- Email: [email protected]
Flame propagation in methane-air mixtures with transverse concentration gradients in horizontal duct
Fuel 265 (2020) 116926
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Flame propagation in methane-air mixtures with transverse concentration gradients in horizontal duct Chunhua Wanga, Jialin Lib, Zesi Tangb, Yanzhen Zhuangb, Jin Guob, a b
T
⁎
College of Environmental and Biological Engineering, Putian University, Putian 351100, PR China College of Environment and Resources, Fuzhou University, Fuzhou 350116, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Methane Concentration gradient Flame Overpressure
Stratified methane-air mixtures are formed in the early stages of methane leakage in confined spaces, and an explosion may occur if a source of ignition were to appear at the appropriate time and location. However, this has not been elucidated thus far. The aim of this study is to measure the concentration gradient of methane injected from the top surface of a duct, and to investigate the effects of ignition delay (tig ), which is the time interval from methane leakage to ignition, on flame behavior and explosion overpressure. Experimental results show that tig significantly affects flame behavior in stratified methane-air mixtures, including the flame shape and speed. The stratified methane-air mixtures cannot be ignited at tig ≤ 3 min. The horizontal flame speed and maximum overpressure (Pmax ) first increase as tig increases from 4 min to 15 min and thereafter remain nearly unchanged with further increase in tig . The value of Pmax at tig in the range 15–25 min is approximately that observed for homogeneous methane-air mixtures. Tulip flames are observed at tig ≥ 10 min. Diffusion and convection flames, which follow the leading premixed flame front, are only formed at tig ≤ 6 min.
1. Introduction Methane is the main component of natural gas, and is used as a raw material in the chemical industry. However, the leakage of methane in confined spaces often poses the serious problem of potential explosions. In general, upon accidental release, methane first accumulates locally and then diffuses gradually to the entire confined space. During this period, the methane-air mixtures are stratified rather than homogeneous. In this situation, an explosion may occur if a source of ignition appears at the right time and location. Therefore, it is extremely important to understand this phenomenon (including flame behavior and pressure buildup) of the explosion of stratified methane-air mixtures, to design suitable protection and mitigation measures against such explosions. Investigations [1–13] have been conducted into the phenomena of flame propagation in methane-air mixtures whose concentration gradients were experimentally measured [1–4] or theoretically predicted [8]. In general, the investigations can be classified into two categories: 1) flame propagation along the concentration gradient of fuel [1–7], and 2) flame propagation normal to the concentration gradient [8–13]. In the first category of investigations, the lower flammability limit (LFL) was found to decrease [1–4,6] owing to heat input to the propagating flame from the burnt gases [1,7], and the upper flammability limit ⁎
(UFL) was found to increase because of hydrogen production through flame propagation into the fuel-rich mixtures [3,14]. With respect to flame speed, Badr and Karim [5], and Karim and Tsang [7] found that the speed of the accelerating flame (i.e., the flame propagated from lean or rich into nearly stoichiometric mixtures) was close to those of the corresponding homogeneous mixtures. Conversely, higher laminar flame speed was observed when the accelerating flame propagated into step-stratified methane-air mixtures [14]. Previous research has demonstrated that the speed of the decelerating flame, i.e., the flame propagating from nearly stoichiometric into lean or rich mixtures, was higher than that of the flame corresponding to a homogeneous mixture of the local equivalence ratio [1–5,7]. For instance, Badr and Karim [5] observed a decelerating flame with up to 70% higher velocity compared to that of homogeneous mixtures. The increase in flame speed resulted from heat feedback from the burnt mixture [2,3,14]. Differently, Cruz et al. [14] found that the laminar flame speed was lower in step-stratified methane-air mixtures (from stoichiometric to rich mixtures) than in homogeneous mixtures. Furthermore, Wang et al. [15] investigated the effect of partially premixed mixture and hydrogen addition on natural gas direct-injection lean combustion under engine-relevant conditions, and it was found that the flame kernel growth was decreased with the increase of the premixed ratio. In the second category of investigations (flame propagation normal
Corresponding author. E-mail address: [email protected] (J. Guo).
https://doi.org/10.1016/j.fuel.2019.116926 Received 19 October 2019; Received in revised form 2 December 2019; Accepted 19 December 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
Fuel 265 (2020) 116926
C. Wang, et al.
to the concentration gradient of fuel), special attention was paid to the relationship between the combustible zone thickness and flame behavior, including flame structure and speed. When flame propagated perpendicular to the concentration gradient, three-zone flames were observed in some works [9,10,12], which comprised the premixed flame in the front, diffusion flame in the middle, and trailing convection flame. Experimental results revealed that the leading front of the premixed flame propagated through a layer where the mixture composition before ignition was closer to that for the maximum flame velocity in a homogeneous mixture. For instance, Liebman et al. [9,10] found that the nose of the premixed flame followed the stoichiometric isopleth (about 10 vol% methane). When the flame propagated perpendicular to a given concentration gradient, its velocity remained nearly unchanged in a duct with one open end [9,12,13], but increased in a closed duct [9]. Some previous research has revealed that the speed of the leading front of the premixed flame is closely related to the thicknesses of the combustible stratified gas mixtures [8,9,12,13]. For instance, Liebman et al. [9] discovered that the flame speed in a semi-confined gallery decreased with combustible zone thickness. Similarly, Ishikawa [8] found that the flame propagating perpendicular to the concentration gradient decelerated and may even extinguish when the combustible layer became narrower. It was also found that the flame width decreased with that of the combustible zone [8,11]. Furthermore, the deflagration-to-detonation transition [16] and explosion venting [17] of stratified methane-air mixtures were also investigated, which are beyond the purview of the current study. The available research is inadequate for elucidating the characteristics of flame propagation in stratified methane-air mixtures. For instance, most previous research was carried out under constant pressure conditions [1–3,5–7,12–14], and little work has been done in closed ducts; especially, the pressure buildup owing to the combustion of stratified methane-air mixtures in closed vessels remains unclear. In this study, the transverse concentration distribution of stratified methaneair mixtures in a 2 m long horizontal duct with observation windows was first measured. Subsequently, the effects of ignition delay (tig ), i.e., the time interval between ignition and supply of methane, on the flame behavior and pressure buildup in the duct were experimentally investigated.
Fig. 2. Schematic diagram of apparatus. PS: pressure sensor; OS: oxygen sensor.
to Dalton's law of partial pressure, which is introduced in Section 3.1. The self-powered, electrochemical oxygen sensors had a published resolution of 0.01% in the range of 0–100 % O2, and T90 response time, which refers to the time taken for the sensor output to decrease from 20.9% O2 to 2.1% O2, was less than 5 s. The sampling rate of oxygen measurement was 1 Hz. The sensors, OS1–OS5, were installed at distances of 25, 87.5, 150, 212.5, and 275 mm from the bottom of the duct, respectively, as shown in Fig. 2. 2.2. Procedure Two groups of tests were conducted in this study. The first group of tests was carried out to measure the concentration distribution of methane with an overall volume fraction of 10%. In detail, the gas chamber with a volume of 90 L was first emptied using a vacuum pump and then filled with methane to the desired pressure. Next, the solenoid valve was opened, and methane was injected horizontally into the duct through 36 nozzles with 12 holes, each 0.8 mm in diameter, in approximately 30 s, as schematically presented in Fig. 3. During the charge of methane, air was squeezed outside the duct through two open ball valves at the bottom of the duct, which were closed along with the solenoid valve after the methane was charged; simultaneously, the oxygen concentration was measured using the sensors OS1–OS5 to calculate the methane concentration. Prior to the subsequent test, the duct was swept thoroughly using dry air. The second group of tests was performed to investigate the effect of tig on the explosion of stratified methane-air mixtures in the closed duct. The experimental procedures used in this group were identical to those used in the first group. Thus, the concentration distribution of methane at the selected time of ignition was obtained according to the results of the first group of tests. The ignition unit, high-speed camera, and data acquisition system were triggered simultaneously by transistor-transistor logic (TTL) signals generated by a signal synchronizer to ignite the methane-air mixtures and record the experimental data accordingly. In this study, all the tests were performed twice at an initial pressure of 101 kPa and temperature of 298 K, and good repeatability was achieved.
2. Experimental 2.1. Apparatus The experiments were conducted in a closed duct, with a length of 2 m and cross-sectional area of 0.3 × 0.3 m2, comprising two 1 m long sections, as shown in Fig. 1. A window, 0.75 m (length) × 0.30 m (width), was installed in the center of each section, through which the explosion flames were recorded using a high-speed camera at a frequency of 500 Hz. A piezoresistive pressure sensor with a measurement range of − 100 to + 1000 kPa (PS) was installed 1,325 mm from the ignition source to measure the internal overpressure, and the signals it generated were recorded using a data acquisition system, as shown in Fig. 2. Stratified methane-air mixtures were ignited by an electric spark with nominal ignition energy of approximately 1 J, which was generated by electrodes mounted at the center of one blind flange of the duct. Five oxygen sensors, OS1–OS5, were used to measure the oxygen concentration, and the concentration of methane was calculated according
3. Results and discussion 3.1. Concentration distribution of methane In this paper, based on the assumption that the duct consisted only of air and methane, the methane concentration was determined from the oxygen concentration according to Dalton's law of partial pressure C [18]. That is, CCH 4 = 1 − 20.9o2% × 100%, where, CCH 4 and Co2 are the volume concentrations of methane and oxygen, respectively; and 20.9% is the volume fraction of oxygen in dry air. Fig. 4 presents the values of
(
Fig. 1. Photo of duct. 2
)
Fuel 265 (2020) 116926
C. Wang, et al.
Fig. 3. Schematic diagram of methane injection.
55
300
H=275 mm (OS5) H=212.5 mm (OS4) H=150 mm (OS3) H=87.5 mm (OS2) H=25 mm (OS1)
50 45 40
250 200
30
H (mm)
CCH4 (%)
35
25 20
100
UFL
15 10
50
LFL
5 0 0.0
2.5
5.0
7.5
Height of ignition source
150
0 0
10.0 12.5 15.0 17.5 20.0 22.5 25.0 Diffusion time (min)
2
4
LFL 6 8
2 min 4 min 6 min 8 min 10 min 15 min 20 min UFL 25 min 10 12 14 16 18 20 22 24 26 28 30
CCH4 (%)
Fig. 5. CCH 4 as functions of H at different tig values.
Fig. 4. CCH 4 at different H as functions of diffusion time.
3.2. Effect of tig on flame behavior
CCH 4 calculated at different distances from the bottom of the duct (H) as functions of diffusion time. When the methane was charged horizontally into the duct, it first accumulated at the top of the duct and then diffused downward. As a result, the CCH 4 -time histories depended on the value of H. The LFL and UFL of the methane-air mixtures were assumed to be 5% and 15%, respectively [6,9]. As shown in Fig. 4, CCH 4 first increased and entered the flammability limits, and then exceeded UFL at H = 212.5 and 275 mm; thereafter, it reached a maximum value and then decreased gradually and entered the flammability limits again. At H = 25 and 87.5 mm, CCH 4 increased nearly monotonically to reach an average volume fraction of 10% approximately 25 min after the supply of methane. The value of CCH 4 first increased and then remained nearly unchanged at the longitudinal central cross-section (H = 150 mm). In the second group of tests, the electrodes were mounted 150 mm from the bottom; tig values in the range 2–25 min were chosen based on the experimental results shown in Fig. 4. The methane concentration profiles as functions of H are presented in Fig. 5. Obviously, the transverse concentration gradient of methane decreased with increase in tig owing to the interdiffusion in the stratified methane-air mixtures; for example, the value of CCH 4 at different values of H was approximately equal to the average volume fraction of 10% at tig = 25 min, i.e., the methane-air mixture became nearly completely homogeneous. As seen in Fig. 5, the thickness of the flammable layer increased significantly as tig increased from 2 min to 10 min and was nearly independent of tig when the latter exceeded 15 min, which was the dominant reason for the influence of tig on flame behavior and explosion overpressure, as will be discussed in Sections 3.2 and 3.3, respectively. Although CCH 4 around the ignition source (H = 150 mm) attig = 2 min was above LFL, as shown in Fig. 5, it was found that the gas mixtures could not be ignited, which may have been owing to the increased minimum ignition energy of extremely lean mixtures [19–21]. In fact, non-ignition always occurred at tig ≤ 3 min under the current experimental conditions.
The experimental results showed that nearly spherical flame bubbles formed shortly after ignition at various tig . However, the diameters of the bubbles at a given time depended on tig . As seen in Fig. 6 (a) and (b), the spherical flame bubbles with relatively smooth surface were of nearly identical diameters at tig ≥ 15 min owing to their low transverse concentration gradients, as given in Fig. 5. However, the flame bubble expanded at a slightly lower speed as tig decreased to 8 min, as shown in Fig. 6 (c), and expanded even more slowly as tig decreased to 6 and 4 min, as presented in Fig. 6 (d) and (e), respectively. The decrease in the size of the flame bubble was closely related with the steep concentration gradient around the electrodes at the time of ignition. In detail, takingtig = 4 min as an example, it can be seen from Fig. 5 that the value of CCH 4 at H = 150 mm was around 8.2%, which became leaner and was approximately equal to LFL as H decreased to around 115 mm. Therefore, as seen in Fig. 6 (e), the flame bubble expanded downward at a much lower speed and its luminosity decreased in contrast with those observed in Fig. 6 (a)–(c). In the upward direction, CCH 4 first approached stoichiometric value and then exceeded UFL as H increased to about 200 mm; hence, the flame first accelerated and then decelerated, which reduced the overall flame speed compared with that achieved at greater tig s . In other words, the steeper the transverse concentration gradient of methane, the more slowly the flame bubble expanded. As seen in Fig. 6 (e), most of the flame bubble was above the horizontal centerline of the duct (white line), which resulted from the discrepancy between the downward and upward flame speeds owing to the steep concentration gradient, in addition to the effect of buoyancy on the flame bubble [22]. No significant discrepancy was observed during further propagation of the flame at tig ≥ 15 min, as shown in Fig. 7 (a). However, as tig decreased from 15 to 4 min, the thickness of the flame decreased gradually owing to the reduction in the thickness of the flammable layer (Fig. 7 (b)–(d)), as discovered in previous research [8,9,11,12]. Besides, 3
Fuel 265 (2020) 116926
C. Wang, et al.
Fig. 6. Flame images at 26 ms after ignition at various tig s (The diameters of the red circles are equal.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
an olive-shaped flame bubble was observed especially at tig = 4 min, as shown in Fig. 7 (d), because the concentration gradient was steep and the horizontal flame speed was the highest along the layer where the value of CCH 4 before ignition was closer to that observed in the case of maximum flame velocity in a homogeneous mixture [9,23]. As seen in Fig. 7, the time taken for the flame to reach the length shown in Fig. 7 (a) was greater at lower values of tig s ; this phenomenon demonstrated that horizontal flame speed depended on tig , as presented in Fig. 8. Here, the flame speed was obtained by dividing the horizontal travel distance of the flame tip by the time interval. In this paper, the flame speed was presented only when the flame traveled through window 1 because the flame surface was smooth for the duration. As seen in Fig. 8, the horizontal flame speed first increased from around 2 to 12 m/s and then decreased as the flame front went away from the ignition spark. Further, tig , at values exceeding 15 min, had a negligible effect on the spatial distribution of the speed of the leading flame front owing to the small discrepancy in the transverse concentration gradient, as presented in Fig. 5. As tig decreased from 15 to 4 min, no significant reduction in flame speed was observed when the flame front was not far from the ignition spark. However, as the flame propagated further, its maximum speed decreased significantly to approximately 8 m/s attig = 10 min and 5 m/s attig = 6 min, as shown in Fig. 8, which accorded with previous findings that flame speed in a semi-confined gallery decreased with combustible zone thickness [8,9]. The flame speed was the sum of the laminar burning speed and the unburnt gas speed in front of the flame [9]; the former was nearly identical when tig ranges from 4 to 25 min because a layer with methane concentration near stoichiometric always exists. Therefore, the variation of the maximum flame speed with tig results from the discrepancy in the unburnt gas speed in front of the flame. For a given travel distance of the flame front, lower quantities of the methane-air mixtures were burnt at smaller tig owing to the thinner combustible layer of the gas mixtures. Hence, less heat was generated, and consequently the speed of the unburnt gas before the flame front owing to the slower expanding speed of the flame bubble. This reduced the maximum horizontal flame speed. At tig ≥ 15 min, the behavior of the flame was nearly independent of tig when it passed through window 2, in addition to window 1, as shown in Figs. 9 and 10. As shown, the flame front always remained smooth as it passed through window 1, which deforms and becomes cellular shortly after the flame front entered window 2. During the period, the flame may experience self-acceleration as discovered in a constant volume vessel [24–26]. After that, a backward pointing cusp was observed, which was known as tulip flame. Previous research [27–30]
Fig. 8. Horizontal flame speed vs. distance to ignition spark.
have shown that the hydrodynamic structure results from the interaction between the flame and the gas flow. However, as seen in Fig. 9 (b)–(d) and Fig. 10 (b)–(d), the tulip flames were not symmetrical; in other words, the upper part of the tulip flame was ahead of its lower part. This was probably because the nozzles protruding 3 mm into the duct accelerated the flame [31–33], as similar phenomena were observed in the same duct with a homogeneous mixture of 10 vol% methane in air. When tig decreased to 10 min, the tulip flame was observed only for a short distance after it entered window 2, and totally disappeared at tig ≤ 8 min.Fig. 11. The three-zone flames, including the leading premixed flame, diffusion flame in the middle, and trailing convection flame, as discovered in former research [9,10,12], were observed in the current experiments only when tig ≤ 6 min, as shown in Fig. 12. In the early stage of flame propagation, only a blue premixed flame was observed, which traveled through the narrow combustible layer, as shown in Fig. 7 (d). As shown in Fig. 12 (a), as the flame bubble expanded, some smooth yellow diffusion flames first appeared at the upper part of the duct, owing to the interdiffusion between the upper rich methane-air mixture that originally exceeded UFL and the hot product that contained excessive oxygen [9,12]. Under the effects of buoyancy and variation in relative velocity [9,12], the smooth yellow diffusion flame became curly, flocculent, and brighter as it moved downward, as shown in Fig. 12 (b)–(c), and, as a result, convection flame was formed, as shown in Fig. 12 (d)–(i). It was also found that the combustion duration increased with
Fig. 7. Flames images at various tig s (The red rectangles are of same size.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4
Fuel 265 (2020) 116926
C. Wang, et al.
Fig. 9. Flame images attig = 25 min.
Fig. 10. Flame images attig = 15 min.
Fig. 11. Flame images attig = 10 min.
Fig. 12. Evolution of bright yellow flame attig = 4 min (window 1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
gradient of methane across height was steeper at lower tig , which reduced the quantity of the methane-air mixture within the flammability limits. Therefore, the flame had to travel a greater horizontal distance to generate the same amount of heat as that attained at greater tig . As seen in Fig. 13, the value of Pmax at tig = 10 min was higher than that attig = 6 min, and the relationship between Pmax , TPmax (time interval from ignition to attaining Pmax ), and tig is summarized in Fig. 14. The value of Pmax first increased quickly from an average value of 221 kPa at tig = 4 min to 393 kPa at tig = 15 min and then slightly increased to 405 kPa as tig increased to 25 min. The relationship of Pmax with tig resulted from the variation of the heat release owing to the combustion of the stratified methane-air mixtures, which depends on the layer thickness of combustible gases. As discussed in Section 3.1, the volume of the combustible gases first increased and then remained nearly unchanged with increase intig . Therefore, heat release and the
decrease in tig especially as tig decreased to 6 min and 4 min because of the slow progress of the yellow diffusion and convection flames. 3.3. Effect of tig on explosion overpressure Similar to the results obtained in the case of homogeneous methaneair mixtures in closed spaces [34,35], the overpressure increased monotonically to a maximum value, Pmax , and then decreased gradually for the current stratified mixtures with tig ranging from 25 to 4 min. As expected, the pressure–time histories were nearly identical at tig ≥ 15 min because of the approximate flame behavior, as discussed in Section 3.2. However, as shown in Fig. 13, at less than 15tig min, the time and distance required attig = 6 min to attain the same overpressure, for instance, 50 kPa and 150 kPa, were greater than those attig = 10 min. The chief reason for this was that the concentration 5
Fuel 265 (2020) 116926
C. Wang, et al.
300
400
(a) t ig = 10 min 350 Pmax
(b) tig = 6 min
Pmax
250
250
Overpressure (kPa)
Overpressure (kPa)
300 0.782 s
200 0.308 s
150 100
0.125 s
50
200
0.978 s
150 0.536 s
100 0.188 s
50 0
0 -0.5 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time after ignition (s)
3.5
4.0
4.5
-0.5 0.0
5.0
0.5
1.0
1.5
2.0
2.5
3.0
Time after ignition (s)
3.5
4.0
4.5
5.0
Fig. 13. Pressure-time histories attig = 10 min and 6 min. 1.3
450
Pmax for homogeneous methane-air mixtures 400
(3) The stratified methane-air mixtures could not be ignited at tig ≤ 3 min. The value of Pmax first increased quickly as tig increased from 4 min to 15 min, and then remained nearly unchanged as tig increased to 25 min. The Pmax s for tig ranging from 15 min to 25 min was approximately that observed for homogeneous methane-air mixtures.
1.2 1.1
Pmax
0.9
Tmax
0.8
T Pmax (s)
300
Non-ignition
P max (kPa)
1.0 350
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
0.7 0.6
250
0.5 200 0
2
4
6
8
10 12 14 16 tig (min)
Acknowledgments
0.4 18 20 22 24 26
This study is supported by the National Key Research and Development Program of China (No. 2016YFE0113400) and the National Natural Science Foundation of China (Grant No. 51604083 and 51704079).
Fig. 14. Pmax and TPmax vs. tig .
Pmax also first increased and then remained nearly unchanged when tig increases from 4 min to 25 min. In addition, at tig in the range 15–25 min, the Pmax s was approximately equal to the maximum overpressure for homogeneous methane-air mixtures because of the low transverse concentration gradient of methane, as presented in Fig. 5. As seen in Fig. 14, TPmax first decreased and then remained nearly unchanged as tig increased from 4 min to 25 min, which was owing to relationship between the horizontal flame speed and tig , as discussed in Section 3.2.
Author Contributions Statement Chunhua Wang and Jin Guo designed the study, performed the research, analysed data, and wrote the paper. Jialin Li, Zesi Tang and Yanzhen Zhuang performed the research. References [1] Kang T, Kyritsis DC. A combined experimental/computational investigation of stratified combustion in methane–air mixtures. Energy Convers Manage 2007;48(11):2769–74. [2] Kang T, Kyritsis DC. Methane flame propagation in compositionally stratified gases. Combust Sci Technol 2005;177(11):2191–210. [3] Kang T, Kyritsis DC. Phenomenology of methane flame propagation into compositionally stratified, gradually richer mixtures. Proc Combust Inst 2009;32(1):979–85. [4] Karim GA, Lam HT. Ignition and flame propagation within stratified methane-air mixtures formed by convective diffusion. Proc Combust Inst 1988;21(1):1909–15. [5] Badr O, Karim G. Flame propagation in stratified methane-air mixtures. J Fire Sci 1984;2(6):415–26. [6] Liebman I, Corry J, Perlee HE. Dynamics of flame propagation through layered methane-air mixtures. Combust Sci Technol 1971;2(5–6):365–75. [7] Karim GA, Tsang P. Flame propagation through atmospheres involving concentration gradients formed by mass transfer phenomena. J Fluids Eng 1975;97(4):615–7. [8] Ishikawa NA. Diffusion combustor and methane-air flame propagation in concentration gradient fields. Combust Sci Technol 1983;30(1–6):185–203. [9] Liebman I, Corry J, Perlee HE. Flame propagation in layered methane-air systems. Combust Sci Technol 1970;1:257–67. [10] Liebman I, Perlee HE, Corry J. Investigation of flame propagation characteristics in layered gas mixtures. U.S. Bureau of Mines 7078, Pittsburgh, PA, 1968. [11] Ishikawa N. Combustion of stratified methane/air layers. Combust Sci Technol 1983;30(1–6):311–25. [12] Phillips H. Flame in a buoyant methane layer. Proc Combust Inst 1965;10(1):1277–83.
4. Conclusions The experiments on the explosions of methane-air mixtures with transverse concentration gradients were conducted in a 2 m long closed duct. The concentration distribution of methane with an overall concentration of 10 vol% (which was charged from the top of the duct) was measured, and the effects of tig variation in the range 2–25 min on the flame behavior and Pmax were investigated. The major conclusions drawn under the current experimental conditions are summarized as follows. (1) The value oftig significantly affected the flame behavior and Pmax . (2) At tig ≥ 15 min, the flame behavior, including flame shape and speed, was similar. Tulip flames were observed at tig ≥ 10 min. The horizontal flame speed decreased as tig decreased from 15 to 4 min. Bright yellow diffusion and convection flames following the blue flame front were observed only at tig ≤ 6 min owing to the steep transverse concentration gradient of methane.
6
Fuel 265 (2020) 116926
C. Wang, et al.
[25] Xie YL, Wang JH, Cai X, Huang ZH. Self-acceleration of cellular flames and laminar flame speed of syngas/air mixtures at elevated pressures. Int J Hydrogen Energy 2016;41(40):18250–8. [26] Cai X, Wang JH, Zhao HR, Zhang M, Huang ZH. Flame morphology and self-acceleration of syngas spherically expanding flames. Int J Hydrogen Energy 2018;43(36):17531–41. [27] Xiao HH, Wang QS, Shen XB, Guo S, Sun JH. An experimental study of distorted tulip flame formation in a closed duct. Combust Flame 2013;160(9):1725–8. [28] Ponizy B, Claverie A, Veyssière B. Tulip flame-the mechanism of flame front inversion. Combust Flame 2014;161(12):3051–62. [29] Xiao HH, Houim RW, Oran ES. Formation and evolution of distorted tulip flames. Combust Flame 2015;162(11):4084–101. [30] Shen XB, He XC, Sun JH. A comparative study on premixed hydrogen-air and propane-air flame propagations with tulip distortion in a closed duct. Fuel 2015;161:248–53. [31] Li HW, Guo J, Yang FQ, Wang CJ, Zhang JQ, Lu SX. Explosion venting of hydrogenair mixtures from a duct to a vented vessel. Int J Hydrogen Energy 2018;43(24):11307–13. [32] Ciccarelli G, Johansen CT, Parravani M. The role of shock-flame interactions on flame acceleration in an obstacle laden channel. Combust Flame 2010;157(11):2125–36. [33] Ciccarelli G, Dorofeev S. Flame acceleration and transition to detonation in ducts. Prog Energy Combust Sci 2008;34(4):499–550. [34] Zhang PP, Zhou YH, Cao XY, Gao XL, Bi MS. Enhancement effects of methane/air explosion caused by water spraying in a sealed vessel. J Loss Prev Process Ind 2014;29:313–8. [35] Ma QJ, Zhang Q, Chen JC, Huang Y, Shi YT. Effects of hydrogen on combustion characteristics of methane in air. Int J Hydrogen Energy 2014;39(21):11291–8.
[13] Feng CC, Lam SH, Glassman I. Flame propagation through layered fuel-air mixtures. Combust Sci Technol 1975;10:59–71. [14] Cruz APD, Dean AM, Grenda JM. A numerical study of the laminar flame speed of stratified methane/air flames. Proc Combust Inst 2000;28(2):1925–32. [15] Wang JH, Huang ZH. Effect of partially premixed and hydrogen addition on natural gas direct injection lean combustion. Int J Hydrogen Energy 2009;34(22):9239–47. [16] Zheng W, Kaplan CR, Houim RW, Oran ES. Flame acceleration and transition to detonation: Effects of a composition gradient in a mixture of methane and air. Proc Combust Inst 2019;37:3521–8. [17] Hjertager BH, Bjørkhaug M, Fuhre K. Explosion propagation of non-homogeneous methane-air clouds inside an obstructed 50 m3 vented vessel. J Hazard Mater 1988;19(2):139–53. [18] Shirvill LC, Roberts P, Butler CJ, Roberts TA, Royle M. Characterization of the hazards from jet releases of hydrogen. 1st International Conference on Hydrogen Safety. 2005. [19] Eckhoff RK, Ngo M, Olsen W. On the minimum ignition energy (MIE) for propane/ air. J Hazard Mater 2010;175(1–3):293–7. [20] Gan Cui, Li ZL, Yang C, Zhou Z, Li JL. Experimental study of minimum ignition energy of methane-air mixtures at low temperatures and elevated pressures. Energy Fuels 2016;30:6738–44. [21] Fernández-Tarrazo E, Sánchez-Sanz M, Sánchez AL, Williams FA. Minimum ignition energy of methanol-air mixtures. Combust Flame 2016;171:234–6. [22] Li Q, Hu E, Cheng Y, Huang Z. Measurements of laminar flame speeds and flame instability analysis of 2-methyl-1-butanoleair mixture. Fuel 2013;112:263–71. [23] Hirano T, Suzuki T, Mashiko I, Iwai K. Flame propagation through mixtures with concentration gradient. Proc Combust Inst 1977;16(1):1307–15. [24] Wang JH, Huang ZH, Tang CL, Zheng JJ. Effect of hydrogen addition on early flame growth of lean burn natural gas-air mixtures. Int J Hydrogen Energy 2010;35(13):7246–52.
7
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Flame propagation in methane-air mixtures with transverse concentration gradients in horizontal duct Chunhua Wanga, Jialin Lib, Zesi Tangb, Yanzhen Zhuangb, Jin Guob, a b
T
⁎
College of Environmental and Biological Engineering, Putian University, Putian 351100, PR China College of Environment and Resources, Fuzhou University, Fuzhou 350116, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Methane Concentration gradient Flame Overpressure
Stratified methane-air mixtures are formed in the early stages of methane leakage in confined spaces, and an explosion may occur if a source of ignition were to appear at the appropriate time and location. However, this has not been elucidated thus far. The aim of this study is to measure the concentration gradient of methane injected from the top surface of a duct, and to investigate the effects of ignition delay (tig ), which is the time interval from methane leakage to ignition, on flame behavior and explosion overpressure. Experimental results show that tig significantly affects flame behavior in stratified methane-air mixtures, including the flame shape and speed. The stratified methane-air mixtures cannot be ignited at tig ≤ 3 min. The horizontal flame speed and maximum overpressure (Pmax ) first increase as tig increases from 4 min to 15 min and thereafter remain nearly unchanged with further increase in tig . The value of Pmax at tig in the range 15–25 min is approximately that observed for homogeneous methane-air mixtures. Tulip flames are observed at tig ≥ 10 min. Diffusion and convection flames, which follow the leading premixed flame front, are only formed at tig ≤ 6 min.
1. Introduction Methane is the main component of natural gas, and is used as a raw material in the chemical industry. However, the leakage of methane in confined spaces often poses the serious problem of potential explosions. In general, upon accidental release, methane first accumulates locally and then diffuses gradually to the entire confined space. During this period, the methane-air mixtures are stratified rather than homogeneous. In this situation, an explosion may occur if a source of ignition appears at the right time and location. Therefore, it is extremely important to understand this phenomenon (including flame behavior and pressure buildup) of the explosion of stratified methane-air mixtures, to design suitable protection and mitigation measures against such explosions. Investigations [1–13] have been conducted into the phenomena of flame propagation in methane-air mixtures whose concentration gradients were experimentally measured [1–4] or theoretically predicted [8]. In general, the investigations can be classified into two categories: 1) flame propagation along the concentration gradient of fuel [1–7], and 2) flame propagation normal to the concentration gradient [8–13]. In the first category of investigations, the lower flammability limit (LFL) was found to decrease [1–4,6] owing to heat input to the propagating flame from the burnt gases [1,7], and the upper flammability limit ⁎
(UFL) was found to increase because of hydrogen production through flame propagation into the fuel-rich mixtures [3,14]. With respect to flame speed, Badr and Karim [5], and Karim and Tsang [7] found that the speed of the accelerating flame (i.e., the flame propagated from lean or rich into nearly stoichiometric mixtures) was close to those of the corresponding homogeneous mixtures. Conversely, higher laminar flame speed was observed when the accelerating flame propagated into step-stratified methane-air mixtures [14]. Previous research has demonstrated that the speed of the decelerating flame, i.e., the flame propagating from nearly stoichiometric into lean or rich mixtures, was higher than that of the flame corresponding to a homogeneous mixture of the local equivalence ratio [1–5,7]. For instance, Badr and Karim [5] observed a decelerating flame with up to 70% higher velocity compared to that of homogeneous mixtures. The increase in flame speed resulted from heat feedback from the burnt mixture [2,3,14]. Differently, Cruz et al. [14] found that the laminar flame speed was lower in step-stratified methane-air mixtures (from stoichiometric to rich mixtures) than in homogeneous mixtures. Furthermore, Wang et al. [15] investigated the effect of partially premixed mixture and hydrogen addition on natural gas direct-injection lean combustion under engine-relevant conditions, and it was found that the flame kernel growth was decreased with the increase of the premixed ratio. In the second category of investigations (flame propagation normal
Corresponding author. E-mail address: [email protected] (J. Guo).
https://doi.org/10.1016/j.fuel.2019.116926 Received 19 October 2019; Received in revised form 2 December 2019; Accepted 19 December 2019 0016-2361/ © 2019 Published by Elsevier Ltd.
Fuel 265 (2020) 116926
C. Wang, et al.
to the concentration gradient of fuel), special attention was paid to the relationship between the combustible zone thickness and flame behavior, including flame structure and speed. When flame propagated perpendicular to the concentration gradient, three-zone flames were observed in some works [9,10,12], which comprised the premixed flame in the front, diffusion flame in the middle, and trailing convection flame. Experimental results revealed that the leading front of the premixed flame propagated through a layer where the mixture composition before ignition was closer to that for the maximum flame velocity in a homogeneous mixture. For instance, Liebman et al. [9,10] found that the nose of the premixed flame followed the stoichiometric isopleth (about 10 vol% methane). When the flame propagated perpendicular to a given concentration gradient, its velocity remained nearly unchanged in a duct with one open end [9,12,13], but increased in a closed duct [9]. Some previous research has revealed that the speed of the leading front of the premixed flame is closely related to the thicknesses of the combustible stratified gas mixtures [8,9,12,13]. For instance, Liebman et al. [9] discovered that the flame speed in a semi-confined gallery decreased with combustible zone thickness. Similarly, Ishikawa [8] found that the flame propagating perpendicular to the concentration gradient decelerated and may even extinguish when the combustible layer became narrower. It was also found that the flame width decreased with that of the combustible zone [8,11]. Furthermore, the deflagration-to-detonation transition [16] and explosion venting [17] of stratified methane-air mixtures were also investigated, which are beyond the purview of the current study. The available research is inadequate for elucidating the characteristics of flame propagation in stratified methane-air mixtures. For instance, most previous research was carried out under constant pressure conditions [1–3,5–7,12–14], and little work has been done in closed ducts; especially, the pressure buildup owing to the combustion of stratified methane-air mixtures in closed vessels remains unclear. In this study, the transverse concentration distribution of stratified methaneair mixtures in a 2 m long horizontal duct with observation windows was first measured. Subsequently, the effects of ignition delay (tig ), i.e., the time interval between ignition and supply of methane, on the flame behavior and pressure buildup in the duct were experimentally investigated.
Fig. 2. Schematic diagram of apparatus. PS: pressure sensor; OS: oxygen sensor.
to Dalton's law of partial pressure, which is introduced in Section 3.1. The self-powered, electrochemical oxygen sensors had a published resolution of 0.01% in the range of 0–100 % O2, and T90 response time, which refers to the time taken for the sensor output to decrease from 20.9% O2 to 2.1% O2, was less than 5 s. The sampling rate of oxygen measurement was 1 Hz. The sensors, OS1–OS5, were installed at distances of 25, 87.5, 150, 212.5, and 275 mm from the bottom of the duct, respectively, as shown in Fig. 2. 2.2. Procedure Two groups of tests were conducted in this study. The first group of tests was carried out to measure the concentration distribution of methane with an overall volume fraction of 10%. In detail, the gas chamber with a volume of 90 L was first emptied using a vacuum pump and then filled with methane to the desired pressure. Next, the solenoid valve was opened, and methane was injected horizontally into the duct through 36 nozzles with 12 holes, each 0.8 mm in diameter, in approximately 30 s, as schematically presented in Fig. 3. During the charge of methane, air was squeezed outside the duct through two open ball valves at the bottom of the duct, which were closed along with the solenoid valve after the methane was charged; simultaneously, the oxygen concentration was measured using the sensors OS1–OS5 to calculate the methane concentration. Prior to the subsequent test, the duct was swept thoroughly using dry air. The second group of tests was performed to investigate the effect of tig on the explosion of stratified methane-air mixtures in the closed duct. The experimental procedures used in this group were identical to those used in the first group. Thus, the concentration distribution of methane at the selected time of ignition was obtained according to the results of the first group of tests. The ignition unit, high-speed camera, and data acquisition system were triggered simultaneously by transistor-transistor logic (TTL) signals generated by a signal synchronizer to ignite the methane-air mixtures and record the experimental data accordingly. In this study, all the tests were performed twice at an initial pressure of 101 kPa and temperature of 298 K, and good repeatability was achieved.
2. Experimental 2.1. Apparatus The experiments were conducted in a closed duct, with a length of 2 m and cross-sectional area of 0.3 × 0.3 m2, comprising two 1 m long sections, as shown in Fig. 1. A window, 0.75 m (length) × 0.30 m (width), was installed in the center of each section, through which the explosion flames were recorded using a high-speed camera at a frequency of 500 Hz. A piezoresistive pressure sensor with a measurement range of − 100 to + 1000 kPa (PS) was installed 1,325 mm from the ignition source to measure the internal overpressure, and the signals it generated were recorded using a data acquisition system, as shown in Fig. 2. Stratified methane-air mixtures were ignited by an electric spark with nominal ignition energy of approximately 1 J, which was generated by electrodes mounted at the center of one blind flange of the duct. Five oxygen sensors, OS1–OS5, were used to measure the oxygen concentration, and the concentration of methane was calculated according
3. Results and discussion 3.1. Concentration distribution of methane In this paper, based on the assumption that the duct consisted only of air and methane, the methane concentration was determined from the oxygen concentration according to Dalton's law of partial pressure C [18]. That is, CCH 4 = 1 − 20.9o2% × 100%, where, CCH 4 and Co2 are the volume concentrations of methane and oxygen, respectively; and 20.9% is the volume fraction of oxygen in dry air. Fig. 4 presents the values of
(
Fig. 1. Photo of duct. 2
)
Fuel 265 (2020) 116926
C. Wang, et al.
Fig. 3. Schematic diagram of methane injection.
55
300
H=275 mm (OS5) H=212.5 mm (OS4) H=150 mm (OS3) H=87.5 mm (OS2) H=25 mm (OS1)
50 45 40
250 200
30
H (mm)
CCH4 (%)
35
25 20
100
UFL
15 10
50
LFL
5 0 0.0
2.5
5.0
7.5
Height of ignition source
150
0 0
10.0 12.5 15.0 17.5 20.0 22.5 25.0 Diffusion time (min)
2
4
LFL 6 8
2 min 4 min 6 min 8 min 10 min 15 min 20 min UFL 25 min 10 12 14 16 18 20 22 24 26 28 30
CCH4 (%)
Fig. 5. CCH 4 as functions of H at different tig values.
Fig. 4. CCH 4 at different H as functions of diffusion time.
3.2. Effect of tig on flame behavior
CCH 4 calculated at different distances from the bottom of the duct (H) as functions of diffusion time. When the methane was charged horizontally into the duct, it first accumulated at the top of the duct and then diffused downward. As a result, the CCH 4 -time histories depended on the value of H. The LFL and UFL of the methane-air mixtures were assumed to be 5% and 15%, respectively [6,9]. As shown in Fig. 4, CCH 4 first increased and entered the flammability limits, and then exceeded UFL at H = 212.5 and 275 mm; thereafter, it reached a maximum value and then decreased gradually and entered the flammability limits again. At H = 25 and 87.5 mm, CCH 4 increased nearly monotonically to reach an average volume fraction of 10% approximately 25 min after the supply of methane. The value of CCH 4 first increased and then remained nearly unchanged at the longitudinal central cross-section (H = 150 mm). In the second group of tests, the electrodes were mounted 150 mm from the bottom; tig values in the range 2–25 min were chosen based on the experimental results shown in Fig. 4. The methane concentration profiles as functions of H are presented in Fig. 5. Obviously, the transverse concentration gradient of methane decreased with increase in tig owing to the interdiffusion in the stratified methane-air mixtures; for example, the value of CCH 4 at different values of H was approximately equal to the average volume fraction of 10% at tig = 25 min, i.e., the methane-air mixture became nearly completely homogeneous. As seen in Fig. 5, the thickness of the flammable layer increased significantly as tig increased from 2 min to 10 min and was nearly independent of tig when the latter exceeded 15 min, which was the dominant reason for the influence of tig on flame behavior and explosion overpressure, as will be discussed in Sections 3.2 and 3.3, respectively. Although CCH 4 around the ignition source (H = 150 mm) attig = 2 min was above LFL, as shown in Fig. 5, it was found that the gas mixtures could not be ignited, which may have been owing to the increased minimum ignition energy of extremely lean mixtures [19–21]. In fact, non-ignition always occurred at tig ≤ 3 min under the current experimental conditions.
The experimental results showed that nearly spherical flame bubbles formed shortly after ignition at various tig . However, the diameters of the bubbles at a given time depended on tig . As seen in Fig. 6 (a) and (b), the spherical flame bubbles with relatively smooth surface were of nearly identical diameters at tig ≥ 15 min owing to their low transverse concentration gradients, as given in Fig. 5. However, the flame bubble expanded at a slightly lower speed as tig decreased to 8 min, as shown in Fig. 6 (c), and expanded even more slowly as tig decreased to 6 and 4 min, as presented in Fig. 6 (d) and (e), respectively. The decrease in the size of the flame bubble was closely related with the steep concentration gradient around the electrodes at the time of ignition. In detail, takingtig = 4 min as an example, it can be seen from Fig. 5 that the value of CCH 4 at H = 150 mm was around 8.2%, which became leaner and was approximately equal to LFL as H decreased to around 115 mm. Therefore, as seen in Fig. 6 (e), the flame bubble expanded downward at a much lower speed and its luminosity decreased in contrast with those observed in Fig. 6 (a)–(c). In the upward direction, CCH 4 first approached stoichiometric value and then exceeded UFL as H increased to about 200 mm; hence, the flame first accelerated and then decelerated, which reduced the overall flame speed compared with that achieved at greater tig s . In other words, the steeper the transverse concentration gradient of methane, the more slowly the flame bubble expanded. As seen in Fig. 6 (e), most of the flame bubble was above the horizontal centerline of the duct (white line), which resulted from the discrepancy between the downward and upward flame speeds owing to the steep concentration gradient, in addition to the effect of buoyancy on the flame bubble [22]. No significant discrepancy was observed during further propagation of the flame at tig ≥ 15 min, as shown in Fig. 7 (a). However, as tig decreased from 15 to 4 min, the thickness of the flame decreased gradually owing to the reduction in the thickness of the flammable layer (Fig. 7 (b)–(d)), as discovered in previous research [8,9,11,12]. Besides, 3
Fuel 265 (2020) 116926
C. Wang, et al.
Fig. 6. Flame images at 26 ms after ignition at various tig s (The diameters of the red circles are equal.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
an olive-shaped flame bubble was observed especially at tig = 4 min, as shown in Fig. 7 (d), because the concentration gradient was steep and the horizontal flame speed was the highest along the layer where the value of CCH 4 before ignition was closer to that observed in the case of maximum flame velocity in a homogeneous mixture [9,23]. As seen in Fig. 7, the time taken for the flame to reach the length shown in Fig. 7 (a) was greater at lower values of tig s ; this phenomenon demonstrated that horizontal flame speed depended on tig , as presented in Fig. 8. Here, the flame speed was obtained by dividing the horizontal travel distance of the flame tip by the time interval. In this paper, the flame speed was presented only when the flame traveled through window 1 because the flame surface was smooth for the duration. As seen in Fig. 8, the horizontal flame speed first increased from around 2 to 12 m/s and then decreased as the flame front went away from the ignition spark. Further, tig , at values exceeding 15 min, had a negligible effect on the spatial distribution of the speed of the leading flame front owing to the small discrepancy in the transverse concentration gradient, as presented in Fig. 5. As tig decreased from 15 to 4 min, no significant reduction in flame speed was observed when the flame front was not far from the ignition spark. However, as the flame propagated further, its maximum speed decreased significantly to approximately 8 m/s attig = 10 min and 5 m/s attig = 6 min, as shown in Fig. 8, which accorded with previous findings that flame speed in a semi-confined gallery decreased with combustible zone thickness [8,9]. The flame speed was the sum of the laminar burning speed and the unburnt gas speed in front of the flame [9]; the former was nearly identical when tig ranges from 4 to 25 min because a layer with methane concentration near stoichiometric always exists. Therefore, the variation of the maximum flame speed with tig results from the discrepancy in the unburnt gas speed in front of the flame. For a given travel distance of the flame front, lower quantities of the methane-air mixtures were burnt at smaller tig owing to the thinner combustible layer of the gas mixtures. Hence, less heat was generated, and consequently the speed of the unburnt gas before the flame front owing to the slower expanding speed of the flame bubble. This reduced the maximum horizontal flame speed. At tig ≥ 15 min, the behavior of the flame was nearly independent of tig when it passed through window 2, in addition to window 1, as shown in Figs. 9 and 10. As shown, the flame front always remained smooth as it passed through window 1, which deforms and becomes cellular shortly after the flame front entered window 2. During the period, the flame may experience self-acceleration as discovered in a constant volume vessel [24–26]. After that, a backward pointing cusp was observed, which was known as tulip flame. Previous research [27–30]
Fig. 8. Horizontal flame speed vs. distance to ignition spark.
have shown that the hydrodynamic structure results from the interaction between the flame and the gas flow. However, as seen in Fig. 9 (b)–(d) and Fig. 10 (b)–(d), the tulip flames were not symmetrical; in other words, the upper part of the tulip flame was ahead of its lower part. This was probably because the nozzles protruding 3 mm into the duct accelerated the flame [31–33], as similar phenomena were observed in the same duct with a homogeneous mixture of 10 vol% methane in air. When tig decreased to 10 min, the tulip flame was observed only for a short distance after it entered window 2, and totally disappeared at tig ≤ 8 min.Fig. 11. The three-zone flames, including the leading premixed flame, diffusion flame in the middle, and trailing convection flame, as discovered in former research [9,10,12], were observed in the current experiments only when tig ≤ 6 min, as shown in Fig. 12. In the early stage of flame propagation, only a blue premixed flame was observed, which traveled through the narrow combustible layer, as shown in Fig. 7 (d). As shown in Fig. 12 (a), as the flame bubble expanded, some smooth yellow diffusion flames first appeared at the upper part of the duct, owing to the interdiffusion between the upper rich methane-air mixture that originally exceeded UFL and the hot product that contained excessive oxygen [9,12]. Under the effects of buoyancy and variation in relative velocity [9,12], the smooth yellow diffusion flame became curly, flocculent, and brighter as it moved downward, as shown in Fig. 12 (b)–(c), and, as a result, convection flame was formed, as shown in Fig. 12 (d)–(i). It was also found that the combustion duration increased with
Fig. 7. Flames images at various tig s (The red rectangles are of same size.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4
Fuel 265 (2020) 116926
C. Wang, et al.
Fig. 9. Flame images attig = 25 min.
Fig. 10. Flame images attig = 15 min.
Fig. 11. Flame images attig = 10 min.
Fig. 12. Evolution of bright yellow flame attig = 4 min (window 1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
gradient of methane across height was steeper at lower tig , which reduced the quantity of the methane-air mixture within the flammability limits. Therefore, the flame had to travel a greater horizontal distance to generate the same amount of heat as that attained at greater tig . As seen in Fig. 13, the value of Pmax at tig = 10 min was higher than that attig = 6 min, and the relationship between Pmax , TPmax (time interval from ignition to attaining Pmax ), and tig is summarized in Fig. 14. The value of Pmax first increased quickly from an average value of 221 kPa at tig = 4 min to 393 kPa at tig = 15 min and then slightly increased to 405 kPa as tig increased to 25 min. The relationship of Pmax with tig resulted from the variation of the heat release owing to the combustion of the stratified methane-air mixtures, which depends on the layer thickness of combustible gases. As discussed in Section 3.1, the volume of the combustible gases first increased and then remained nearly unchanged with increase intig . Therefore, heat release and the
decrease in tig especially as tig decreased to 6 min and 4 min because of the slow progress of the yellow diffusion and convection flames. 3.3. Effect of tig on explosion overpressure Similar to the results obtained in the case of homogeneous methaneair mixtures in closed spaces [34,35], the overpressure increased monotonically to a maximum value, Pmax , and then decreased gradually for the current stratified mixtures with tig ranging from 25 to 4 min. As expected, the pressure–time histories were nearly identical at tig ≥ 15 min because of the approximate flame behavior, as discussed in Section 3.2. However, as shown in Fig. 13, at less than 15tig min, the time and distance required attig = 6 min to attain the same overpressure, for instance, 50 kPa and 150 kPa, were greater than those attig = 10 min. The chief reason for this was that the concentration 5
Fuel 265 (2020) 116926
C. Wang, et al.
300
400
(a) t ig = 10 min 350 Pmax
(b) tig = 6 min
Pmax
250
250
Overpressure (kPa)
Overpressure (kPa)
300 0.782 s
200 0.308 s
150 100
0.125 s
50
200
0.978 s
150 0.536 s
100 0.188 s
50 0
0 -0.5 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Time after ignition (s)
3.5
4.0
4.5
-0.5 0.0
5.0
0.5
1.0
1.5
2.0
2.5
3.0
Time after ignition (s)
3.5
4.0
4.5
5.0
Fig. 13. Pressure-time histories attig = 10 min and 6 min. 1.3
450
Pmax for homogeneous methane-air mixtures 400
(3) The stratified methane-air mixtures could not be ignited at tig ≤ 3 min. The value of Pmax first increased quickly as tig increased from 4 min to 15 min, and then remained nearly unchanged as tig increased to 25 min. The Pmax s for tig ranging from 15 min to 25 min was approximately that observed for homogeneous methane-air mixtures.
1.2 1.1
Pmax
0.9
Tmax
0.8
T Pmax (s)
300
Non-ignition
P max (kPa)
1.0 350
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
0.7 0.6
250
0.5 200 0
2
4
6
8
10 12 14 16 tig (min)
Acknowledgments
0.4 18 20 22 24 26
This study is supported by the National Key Research and Development Program of China (No. 2016YFE0113400) and the National Natural Science Foundation of China (Grant No. 51604083 and 51704079).
Fig. 14. Pmax and TPmax vs. tig .
Pmax also first increased and then remained nearly unchanged when tig increases from 4 min to 25 min. In addition, at tig in the range 15–25 min, the Pmax s was approximately equal to the maximum overpressure for homogeneous methane-air mixtures because of the low transverse concentration gradient of methane, as presented in Fig. 5. As seen in Fig. 14, TPmax first decreased and then remained nearly unchanged as tig increased from 4 min to 25 min, which was owing to relationship between the horizontal flame speed and tig , as discussed in Section 3.2.
Author Contributions Statement Chunhua Wang and Jin Guo designed the study, performed the research, analysed data, and wrote the paper. Jialin Li, Zesi Tang and Yanzhen Zhuang performed the research. References [1] Kang T, Kyritsis DC. A combined experimental/computational investigation of stratified combustion in methane–air mixtures. Energy Convers Manage 2007;48(11):2769–74. [2] Kang T, Kyritsis DC. Methane flame propagation in compositionally stratified gases. Combust Sci Technol 2005;177(11):2191–210. [3] Kang T, Kyritsis DC. Phenomenology of methane flame propagation into compositionally stratified, gradually richer mixtures. Proc Combust Inst 2009;32(1):979–85. [4] Karim GA, Lam HT. Ignition and flame propagation within stratified methane-air mixtures formed by convective diffusion. Proc Combust Inst 1988;21(1):1909–15. [5] Badr O, Karim G. Flame propagation in stratified methane-air mixtures. J Fire Sci 1984;2(6):415–26. [6] Liebman I, Corry J, Perlee HE. Dynamics of flame propagation through layered methane-air mixtures. Combust Sci Technol 1971;2(5–6):365–75. [7] Karim GA, Tsang P. Flame propagation through atmospheres involving concentration gradients formed by mass transfer phenomena. J Fluids Eng 1975;97(4):615–7. [8] Ishikawa NA. Diffusion combustor and methane-air flame propagation in concentration gradient fields. Combust Sci Technol 1983;30(1–6):185–203. [9] Liebman I, Corry J, Perlee HE. Flame propagation in layered methane-air systems. Combust Sci Technol 1970;1:257–67. [10] Liebman I, Perlee HE, Corry J. Investigation of flame propagation characteristics in layered gas mixtures. U.S. Bureau of Mines 7078, Pittsburgh, PA, 1968. [11] Ishikawa N. Combustion of stratified methane/air layers. Combust Sci Technol 1983;30(1–6):311–25. [12] Phillips H. Flame in a buoyant methane layer. Proc Combust Inst 1965;10(1):1277–83.
4. Conclusions The experiments on the explosions of methane-air mixtures with transverse concentration gradients were conducted in a 2 m long closed duct. The concentration distribution of methane with an overall concentration of 10 vol% (which was charged from the top of the duct) was measured, and the effects of tig variation in the range 2–25 min on the flame behavior and Pmax were investigated. The major conclusions drawn under the current experimental conditions are summarized as follows. (1) The value oftig significantly affected the flame behavior and Pmax . (2) At tig ≥ 15 min, the flame behavior, including flame shape and speed, was similar. Tulip flames were observed at tig ≥ 10 min. The horizontal flame speed decreased as tig decreased from 15 to 4 min. Bright yellow diffusion and convection flames following the blue flame front were observed only at tig ≤ 6 min owing to the steep transverse concentration gradient of methane.
6
Fuel 265 (2020) 116926
C. Wang, et al.
[25] Xie YL, Wang JH, Cai X, Huang ZH. Self-acceleration of cellular flames and laminar flame speed of syngas/air mixtures at elevated pressures. Int J Hydrogen Energy 2016;41(40):18250–8. [26] Cai X, Wang JH, Zhao HR, Zhang M, Huang ZH. Flame morphology and self-acceleration of syngas spherically expanding flames. Int J Hydrogen Energy 2018;43(36):17531–41. [27] Xiao HH, Wang QS, Shen XB, Guo S, Sun JH. An experimental study of distorted tulip flame formation in a closed duct. Combust Flame 2013;160(9):1725–8. [28] Ponizy B, Claverie A, Veyssière B. Tulip flame-the mechanism of flame front inversion. Combust Flame 2014;161(12):3051–62. [29] Xiao HH, Houim RW, Oran ES. Formation and evolution of distorted tulip flames. Combust Flame 2015;162(11):4084–101. [30] Shen XB, He XC, Sun JH. A comparative study on premixed hydrogen-air and propane-air flame propagations with tulip distortion in a closed duct. Fuel 2015;161:248–53. [31] Li HW, Guo J, Yang FQ, Wang CJ, Zhang JQ, Lu SX. Explosion venting of hydrogenair mixtures from a duct to a vented vessel. Int J Hydrogen Energy 2018;43(24):11307–13. [32] Ciccarelli G, Johansen CT, Parravani M. The role of shock-flame interactions on flame acceleration in an obstacle laden channel. Combust Flame 2010;157(11):2125–36. [33] Ciccarelli G, Dorofeev S. Flame acceleration and transition to detonation in ducts. Prog Energy Combust Sci 2008;34(4):499–550. [34] Zhang PP, Zhou YH, Cao XY, Gao XL, Bi MS. Enhancement effects of methane/air explosion caused by water spraying in a sealed vessel. J Loss Prev Process Ind 2014;29:313–8. [35] Ma QJ, Zhang Q, Chen JC, Huang Y, Shi YT. Effects of hydrogen on combustion characteristics of methane in air. Int J Hydrogen Energy 2014;39(21):11291–8.
[13] Feng CC, Lam SH, Glassman I. Flame propagation through layered fuel-air mixtures. Combust Sci Technol 1975;10:59–71. [14] Cruz APD, Dean AM, Grenda JM. A numerical study of the laminar flame speed of stratified methane/air flames. Proc Combust Inst 2000;28(2):1925–32. [15] Wang JH, Huang ZH. Effect of partially premixed and hydrogen addition on natural gas direct injection lean combustion. Int J Hydrogen Energy 2009;34(22):9239–47. [16] Zheng W, Kaplan CR, Houim RW, Oran ES. Flame acceleration and transition to detonation: Effects of a composition gradient in a mixture of methane and air. Proc Combust Inst 2019;37:3521–8. [17] Hjertager BH, Bjørkhaug M, Fuhre K. Explosion propagation of non-homogeneous methane-air clouds inside an obstructed 50 m3 vented vessel. J Hazard Mater 1988;19(2):139–53. [18] Shirvill LC, Roberts P, Butler CJ, Roberts TA, Royle M. Characterization of the hazards from jet releases of hydrogen. 1st International Conference on Hydrogen Safety. 2005. [19] Eckhoff RK, Ngo M, Olsen W. On the minimum ignition energy (MIE) for propane/ air. J Hazard Mater 2010;175(1–3):293–7. [20] Gan Cui, Li ZL, Yang C, Zhou Z, Li JL. Experimental study of minimum ignition energy of methane-air mixtures at low temperatures and elevated pressures. Energy Fuels 2016;30:6738–44. [21] Fernández-Tarrazo E, Sánchez-Sanz M, Sánchez AL, Williams FA. Minimum ignition energy of methanol-air mixtures. Combust Flame 2016;171:234–6. [22] Li Q, Hu E, Cheng Y, Huang Z. Measurements of laminar flame speeds and flame instability analysis of 2-methyl-1-butanoleair mixture. Fuel 2013;112:263–71. [23] Hirano T, Suzuki T, Mashiko I, Iwai K. Flame propagation through mixtures with concentration gradient. Proc Combust Inst 1977;16(1):1307–15. [24] Wang JH, Huang ZH, Tang CL, Zheng JJ. Effect of hydrogen addition on early flame growth of lean burn natural gas-air mixtures. Int J Hydrogen Energy 2010;35(13):7246–52.
7