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Detonation propagation characteristics for CH4-2H2-3O2 mixtures in a tube filled with orifice plates
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 7 6 1 6 e7 6 2 7
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Detonation propagation characteristics for CH42H2-3O2 mixtures in a tube filled with orifice plates Xuxu Sun, Quan Li, Changhai Li, Shouxiang Lu* State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027, PR China
article info
abstract
Article history:
In this study, the detonation propagation characteristics of stoichiometric CH4-2H2-3O2
Received 10 October 2018
mixture are investigated comprehensively in a round tube with an inner diameter of 90-
Received in revised form
mm and 6-m in length. Three different orifice plates with the blockage ratios (BR) of 0.7
25 January 2019
and 0.8 including circular, triangular and square orifice, are considered for the first time to
Accepted 30 January 2019
investigate the effect of obstacle geometries on the detonation evolution. Eight high-speed
Available online 20 February 2019
piezoelectric pressure transducers are mounted on the outer wall to obtain the detonation velocity while the smoked foil technique is adopted to record the detonation cellular
Keywords:
patterns. The results indicate that well within the limit, the detonation can propagate at
Detonation
about the theoretical CJ velocity (VCJ). Near the limit, the velocity deficit is sharply
Orifice plates
enhanced but the detonation still can propagate at about 0.6VCJ, which seems to be a
Geometries
universal phenomenon before the failure of the detonation. In the smooth tube, a sudden
Velocity deficit
velocity drop and the single-headed spin can be seen near the critical condition, and the
Critical condition
critical pressure (Pc) is 3 kPa. In the tube filled with obstacles, the effect of obstacle geometries on the detonation transmission can be ignored approximately for the BR ¼ 0.7 case, and the critical pressures are increased to 7, 7 and 10 kPa, respectively. In the case of BR ¼ 0.8, the effect of the orifice plates structures on the detonation propagation becomes more significant. The square orifice has the most serious impact on the detonation transmission, followed by triangular ones and the round hole has the least impact. The critical pressures are sharply enhanced to 10, 12 and 18 kPa, respectively. Finally, the effective diameter (deff) and the characteristic parameter (L) are introduced to analyze the critical condition of the detonation propagation. The critical condition can quantified as deff/l > 1 and L/l > 7 where l is the detonation cell size. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, binary fuel blends (e.g., hydrogen and methane) have absorbed a lot of attention due to their excellent combustion performance in engines. This is mainly contributed to the addition of hydrogen into the natural gas
(the main component is methane), which greatly enhance the ignition and combustion properties because hydrogen has the fastest flame velocity among practical fuels [1e6]. Moreover, binary fuels of methane-hydrogen used in combustion engines can improve the lean-burn ability and alleviate environmental pollution [7e12] as well, which makes up the weakness of the methane combustion in engines.
* Corresponding author. E-mail address: [email protected] (S. Lu). https://doi.org/10.1016/j.ijhydene.2019.01.283 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Latin BR CJ d deff D DH l L P0 Pc S TOA V VCJ Vdef
blockage ratio Chapman-Jouguet orifice plate size effective diameter inner diameter of tube hydraulic diameter length of tube characteristic dimension initial pressure critical pressure orifice plate spacing time-of-arrival measured average velocity theoretical CJ detonation velocity velocity deficit
Greek l
detonation cell size
In the past, to better understand the combustion behaviors in methane-hydrogen mixtures, a number of works have been conducted to investigate its combustion characteristics. Yan et al. [13,14] comparatively investigated the micro-scale combustion mechanism in hydrogen-methane-air mixtures. Hu et al. [1,2,15], Halter et al. [16] and Di Sarli et al. [17] systematically investigated the laminar burning speed of methane-hydrogen mixtures. Furthermore, the effect of the fraction of hydrogen addition on the properties and emissions of a spark-ignition engine fueled with methane-hydrogen mixtures was studied by Hu et al. [1,2]. Wang et al. [3] further numerically studied the influence of hydrogen addition on the characteristics of methane-air combustion. Zheng et al. [18] theoretically and experimentally analyzed the propagation of premixed flame in methane-hydrogen mixtures, and discovered that the agreement is good between the theoretical model and experimental results. Moreover, the ignition abilities of methane-hydrogen mixtures were further experimentally studied by Gersen et al. [19], and the ignition delay times were also compared with the simulation results obtained from different chemical mechanism. Recently, the detonation features of methane-hydrogen mixtures get more attention due to its more seriously destructive effect, which threatens personal and property safety. Porowski and Teodorczyk [20] experimentally investigated the mechanism of deflagration to detonation transition (DDT) in stoichiometric hydrogen-methane-air mixture. Zhang et al. [6,21] experimentally investigated the detonation limits of methanehydrogen mixtures with various compositions in a round tube. The results showed that the addition of hydrogen produces an important influence on the detonation limit, and six different propagation modes can be observed, i.e., steady detonation, fast fluctuation-stable detonation, stuttering detonation, stuttering detonation-fast deflagration, fast
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deflagration and flame propagation. Subsequently, the deflagration to detonation transition (DDT) limit in hydrogenmethane-air mixtures was experimentally investigated by Wang et al. [22], who suggested that the ratio of the height of the square orifice plate (d) and the detonation cell size (l) increases with the increases of the blockage ratio (BR) and the decrease of obstacle spacing. Based on the studies discussed above, it can be seen that although the combustion characteristics of hydrogenmethane mixtures have been investigated by many researchers, little attention is paid to its detonation behaviors, especially for the investigation of detonation limits. In fact, the detonation limit is an important parameter in the deflagration or detonation hazard assessment [23e28]. In the past, a closed round or square tube filled with orifice plates is one of the most common apparatus to investigate the detonation propagation mechanisms [29,30]. Moreover, the effects of blockage ratio (BR) and obstacle spacing were considered well, and the details can refer in Refs. [31e35]. However, the influences of the shapes of obstacle on the detonation propagation are still lacking, despite it is a more common and practical problem in industry safety. Liu et al. [36] investigated the effect of geometries of a single orifice plate on the detonation transmission, including square, triangular, elliptical and circular orifice. The results indicate that the critical diameter is identical to the empirical correlation of dc z 13l. Mehrjoo et al. [37,38] systematically investigated the effect of blockage geometries on the detonation transmission with the blockage ratio (BR) in the range of 0.05e0.25. The results demonstrate that the detonation propagation is independent of the obstacle geometry, and indicate that it is only a function of its BR. But it is a pity that the effect of continuous obstacles is not considered, and the values of BR employed in these studies are relatively small. In practice, it is reasonably anticipated that an adverse impact can be observed in the cases of large BRs because of excess momentum losses induced by the obstacles and reduction of the “effective” tube diameter [38]. Therefore, for the cases of large BRs, the effect of obstacle geometries on the detonation propagation is worth exploring and the in-depth mechanisms are still not clear. To further understand the effect of the geometries of repeating orifice plates on the detonation propagation in a tube, more investigations are still required. This work aims to investigate the detonation propagation characteristics for stoichiometric hydrogen-methane-oxygen mixtures in a tube filled orifice plates with various shapes. The round, triangular, and square orifice plates are adopted and the larger BRs (0.7 and 0.8) are also considered. Highspeed piezoelectric pressure transducers (PCB102B06) are used to record the time-of-arrival of the detonation wave, from which the average velocity can be obtained. Smoked foils technique is adopted to register the detonation cellular structures. The effect of perturbation induced by orifice plates on the detonation velocity deficit is explored, and compared with the results obtained from the smooth tube. Moreover, the analysis of the critical condition of detonation propagation are conducted as well.
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Experimental details
Table 1 e Detailed sizes of orifice plates were adopted in this study.
Experimental setup
BR
Plate spacing (S/mm)
0.7 0.8
180
Detonation experiments were performed in a circular tube with an inner diameter of 90 mm (D) and 6.0 m in length (l). The tube consisted of two 3 m-long sections, and they were connected together by flanges, as shown in Fig. 1a. The former is the driver section used to accelerate the formation of a planner detonation wave. The latter is the test section filled with obstacles, including circular, triangular and square orifice, as shown in Fig. 1b. These orifice plates were fixed in the tube with equal spacing by three 5 mm diameter stainless rods and numerous drivepipes to prevent the mobilization of orifice plates array. In this study, the blockage ratios (BR) of orifice plates are 0.7 and 0.8, and the orifice plates spacing (S) is fixed to twice inner diameter of the tube, i.e., S ¼ 2D. The detailed sizes of the orifice plates were tabulated in Table 1.
Mixture preparation In this study, the typical stoichiometric CH4-2H2-3O2 mixtures were adopted as the test gas. The stoichiometric mixtures mean that in theory the fuel and oxygen can be reacted completely. The purities of methane and hydrogen are both 99.99%. The initial temperature for all these mixtures is 298 K. The specific properties of these gases were tabulated in Table 2. The mixtures were introduced into a 150 L mixed tank for at least 24 h to ensure homogeneity prior to each experiment. The detonation tube was evacuated by a vacuum pump to lower than 100 pa, and then the mixtures were released into
Orifice plate size (d/mm) Round
Triangular
Square
49 40
66 54
44 36
the tube to the desired initial pressure by the method of partial pressure. The initial pressure in the tube or the mixing tank was monitored by a manometer (SXT-4A, 0e150 kPa) with an accuracy of ±0.067% full scale. The mixtures are ignited 120 s after the mixtures were introduced into the tube by an electric spark from the discharge of a capacitor bank.
Technique of data collection In the test section, six high-speed piezoelectric pressure transducers (PCB102B06) with 50 cm interval are located at the tube wall to register the time-of-arrival (TOA) of detonation wave, from which the averaged velocity can be determined. Two pressure sensors (PCB102B06) with 40 cm interval were mounted at the driver section to verify a CJ detonation wave has been created prior to its transmission into the test section.
Records of detonation cellular structure Smoked foil technique was used to register the detonation cellular structures. This is accomplished by inserting a 0.1 mm thick stainless steel plate covered with uniform soot into the tube from the end before each experiment.
Fig. 1 e Schematic diagram of experimental set-up (a), various-shaped orifice plate structures (b).
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Table 2 e The specific properties of these gases used in this study. Gases CH4 H2 CH4-2H2-3O2
Purity %
Temperature K
Methane vol%
Hydrogen vol%
Oxygen vol%
Equivalence ratio, F
99.99 99.99 e
298 298 298
e e 16.67
e e 33.33
e e 50.00
e e 1
Results and discussion Smooth tube Fig. 2 presents the variation of the normalized detonation velocity with CJ value as a function of initial pressure (P0) in a 90 mm inner diameter tube for stoichiometric CH4-2H2-3O2 mixture. In this study, the theoretical CJ detonation velocity (VCJ) calculated by the CHEMKIN package [39] was also given for comparison. The critical pressure (Pc) below which the selfsustained steady detonation wave can no longer be obtained, and the experimental values of Pc are also determined. The experimental results indicate that the value of Pc is 3 kPa in the smooth tube. At pressures greater than Pc, the detonation can propagate at about theoretical CJ velocity. With the decreasing of the initial pressure, the detonation velocity deficit increases gradually, and a sudden velocity drop can be seen as approaching to the limits, see the vertical dashed line in Fig. 2. Here, the detonation velocity deficit (Vdef) is defined as: Vdef ¼
VCJ V VCJ
(1)
where is VCJ the theoretical CJ detonation velocity, m/s; V is the experimental measured average velocity, m/s. It also can be seen from the curve fit that, with the initial pressure decreases to the critical value, the detonation velocity gradually reduces and deviates from the CJ value. Below the critical pressure, the detonation propagation cannot maintain a steady state. To further investigate the detonation
Fig. 2 e Variation of normalized detonation velocity as a function of initial pressure in 90 mm diameter round tube.
propagation behaviors in the smooth tube, the variation of the normalized detonation velocity as a function of distance is also given in Fig. 3. It can be observed that, in the test section, the values of V/VCJ are all closed 1 with some fluctuation when P0 is greater than Pc. As P0 approaches to Pc, the detonation velocity is far smaller than VCJ, similar to that be observed in Fig. 2. Of note is that the detonation sensitivity closely depends on the values of initial pressure as well. It can be reflected from the detonation cell size [40]. In the studies of Zhang et al. [6,41e43], Gao et al. [44] and Ng et al. [45], it has been confirmed that the cell size and the critical energy of directing detonation initiation are both obviously increased by gradually reducing the initial pressure, which indicate the detonation sensitivity is reduced. Moreover, the cell size also is an important parameter which is related to some dynamic parameters of the detonation, e.g., critical ignition energy and critical tube diameter etc. [46]. Although the numerical simulation has been developed well and many researchers obtain the cellular structures by the numerical methods, the smoked foil technique still is one of the most reliable and convenient ways [47,48] to directly measure the cell size. In this paper, the detonation cell size was measured experimentally, as shown in Fig. 4. To minimize the subjectivity of measurement, five different experiments were conducted at the same initial pressure. In Fig. 4, the data points represent the mean values and the error bars are the standard deviations. Typical cellular patterns at different initial pressures are shown in Fig. 5, and the detonation propagates from left to right. The smoked foil is fixed at the end of the tube. For
Fig. 3 e Variation of normalized detonation velocity as a function of distance at different initial pressures for CH4H2-O2 mixtures.
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stoichiometric CH4-2H2-3O2 mixture, spinning detonation with some trivial transverse waves can be observed at P0 ¼ 3 kPa. As the initial pressure further decreases to smaller than Pc (e.g., P0 ¼ 2.5 kPa), no cellular structures can be recorded, which shows that the detonation fails eventually. At P0 ¼ 4 kPa, the transition from multi-headed detonation to double-headed detonation can be observed, and the cell size is consistent with the criterion proposed by Lee [48] and Dupre et al. [49], i.e., l ¼ pd. At P0 ¼ 5 kPa, multiple-headed detonation structure occurs. In the present study, no cell structure and steady velocity can be obtained at P0 lower than Pc, Therefore, for stoichiometric CH4-2H2-3O2 mixture, the detonation limits correspond to the critical condition of the detonation propagation.
Repeating orifice plates Fig. 4 e Cell size as a function of initial pressure in a 90 mm diameter round tube.
Fig. 5 e Cellular structure records in a 90 mm diameter round tube.
As the detonation propagates from circular tube into the section of repeating orifice plates, it can be observed that the velocity decreases and the critical pressure increases significantly due to the effects of diffraction wave and the heat and momentum losses from the wall. Figs. 6 and 7 show the normalized detonation velocity as a function of initial pressure in the cases of BR ¼ 0.7 and 0.8. Herein, the error bars represent the standard deviations. In Fig. 6, the velocity deficit becomes more serious, and the detonation velocity sharply decreases to about 0.6VCJ at the critical condition for all three different-shaped orifice plates. The values of critical pressure are increased to 7 kPa, 7 kPa and 10 kPa for circular, triangular and square orifice, respectively. As the BR value is further increased to 0.8, the velocity deficit and the values of the critical pressure are both enhanced significantly due to the stronger diffraction effect, as shown in Fig. 7. It can be observed that the values of critical pressure are sharply enhanced to 10 kPa, 12 kPa and 18 kPa for circular, triangular and square orifice, respectively. However, when P0 approaches to Pc, the values of detonation velocity still can fluctuate in the vicinity of 0.6VCJ, which seems to be a universal phenomenon just before the failure of detonation in the present study. This is similar to the phenomenon observed by Teodorczyk et al. [50], Radulescu et al. [51] and Zhu et al. [52], i.e., when a detonation fails, the wave velocity is about half the CJ detonation velocity. This deviation can be contributed to the larger BR of the orifice plate used in this study. From the results obtained from Figs. 6 and 7, it can be found that, in the case of BR ¼ 0.7, the effect of the orifice plates geometries on the detonation propagation can be ignored approximately. As the BR value further increases to 0.8, the difference of critical pressure values among three various orifice plates becomes more significant. The square orifice has the most serious impact on the detonation transmission, followed by triangular ones and the round hole has the least impact. This suggested that the geometries of the obstacles have an important influence on the detonation propagation in higher BR's cases. In the future, the impact of geometries of obstacles on the detonation transmission should get more attention, especially for the cases of larger BRs. In the past, the hydraulic diameter (DH ¼ 4 area/perimeter) was often used as an appropriate parameter to investigate the detonation propagation limit in circular tube or
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Fig. 6 e Variation of normalized detonation velocity as a function of initial pressure in the case of BR ¼ 0.7.
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Fig. 7 e Variation of normalized detonation velocity as a function of initial pressure in the case of BR ¼ 0.8.
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Table 3 e Parameters of the orifice plates adopted in this study. Effective diameter (deff/mm)
BR
0.7 0.8
L (mm)
Round
Triangle
Square
Round
Triangle
Square
49 40
61.58 50.38
53.11 43.56
296 243
506 337
264 225
annular channels [53,54]. However, in this study, adopting the hydraulic diameter of a triangular and a square orifice as a length scale may lead to an improper results of the detonation limit. For example, for circular and square hole, the perimeter of the cross-section of the square orifice is larger than the round orifice at the same hydraulic diameter, which will result in a wider detonable range [29]. To avoid this problem, the effective diameter (deff) was introduced to predict the limit. It was first proposed by Liu et al. [36], which used to investigate the effect of geometries of an orifice plate on the detonation transmission. Liu et al. [36] suggested that, for a round orifice, there is only one linear dimension (i.e., the tube diameter or orifice diameter), but for noncircular geometries (e.g., triangular and square hole) there are two linear dimension impacting the detonation wave propagation. Thus, the effective diameter gives an appropriate length scale to characterize the geometry of orifice plates. In the study of Liu et al. [36], the effective diameter is defined as the mean value of the longest and shortest dimensions of the orifice shape. In this study, the effective diameter can be written as:
Fig. 8 e Variation of normalized detonation velocity as a function of deff/l.
deff
8 d > > > pffiffiffi > > > 3 > > d < 1þ 2 ¼ 2 > > > pffiffiffi > > > > 1þ 2 d > : 2
for circular orifice
for triangular orifice
(2)
for square orifice
Table 3 shows the specific values of the effective diameter among different orifice plates. Fig. 8 presents the normalized detonation velocity as a function of deff/l. The deff/l represents the cell numbers across the effective diameter, which is a common characteristic parameter used to investigate the detonation limit in the past [27,55]. In Fig. 8, it can be seen that, for all orifice plates, the velocity tendency collapses to a single curve when the initial pressure approaches to the critical value. Moreover, the values of deff/l are all greater than 1 at the critical condition, which is consistent with the criterion of d/l > 1 proposed by Cross and Ciccarelli [34], Peraldi et al. [32] and Knystautas et al. [56]. This also indicates that the effective diameter is exactly an appropriate length scale to investigate the detonation limit. Furthermore, Teodorczyk et al. [57] found that the critical condition of detonation propagation also depends on the
Fig. 9 e Variation of normalized detonation velocity as a function of L/l.
Table 4 e Experimental results in the tube filled with obstacles.
0.7 0.8
l (mm)
Pc (kPa)
BR
deff/l
L/l
Round
Triangle
Square
Round
Triangle
Square
Round
Triangle
Square
Round
Triangle
Square
7 10
7 12
10 18
47.6 37.8
47.6 24.5
37.8 23.6
1.03 1.30
1.29 2.10
1.41 1.85
6.22 7.83
10.63 13.75
6.98 9.53
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Fig. 10 e Detonation cellular structure records in the case of BR ¼ 0.7.
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obstacle spacing. Therefore, to quantify the critical condition of detonation propagation, Dorofeev et al. [58] proposed a criterion considering the obstacle spacing, i.e., L/l > 7 where L is a characteristic length-scale. For a tube filled with continuous obstacles, the L is defined as: L ¼ (SþD)/2/(1-d/D)
(3)
Where S e the spacing between obstacles, mm; D e the inner diameter of tube, mm; d e the hole diameter of orifice plate, mm.
As shown in Table 3, gives the specific values of L among different orifice plates. Fig. 9 presents the normalized detonation velocity as a function of L/l. The experimental results are similar to that be observed in Fig. 8, i.e., the data points seem to follow the same tendency near the limit. Moreover, it can be observed that the critical values of L/l fluctuate in the vicinity of 7 for all these cases, which is consistent with the criterion proposed by Dorofeev et al. [58] if the subjectivity of the measurement of cell size is considered, i.e., L/l > 7. Table 4 gives the experimental results in the tube filled with obstacles,
Fig. 11 e Detonation cellular structure records in the case of BR ¼ 0.8.
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including the critical pressure, detonation cell sizes under the critical pressure and the critical values of deff/l and L/l. Figs. 10 and 11 present the detonation cellular structures of CH4-2H2-3O2 mixture, which are taken in a tube filled with orifice plates. The smoked foil with 180 mm long was installed between the penultimate orifice plates and the antepenultimate orifice plates, and the distance far from the ignitor is 5.64 m and 5.82 m, respectively. The detonation propagates from left to right. As the initial pressure decreases, the trajectory of the three-wave point becomes more blurred due to lower detonation sensitivity. Meanwhile, some trivial transverse waves from the shock reflection also can be observed. However, the single-headed spin propagation mode is not to occur at the critical condition. For example, in the case of round orifice with BR ¼ 0.7 (see Fig. 10a), at P0 ¼ 9 kPa, an imprint of only a cellular pattern was left on the soot foil at beginning, followed by the fine cell region, as shown in the red dotted line. This can be explained that the local explosion centers (cone-shaped zone in Fig. 10a) are produced at the intersection of the two spirals, resulting in over-driven detonation. With the P0 decreases to 8 kPa, only one cellular structure was printed on the entire smoked foil without the fine cell zone, indicating that the local explosion centers cannot be produced in this condition. As the initial pressure further decreased to 7 kPa, a blurred trajectory of the threewave point can be seen and disappear eventually. Below the limiting pressure, no detonation cellular structures and steady detonation velocity can be obtained. For the cases of BR ¼ 0.8, a similar results also can be observed near the limit, which are shown in Fig. 11. But the corresponding critical pressure is higher, see Fig. 11a for an example. At P0 ¼ 12 kPa, a cellular pattern was presented on the first half of the soot foils. The cone-shaped region followed by fine cellular structures can be seen obviously, i.e., forming the over-driven detonation. The cell size gradually grows with the propagation of the detonation, and the over-driven detonation progressively decays to the normal CJ detonation. As the initial pressure decreases to 10 kPa, a detonation cellular structure with indistinct trajectory of the three-wave points was printed on the entire soot foil.
Conclusions In this study, a systematic experimental investigations of the detonation propagation characteristics in stoichiometric CH42H2-3O2 mixtures were performed in a round tube filled with orifice plates. The effects of the shapes of orifice plate were considered well (including circular, triangular and square orifice). Experimental results obtained in a smooth tube were also given for comparison. Some interesting discoveries are as follows: (1) Well within the limit, the detonation can propagate with a small velocity deficit. By decreasing the initial pressure, the velocity deficit increases gradually until the critical condition was approached. When P0 approaches to Pc, the detonation can propagate at about 0.6VCJ for all cases, it seems to be a universal phenomenon before the detonation failure. As the initial pressure further decreases to smaller than the critical value,
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no cell structures and steady velocity can be achieved, i.e., the detonation fails eventually. (2) In the smooth tube, the value of critical pressure is 3 kPa, and a sudden velocity drop can be observed at the critical condition. The detonation cell size was obtained experimentally, increasing with the decreases of initial pressure. The detonation cellular patterns were also given near the limit, and the spinning detonation with some trivial transverse waves can be observed. (3) In the tube filled with obstacles, the velocity deficit and the critical pressure are both enhanced sharply due to the diffraction effect and the heat and momentum loss from the wall. The geometries of obstacles nearly have no effect on the detonation propagation in the case of BR ¼ 0.7. As the BR's value increases to 0.8, the effect of the structures of orifice plates on the detonation transmission becomes more obvious. The square orifice has the most serious impact, followed by triangular ones and the round hole has the least impact. The detonation limit was approached by decreasing the initial pressure gradually as well. At pressures near Pc, only one cellular structure with indistinct trajectory of the three-wave points can be observed, and the singleheaded spin is not to occur. To determine the detonation limit, the effective diameter (deff) and the characteristic parameter (L) were introduced to analyze the critical condition of the detonation propagation. The experimental results indicate that the critical condition is consistent with the criterions proposed in Refs. [32,34,56,58], i.e., deff/l > 1 and L/l > 7.
Acknowledgements This work was supported by the National Key R&D Program of China [No.2016YFC0802101].
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Detonation propagation characteristics for CH42H2-3O2 mixtures in a tube filled with orifice plates Xuxu Sun, Quan Li, Changhai Li, Shouxiang Lu* State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027, PR China
article info
abstract
Article history:
In this study, the detonation propagation characteristics of stoichiometric CH4-2H2-3O2
Received 10 October 2018
mixture are investigated comprehensively in a round tube with an inner diameter of 90-
Received in revised form
mm and 6-m in length. Three different orifice plates with the blockage ratios (BR) of 0.7
25 January 2019
and 0.8 including circular, triangular and square orifice, are considered for the first time to
Accepted 30 January 2019
investigate the effect of obstacle geometries on the detonation evolution. Eight high-speed
Available online 20 February 2019
piezoelectric pressure transducers are mounted on the outer wall to obtain the detonation velocity while the smoked foil technique is adopted to record the detonation cellular
Keywords:
patterns. The results indicate that well within the limit, the detonation can propagate at
Detonation
about the theoretical CJ velocity (VCJ). Near the limit, the velocity deficit is sharply
Orifice plates
enhanced but the detonation still can propagate at about 0.6VCJ, which seems to be a
Geometries
universal phenomenon before the failure of the detonation. In the smooth tube, a sudden
Velocity deficit
velocity drop and the single-headed spin can be seen near the critical condition, and the
Critical condition
critical pressure (Pc) is 3 kPa. In the tube filled with obstacles, the effect of obstacle geometries on the detonation transmission can be ignored approximately for the BR ¼ 0.7 case, and the critical pressures are increased to 7, 7 and 10 kPa, respectively. In the case of BR ¼ 0.8, the effect of the orifice plates structures on the detonation propagation becomes more significant. The square orifice has the most serious impact on the detonation transmission, followed by triangular ones and the round hole has the least impact. The critical pressures are sharply enhanced to 10, 12 and 18 kPa, respectively. Finally, the effective diameter (deff) and the characteristic parameter (L) are introduced to analyze the critical condition of the detonation propagation. The critical condition can quantified as deff/l > 1 and L/l > 7 where l is the detonation cell size. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, binary fuel blends (e.g., hydrogen and methane) have absorbed a lot of attention due to their excellent combustion performance in engines. This is mainly contributed to the addition of hydrogen into the natural gas
(the main component is methane), which greatly enhance the ignition and combustion properties because hydrogen has the fastest flame velocity among practical fuels [1e6]. Moreover, binary fuels of methane-hydrogen used in combustion engines can improve the lean-burn ability and alleviate environmental pollution [7e12] as well, which makes up the weakness of the methane combustion in engines.
* Corresponding author. E-mail address: [email protected] (S. Lu). https://doi.org/10.1016/j.ijhydene.2019.01.283 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Latin BR CJ d deff D DH l L P0 Pc S TOA V VCJ Vdef
blockage ratio Chapman-Jouguet orifice plate size effective diameter inner diameter of tube hydraulic diameter length of tube characteristic dimension initial pressure critical pressure orifice plate spacing time-of-arrival measured average velocity theoretical CJ detonation velocity velocity deficit
Greek l
detonation cell size
In the past, to better understand the combustion behaviors in methane-hydrogen mixtures, a number of works have been conducted to investigate its combustion characteristics. Yan et al. [13,14] comparatively investigated the micro-scale combustion mechanism in hydrogen-methane-air mixtures. Hu et al. [1,2,15], Halter et al. [16] and Di Sarli et al. [17] systematically investigated the laminar burning speed of methane-hydrogen mixtures. Furthermore, the effect of the fraction of hydrogen addition on the properties and emissions of a spark-ignition engine fueled with methane-hydrogen mixtures was studied by Hu et al. [1,2]. Wang et al. [3] further numerically studied the influence of hydrogen addition on the characteristics of methane-air combustion. Zheng et al. [18] theoretically and experimentally analyzed the propagation of premixed flame in methane-hydrogen mixtures, and discovered that the agreement is good between the theoretical model and experimental results. Moreover, the ignition abilities of methane-hydrogen mixtures were further experimentally studied by Gersen et al. [19], and the ignition delay times were also compared with the simulation results obtained from different chemical mechanism. Recently, the detonation features of methane-hydrogen mixtures get more attention due to its more seriously destructive effect, which threatens personal and property safety. Porowski and Teodorczyk [20] experimentally investigated the mechanism of deflagration to detonation transition (DDT) in stoichiometric hydrogen-methane-air mixture. Zhang et al. [6,21] experimentally investigated the detonation limits of methanehydrogen mixtures with various compositions in a round tube. The results showed that the addition of hydrogen produces an important influence on the detonation limit, and six different propagation modes can be observed, i.e., steady detonation, fast fluctuation-stable detonation, stuttering detonation, stuttering detonation-fast deflagration, fast
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deflagration and flame propagation. Subsequently, the deflagration to detonation transition (DDT) limit in hydrogenmethane-air mixtures was experimentally investigated by Wang et al. [22], who suggested that the ratio of the height of the square orifice plate (d) and the detonation cell size (l) increases with the increases of the blockage ratio (BR) and the decrease of obstacle spacing. Based on the studies discussed above, it can be seen that although the combustion characteristics of hydrogenmethane mixtures have been investigated by many researchers, little attention is paid to its detonation behaviors, especially for the investigation of detonation limits. In fact, the detonation limit is an important parameter in the deflagration or detonation hazard assessment [23e28]. In the past, a closed round or square tube filled with orifice plates is one of the most common apparatus to investigate the detonation propagation mechanisms [29,30]. Moreover, the effects of blockage ratio (BR) and obstacle spacing were considered well, and the details can refer in Refs. [31e35]. However, the influences of the shapes of obstacle on the detonation propagation are still lacking, despite it is a more common and practical problem in industry safety. Liu et al. [36] investigated the effect of geometries of a single orifice plate on the detonation transmission, including square, triangular, elliptical and circular orifice. The results indicate that the critical diameter is identical to the empirical correlation of dc z 13l. Mehrjoo et al. [37,38] systematically investigated the effect of blockage geometries on the detonation transmission with the blockage ratio (BR) in the range of 0.05e0.25. The results demonstrate that the detonation propagation is independent of the obstacle geometry, and indicate that it is only a function of its BR. But it is a pity that the effect of continuous obstacles is not considered, and the values of BR employed in these studies are relatively small. In practice, it is reasonably anticipated that an adverse impact can be observed in the cases of large BRs because of excess momentum losses induced by the obstacles and reduction of the “effective” tube diameter [38]. Therefore, for the cases of large BRs, the effect of obstacle geometries on the detonation propagation is worth exploring and the in-depth mechanisms are still not clear. To further understand the effect of the geometries of repeating orifice plates on the detonation propagation in a tube, more investigations are still required. This work aims to investigate the detonation propagation characteristics for stoichiometric hydrogen-methane-oxygen mixtures in a tube filled orifice plates with various shapes. The round, triangular, and square orifice plates are adopted and the larger BRs (0.7 and 0.8) are also considered. Highspeed piezoelectric pressure transducers (PCB102B06) are used to record the time-of-arrival of the detonation wave, from which the average velocity can be obtained. Smoked foils technique is adopted to register the detonation cellular structures. The effect of perturbation induced by orifice plates on the detonation velocity deficit is explored, and compared with the results obtained from the smooth tube. Moreover, the analysis of the critical condition of detonation propagation are conducted as well.
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Experimental details
Table 1 e Detailed sizes of orifice plates were adopted in this study.
Experimental setup
BR
Plate spacing (S/mm)
0.7 0.8
180
Detonation experiments were performed in a circular tube with an inner diameter of 90 mm (D) and 6.0 m in length (l). The tube consisted of two 3 m-long sections, and they were connected together by flanges, as shown in Fig. 1a. The former is the driver section used to accelerate the formation of a planner detonation wave. The latter is the test section filled with obstacles, including circular, triangular and square orifice, as shown in Fig. 1b. These orifice plates were fixed in the tube with equal spacing by three 5 mm diameter stainless rods and numerous drivepipes to prevent the mobilization of orifice plates array. In this study, the blockage ratios (BR) of orifice plates are 0.7 and 0.8, and the orifice plates spacing (S) is fixed to twice inner diameter of the tube, i.e., S ¼ 2D. The detailed sizes of the orifice plates were tabulated in Table 1.
Mixture preparation In this study, the typical stoichiometric CH4-2H2-3O2 mixtures were adopted as the test gas. The stoichiometric mixtures mean that in theory the fuel and oxygen can be reacted completely. The purities of methane and hydrogen are both 99.99%. The initial temperature for all these mixtures is 298 K. The specific properties of these gases were tabulated in Table 2. The mixtures were introduced into a 150 L mixed tank for at least 24 h to ensure homogeneity prior to each experiment. The detonation tube was evacuated by a vacuum pump to lower than 100 pa, and then the mixtures were released into
Orifice plate size (d/mm) Round
Triangular
Square
49 40
66 54
44 36
the tube to the desired initial pressure by the method of partial pressure. The initial pressure in the tube or the mixing tank was monitored by a manometer (SXT-4A, 0e150 kPa) with an accuracy of ±0.067% full scale. The mixtures are ignited 120 s after the mixtures were introduced into the tube by an electric spark from the discharge of a capacitor bank.
Technique of data collection In the test section, six high-speed piezoelectric pressure transducers (PCB102B06) with 50 cm interval are located at the tube wall to register the time-of-arrival (TOA) of detonation wave, from which the averaged velocity can be determined. Two pressure sensors (PCB102B06) with 40 cm interval were mounted at the driver section to verify a CJ detonation wave has been created prior to its transmission into the test section.
Records of detonation cellular structure Smoked foil technique was used to register the detonation cellular structures. This is accomplished by inserting a 0.1 mm thick stainless steel plate covered with uniform soot into the tube from the end before each experiment.
Fig. 1 e Schematic diagram of experimental set-up (a), various-shaped orifice plate structures (b).
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Table 2 e The specific properties of these gases used in this study. Gases CH4 H2 CH4-2H2-3O2
Purity %
Temperature K
Methane vol%
Hydrogen vol%
Oxygen vol%
Equivalence ratio, F
99.99 99.99 e
298 298 298
e e 16.67
e e 33.33
e e 50.00
e e 1
Results and discussion Smooth tube Fig. 2 presents the variation of the normalized detonation velocity with CJ value as a function of initial pressure (P0) in a 90 mm inner diameter tube for stoichiometric CH4-2H2-3O2 mixture. In this study, the theoretical CJ detonation velocity (VCJ) calculated by the CHEMKIN package [39] was also given for comparison. The critical pressure (Pc) below which the selfsustained steady detonation wave can no longer be obtained, and the experimental values of Pc are also determined. The experimental results indicate that the value of Pc is 3 kPa in the smooth tube. At pressures greater than Pc, the detonation can propagate at about theoretical CJ velocity. With the decreasing of the initial pressure, the detonation velocity deficit increases gradually, and a sudden velocity drop can be seen as approaching to the limits, see the vertical dashed line in Fig. 2. Here, the detonation velocity deficit (Vdef) is defined as: Vdef ¼
VCJ V VCJ
(1)
where is VCJ the theoretical CJ detonation velocity, m/s; V is the experimental measured average velocity, m/s. It also can be seen from the curve fit that, with the initial pressure decreases to the critical value, the detonation velocity gradually reduces and deviates from the CJ value. Below the critical pressure, the detonation propagation cannot maintain a steady state. To further investigate the detonation
Fig. 2 e Variation of normalized detonation velocity as a function of initial pressure in 90 mm diameter round tube.
propagation behaviors in the smooth tube, the variation of the normalized detonation velocity as a function of distance is also given in Fig. 3. It can be observed that, in the test section, the values of V/VCJ are all closed 1 with some fluctuation when P0 is greater than Pc. As P0 approaches to Pc, the detonation velocity is far smaller than VCJ, similar to that be observed in Fig. 2. Of note is that the detonation sensitivity closely depends on the values of initial pressure as well. It can be reflected from the detonation cell size [40]. In the studies of Zhang et al. [6,41e43], Gao et al. [44] and Ng et al. [45], it has been confirmed that the cell size and the critical energy of directing detonation initiation are both obviously increased by gradually reducing the initial pressure, which indicate the detonation sensitivity is reduced. Moreover, the cell size also is an important parameter which is related to some dynamic parameters of the detonation, e.g., critical ignition energy and critical tube diameter etc. [46]. Although the numerical simulation has been developed well and many researchers obtain the cellular structures by the numerical methods, the smoked foil technique still is one of the most reliable and convenient ways [47,48] to directly measure the cell size. In this paper, the detonation cell size was measured experimentally, as shown in Fig. 4. To minimize the subjectivity of measurement, five different experiments were conducted at the same initial pressure. In Fig. 4, the data points represent the mean values and the error bars are the standard deviations. Typical cellular patterns at different initial pressures are shown in Fig. 5, and the detonation propagates from left to right. The smoked foil is fixed at the end of the tube. For
Fig. 3 e Variation of normalized detonation velocity as a function of distance at different initial pressures for CH4H2-O2 mixtures.
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stoichiometric CH4-2H2-3O2 mixture, spinning detonation with some trivial transverse waves can be observed at P0 ¼ 3 kPa. As the initial pressure further decreases to smaller than Pc (e.g., P0 ¼ 2.5 kPa), no cellular structures can be recorded, which shows that the detonation fails eventually. At P0 ¼ 4 kPa, the transition from multi-headed detonation to double-headed detonation can be observed, and the cell size is consistent with the criterion proposed by Lee [48] and Dupre et al. [49], i.e., l ¼ pd. At P0 ¼ 5 kPa, multiple-headed detonation structure occurs. In the present study, no cell structure and steady velocity can be obtained at P0 lower than Pc, Therefore, for stoichiometric CH4-2H2-3O2 mixture, the detonation limits correspond to the critical condition of the detonation propagation.
Repeating orifice plates Fig. 4 e Cell size as a function of initial pressure in a 90 mm diameter round tube.
Fig. 5 e Cellular structure records in a 90 mm diameter round tube.
As the detonation propagates from circular tube into the section of repeating orifice plates, it can be observed that the velocity decreases and the critical pressure increases significantly due to the effects of diffraction wave and the heat and momentum losses from the wall. Figs. 6 and 7 show the normalized detonation velocity as a function of initial pressure in the cases of BR ¼ 0.7 and 0.8. Herein, the error bars represent the standard deviations. In Fig. 6, the velocity deficit becomes more serious, and the detonation velocity sharply decreases to about 0.6VCJ at the critical condition for all three different-shaped orifice plates. The values of critical pressure are increased to 7 kPa, 7 kPa and 10 kPa for circular, triangular and square orifice, respectively. As the BR value is further increased to 0.8, the velocity deficit and the values of the critical pressure are both enhanced significantly due to the stronger diffraction effect, as shown in Fig. 7. It can be observed that the values of critical pressure are sharply enhanced to 10 kPa, 12 kPa and 18 kPa for circular, triangular and square orifice, respectively. However, when P0 approaches to Pc, the values of detonation velocity still can fluctuate in the vicinity of 0.6VCJ, which seems to be a universal phenomenon just before the failure of detonation in the present study. This is similar to the phenomenon observed by Teodorczyk et al. [50], Radulescu et al. [51] and Zhu et al. [52], i.e., when a detonation fails, the wave velocity is about half the CJ detonation velocity. This deviation can be contributed to the larger BR of the orifice plate used in this study. From the results obtained from Figs. 6 and 7, it can be found that, in the case of BR ¼ 0.7, the effect of the orifice plates geometries on the detonation propagation can be ignored approximately. As the BR value further increases to 0.8, the difference of critical pressure values among three various orifice plates becomes more significant. The square orifice has the most serious impact on the detonation transmission, followed by triangular ones and the round hole has the least impact. This suggested that the geometries of the obstacles have an important influence on the detonation propagation in higher BR's cases. In the future, the impact of geometries of obstacles on the detonation transmission should get more attention, especially for the cases of larger BRs. In the past, the hydraulic diameter (DH ¼ 4 area/perimeter) was often used as an appropriate parameter to investigate the detonation propagation limit in circular tube or
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 7 6 1 6 e7 6 2 7
Fig. 6 e Variation of normalized detonation velocity as a function of initial pressure in the case of BR ¼ 0.7.
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Fig. 7 e Variation of normalized detonation velocity as a function of initial pressure in the case of BR ¼ 0.8.
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Table 3 e Parameters of the orifice plates adopted in this study. Effective diameter (deff/mm)
BR
0.7 0.8
L (mm)
Round
Triangle
Square
Round
Triangle
Square
49 40
61.58 50.38
53.11 43.56
296 243
506 337
264 225
annular channels [53,54]. However, in this study, adopting the hydraulic diameter of a triangular and a square orifice as a length scale may lead to an improper results of the detonation limit. For example, for circular and square hole, the perimeter of the cross-section of the square orifice is larger than the round orifice at the same hydraulic diameter, which will result in a wider detonable range [29]. To avoid this problem, the effective diameter (deff) was introduced to predict the limit. It was first proposed by Liu et al. [36], which used to investigate the effect of geometries of an orifice plate on the detonation transmission. Liu et al. [36] suggested that, for a round orifice, there is only one linear dimension (i.e., the tube diameter or orifice diameter), but for noncircular geometries (e.g., triangular and square hole) there are two linear dimension impacting the detonation wave propagation. Thus, the effective diameter gives an appropriate length scale to characterize the geometry of orifice plates. In the study of Liu et al. [36], the effective diameter is defined as the mean value of the longest and shortest dimensions of the orifice shape. In this study, the effective diameter can be written as:
Fig. 8 e Variation of normalized detonation velocity as a function of deff/l.
deff
8 d > > > pffiffiffi > > > 3 > > d < 1þ 2 ¼ 2 > > > pffiffiffi > > > > 1þ 2 d > : 2
for circular orifice
for triangular orifice
(2)
for square orifice
Table 3 shows the specific values of the effective diameter among different orifice plates. Fig. 8 presents the normalized detonation velocity as a function of deff/l. The deff/l represents the cell numbers across the effective diameter, which is a common characteristic parameter used to investigate the detonation limit in the past [27,55]. In Fig. 8, it can be seen that, for all orifice plates, the velocity tendency collapses to a single curve when the initial pressure approaches to the critical value. Moreover, the values of deff/l are all greater than 1 at the critical condition, which is consistent with the criterion of d/l > 1 proposed by Cross and Ciccarelli [34], Peraldi et al. [32] and Knystautas et al. [56]. This also indicates that the effective diameter is exactly an appropriate length scale to investigate the detonation limit. Furthermore, Teodorczyk et al. [57] found that the critical condition of detonation propagation also depends on the
Fig. 9 e Variation of normalized detonation velocity as a function of L/l.
Table 4 e Experimental results in the tube filled with obstacles.
0.7 0.8
l (mm)
Pc (kPa)
BR
deff/l
L/l
Round
Triangle
Square
Round
Triangle
Square
Round
Triangle
Square
Round
Triangle
Square
7 10
7 12
10 18
47.6 37.8
47.6 24.5
37.8 23.6
1.03 1.30
1.29 2.10
1.41 1.85
6.22 7.83
10.63 13.75
6.98 9.53
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 7 6 1 6 e7 6 2 7
Fig. 10 e Detonation cellular structure records in the case of BR ¼ 0.7.
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obstacle spacing. Therefore, to quantify the critical condition of detonation propagation, Dorofeev et al. [58] proposed a criterion considering the obstacle spacing, i.e., L/l > 7 where L is a characteristic length-scale. For a tube filled with continuous obstacles, the L is defined as: L ¼ (SþD)/2/(1-d/D)
(3)
Where S e the spacing between obstacles, mm; D e the inner diameter of tube, mm; d e the hole diameter of orifice plate, mm.
As shown in Table 3, gives the specific values of L among different orifice plates. Fig. 9 presents the normalized detonation velocity as a function of L/l. The experimental results are similar to that be observed in Fig. 8, i.e., the data points seem to follow the same tendency near the limit. Moreover, it can be observed that the critical values of L/l fluctuate in the vicinity of 7 for all these cases, which is consistent with the criterion proposed by Dorofeev et al. [58] if the subjectivity of the measurement of cell size is considered, i.e., L/l > 7. Table 4 gives the experimental results in the tube filled with obstacles,
Fig. 11 e Detonation cellular structure records in the case of BR ¼ 0.8.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 7 6 1 6 e7 6 2 7
including the critical pressure, detonation cell sizes under the critical pressure and the critical values of deff/l and L/l. Figs. 10 and 11 present the detonation cellular structures of CH4-2H2-3O2 mixture, which are taken in a tube filled with orifice plates. The smoked foil with 180 mm long was installed between the penultimate orifice plates and the antepenultimate orifice plates, and the distance far from the ignitor is 5.64 m and 5.82 m, respectively. The detonation propagates from left to right. As the initial pressure decreases, the trajectory of the three-wave point becomes more blurred due to lower detonation sensitivity. Meanwhile, some trivial transverse waves from the shock reflection also can be observed. However, the single-headed spin propagation mode is not to occur at the critical condition. For example, in the case of round orifice with BR ¼ 0.7 (see Fig. 10a), at P0 ¼ 9 kPa, an imprint of only a cellular pattern was left on the soot foil at beginning, followed by the fine cell region, as shown in the red dotted line. This can be explained that the local explosion centers (cone-shaped zone in Fig. 10a) are produced at the intersection of the two spirals, resulting in over-driven detonation. With the P0 decreases to 8 kPa, only one cellular structure was printed on the entire smoked foil without the fine cell zone, indicating that the local explosion centers cannot be produced in this condition. As the initial pressure further decreased to 7 kPa, a blurred trajectory of the threewave point can be seen and disappear eventually. Below the limiting pressure, no detonation cellular structures and steady detonation velocity can be obtained. For the cases of BR ¼ 0.8, a similar results also can be observed near the limit, which are shown in Fig. 11. But the corresponding critical pressure is higher, see Fig. 11a for an example. At P0 ¼ 12 kPa, a cellular pattern was presented on the first half of the soot foils. The cone-shaped region followed by fine cellular structures can be seen obviously, i.e., forming the over-driven detonation. The cell size gradually grows with the propagation of the detonation, and the over-driven detonation progressively decays to the normal CJ detonation. As the initial pressure decreases to 10 kPa, a detonation cellular structure with indistinct trajectory of the three-wave points was printed on the entire soot foil.
Conclusions In this study, a systematic experimental investigations of the detonation propagation characteristics in stoichiometric CH42H2-3O2 mixtures were performed in a round tube filled with orifice plates. The effects of the shapes of orifice plate were considered well (including circular, triangular and square orifice). Experimental results obtained in a smooth tube were also given for comparison. Some interesting discoveries are as follows: (1) Well within the limit, the detonation can propagate with a small velocity deficit. By decreasing the initial pressure, the velocity deficit increases gradually until the critical condition was approached. When P0 approaches to Pc, the detonation can propagate at about 0.6VCJ for all cases, it seems to be a universal phenomenon before the detonation failure. As the initial pressure further decreases to smaller than the critical value,
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no cell structures and steady velocity can be achieved, i.e., the detonation fails eventually. (2) In the smooth tube, the value of critical pressure is 3 kPa, and a sudden velocity drop can be observed at the critical condition. The detonation cell size was obtained experimentally, increasing with the decreases of initial pressure. The detonation cellular patterns were also given near the limit, and the spinning detonation with some trivial transverse waves can be observed. (3) In the tube filled with obstacles, the velocity deficit and the critical pressure are both enhanced sharply due to the diffraction effect and the heat and momentum loss from the wall. The geometries of obstacles nearly have no effect on the detonation propagation in the case of BR ¼ 0.7. As the BR's value increases to 0.8, the effect of the structures of orifice plates on the detonation transmission becomes more obvious. The square orifice has the most serious impact, followed by triangular ones and the round hole has the least impact. The detonation limit was approached by decreasing the initial pressure gradually as well. At pressures near Pc, only one cellular structure with indistinct trajectory of the three-wave points can be observed, and the singleheaded spin is not to occur. To determine the detonation limit, the effective diameter (deff) and the characteristic parameter (L) were introduced to analyze the critical condition of the detonation propagation. The experimental results indicate that the critical condition is consistent with the criterions proposed in Refs. [32,34,56,58], i.e., deff/l > 1 and L/l > 7.
Acknowledgements This work was supported by the National Key R&D Program of China [No.2016YFC0802101].
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