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Effects of concentration and initial turbulence on the vented explosion characteristics of methane-air mixtures
Fuel 267 (2020) 117103
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Full Length Article
Effects of concentration and initial turbulence on the vented explosion characteristics of methane-air mixtures Song Suna, Yanyu Qiua, Huadao Xingb, Mingyang Wanga, a b
T
⁎
State Key Laboratory for Disaster Prevention & Mitigation of Explosion & Impact, The Army Engineering University of PLA, Nanjing 210007, Jiangsu, China School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210000, Jiangsu, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Vented explosion Methane concentration Pressure characteristics Internal flame development Initial turbulence
The testing of vented explosion of methane-air mixtures was conducted in a custom-designed 4.5-m3 steel chamber. Methane concentrations in the mixed gas varied between 7 and 13 vol% and the static hysteresis time was set as 0, 1, and 5 min to characterize strong, medium and weak turbulence levels. The effects of concentration and initial turbulence on pressure characteristics and flame development were investigated. It is found that the quasi-static pressure in the chamber during vented explosion can be divided into three stages: pressure release, Helmholtz oscillation, and acoustic oscillation. The Helmholtz oscillation at a frequency of about 20–40 Hz is produced after the peak pressure P1 caused by the pressure release at all concentrations; however, the acoustic oscillation at a frequency of about 300 Hz only occurs after Helmholtz oscillation when the concentration is near the optimum. In the early stage of internal flame development, the cellular structure is generated due to hydrodynamic instability and diffusive-thermal instability, and the flame front is distorted owing to the instability caused by venting. Initial turbulence distorts the flame front, significantly increases the peak pressure, and stimulates the generation of acoustic oscillation at the upper and lower limits of concentration. Under the influence of initial high turbulence, the internal flame propagation distance has a power function relationship with time, and the turbulence acceleration factor and distance also show a power function relationship.
1. Introduction With the development of economy and the adjustment of the global energy structure, gas fuel has been increasingly widely used due to its characteristics, such as high thermal energy, lower pollution, and lower cost. BP, in the United Kingdom, pointed out in their latest annual report [1] (2019 BP Energy Outlook) that 2018 was a bonanza year for natural gas, with both global consumption and production increasing by over 5%, which was one of the strongest growth rates in either gas demand or output for over 30 years due to the growth of energy demand and the pursuit of environmental protection. Furthermore, gases tend to have a low ignition point and explode readily, which makes explosion accidents likely once leakage during production, transportation, or use occurs. By taking China as an example, there were 814 gas explosion accidents in China in 2018 and explosion disasters involving combustible gas have become one of the main safety threats to both life and productivity. For gas explosion disasters occurring in buildings, protection is
generally applied in three aspects: anti-explosion [2,3], explosion suppression [4,5], and vented explosion measures [6,7]. Of them, the vented explosion has become the most commonly used method of protection because of its convenience and low cost: much research has been undertaken into the characteristics of vented gas explosions. Cooper [8], Bauwens [9], and Chao et al. [10] studied pressure development during gas vented explosions experimentally and obtained the typical multi-peak pressure curve of a gas vented explosion. The experiment shows that phenomena such as component breakdown, external explosion, the maximum internal flame area, and acoustic oscillation are likely to produce peak pressure in the space. By changing the ignition position, gas concentration, and venting area, the influences of different environmental factors on pressure characteristics during a vented explosion have been analysed. By performing tests in a stainless steel explosion vessel with a diameter of 250 mm, Guo [11,12] studied the effects of burst pressure on vented explosion of different combustible gases. In addition, the flame propagation process during venting was observed and analysed using a high-speed camera and the
⁎ Corresponding author at: State Key Laboratory of Disaster Prevention & Mitigation of Explosion & Impact, The Army Engineering University of PLA, No.1, Haifu Lane, Qinghuai District, Nanjing City, Jiangsu Province, China. E-mail address: [email protected] (M. Wang).
https://doi.org/10.1016/j.fuel.2020.117103 Received 11 December 2019; Received in revised form 10 January 2020; Accepted 13 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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Nomenclature
σ
P S L t K
Subscripts
peak overpressure (kPa) propagation velocity of flame (m/s) flame propagation distance (m) propagation time (s) acceleration factor
l t d
expansion rate
laminar environmental turbulence self-accelerating action
Greek Letters φ
equivalence ratio
Helmholtz oscillation caused by flame propagation was studied: multiple peaks also appeared during the vented explosion and it was found that the reciprocating motion of the flame during venting is the main cause of this Helmholtz oscillation. Bao [13] exploded various methane–air mixtures in a concrete chamber with a volume of 12 m3 and investigated the influences of gas concentration and venting pressure on peak pressure and rate of pressure rise by changing the types of vent covers used. Qi [14] conducted an experimental gas vented explosion in a small rectangular chamber with a volume of 2 L, so as to study effects of vent size on pressure development and flame propagation. Based on the experiment, it is found that flame propagation distance in the vessel is mainly controlled by the vent size. By changing vent covers, Molkov [15–17], Sun [18], and Tamanini [19] investigated changes in internal pressure with the use of inertial vented component. It was found that the use of inertial vented components can reduce the turbulence generated during a vented explosion. Mitu and Razus studied the characteristics of mixture explosion and flame propagation in space. And the ranges of validity, physical background and applicability of empirical calculation methods for gas venting explosions were compared and verified by them [20–24]. Based on the above analysis, current experiments involving gas vented explosion are mainly conducted in a tube or a spherical chamber with a small volume to study pressure changes in the chamber or flame propagation characteristics in a transparent tube [12,25–27]. At present, there are few large-scale field experiments on vented explosion and existing field experiments generally focus on explosion overpressure and external flames, while flame development processes in the space for vented explosion are rarely studied [13,28–30]. Initial turbulence can exert important influences on gas vented explosion, while
less attention is paid on the effects of initial turbulence on explosion characteristics. Moreover, experimental research into the influences of initial turbulence on flame development and pressure characteristics in a large space remains rare. In view of the above situations, a field experiment was conducted in a self-designed explosion chamber with a volume of 4.5 m3 to study the characteristics of the large-scale vented explosion of methane–air mixtures. By changing the methane concentration and initial turbulence level, the pressure characteristics and internal flame development under different conditions were studied and the causes of development and control mechanisms were analysed. This study is a supplement to existing research, and provides a reference for analysing mechanisms of development of gas vented explosions. 2. Experimental work 2.1. Test set-up The self-designed experimental platform is shown in Fig. 1(a). It consisted of an explosion chamber, an explosion venting system, an ignition system, a gas distribution system, a flame observation system, and a data acquisition system. A rectangular metal chamber measuring 2 m × 1.5 m × 1.5 m was used as the main chamber in the experiment (it could bear an internal pressure of 1 MPa). A vent measuring 0.8 m × 0.8 m was set in the centre of the front face of the experimental chamber and a vent cover was installed on the vent by means of a self-designed flange structure. The better to describe gas explosion disasters in actual building structures, a calcium silicate board generally used in industrial production
Fig. 1. Schematic diagram of the layout of the test system. (a) Three-dimensional schematic diagram of the test system (blue and yellow circles represent sensors and gas distribution holes, respectively); (b) Plane diagram of the test system (top view). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2
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Interestingly, it was found that methane concentration could affect pressure oscillation in the experiment. In Fig. 3, P1 was the peak pressure when the vent cover was broken down, accompanied with post-peak pressure oscillation under all experimental concentrations; however, only when the methane equivalence ratio is 1.05 did the phenomenon of high-frequency oscillation occur in the later stage of the explosion. When the concentration is close to the upper or lower limit, the high-frequency oscillation would not occur. In general, pressure easily reaches its equilibrium state in the chamber and the pressure oscillation occurs in the non-equilibrium state [34,35]. From Fig. 3 it was found that the venting caused the internal pressure to be non-equilibrium state. Based on the above phenomenon, the pressure development during vented explosion was divided into three stages (Fig. 4). In the first stage, combustible gas in the confined space was ignited by the electric spark and started to burn and expand. At the same time, energy was released, which increased the pressure in the space. When reaching the venting threshold, owing to the vent cover being fractured, gas in the space began to flow out. Due to a large vent area in the experiment, the outflow of gas was so fast that pressure decreased to normal pressure in a very short time and the first stage ended. In the second stage, namely the Helmholtz oscillation stage [11,36], the pressure dropped after failure of the vent cover and the vented gas would cause heat loss along the direction of the vent. The heat loss and inertia effect of the vented gas allowed a vacuum to be formed inside the chamber and the gas outside refluxed. The venting-induced disturbance and backflow of gas would produce turbulent disturbance of the flame front, thus generating a new peak pressure in the chamber. Afterwards, rapid combustion and gas expansion could lead to venting and backflow of more gases and lowfrequency pressure oscillation was generated by repeating the above process. The third stage was characterised by high-frequency oscillation caused by acoustic instability [13,37]. In this stage, pressure waves generated by combustion could stimulate acoustic oscillation of the structure and the coupling of pressure waves with acoustic waves produced by natural oscillation of the structure could result in acoustic instability in the chamber. Meanwhile, gas outflow changed the thermodynamic parameters of the gas in the chamber, which caused the rate of heat release to fluctuate and promoted generation of acoustic oscillations. The reflection of acoustic waves enhanced their oscillation, which promoted the generation of a cellular structure at the flame front and enlarged the flame area. Therefore, the effect of acoustic oscillation was related to the rate of energy release from the combustible gas: only when the reaction rate reached a certain level could this occur, so concentrations close to the upper or lower limit could not lead to acoustic oscillation. There are many published reports describing pressure oscillation
was used as the vent cover in the experiment, with a relieving pressure of about 6 kPa. An observation window with the diameter of 0.3 m was set in the centre of the back of the experimental chamber and an explosion-proof glass panel was installed to form an observation window through the flange structure. By using a high-speed camera, internal flame propagation was recorded from the observation window at a frame rate of 1000 fps. In the experiment, a spark igniter, with its ignition head installed in the centre of a side-face of the chamber, was adopted and the ignition energy was about 500 mJ. In the experiment, methane, the most commonly used gaseous fuel, was selected as the combustible gas and its concentration ranged from 7 to 15 vol% (equivalence ratio ranged from 0.74 to 1.37). Inlets and outlets were separately set on two sides of the experimental chamber at equal intervals for even gas distribution purposes. An explosion-proof fan was installed in the chamber, so as to ensure that methane was uniformly mixed. Gas concentration was monitored by an infrared concentration analyser. Eight pressure measuring points were established: of them, S1 and S2 were located above and below the vent, S3 to S7 were mounted on the top of the chamber, and S8 was set above the observation window (Fig. 1(b)). Piezoresistive sensors with circulatory water cooling systems were used to ensure that the measured data were unaffected by the high temperatures induced by the explosion. Pressure data were recorded at a frequency of 50 kHz through the data acquisition system (Donghua DH5922N, Donghua Texting Technology Co., Ltd, China). 2.2. Test procedures When conducting the experiment, after confirming correct installation of the ignition head, the sensors, explosion-proof fan, and vent cover were installed through the flange structure, so as to allow us to seal the vent. Thereafter, valves at the inlets and outlets were opened and methane with a purity of 99.9% was injected into the explosion chamber through a high-pressure gas cylinder. Methane concentrations in the chamber were monitored by the infrared concentration analyser. After complete gas distribution, valves of inlets and outlets were closed and the fan was turned on for one minute to ensure uniform mixing of the gases inside the chamber. After turning off the fan, the chamber was allowed to stay in temporal stasis for one minute, so that the internal initial turbulence reached a low uniform level. Finally, after determining that the internal methane concentration had reached the preset value through the infrared concentration analyser, the high-speed camera, pressure measuring system, and data acquisition system were turned on and the ignition system was triggered. In this way, the methane–air mixtures were ignited and an explosion triggered. To ensure the stability and repeatability of the experimental data, the experiment was conducted at least three times under each set of conditions. 3. Results and discussion 3.1. Concentration effects During vented explosion, the pressure distribution at different measuring points in the chamber is as shown in Fig. 2. Pressure curves measured by each sensor overlapped and the pressure presented quasistatic characteristics [13,31]. When analysing the influences of methane concentration on explosion-induced pressure, time-history curves of pressures under different equivalence ratios are plotted by taking sensor T1 as an example (Fig. 3). Fig. 3 shows that the peak pressure first increased, then decreased with increasing methane equivalence ratio. When the methane equivalence ratio is 1.05, the peak pressure reached its highest and the pressure rise-time was the shortest because the reaction of methane–air mixtures was the most intense and the energy release rate was the highest at the optimal methane concentration. This has been observed and analysed in many studies [32,33] and, as such, is not discussed further.
Fig. 2. Pressure-time histories at various measurement points. 3
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Table 1, Helmholtz oscillation occurred at a frequency of between 20 and 40 Hz after venting and lasted for about 300–500 ms: this was consistent with flame propagation velocities observed during venting. In other words, oscillations disappeared after multiple reciprocating motions of the flame. In addition, it could be obtained from Table 1 that, as the methane equivalence ratio changed from 0.74 to 1.37, the frequency of Helmholtz oscillation first increased, then decreased, which was consistent with the trend in combustion velocity of the flame. This verified the previous analysis once again. In terms of acoustic oscillation, differing from the frequency of 700 Hz (Bauwens [37]), data in Table 1 show that oscillation frequency was about 300 Hz and oscillation time was about 1000 ms. Oscillation at such a frequency had similar propagation velocity to acoustic waves, which indicated that the phenomenon was mainly caused by wave effects. In the late stage of a vented explosion, unburned gases remained in the space were burned to produce combustion waves which were coupled with acoustic oscillation induced by vibration of the structure, thus leading to the high frequency. The methane remaining in the chamber burned for a long time in the later stage of the vented explosion, so acoustic oscillations lasted for a long time. Bao [13] changed the reflected path of acoustic waves by adding asbestos shingles to the inner wall of the structure, which inhibited the development of acoustic oscillations. Some researchers considered that although acoustic oscillation, such a high-frequency oscillation, shows a high peak amplitude, its duration is very short, so the resulting impulse is small and the damage caused thereby to building structures can be ignored [9,10].
Fig. 3. Comparison of pressure-time histories under different methane equivalence ratios.
3.2. Internal flame development Flame development in the chamber was studied from the observation window and analysed from two aspects: flame colour and flame shape. By taking the moment when forming a spherical flame as the start time, the flame development process in the chamber is illustrated in Fig. 5. The analysis started from an assessment of the colour changes in the flame within the chamber. A flame is a high-temperature gas or ionic substance in nature and its colour is mainly determined by factors, such as temperature and the intrinsic spectrum of ionic elements. The flame was blue at the beginning, mainly because methane reacted with oxygen in the initial stage of combustion. Some carbon monoxide was produced because methane cannot be burned completely under oxygendeficient conditions. Moreover, carbon monoxide reacted fully with oxygen again in the flame tail to produce a large number of OH and CH free radicals. These free radicals were light blue, so a bluish-green flame was generated (Fig. 5(A and B)). With the development of the spherical flame, the oxygen inside the fireball was largely exhausted, and the unburned gas remaining in the fireball was partially burned with a small amount of residual air. Incomplete combustion of methane generated soot and soot continued to burn to produce yellow light, resulting in yellow patches inside the flame (Fig. 5C). Different flame colours corresponded to different temperatures, and the distribution of the flame colour roughly followed the trend whereby the higher the
Fig. 4. Stages in the pressure development process (φ = 1.16).
after venting [38,39]. From the perspective of oscillation frequency, we analysed two oscillations for which all characteristic parameters are listed in Table 1. It was found that the fundamental parameter distinguishing the two oscillations was the oscillation frequency. By conducting a venting experiment in a tube in the laboratory, Guo [11] studied flame profiles through the use of a high-speed camera. He found that Helmholtz oscillation is accompanied by reciprocating motion of the flame and interpreted this as the effects of R-T and K-H instability thereon. By utilising a high-speed camera to image microscopic flame shapes during venting, McCann [38] found that Helmholtz oscillation is mainly correlated with propagation velocity and shape of the flame. As shown in Table 1 Statistics related to pressure oscillation parameters. Methane concentration (vol.%)
7 8 9 10 11 12 13
Methane equivalence ratio (φ)
0.74 0.84 0.95 1.05 1.16 1.26 1.37
Helmholtz oscillation
Acoustic oscillation
Frequency (Hz)
Duration (ms)
Frequency (Hz)
Duration (ms)
24 26 32 39 41 38 35
520 311 344 320 366 286 300
– – 250 396 378 294 –
– – 1,295 1,160 1,227 956 –
4
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Fig. 5. Development of the flame front (φ = 1.37).
temperature, the longer the wavelength and the bluer the flame. After bursting the vent cover, much of the gas carrying heat energy escaped, which reduced the internal pressure and temperature, so the blue flame gradually changed into a yellow flame (Fig. 5(D–G)). The venting distorted the flame front and the air outside entered the chamber to react with the remaining methane. In the late stage of the reaction, many soot particles were generated due to incomplete reaction of residual methane in the chamber, so the flame became yellow and white [22] (Fig. 5(H)). In the later stage of the reaction, a large amount of heat energy was dissipated and the internal temperature of the space decreased, rendering the flame orange-red (Fig. 5(I)). It is worth noting that the analysis above is based on the colour changes of the flame, and GC-Analyse is needed to get the definite result. Based on the folding of the lame front, the influence of turbulence disturbance on flame development was studied. At the beginning of flame development, the flame propagated in the form of a regular hemisphere (Fig. 5(A)). In the process of initial flame development, cellular structures would still be generated even without disturbance (Fig. 5(B)), which resulted from hydrodynamic instability and diffusivethermal instability of the flame [40]. Upon flame propagation, the burned gases expanded, which spontaneously led to disturbance of the flame front, rendering it equivalent to a plane vortex, showing antigradient diffusion. This led to larger curvature of the curved part and generated a pressure gradient. Furthermore, stable honeycomb structures of regular size and shape were generated on the flame front. The flame underwent molecular diffusion and heat diffusion, and the
difference between the molecular conductivity and the molecular diffusivity of reactants resulted in diffusive-thermal instability which was determined by the Lewis number Le. When Le < 1, diffusive-thermal instability and hydrodynamic instability reinforced each other and the disturbance of the flame front increased, so that an irregular honeycomb flame was generated on the surface. The pressure wave generated by the explosion was reflected when it encountered the wall. The reflected pressure wave caused the pressure gradient of the flame front to no longer be parallel to the density gradient, which generated a baroclinic effect and induced turbulence, thus generating a Rayleigh–Taylor instability (Fig. 5(C)). After bursting the vent cover, a flame was ejected (Fig. 5(D)–(G)). In the process of flame outflow, violent disturbance was generated and the flame front was distorted. When the low-density combustion products flowed towards, and into, the denser air, a Rayleigh–Taylor instability was generated, and K-H instability could also occur in continuous fluid with shear force and a fluid interface across which a velocity difference existed. These two types of instability exerted significant influences on the flame during venting [41]. In the later stage of venting, the spherical flame in the chamber burst and most of the flame and combustion products flowed out of the space, so only the remaining unburned gases were left in the space for combustion (Fig. 5(H) and (I)).
3.3. Initial turbulence effects The initial turbulence in the chamber affected these gas explosions:
Table 2 Test conditions. Turbulence level: (static hysteresis time)
Strong turbulence Medium turbulence Weak turbulence
No temporal stasis Temporal stasis for 1 min Temporal stasis for 5 min
Ignition
Fan blowing time
resistance wire
3 min
5
Methane equivalence ratio (φ) 0.74
1.05
1.37
S1 M1 W1
S2 M2 W1
S3 M3 W3
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velocity, the ignition head was replaced with a resistance wire with a length of 20 mm. Taking this as a reference, the flame propagation distance was calculated. Under different equivalence ratios, the most obvious pictures of flame folds under strong initial turbulence is shown in Fig. 7: at a methane equivalence ratio of 0.74, the flame was lighter in colour and it was difficult to observe the outline of the light blue flame. The higher the methane concentration, the clearer the outline of the flame and the more obvious the folding of the flame front. To facilitate observation and analysis, by taking a methane equivalence ratio of 1.37 as an example, the flame development at different initial turbulence levels was analysed (Fig. 8). As demonstrated therein, the higher the level of turbulence, the faster the flame propagation velocity and the more obvious the cellular structure. Under strong initial turbulence, the internal flow field was strongly disturbed and the flame front distorted. Moreover, a cellular structure was produced and the flame propagation velocity increased markedly. Under medium and weak turbulence conditions, it can be observed that changes in flame propagation were similar, indicating that turbulence decreased with time. Once static hysteresis occurred, irregular motion of internal gas decreased and turbulence (and the allied disturbance) diminished. By observing flame propagation under medium and weak levels of turbulence, a buoyancy effect was observed. In the experiment, the resistance wire was heated during the explosion, and heat was released continuously. After static hysteresis, the turbulence diminished, making the buoyancy effect more obvious. The elevation of heat under buoyancy effects resulted in a lower air density above the resistance wire, so the flame propagated faster in the vertical direction under buoyancy effects and showed a teardrop shape. Fig. 9 shows flame propagation distances over time at methane equivalence ratio of 1.05 and 1.37. Since strong initial turbulence increased the rate of folding of the flame, and accelerated both combustion rate and flame propagation by enlarging the reaction area, the flame propagated fastest in these conditions. The internal turbulence decreased rapidly and the allied disturbance was rapidly attenuated with increasing static hysteresis time. Through observation, flames under weak and medium initial levels of turbulence propagated at a constant velocity, while flame propagation showed a power function relationship with time under strong turbulence. The burning velocity of methane-air flames has been extensively studied [43–45]. In numerical calculation, turbulence factor was generally introduced to express the acceleration effects caused by turbulence and the flame propagation velocity was obtained as follows [46]:
to study the influence of initial turbulence on explosion characteristics, it was necessary to control the turbulence in the chamber before ignition. Limited by experimental conditions, it was impossible to measure the turbulence within, therefore, the most commonly used method, that is, changing static hysteresis time between turning off the fan and ignition [28,42], was utilised to adjust the initial level turbulence on a qualitative basis. After the gas distribution was completed, the fan was turned on to mix the internal mixture and increase the internal turbulence disturbance. After blowing for three minutes, the fan was turned off. By selecting three static hysteresis times (0, 1, and 5 min) between turning off the fan and ignition, strong, medium, and weak turbulence in the space were qualitatively characterised (Table 2). At different methane equivalence ratios, the effects of initial turbulence on pressure curves are as shown in Fig. 6: the higher the initial level of turbulence, the greater the measured peak pressure and the shorter the rise-time thereto. The main reason for this was that stronger initially turbulent, the greater the increased rate of stretching of the flame. The increase in the energy release rate and flame propagation velocity was translated into a higher peak pressure and faster rate of pressure rise. By comparing Fig. 6(a), (b), and (c), it was found that methane concentration and turbulence level had synergistic effects on overpressure and the turbulence had different effects under different methane concentrations. To be specific, the higher the concentration of combustible gas, the greater the enhancement of turbulence on the peak overpressure and the stronger the response of pressure to turbulence. This may be due to the fact that the higher the methane concentration, the more combustible gases participated in the reaction under the disturbance caused by the initial turbulence, which led to greater increases of peak pressure. An interesting phenomenon is demonstrated in Fig. 6: strong initial turbulence could stimulate acoustic oscillation. By comparing Fig. 6(a), (b), and (c), Helmholtz oscillation occurred under all experimental conditions, however, when the methane equivalence ratio were 0.74 and 1.37, acoustic oscillation occurred after ending Helmholtz oscillation only under conditions of strong initial turbulence. It was considered that when the initial turbulence was weak, and the concentration approached the upper and lower limits of combustion, the reaction intensity of the methane remaining was weak in the later stage of the reaction, thus it failed to stimulate acoustic instability: however, when the initial turbulence was strong, the turbulence could disturb the internal flow field and increase the rate of stretching of the flame. With the increase of the stretching rate and the combustion rate, the D-L instability and diffusional-thermal instability of flame were intensified and the aforementioned cellular structure appeared on the flame front. This promoted the remaining unburned gas to participate in the reaction and increased the rate of energy release, thus stimulating acoustic oscillation effects. This section describes our study of the effects of initial turbulence on internal flame development. To quantify the flame propagation
(a)
=0.74
S = Kt ·K d·σ·Sl where, Kt , K d , σ , and Sl indicate the accelerating action attributed to environmental turbulence, the self-accelerating action of the flame, the expansion rate during combustion, and the laminar propagation velocity of the flame, respectively. At the same methane concentration, laminar burning velocity Sl ,
(b)
=1.05
Fig. 6. Pressure-time-history under different initial levels of turbulence. 6
(c)
=1.37
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Fig. 7. Comparison of flames under different methane equivalence ratios.
Fig. 8. Flame development under different initial levels of turbulence (φ = 1.37).
behaviour whereby flame propagation had a power function relationship with time under strong turbulence. With increasing propagation distance (Fig. 10), the flame front area increased and more folds were present on the surface. The turbulent disturbance accelerated the rate of reaction by increasing the number of folds of the flame front, so turbulence factor Kt increased with propagation distance.
expansion rate σ , and self-accelerating factor K d were similar and the external environment mainly influenced the flame propagation by changing Kt . By using the ratio of propagation velocities under strong and weak turbulence to separate the turbulence factor Kt induced by the environment, the relationship between turbulence factor and propagation distance was obtained (Fig. 10). In Fig. 10, the turbulent factor did not remain the same, but increased with increasing propagation distance. The turbulence factor Kt showed a power function relationship with propagation distance. This was consistent with the observed
(a)
=1.05
(b)
Fig. 9. Flame propagation distances under different levels of turbulence. 7
=1.37
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environment also presented a power function relationship with distance. Funding This work was supported by the National Key Research and Development Program of China [No: 2017YFC0804702]. CRediT authorship contribution statement Song Sun: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Yanyu Qiu: Conceptualization, Resources. Huadao Xing: Methodology, Validation, Investigation. Mingyang Wang: Resources, Writing - review & editing, Supervision. 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.
Fig. 10. Fitted curves relating turbulent factor to propagation distance.
4. Conclusion
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The testing of vented explosions of methane–air mixtures was conducted in an explosion chamber with a volume of 4.5 m3, so as to study effects of methane concentration and initial turbulence level on pressure characteristics and flame development during a vented explosion. The conclusions were as follows:
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(1) There was a quasi-static pressure in the space during vented explosion. As the methane equivalence ratio increased from 0.74 to 1.05 and finally to 1.37, the peak overpressure first increased, then decreased. (2) Under all experimental concentrations, peak pressure P1 was generated after breakdown of the vent cover, accompanied by Helmholtz oscillation. The oscillation lasted for about 300–500 ms at a frequency of about 20–40 Hz, which was mainly because R-T instability and K-H instability (after venting) resulted in reciprocating oscillation of the flame. Only when the concentration approached the optimum value could acoustic oscillation occur for about 1000 ms at a frequency of 300 Hz. This was mainly because the pressure wave produced by combustion was coupled with acoustic oscillation of the chamber itself. (3) In the early stage of flame combustion, because hydrodynamic instability and diffusive-thermal instability could lead to the formation of a cellular structure, many OH and CH free radicals were generated by flame combustion and the flame appeared light blue. Due to incomplete combustion, yellow patches were seen in the interior. After venting, the flame flowed out and the resulting R-T and K-H instabilities disturbed the flame. In this case, a large amount of heat energy leaked from the system, accompanied by external air entering the chamber to trigger a further reaction, so that soot was produced and the flame changed from blue to yellowish-white and finally to orange-red in colour. (4) The increase in initial level of turbulence could significantly increase the peak pressure and stimulate acoustic oscillation at the upper and lower limits of combustion concentration. The experiment demonstrated that the higher the methane concentration, the stronger the disturbing effect of initial turbulence on the flame. Moreover, the folding of the flame front was more obvious and the enhancement of turbulence on overpressure was more significant. Initial turbulence reduced quickly with increasing static hysteresis time. Under strong initial turbulence, flame propagation distances showed a power function relationship with time, while a linear relationship was found between them under weak initial turbulence. In addition, the turbulent factor caused by the initial 8
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[36] Ambrosi G, Bosco L, Camosso ME. Experimental and Numerical Study of Methaneair Deflagrations in a Vented Enclosure. Fire Safety Sci 2009;9(2):1043–54. [37] Bauwens CR, Chaffee J, Dorofeev SB. Vented explosion overpressures from combustion of hydrogen and hydrocarbon mixtures. Int J Hydrogen Energ 2011;36:2329–36. [38] Mccann DPJ, Thomas GO, Edwards DH. Gasdynamics of vented explosions part I: Experimental studies. Combust Flame 1985;59(3):233–50. [39] Li G, Yang D, Wang S, Qi S, Zhang P, Chen W. Large eddy simulation and experimental study on vented gasoline-air mixture explosions in a semi-confined obstructed pipe. J Hazard Mater 2017;339:131–42. [40] Li Y, Bi M, Gao W. Theoretical pressure prediction of confined hydrogen explosion considering flame instabilities. J Loss Prevent Proc 2019;57:320–6. [41] Xiao H, Makarov D, Sun J, Molkov V. Experimental and numerical investigation of premixed flame propagation with distorted tulip shape in a closed duct. Combust Flame 2012;159(4):1523–38. [42] Benedetto AD, Garcia-Agreda A, Russo P, Sanchirico R. Combined effect of ignition energy and initial turbulence on the explosion behavior of lean gas/dust-air mixtures. Ind Eng Chem Res 2012;51(22):7663–70. [43] Hu E, Li X, Meng X, Chen Y, Chen Y, Xie Y, et al. Laminar flame speeds and ignition delay times of methane–air mixtures at elevated temperatures and pressures. Fuel 2015;158:1–10. [44] Hu E, Jiang X, Huang Z, Lida N. Numerical study on the effects of diluents on the laminar burning velocity of methane–air mixtures. Energ Fuel 2012;26:4242–52. [45] Cai X, Wang J, Bian Z, Zhao H, Zhang M, Huang Z. Self-similar propagation and turbulent burning velocity of CH4/H2/air expanding flames: Effect of Lewis number. Combust Flame 2020;212:1–12. [46] Fire National. Protection Association. NFPA 68, Standard on explosion protection by deflagration. venting 2007.
[25] Chen X, Li J, Feng M, Zhao D, Shi B, Wang N. Flame stability and combustion characteristics of liquid fuel in a meso-scale burner with porous media. Fuel 2019;251:249–59. [26] Yan X, Yu J, Gao W. Flame behaviors and pressure characteristics of vented dust explosions at elevated static activation overpressures. J Loss Prevent Proc 2015;33:101–8. [27] Wei H, Gao D, Zhou L, Pan J, Tao K, Pei Z. Experimental observations of turbulent flame propagation effected by flame acceleration in the end gas of closed combustion chamber. Fuel 2016;180:157–63. [28] Liang Z. Scaling effects of vented deflagrations for near lean flammability limit hydrogen-air mixtures in large scale rectangular volumes. Int J Hydrogen Energ 2017;42(20):14321–31. [29] Diakow PA, Thomas JK, Parsons PJ. Large-scale vented deflagration tests. J Loss Prevent Proc 2017;50:121–30. [30] Jérôme D, Christophe P, Guillaume L. Propagation of a confined explosion to an external cloud. J Loss Prev Process Ind 2017;49:805–13. https://doi.org/10.1016/j. jlp.2017.03.009. [31] Sun S, Wang MY, Gao KH, Zhao TH, Guo Q. Effect of vent conditions on internal overpressure time-history during a vented explosion. J Loss Prevent Proc 2018;54:85–92. [32] Zhang B, Ng HD. An experimental investigation of the explosion characteristics of dimethyl ether-air mixtures. Energy 2016;107:1–8. [33] Kim WK, Mogi T, Dobashi R. Fundamental study on accidental explosion behavior of hydrogen-air mixtures in an open space. Int J Hydrogen Energ 2013;38(19):8024–9. [34] Tang C, Zhang S, Si Z, Huang Z, Zhang K, Jin Z. High methane natural gas/air explosion characteristics in confined vessel. J Hazard Mater 2014;278:520–8. [35] Xie Y, Wang J, Cai X, Huang Z. Pressure history in the explosion of moist syngas/air mixtures. Fuel 2016;185:18–25.
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Full Length Article
Effects of concentration and initial turbulence on the vented explosion characteristics of methane-air mixtures Song Suna, Yanyu Qiua, Huadao Xingb, Mingyang Wanga, a b
T
⁎
State Key Laboratory for Disaster Prevention & Mitigation of Explosion & Impact, The Army Engineering University of PLA, Nanjing 210007, Jiangsu, China School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210000, Jiangsu, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Vented explosion Methane concentration Pressure characteristics Internal flame development Initial turbulence
The testing of vented explosion of methane-air mixtures was conducted in a custom-designed 4.5-m3 steel chamber. Methane concentrations in the mixed gas varied between 7 and 13 vol% and the static hysteresis time was set as 0, 1, and 5 min to characterize strong, medium and weak turbulence levels. The effects of concentration and initial turbulence on pressure characteristics and flame development were investigated. It is found that the quasi-static pressure in the chamber during vented explosion can be divided into three stages: pressure release, Helmholtz oscillation, and acoustic oscillation. The Helmholtz oscillation at a frequency of about 20–40 Hz is produced after the peak pressure P1 caused by the pressure release at all concentrations; however, the acoustic oscillation at a frequency of about 300 Hz only occurs after Helmholtz oscillation when the concentration is near the optimum. In the early stage of internal flame development, the cellular structure is generated due to hydrodynamic instability and diffusive-thermal instability, and the flame front is distorted owing to the instability caused by venting. Initial turbulence distorts the flame front, significantly increases the peak pressure, and stimulates the generation of acoustic oscillation at the upper and lower limits of concentration. Under the influence of initial high turbulence, the internal flame propagation distance has a power function relationship with time, and the turbulence acceleration factor and distance also show a power function relationship.
1. Introduction With the development of economy and the adjustment of the global energy structure, gas fuel has been increasingly widely used due to its characteristics, such as high thermal energy, lower pollution, and lower cost. BP, in the United Kingdom, pointed out in their latest annual report [1] (2019 BP Energy Outlook) that 2018 was a bonanza year for natural gas, with both global consumption and production increasing by over 5%, which was one of the strongest growth rates in either gas demand or output for over 30 years due to the growth of energy demand and the pursuit of environmental protection. Furthermore, gases tend to have a low ignition point and explode readily, which makes explosion accidents likely once leakage during production, transportation, or use occurs. By taking China as an example, there were 814 gas explosion accidents in China in 2018 and explosion disasters involving combustible gas have become one of the main safety threats to both life and productivity. For gas explosion disasters occurring in buildings, protection is
generally applied in three aspects: anti-explosion [2,3], explosion suppression [4,5], and vented explosion measures [6,7]. Of them, the vented explosion has become the most commonly used method of protection because of its convenience and low cost: much research has been undertaken into the characteristics of vented gas explosions. Cooper [8], Bauwens [9], and Chao et al. [10] studied pressure development during gas vented explosions experimentally and obtained the typical multi-peak pressure curve of a gas vented explosion. The experiment shows that phenomena such as component breakdown, external explosion, the maximum internal flame area, and acoustic oscillation are likely to produce peak pressure in the space. By changing the ignition position, gas concentration, and venting area, the influences of different environmental factors on pressure characteristics during a vented explosion have been analysed. By performing tests in a stainless steel explosion vessel with a diameter of 250 mm, Guo [11,12] studied the effects of burst pressure on vented explosion of different combustible gases. In addition, the flame propagation process during venting was observed and analysed using a high-speed camera and the
⁎ Corresponding author at: State Key Laboratory of Disaster Prevention & Mitigation of Explosion & Impact, The Army Engineering University of PLA, No.1, Haifu Lane, Qinghuai District, Nanjing City, Jiangsu Province, China. E-mail address: [email protected] (M. Wang).
https://doi.org/10.1016/j.fuel.2020.117103 Received 11 December 2019; Received in revised form 10 January 2020; Accepted 13 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
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Nomenclature
σ
P S L t K
Subscripts
peak overpressure (kPa) propagation velocity of flame (m/s) flame propagation distance (m) propagation time (s) acceleration factor
l t d
expansion rate
laminar environmental turbulence self-accelerating action
Greek Letters φ
equivalence ratio
Helmholtz oscillation caused by flame propagation was studied: multiple peaks also appeared during the vented explosion and it was found that the reciprocating motion of the flame during venting is the main cause of this Helmholtz oscillation. Bao [13] exploded various methane–air mixtures in a concrete chamber with a volume of 12 m3 and investigated the influences of gas concentration and venting pressure on peak pressure and rate of pressure rise by changing the types of vent covers used. Qi [14] conducted an experimental gas vented explosion in a small rectangular chamber with a volume of 2 L, so as to study effects of vent size on pressure development and flame propagation. Based on the experiment, it is found that flame propagation distance in the vessel is mainly controlled by the vent size. By changing vent covers, Molkov [15–17], Sun [18], and Tamanini [19] investigated changes in internal pressure with the use of inertial vented component. It was found that the use of inertial vented components can reduce the turbulence generated during a vented explosion. Mitu and Razus studied the characteristics of mixture explosion and flame propagation in space. And the ranges of validity, physical background and applicability of empirical calculation methods for gas venting explosions were compared and verified by them [20–24]. Based on the above analysis, current experiments involving gas vented explosion are mainly conducted in a tube or a spherical chamber with a small volume to study pressure changes in the chamber or flame propagation characteristics in a transparent tube [12,25–27]. At present, there are few large-scale field experiments on vented explosion and existing field experiments generally focus on explosion overpressure and external flames, while flame development processes in the space for vented explosion are rarely studied [13,28–30]. Initial turbulence can exert important influences on gas vented explosion, while
less attention is paid on the effects of initial turbulence on explosion characteristics. Moreover, experimental research into the influences of initial turbulence on flame development and pressure characteristics in a large space remains rare. In view of the above situations, a field experiment was conducted in a self-designed explosion chamber with a volume of 4.5 m3 to study the characteristics of the large-scale vented explosion of methane–air mixtures. By changing the methane concentration and initial turbulence level, the pressure characteristics and internal flame development under different conditions were studied and the causes of development and control mechanisms were analysed. This study is a supplement to existing research, and provides a reference for analysing mechanisms of development of gas vented explosions. 2. Experimental work 2.1. Test set-up The self-designed experimental platform is shown in Fig. 1(a). It consisted of an explosion chamber, an explosion venting system, an ignition system, a gas distribution system, a flame observation system, and a data acquisition system. A rectangular metal chamber measuring 2 m × 1.5 m × 1.5 m was used as the main chamber in the experiment (it could bear an internal pressure of 1 MPa). A vent measuring 0.8 m × 0.8 m was set in the centre of the front face of the experimental chamber and a vent cover was installed on the vent by means of a self-designed flange structure. The better to describe gas explosion disasters in actual building structures, a calcium silicate board generally used in industrial production
Fig. 1. Schematic diagram of the layout of the test system. (a) Three-dimensional schematic diagram of the test system (blue and yellow circles represent sensors and gas distribution holes, respectively); (b) Plane diagram of the test system (top view). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2
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Interestingly, it was found that methane concentration could affect pressure oscillation in the experiment. In Fig. 3, P1 was the peak pressure when the vent cover was broken down, accompanied with post-peak pressure oscillation under all experimental concentrations; however, only when the methane equivalence ratio is 1.05 did the phenomenon of high-frequency oscillation occur in the later stage of the explosion. When the concentration is close to the upper or lower limit, the high-frequency oscillation would not occur. In general, pressure easily reaches its equilibrium state in the chamber and the pressure oscillation occurs in the non-equilibrium state [34,35]. From Fig. 3 it was found that the venting caused the internal pressure to be non-equilibrium state. Based on the above phenomenon, the pressure development during vented explosion was divided into three stages (Fig. 4). In the first stage, combustible gas in the confined space was ignited by the electric spark and started to burn and expand. At the same time, energy was released, which increased the pressure in the space. When reaching the venting threshold, owing to the vent cover being fractured, gas in the space began to flow out. Due to a large vent area in the experiment, the outflow of gas was so fast that pressure decreased to normal pressure in a very short time and the first stage ended. In the second stage, namely the Helmholtz oscillation stage [11,36], the pressure dropped after failure of the vent cover and the vented gas would cause heat loss along the direction of the vent. The heat loss and inertia effect of the vented gas allowed a vacuum to be formed inside the chamber and the gas outside refluxed. The venting-induced disturbance and backflow of gas would produce turbulent disturbance of the flame front, thus generating a new peak pressure in the chamber. Afterwards, rapid combustion and gas expansion could lead to venting and backflow of more gases and lowfrequency pressure oscillation was generated by repeating the above process. The third stage was characterised by high-frequency oscillation caused by acoustic instability [13,37]. In this stage, pressure waves generated by combustion could stimulate acoustic oscillation of the structure and the coupling of pressure waves with acoustic waves produced by natural oscillation of the structure could result in acoustic instability in the chamber. Meanwhile, gas outflow changed the thermodynamic parameters of the gas in the chamber, which caused the rate of heat release to fluctuate and promoted generation of acoustic oscillations. The reflection of acoustic waves enhanced their oscillation, which promoted the generation of a cellular structure at the flame front and enlarged the flame area. Therefore, the effect of acoustic oscillation was related to the rate of energy release from the combustible gas: only when the reaction rate reached a certain level could this occur, so concentrations close to the upper or lower limit could not lead to acoustic oscillation. There are many published reports describing pressure oscillation
was used as the vent cover in the experiment, with a relieving pressure of about 6 kPa. An observation window with the diameter of 0.3 m was set in the centre of the back of the experimental chamber and an explosion-proof glass panel was installed to form an observation window through the flange structure. By using a high-speed camera, internal flame propagation was recorded from the observation window at a frame rate of 1000 fps. In the experiment, a spark igniter, with its ignition head installed in the centre of a side-face of the chamber, was adopted and the ignition energy was about 500 mJ. In the experiment, methane, the most commonly used gaseous fuel, was selected as the combustible gas and its concentration ranged from 7 to 15 vol% (equivalence ratio ranged from 0.74 to 1.37). Inlets and outlets were separately set on two sides of the experimental chamber at equal intervals for even gas distribution purposes. An explosion-proof fan was installed in the chamber, so as to ensure that methane was uniformly mixed. Gas concentration was monitored by an infrared concentration analyser. Eight pressure measuring points were established: of them, S1 and S2 were located above and below the vent, S3 to S7 were mounted on the top of the chamber, and S8 was set above the observation window (Fig. 1(b)). Piezoresistive sensors with circulatory water cooling systems were used to ensure that the measured data were unaffected by the high temperatures induced by the explosion. Pressure data were recorded at a frequency of 50 kHz through the data acquisition system (Donghua DH5922N, Donghua Texting Technology Co., Ltd, China). 2.2. Test procedures When conducting the experiment, after confirming correct installation of the ignition head, the sensors, explosion-proof fan, and vent cover were installed through the flange structure, so as to allow us to seal the vent. Thereafter, valves at the inlets and outlets were opened and methane with a purity of 99.9% was injected into the explosion chamber through a high-pressure gas cylinder. Methane concentrations in the chamber were monitored by the infrared concentration analyser. After complete gas distribution, valves of inlets and outlets were closed and the fan was turned on for one minute to ensure uniform mixing of the gases inside the chamber. After turning off the fan, the chamber was allowed to stay in temporal stasis for one minute, so that the internal initial turbulence reached a low uniform level. Finally, after determining that the internal methane concentration had reached the preset value through the infrared concentration analyser, the high-speed camera, pressure measuring system, and data acquisition system were turned on and the ignition system was triggered. In this way, the methane–air mixtures were ignited and an explosion triggered. To ensure the stability and repeatability of the experimental data, the experiment was conducted at least three times under each set of conditions. 3. Results and discussion 3.1. Concentration effects During vented explosion, the pressure distribution at different measuring points in the chamber is as shown in Fig. 2. Pressure curves measured by each sensor overlapped and the pressure presented quasistatic characteristics [13,31]. When analysing the influences of methane concentration on explosion-induced pressure, time-history curves of pressures under different equivalence ratios are plotted by taking sensor T1 as an example (Fig. 3). Fig. 3 shows that the peak pressure first increased, then decreased with increasing methane equivalence ratio. When the methane equivalence ratio is 1.05, the peak pressure reached its highest and the pressure rise-time was the shortest because the reaction of methane–air mixtures was the most intense and the energy release rate was the highest at the optimal methane concentration. This has been observed and analysed in many studies [32,33] and, as such, is not discussed further.
Fig. 2. Pressure-time histories at various measurement points. 3
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Table 1, Helmholtz oscillation occurred at a frequency of between 20 and 40 Hz after venting and lasted for about 300–500 ms: this was consistent with flame propagation velocities observed during venting. In other words, oscillations disappeared after multiple reciprocating motions of the flame. In addition, it could be obtained from Table 1 that, as the methane equivalence ratio changed from 0.74 to 1.37, the frequency of Helmholtz oscillation first increased, then decreased, which was consistent with the trend in combustion velocity of the flame. This verified the previous analysis once again. In terms of acoustic oscillation, differing from the frequency of 700 Hz (Bauwens [37]), data in Table 1 show that oscillation frequency was about 300 Hz and oscillation time was about 1000 ms. Oscillation at such a frequency had similar propagation velocity to acoustic waves, which indicated that the phenomenon was mainly caused by wave effects. In the late stage of a vented explosion, unburned gases remained in the space were burned to produce combustion waves which were coupled with acoustic oscillation induced by vibration of the structure, thus leading to the high frequency. The methane remaining in the chamber burned for a long time in the later stage of the vented explosion, so acoustic oscillations lasted for a long time. Bao [13] changed the reflected path of acoustic waves by adding asbestos shingles to the inner wall of the structure, which inhibited the development of acoustic oscillations. Some researchers considered that although acoustic oscillation, such a high-frequency oscillation, shows a high peak amplitude, its duration is very short, so the resulting impulse is small and the damage caused thereby to building structures can be ignored [9,10].
Fig. 3. Comparison of pressure-time histories under different methane equivalence ratios.
3.2. Internal flame development Flame development in the chamber was studied from the observation window and analysed from two aspects: flame colour and flame shape. By taking the moment when forming a spherical flame as the start time, the flame development process in the chamber is illustrated in Fig. 5. The analysis started from an assessment of the colour changes in the flame within the chamber. A flame is a high-temperature gas or ionic substance in nature and its colour is mainly determined by factors, such as temperature and the intrinsic spectrum of ionic elements. The flame was blue at the beginning, mainly because methane reacted with oxygen in the initial stage of combustion. Some carbon monoxide was produced because methane cannot be burned completely under oxygendeficient conditions. Moreover, carbon monoxide reacted fully with oxygen again in the flame tail to produce a large number of OH and CH free radicals. These free radicals were light blue, so a bluish-green flame was generated (Fig. 5(A and B)). With the development of the spherical flame, the oxygen inside the fireball was largely exhausted, and the unburned gas remaining in the fireball was partially burned with a small amount of residual air. Incomplete combustion of methane generated soot and soot continued to burn to produce yellow light, resulting in yellow patches inside the flame (Fig. 5C). Different flame colours corresponded to different temperatures, and the distribution of the flame colour roughly followed the trend whereby the higher the
Fig. 4. Stages in the pressure development process (φ = 1.16).
after venting [38,39]. From the perspective of oscillation frequency, we analysed two oscillations for which all characteristic parameters are listed in Table 1. It was found that the fundamental parameter distinguishing the two oscillations was the oscillation frequency. By conducting a venting experiment in a tube in the laboratory, Guo [11] studied flame profiles through the use of a high-speed camera. He found that Helmholtz oscillation is accompanied by reciprocating motion of the flame and interpreted this as the effects of R-T and K-H instability thereon. By utilising a high-speed camera to image microscopic flame shapes during venting, McCann [38] found that Helmholtz oscillation is mainly correlated with propagation velocity and shape of the flame. As shown in Table 1 Statistics related to pressure oscillation parameters. Methane concentration (vol.%)
7 8 9 10 11 12 13
Methane equivalence ratio (φ)
0.74 0.84 0.95 1.05 1.16 1.26 1.37
Helmholtz oscillation
Acoustic oscillation
Frequency (Hz)
Duration (ms)
Frequency (Hz)
Duration (ms)
24 26 32 39 41 38 35
520 311 344 320 366 286 300
– – 250 396 378 294 –
– – 1,295 1,160 1,227 956 –
4
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Fig. 5. Development of the flame front (φ = 1.37).
temperature, the longer the wavelength and the bluer the flame. After bursting the vent cover, much of the gas carrying heat energy escaped, which reduced the internal pressure and temperature, so the blue flame gradually changed into a yellow flame (Fig. 5(D–G)). The venting distorted the flame front and the air outside entered the chamber to react with the remaining methane. In the late stage of the reaction, many soot particles were generated due to incomplete reaction of residual methane in the chamber, so the flame became yellow and white [22] (Fig. 5(H)). In the later stage of the reaction, a large amount of heat energy was dissipated and the internal temperature of the space decreased, rendering the flame orange-red (Fig. 5(I)). It is worth noting that the analysis above is based on the colour changes of the flame, and GC-Analyse is needed to get the definite result. Based on the folding of the lame front, the influence of turbulence disturbance on flame development was studied. At the beginning of flame development, the flame propagated in the form of a regular hemisphere (Fig. 5(A)). In the process of initial flame development, cellular structures would still be generated even without disturbance (Fig. 5(B)), which resulted from hydrodynamic instability and diffusivethermal instability of the flame [40]. Upon flame propagation, the burned gases expanded, which spontaneously led to disturbance of the flame front, rendering it equivalent to a plane vortex, showing antigradient diffusion. This led to larger curvature of the curved part and generated a pressure gradient. Furthermore, stable honeycomb structures of regular size and shape were generated on the flame front. The flame underwent molecular diffusion and heat diffusion, and the
difference between the molecular conductivity and the molecular diffusivity of reactants resulted in diffusive-thermal instability which was determined by the Lewis number Le. When Le < 1, diffusive-thermal instability and hydrodynamic instability reinforced each other and the disturbance of the flame front increased, so that an irregular honeycomb flame was generated on the surface. The pressure wave generated by the explosion was reflected when it encountered the wall. The reflected pressure wave caused the pressure gradient of the flame front to no longer be parallel to the density gradient, which generated a baroclinic effect and induced turbulence, thus generating a Rayleigh–Taylor instability (Fig. 5(C)). After bursting the vent cover, a flame was ejected (Fig. 5(D)–(G)). In the process of flame outflow, violent disturbance was generated and the flame front was distorted. When the low-density combustion products flowed towards, and into, the denser air, a Rayleigh–Taylor instability was generated, and K-H instability could also occur in continuous fluid with shear force and a fluid interface across which a velocity difference existed. These two types of instability exerted significant influences on the flame during venting [41]. In the later stage of venting, the spherical flame in the chamber burst and most of the flame and combustion products flowed out of the space, so only the remaining unburned gases were left in the space for combustion (Fig. 5(H) and (I)).
3.3. Initial turbulence effects The initial turbulence in the chamber affected these gas explosions:
Table 2 Test conditions. Turbulence level: (static hysteresis time)
Strong turbulence Medium turbulence Weak turbulence
No temporal stasis Temporal stasis for 1 min Temporal stasis for 5 min
Ignition
Fan blowing time
resistance wire
3 min
5
Methane equivalence ratio (φ) 0.74
1.05
1.37
S1 M1 W1
S2 M2 W1
S3 M3 W3
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velocity, the ignition head was replaced with a resistance wire with a length of 20 mm. Taking this as a reference, the flame propagation distance was calculated. Under different equivalence ratios, the most obvious pictures of flame folds under strong initial turbulence is shown in Fig. 7: at a methane equivalence ratio of 0.74, the flame was lighter in colour and it was difficult to observe the outline of the light blue flame. The higher the methane concentration, the clearer the outline of the flame and the more obvious the folding of the flame front. To facilitate observation and analysis, by taking a methane equivalence ratio of 1.37 as an example, the flame development at different initial turbulence levels was analysed (Fig. 8). As demonstrated therein, the higher the level of turbulence, the faster the flame propagation velocity and the more obvious the cellular structure. Under strong initial turbulence, the internal flow field was strongly disturbed and the flame front distorted. Moreover, a cellular structure was produced and the flame propagation velocity increased markedly. Under medium and weak turbulence conditions, it can be observed that changes in flame propagation were similar, indicating that turbulence decreased with time. Once static hysteresis occurred, irregular motion of internal gas decreased and turbulence (and the allied disturbance) diminished. By observing flame propagation under medium and weak levels of turbulence, a buoyancy effect was observed. In the experiment, the resistance wire was heated during the explosion, and heat was released continuously. After static hysteresis, the turbulence diminished, making the buoyancy effect more obvious. The elevation of heat under buoyancy effects resulted in a lower air density above the resistance wire, so the flame propagated faster in the vertical direction under buoyancy effects and showed a teardrop shape. Fig. 9 shows flame propagation distances over time at methane equivalence ratio of 1.05 and 1.37. Since strong initial turbulence increased the rate of folding of the flame, and accelerated both combustion rate and flame propagation by enlarging the reaction area, the flame propagated fastest in these conditions. The internal turbulence decreased rapidly and the allied disturbance was rapidly attenuated with increasing static hysteresis time. Through observation, flames under weak and medium initial levels of turbulence propagated at a constant velocity, while flame propagation showed a power function relationship with time under strong turbulence. The burning velocity of methane-air flames has been extensively studied [43–45]. In numerical calculation, turbulence factor was generally introduced to express the acceleration effects caused by turbulence and the flame propagation velocity was obtained as follows [46]:
to study the influence of initial turbulence on explosion characteristics, it was necessary to control the turbulence in the chamber before ignition. Limited by experimental conditions, it was impossible to measure the turbulence within, therefore, the most commonly used method, that is, changing static hysteresis time between turning off the fan and ignition [28,42], was utilised to adjust the initial level turbulence on a qualitative basis. After the gas distribution was completed, the fan was turned on to mix the internal mixture and increase the internal turbulence disturbance. After blowing for three minutes, the fan was turned off. By selecting three static hysteresis times (0, 1, and 5 min) between turning off the fan and ignition, strong, medium, and weak turbulence in the space were qualitatively characterised (Table 2). At different methane equivalence ratios, the effects of initial turbulence on pressure curves are as shown in Fig. 6: the higher the initial level of turbulence, the greater the measured peak pressure and the shorter the rise-time thereto. The main reason for this was that stronger initially turbulent, the greater the increased rate of stretching of the flame. The increase in the energy release rate and flame propagation velocity was translated into a higher peak pressure and faster rate of pressure rise. By comparing Fig. 6(a), (b), and (c), it was found that methane concentration and turbulence level had synergistic effects on overpressure and the turbulence had different effects under different methane concentrations. To be specific, the higher the concentration of combustible gas, the greater the enhancement of turbulence on the peak overpressure and the stronger the response of pressure to turbulence. This may be due to the fact that the higher the methane concentration, the more combustible gases participated in the reaction under the disturbance caused by the initial turbulence, which led to greater increases of peak pressure. An interesting phenomenon is demonstrated in Fig. 6: strong initial turbulence could stimulate acoustic oscillation. By comparing Fig. 6(a), (b), and (c), Helmholtz oscillation occurred under all experimental conditions, however, when the methane equivalence ratio were 0.74 and 1.37, acoustic oscillation occurred after ending Helmholtz oscillation only under conditions of strong initial turbulence. It was considered that when the initial turbulence was weak, and the concentration approached the upper and lower limits of combustion, the reaction intensity of the methane remaining was weak in the later stage of the reaction, thus it failed to stimulate acoustic instability: however, when the initial turbulence was strong, the turbulence could disturb the internal flow field and increase the rate of stretching of the flame. With the increase of the stretching rate and the combustion rate, the D-L instability and diffusional-thermal instability of flame were intensified and the aforementioned cellular structure appeared on the flame front. This promoted the remaining unburned gas to participate in the reaction and increased the rate of energy release, thus stimulating acoustic oscillation effects. This section describes our study of the effects of initial turbulence on internal flame development. To quantify the flame propagation
(a)
=0.74
S = Kt ·K d·σ·Sl where, Kt , K d , σ , and Sl indicate the accelerating action attributed to environmental turbulence, the self-accelerating action of the flame, the expansion rate during combustion, and the laminar propagation velocity of the flame, respectively. At the same methane concentration, laminar burning velocity Sl ,
(b)
=1.05
Fig. 6. Pressure-time-history under different initial levels of turbulence. 6
(c)
=1.37
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Fig. 7. Comparison of flames under different methane equivalence ratios.
Fig. 8. Flame development under different initial levels of turbulence (φ = 1.37).
behaviour whereby flame propagation had a power function relationship with time under strong turbulence. With increasing propagation distance (Fig. 10), the flame front area increased and more folds were present on the surface. The turbulent disturbance accelerated the rate of reaction by increasing the number of folds of the flame front, so turbulence factor Kt increased with propagation distance.
expansion rate σ , and self-accelerating factor K d were similar and the external environment mainly influenced the flame propagation by changing Kt . By using the ratio of propagation velocities under strong and weak turbulence to separate the turbulence factor Kt induced by the environment, the relationship between turbulence factor and propagation distance was obtained (Fig. 10). In Fig. 10, the turbulent factor did not remain the same, but increased with increasing propagation distance. The turbulence factor Kt showed a power function relationship with propagation distance. This was consistent with the observed
(a)
=1.05
(b)
Fig. 9. Flame propagation distances under different levels of turbulence. 7
=1.37
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environment also presented a power function relationship with distance. Funding This work was supported by the National Key Research and Development Program of China [No: 2017YFC0804702]. CRediT authorship contribution statement Song Sun: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Yanyu Qiu: Conceptualization, Resources. Huadao Xing: Methodology, Validation, Investigation. Mingyang Wang: Resources, Writing - review & editing, Supervision. 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.
Fig. 10. Fitted curves relating turbulent factor to propagation distance.
4. Conclusion
References
The testing of vented explosions of methane–air mixtures was conducted in an explosion chamber with a volume of 4.5 m3, so as to study effects of methane concentration and initial turbulence level on pressure characteristics and flame development during a vented explosion. The conclusions were as follows:
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(1) There was a quasi-static pressure in the space during vented explosion. As the methane equivalence ratio increased from 0.74 to 1.05 and finally to 1.37, the peak overpressure first increased, then decreased. (2) Under all experimental concentrations, peak pressure P1 was generated after breakdown of the vent cover, accompanied by Helmholtz oscillation. The oscillation lasted for about 300–500 ms at a frequency of about 20–40 Hz, which was mainly because R-T instability and K-H instability (after venting) resulted in reciprocating oscillation of the flame. Only when the concentration approached the optimum value could acoustic oscillation occur for about 1000 ms at a frequency of 300 Hz. This was mainly because the pressure wave produced by combustion was coupled with acoustic oscillation of the chamber itself. (3) In the early stage of flame combustion, because hydrodynamic instability and diffusive-thermal instability could lead to the formation of a cellular structure, many OH and CH free radicals were generated by flame combustion and the flame appeared light blue. Due to incomplete combustion, yellow patches were seen in the interior. After venting, the flame flowed out and the resulting R-T and K-H instabilities disturbed the flame. In this case, a large amount of heat energy leaked from the system, accompanied by external air entering the chamber to trigger a further reaction, so that soot was produced and the flame changed from blue to yellowish-white and finally to orange-red in colour. (4) The increase in initial level of turbulence could significantly increase the peak pressure and stimulate acoustic oscillation at the upper and lower limits of combustion concentration. The experiment demonstrated that the higher the methane concentration, the stronger the disturbing effect of initial turbulence on the flame. Moreover, the folding of the flame front was more obvious and the enhancement of turbulence on overpressure was more significant. Initial turbulence reduced quickly with increasing static hysteresis time. Under strong initial turbulence, flame propagation distances showed a power function relationship with time, while a linear relationship was found between them under weak initial turbulence. In addition, the turbulent factor caused by the initial 8
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