33
Position-I
Position-II
Position-III
Pc (kPa)
l (mm)
DH/l
Pc (kPa)
l (mm)
DH/l
Pc (kPa)
l (mm)
DH/l
17 13 15
20.10 32.40 24.50
1.93 0.99 1.12
12 12 12
34.50 34.50 34.50
1.12 0.93 0.79
25 29 32
5.50 3.10 1.76
7.04 10.37 15.53
Fig. 9. DH/l as a function of pipe bundle position.
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