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Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results
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Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results M. Schiavetti*, M. Carcassi University of Pisa, Department of Civil and Industrial Engineering (DICI), Largo Lucio Lazzarino, 2, 56126, Pisa, Italy
article info
abstract
Article history:
Explosion relief panels or doors are often used in industrial buildings to reduce damages
Received 28 January 2016
caused by gas explosions. Decades of research have contributed to the understanding of
Accepted 29 March 2016
the phenomena involved in gas explosions in order to establish an effective method to
Available online xxx
predict reliably the explosion overpressure. All the methods predict a monotonic increase of the overpressure with the concentration of the gas in the range from the lower explosion
Keywords:
limit to the stoichiometric one. Nevertheless, in few cases, a non-monotonic behaviour of
Hydrogen
the maximum developed pressure as a function of hydrogen concentration was reported in
Vent
the literature. The non-monotonic behaviour was also observed during experimental tests
Deflagration
performed at the Scalbatraio laboratory at the University of Pisa, in a 25 m3 vented combustion test facility, with a vent area of 1,12 m2. This paper presents the results obtained during the tests and investigates the possible explanations of the phenomena. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A deflagration involves essentially an unsteady premixed flame front that develops from an ignition source and travels through a medium which may involve complex boundary conditions and obstruction of various geometries, generating an overpressure that can cause damages to personnel and structures. Venting is normally the less expensive protection method against deflagration. Finding the appropriate design of the vent area is the key problem. Providing correct estimates of the overpressures which may result from various deflagration scenarios, is therefore critical to the assessment of the plant's safety. The study of confined vented deflagrations is very complex, as many parameters affect the phenomena, i.e.:
inhomogeneous concentration of the gas in the environment, volume's geometry, presence of obstacles within the environment, location, size and strength of the vent, position of the ignition source, pre-ignition turbulence, etc. In hydrogen deflagrations, the turbulence created by the venting process and/ or induced by the flow of unburnt fuel over and around obstacles has been generally acknowledged as being a major factor in the development of explosion overpressure, nevertheless the literature reports poor number of experiments with quantitative measurements of the phenomena [1e4,8,11]. Nevertheless, extensive research performed in the previous century has produced a significant contribution to the understanding of the vented gas explosions in empty rooms. Harris [13] showed that the pressure as function of time can be described in terms of four distinct pressure peaks which can,
* Corresponding author. E-mail address: [email protected] (M. Schiavetti). http://dx.doi.org/10.1016/j.ijhydene.2016.03.180 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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but do not necessarily have to, occur (four peak model). Each peak is produced by different physical processes at successive stages during a vented explosion; the four peaks are (see Fig. 3): 1. P1: The first peak is associated with the pressure drop following the removal of the explosion relief panel and the subsequent venting of the unburned gases. Before the vent opens, the pressure increase is caused by the production of hot combustion products generated by the flame front travelling at the flame speed Sf. The pressure in the room is equalized by compression waves travelling at sound speed and reflecting from the walls of the room, thus, at any moment the internal pressure is the same throughout the room. When the vent is removed, the pressure drops, generating the first peak (P1) and provoking the venting of the unburned gases and hence inducing a flow field inside the vented volume. The first peak is often followed by Helmholtz oscillations that eventually decay as the flame continues to expand. 2. P2: The second peak occurs when the flame front reaches the vent. If the vent opens early, the flame front radius rf continues to increase during a significant time after the first peak. The rate of combustion products generated, increased by the interaction with the turbulent flow field caused by the vent opening, become soon larger than the outflow rate, and, consequently, the internal pressure starts to rise. The pressure rises until the flame front reaches the vent. When the vent is fully open the flow of gases can be calculated by the formula [13]: 1 dV 2DP 2 ¼ Cd $Av $ dt r Thus a first explanation of the second pressure peak suggested that when the hot combustion products start to flow out of the vent, their density being the density of the unburned mixture divided by the expansion factor E, which is defined as the ratio between the specific volume of the burned and unburned gases, the outflow rate is suddenly increased by a factor E1/2 and the outflow rate becomes again larger than the rate of combustion products generation, hence resulting in the second pressure peak. However the second pressure peak may also be caused, as stated by many authors, by the external explosion caused by the ignition of the previously vented unburned mixture by the flame emerging from the vent. The unburned mixture released through the vent forms a non-fully developed turbulent momentum jet, with a vortex ring at its head. Consequently when the flame front emerges from the vent it proceeds to propagate in a highly turbulent flow triggering the so called external explosion which creates a back pressure at the vent that results in an higher internal peak pressure. 3. P3: The third pressure peak is caused by the reduction of the flame front area towards the end of the explosion when it reaches the walls, the flame front area has been increasing constantly until this moment, the flame front continues to propagate in the isolated pockets of unburned mixture while its area is decreasing. The third peak, caused by the decrease of the combustion products generation,
usually is not the dominant one (the tests performed in the CVE facility with hydrogen, generated this peak only at very lean concentrations, typically less than 9%). 4. P4: The fourth pressure peak is an oscillatory pressure peak attributed to the interaction of the flame front with the acoustic waves, generated within the room. It appears that this peak is controlled by the resonant coupling between the flame and acoustic waves which can be related to the physical response of the enclosure. The pressure oscillations induce very high combustion rates and produce a significant net overpressure within the vented volume. The fourth pressure peak, as well as the third, reaches its maximum amplitude when the flame reaches the walls. It may be the case that the third and the fourth peaks are generated by the same cause, i.e. the flame reaching its maximum area, with the only difference being the interaction of the flame front with acoustic oscillations in the fourth peak. Based on the results obtained by experiments, usually performed without internal obstacles, a large number of equations have been derived [5e7], whose predictive ability has been compared [6,8]. However correspondence between predicted pressure history and experimental results still owes more to adjustable model parameters than to an exact understanding of the physics involved. The CVE (Chambre View Explosion) experimental facility [9] was built at the “Scalbatraio” laboratory owned by DICI department of University of Pisa to study the confined vented explosion phenomena in real environments. Several experimental campaigns of vented deflagration were performed with the presence of internal obstacles of various shapes in an attempt to collect useful experimental data for the phenomena considered [3,12]. In this paper, some of the results were reviewed, focussing the attention to the non-monotonic behaviour of the maximum peak pressure recorded inside the enclosure as a function of the H2concentration.
Experimental set-up The CVE (Chamber View Explosion) experimental facility has a nearly cubic structure characterized by an internal volume of about 25 m3; the roof and one side face are entirely covered with glass panels which allows video recording of the flame. All other sides are covered with steel panels having different functions. The bottom and one side face are entirely made of steel strengthened panels which are not removable, while the other two lateral faces, on opposite sides, are the test vent and the safety vent respectively [3,12]. CVE has been built in order to perform vented deflagration tests. The design pressure of the test facility is 350 mbar (35 kPa), while the safety vent is designed to open at 300 mbar (30 kPa), which determines the maximum allowed internal pressure's peak [9]. In the experimental campaigns, whose results are presented in this paper, the vent dimensions were kept fixed and characterized by a width of 0.62 m and a height of 1.62 m (vent area 1.004 m2). The vent area was closed with a plastic sheet characterized by an opening pressure of approximately 2.4 kPa.
Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Inside the CVE test facility different kind of obstacles were placed (see Fig. 1(b)). The hydrogen concentration under investigation ranged from 7% vol. to 13% vol. During the release of hydrogen inside the enclosure the concentration was homogenized through the use of a fan. The fan was turned off at the end of the release conveniently earlier with respect to the ignition in order to prevent initial turbulence inside the vented volume. Despite the use of the fan, hydrogen concentration showed a stratification behaviour inside the facility, with lower values at the bottom and higher concentration under the ceiling. The difference of measured concentrations between the lower and the upper sampling points was about 1.5% vol. in every test. The results presented in this paper are the average of the measured values sampled at 5 different heights between the floor and the ceiling. For all the tests analysed in this paper the ignition was set in the middle of the CVE's wall opposite the vent at 1 m high from the floor. The trigger was connected to a remote driven circuit and designed to prevent accidental sparks. Pressure transducers were placed in the centre of the wall opposite the vent and in the centre of the wall opposite the glass wall. Pressure readings were recorded at frequency of 5 kHz.
Results and discussion Results in terms of maximum overpressures measured inside the vented volume are listed in Table 1 for 92 selected tests. Fig. 2 shows the maximum peak overpressure attained in the CVE test facility as a function of average hydrogen concentration for all the tests included in this paper. As shown in Fig. 2, the peak overpressure does not exceed the vent opening pressure for concentrations lower than 10% vol.
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For all the tests performed, except the ones with configuration n.8 (see Fig. 1), the maximum peak pressure developed inside the enclosure exhibits non-monotonic behaviour in the range of concentrations under investigation. The maximum is reached in the range between 11% vol. to 11.5% vol. of hydrogen concentration, then decreases for concentrations higher than 11.5% vol. Beyond 12% vol. H2, peak pressure increases again monotonically with hydrogen concentration. A similar result was described by Kumar [10] during tests of vented hydrogen deflagration in a rectangular 120 m3 facility. Kumar found that with far vent ignition the peak pressure increased at first with increasing hydrogen concentration, reached a peak at 9% vol., and then decreased. Beyond 10% vol. H2, peak pressure increases again monotonically with hydrogen concentration. A higher maximum peak pressure was found in most cases with obstacles as compared to the case with the chamber empty (blue (in the web version) squares in Fig. 2). The cases where the maximum pressure attained in the empty chamber was higher or comparable to the ones obtained with obstacles were with H2 concentration ranging from 11% to 11.5% which is the range of concentrations at which the non-monotonic maximum peak pressure behaviour was recorded. The overpressure time history graph of each test performed showed the characteristics of the gas explosion theory summarized in the introduction. Fig. 3 shows the unfiltered pressure time history of six different tests performed at increasing hydrogen concentrations. These tests were selected as an example, in order to discuss the main physical effects responsible for the pressure build-up in the vented volume. In all tests, the first peak (P1) corresponds to the deployment of the vent's plastic sheet. Before the vent opens, the
Fig. 1 e (a) Photo of the CVE test facility and (b) internal obstacle configurations. Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Table 1 e List of the experimental tests. #
Test ID
Obstacle config.
H2 conc. [%vol.]
Maximum overpressure [kPa]a
#
Test ID
Obstacle config.
H2 average conc. [%vol.]
Maximum overpressure [kPa]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46
SM02 SM03 SM07 SM08 SM09 SM10 SM11 SM12 SM13 SM14 SM15 SM16 SM17 SM18 SM19 SM20 SM21 SM22 SM23 SM24 SM25 SM26 SM27 SM28 SM29 SM30 SM34 SM35 SM36 SM37 SM38 SM39 SM43 SM44 SM45 SM46 SM47 SM48 SM49 SM50 SM51 SM52 SM53 SM54 SM55 SM56
1 1 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 2 2 2 2 2 2 2 2 3 4 4 4 4 4 5 5 5 5 6 6 6 6 8 8 8 8 2 2
7.4 10.4 7.5 7.4 8.4 8.5 8.8 9.4 8.2 8.4 9.3 10.4 11.2 11.2 12.2 12.3 12.8 12.8 10.7 10.5 11.9 11.9 11.5 11.6 12.4 12.6 7.5 9.5 10.5 11.5 12.6 9.5 9.4 10.4 11.3 12.5 9.5 10.6 11.5 12.2 9.5 11 11.4 12.3 10.3 11
Vent Vent Vent Vent Vent Vent Vent Vent Vent Vent Vent 5 19.4 19.8 11.1 12.2 12.9 9.8 11.7 5.5 12.5 6.9 10.4 6.7 6.9 7.3 Vent Vent 7.7 10.2 9.0 Vent Vent 5.6 12.0 12.7 Vent 12.0 20.5 19.3 Vent 15.4 20.7 33.7 6.4 23.2
47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92
SM57 SM58 SM59 SM60 ISP25 ISP26 RED1 RED2 RED3 RED4 RED5 RED6 RED7 RED8 RED9 RED10 RED11 RED12 RED13 RED14 RED15 RED16 RED17 RED18 RED19 RED20 RED21 RED22 RED29 RED33 RED36 RED37 RED38 RED39 RED40 RED41 RED42 RED43 RED44 RED45 RED46 RED47 RED48 RED49 RED50 RED51
2 4 4 3 4 4 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 1 1 1 1 1 1 1 1
12.4 10.5 12.5 12.1 11.4 12.4 10.4 10.5 10.5 11.6 11.3 11.2 11 10.5 10 10.5 10.1 10.5 11.1 11.1 9.7 9.6 8.6 10.6 10 10 11.1 11.6 12 12 11 10.6 11.2 11.4 10.4 11.4 11.4 11.4 10.8 11.3 11.4 12.3 10.8 11 12.8 10.2
12 8 12.6 17 12.2 8.6 Vent 3.2 3 6 5.4 12.5 12.6 8 Vent 3.4 Vent Vent 12 11.8 Vent Vent Vent 5 Vent Vent 8.5 5.7 6.3 6.3 12.4 Vent 12.6 15 3.5 12.1 12.5 11.3 6 16.3 15 4 4 10 6 2.6
a
Vent opening pressure is about 2.4 kPa but can slightly vary in every tests and as a general rule is slightly lower for tests at higher concentration.
increase of pressure is caused by the production of hot combustion products within the room; this production depends on the laminar flame velocity and hence on hydrogen concentration. The time of vent deployment decreases with increasing hydrogen concentration, reaching an asymptote at 0.245 s after the ignition at concentrations from approximately 12% and higher (see Fig. 4). In tests characterized by very lean hydrogen concentrations, i.e. less than 9% vol., a second minor peak is present just after the first peak (see Fig. 3(a)). Under these conditions, when the vent is deployed the flame front has already travelled a
considerable distance from the point where the mixture is ignited, since the event occurs several seconds after the ignition. Hence, the turbulent flow field generated by the deployment of the vent suddenly affects the flame front, causing a fast increase of pressure as the flame approaches the vent area. The pressure time history recorded from the transducer opposite the vent (see Fig. 3(a)) shows a pressure drop after the flame front reaches the vent area (P2), at the same time the pressure recorded from the transducer opposite the glass wall continued to increase. In fact at this moment, the combustion products occupy a sort of cone which extends from the
Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Fig. 2 e Maximum overpressure's peak attained inside the CVE test facility vs. average hydrogen concentration for all the test performed.
opposite wall (were the sensor is located) to the vent area and the unburned mixture occupies only the sides and the corners of the vented volume, so the flow of the burned mixture towards the vent produces the slight decrease of pressure at the wall opposite the vent area. Also, the slight difference of readings between the two pressure transducers, some kPa, has been observed at the end of the explosion in every test. For the tests performed at higher concentrations (Fig. 3(b)e(f)), whose pressure time history are showed in Fig. 3 and where pressure oscillations are involved, only one pressure reading is plotted to provide a more clear view. Tests with very lean hydrogen concentration (i.e. under 9% vol., for example see Fig. 3(a)) show a third peak (P3) which was described in the introduction. This can be explained as follows: after the flame front reaches the vent, the flame area continues to increase for a period of time in the rest of the chamber due to the low burning velocity of the mixture in direction perpendicular to the vent The reduction of the flame front area towards the end of the explosion, as it reaches the walls and burns in the isolated pockets of unburned mixture, generates the third peak (P3). In other words the third peak P3 is caused by the decrease of the combustion products generation. For tests at higher concentrations (Fig. 3(b)e(f)), this peak is substituted by the oscillatory pressure peak (P4). In tests were the vent opens when the flame front is not close to the vent area, Helmholtz oscillations were observed after the vent deployment. These oscillations eventually decay as the flame continues to expand. After the vent opens, a flow field is introduced into the vented volume which generates turbulence that in turn interacts with the flame front as soon as it approaches the vent
opening. The flame radius keeps increasing and eventually the flame stretches, driven by the turbulent flow of unburned gases carried out through the vent area (see Fig. 5). In tests were H2 concentration exceeds 11% vol., the rate of combustion products generated becomes soon larger than the outflow rate, and, consequently, the internal pressure starts to rise dumping in the early stage Helmholtz oscillations generated by the vent deployment (see Fig. 3(e) and (f)). The pressure rises until the flame front reaches the vent, then due to the difference of density of the vented gases, or to an external explosion, a second pressure peak is generated (P2). Concerning the second peak, in tests involving lean hydrogen mixtures as the ones performed in the present experimental tests, in the opinion of the author, the possibility that this may be generated by the external explosion is unlikely, since the turbulent flow of unburned gases exiting the vent should enhance the mixing with external air reducing the already lean H2 concentration outside the vent. Moreover, when the flame front approaches the vent area, pressure oscillations are triggered inside the vented volume. According to Harris formula, the volumetric flow rate through the vent area has a sudden increase when the burned gases start to flow out of the enclosure. This discontinuity may then be responsible of generating the physical response of the chamber as well as affecting the burning behaviour of the expanding flame front making it more susceptible to interactions with acoustic oscillations generated inside the vented volume. In fact, before reaching the vent area, the flammable envelope is accelerated by the production of combustion products. When the flame reaches the vent area and the combustion products start to flow out of
Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Fig. 3 e Pressure time history observed in some of the tests.
the vent their productions does not contribute to the acceleration of the flame front as they were doing before, so the flame velocity may drop. The conditions may become more close to the one of a static flame which most likely may interact with acoustic waves, as extensively studied in the literature. The measured oscillations have a variable frequency, which tends to increase as the deflagration progresses, since the vented volume is progressively filled with combustion products having higher temperature and lower density
compared to the unburned mixture. These oscillations, generated by the physical response of the enclosure [14,15], provoke a resonant coupling with the combustion inside the vented volume, giving rise to very high combustion rates and creating a significant net overpressure within the enclosure. The coupling of the two phenomena seems to generate stronger effects when the flame approaches the walls of the enclosure, in fact the amplitude of the oscillation was observed to increase by two orders of magnitude as the flame approaches the walls.
Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Fig. 4 e Time of vent deployment after the ignition.
Fig. 5 e Photogram of the video recorded during a test.
In respect to the observed non-monotonic behaviour of the maximum overpressure, the following scenario is suggested: pressure oscillations are triggered as the flame reaches the vent area and the density of flowing gases through it, suffers a discontinuity, at the first stage acoustic waves are only superimposed to the flame front and the interaction is weak. When the flame front approaches the walls the interaction increases and so does the amplitude of the pressure oscillations generating a positive overpressure. If the flame front reaches the vent area generating the second peak (P2) and triggering the oscillations when the flame front in radial direction is far from the walls, then the oscillations than the interaction between the flame front and the acoustic waves becomes strong enough to affect the pressure after a delay of time, generating a distinctive peak (P4) that occurs when the
effect of the overpressure generated by the second peak is already decreased (see Fig. 3(b)e(d), (f)). Instead, when the flame front reaches the vent while in radial direction is close to the walls, the oscillatory peak may completely overlap with the second peak generating a higher pressure (see Fig. 3(e)). In tests performed at H2 concentrations between 11% and 11.5% vol. the two peaks were recorded very close to each other, hence, producing a higher overpressure inside the facility. In tests were the H2 concentration is higher than 12% vol., the flame front expands faster in radial direction and reaches the walls before the oscillations are triggered. In this case, more than one oscillatory peak may be observed, each one representing an unburned pocket of gas reaching the walls. Hence, a possible explanation to the non-monotonic behaviour observed in the tests is the superimposition of the two peaks generated by the flame reaching the vent area (P2) and by the resonant coupling between the flame and the acoustic waves (P4) respectively. As reported in the literature by some authors, the presence of the obstacles may reduce the effect of the interaction between the flame front and the oscillations produced during the deflagration. In fact, the maximum peak pressure was found to be higher for tests performed in the empty room with respect to the ones performed with obstacles, particularly at concentrations were the two peaks P2 and P4 overlap. This behaviour was not supported for all obstacle configurations. For example, in configuration 3 and 6 the maximum overpressure generated at H2 concentration around 11% vol. was comparable with the one generated inside the empty room. It should be underlined that for these tests the pressure build up attributed to the second peak is higher than the one obtained in the empty room or in other obstacle configurations, probably due to the fact that the flame is forced to travel towards the vent passing through the higher part of the facility were the concentration is higher (see Fig. 7). The acoustic flame
Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Fig. 6 e Maximum overpressure's peak attained inside the CVE test facility vs. average hydrogen concentration for 4 selected setups.
interaction generates a lower net overpressure if compared with the empty room case, even though the superimposition of the two effects gives similar results. The reduction of the effect of the interaction between the flame front and the oscillations was particularly effective in configuration 7 (see Fig. 6) were the obstacle had a surface facing the walls perpendicular to the vent. In the configuration # 8, the flame front undergoes a stronger acceleration approaching the vent due to the
obstacles configuration, with this acceleration being responsible for the higher second peak (P2) than the one attained in other configurations for the same H2 concentration. In this configuration, the non-monotonic behaviour was not confirmed by the results. In fact, the main contribution to the pressure generation is the acceleration of the flame front towards the vent at higher H2 concentration, overcoming the contribution of the overlap of the two peaks, which was observed during the tests (see Fig. 8).
Fig. 7 e Comparison of overpressure's peak attained inside the empty CVE test facility and in configuration 3. Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Fig. 8 e Comparison of overpressure's peak attained inside the CVE test facility in configuration 8.
Conclusions Experiments were performed at Scalbatraio laboratory, DICI Department of University of Pisa, in a 25 m3 vented combustion test facility that hosted internally different obstacle configurations, to study the combustion behaviour of hydrogen-air mixtures in vented environments. The test facility vent dimension was set to a value of 1,004 m2, while the ignition location was in the middle of the wall opposite the vent for all experiments. The study included hydrogen concentrations from 8.5% to 12.5%. A general description of the pressure-time history obtained during the tests was given following the four peak theory. Peak overpressure developed by far-vent ignition exhibits non-monotonic behaviour in the range of concentrations under investigation. It reached a maximum at 11% vol. of hydrogen concentration and then decreased. At higher than 11.5% vol. H2 concentrations, peak pressure increases again monotonically with hydrogen concentration. The same nonmonotonic behaviour was described by Kumar [10] during tests of vented deflagration in a rectangular 120 m3 facility, but the peak was reached at 9% vol. instead of 11% vol. In respect of the observed non-monotonic behaviour, the following scenario is derived: pressure oscillations are triggered as the flame reaches the vent area and the flow through it suffers a discontinuity. At the first stage, acoustic waves are only superimposed to the flame front and the interaction is weak. When the flame front approaches the walls, the interaction increases and so does the amplitude of the pressure oscillations which generate a positive overpressure. If the flame front reaches the vent area, thus producing the second peak and triggering the acoustic response of the chamber, at the time when the flammable envelope in radial direction is
still far from the walls, then the interaction with the flame front starts blandly and becomes high enough to affect the net overpressure, after a time delay, generating a distinctive peak. The oscillatory peak occurs when the effect of the overpressure generated by the second peak has already decreased. If the flame front reaches the vent when the flammable envelope is close to the wall then the interaction between the acoustic waves and the flame front immediately produce a net overpressure inside the enclosure and the two peaks overlap, being the superimposition dependent on H2 concentration, geometry of the enclosure, obstacle configuration and all the other variables that affect the time history of a deflagration. The non-monotonic behaviour was not observed in configuration 8. In this case, the turbulent flow field generated by the vent deployment provokes a stronger acceleration of the flame front. In fact, in this configuration the main contribution to the pressure generation is the turbulent combustion generated by the acceleration of the flame front towards the vent, which increases with H2 concentration, overcoming the contribution of the acoustic peak whose overlap was observed in the range from 11% to 11.5% vol. of H2 concentrations.
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[10] Kumar RK. Vented combustion of hydrogen-air mixtures in a large rectangular volume. In: 44th AIAA Aerospace Sciences Meeting and Exhibit, 9e12 January 2006, Reno, USA, Paper AIAA2006-375. [11] Carcassi MN, Fineschi F. A theoretical and experimental study on the hydrogen vented deflagration. Nucl Eng Des 1993;145:355e64. [12] Schiavetti M, Marangon A, Carcassi M. Experimental study of vented hydrogen deflagration with ignition inside and outside the vented volume. Int J Hydrogen Energy 2014;39(35):2046e55. December 2014. [13] Harris RJ. The investigation and control of gas explosion in buildings and heating plants. British Gas Corporation; 1983. [14] Bauwens CR, Dorofeev SB. Effect of initial turbulence on vented explosion overpressures from lean hydrogen-air deflagrations. Int J Hydrogen Energy December 2014;39:20509e15. [15] Tamanini F, Chaffee JL. Turbulent vented gas explosions with and without acoustically-induced instabilities. Proceeding Combust Inst 1992;24:1845. http://dx.doi.org/ 10.1016/S0082-0784(06)80216-X.
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Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results M. Schiavetti*, M. Carcassi University of Pisa, Department of Civil and Industrial Engineering (DICI), Largo Lucio Lazzarino, 2, 56126, Pisa, Italy
article info
abstract
Article history:
Explosion relief panels or doors are often used in industrial buildings to reduce damages
Received 28 January 2016
caused by gas explosions. Decades of research have contributed to the understanding of
Accepted 29 March 2016
the phenomena involved in gas explosions in order to establish an effective method to
Available online xxx
predict reliably the explosion overpressure. All the methods predict a monotonic increase of the overpressure with the concentration of the gas in the range from the lower explosion
Keywords:
limit to the stoichiometric one. Nevertheless, in few cases, a non-monotonic behaviour of
Hydrogen
the maximum developed pressure as a function of hydrogen concentration was reported in
Vent
the literature. The non-monotonic behaviour was also observed during experimental tests
Deflagration
performed at the Scalbatraio laboratory at the University of Pisa, in a 25 m3 vented combustion test facility, with a vent area of 1,12 m2. This paper presents the results obtained during the tests and investigates the possible explanations of the phenomena. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A deflagration involves essentially an unsteady premixed flame front that develops from an ignition source and travels through a medium which may involve complex boundary conditions and obstruction of various geometries, generating an overpressure that can cause damages to personnel and structures. Venting is normally the less expensive protection method against deflagration. Finding the appropriate design of the vent area is the key problem. Providing correct estimates of the overpressures which may result from various deflagration scenarios, is therefore critical to the assessment of the plant's safety. The study of confined vented deflagrations is very complex, as many parameters affect the phenomena, i.e.:
inhomogeneous concentration of the gas in the environment, volume's geometry, presence of obstacles within the environment, location, size and strength of the vent, position of the ignition source, pre-ignition turbulence, etc. In hydrogen deflagrations, the turbulence created by the venting process and/ or induced by the flow of unburnt fuel over and around obstacles has been generally acknowledged as being a major factor in the development of explosion overpressure, nevertheless the literature reports poor number of experiments with quantitative measurements of the phenomena [1e4,8,11]. Nevertheless, extensive research performed in the previous century has produced a significant contribution to the understanding of the vented gas explosions in empty rooms. Harris [13] showed that the pressure as function of time can be described in terms of four distinct pressure peaks which can,
* Corresponding author. E-mail address: [email protected] (M. Schiavetti). http://dx.doi.org/10.1016/j.ijhydene.2016.03.180 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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but do not necessarily have to, occur (four peak model). Each peak is produced by different physical processes at successive stages during a vented explosion; the four peaks are (see Fig. 3): 1. P1: The first peak is associated with the pressure drop following the removal of the explosion relief panel and the subsequent venting of the unburned gases. Before the vent opens, the pressure increase is caused by the production of hot combustion products generated by the flame front travelling at the flame speed Sf. The pressure in the room is equalized by compression waves travelling at sound speed and reflecting from the walls of the room, thus, at any moment the internal pressure is the same throughout the room. When the vent is removed, the pressure drops, generating the first peak (P1) and provoking the venting of the unburned gases and hence inducing a flow field inside the vented volume. The first peak is often followed by Helmholtz oscillations that eventually decay as the flame continues to expand. 2. P2: The second peak occurs when the flame front reaches the vent. If the vent opens early, the flame front radius rf continues to increase during a significant time after the first peak. The rate of combustion products generated, increased by the interaction with the turbulent flow field caused by the vent opening, become soon larger than the outflow rate, and, consequently, the internal pressure starts to rise. The pressure rises until the flame front reaches the vent. When the vent is fully open the flow of gases can be calculated by the formula [13]: 1 dV 2DP 2 ¼ Cd $Av $ dt r Thus a first explanation of the second pressure peak suggested that when the hot combustion products start to flow out of the vent, their density being the density of the unburned mixture divided by the expansion factor E, which is defined as the ratio between the specific volume of the burned and unburned gases, the outflow rate is suddenly increased by a factor E1/2 and the outflow rate becomes again larger than the rate of combustion products generation, hence resulting in the second pressure peak. However the second pressure peak may also be caused, as stated by many authors, by the external explosion caused by the ignition of the previously vented unburned mixture by the flame emerging from the vent. The unburned mixture released through the vent forms a non-fully developed turbulent momentum jet, with a vortex ring at its head. Consequently when the flame front emerges from the vent it proceeds to propagate in a highly turbulent flow triggering the so called external explosion which creates a back pressure at the vent that results in an higher internal peak pressure. 3. P3: The third pressure peak is caused by the reduction of the flame front area towards the end of the explosion when it reaches the walls, the flame front area has been increasing constantly until this moment, the flame front continues to propagate in the isolated pockets of unburned mixture while its area is decreasing. The third peak, caused by the decrease of the combustion products generation,
usually is not the dominant one (the tests performed in the CVE facility with hydrogen, generated this peak only at very lean concentrations, typically less than 9%). 4. P4: The fourth pressure peak is an oscillatory pressure peak attributed to the interaction of the flame front with the acoustic waves, generated within the room. It appears that this peak is controlled by the resonant coupling between the flame and acoustic waves which can be related to the physical response of the enclosure. The pressure oscillations induce very high combustion rates and produce a significant net overpressure within the vented volume. The fourth pressure peak, as well as the third, reaches its maximum amplitude when the flame reaches the walls. It may be the case that the third and the fourth peaks are generated by the same cause, i.e. the flame reaching its maximum area, with the only difference being the interaction of the flame front with acoustic oscillations in the fourth peak. Based on the results obtained by experiments, usually performed without internal obstacles, a large number of equations have been derived [5e7], whose predictive ability has been compared [6,8]. However correspondence between predicted pressure history and experimental results still owes more to adjustable model parameters than to an exact understanding of the physics involved. The CVE (Chambre View Explosion) experimental facility [9] was built at the “Scalbatraio” laboratory owned by DICI department of University of Pisa to study the confined vented explosion phenomena in real environments. Several experimental campaigns of vented deflagration were performed with the presence of internal obstacles of various shapes in an attempt to collect useful experimental data for the phenomena considered [3,12]. In this paper, some of the results were reviewed, focussing the attention to the non-monotonic behaviour of the maximum peak pressure recorded inside the enclosure as a function of the H2concentration.
Experimental set-up The CVE (Chamber View Explosion) experimental facility has a nearly cubic structure characterized by an internal volume of about 25 m3; the roof and one side face are entirely covered with glass panels which allows video recording of the flame. All other sides are covered with steel panels having different functions. The bottom and one side face are entirely made of steel strengthened panels which are not removable, while the other two lateral faces, on opposite sides, are the test vent and the safety vent respectively [3,12]. CVE has been built in order to perform vented deflagration tests. The design pressure of the test facility is 350 mbar (35 kPa), while the safety vent is designed to open at 300 mbar (30 kPa), which determines the maximum allowed internal pressure's peak [9]. In the experimental campaigns, whose results are presented in this paper, the vent dimensions were kept fixed and characterized by a width of 0.62 m and a height of 1.62 m (vent area 1.004 m2). The vent area was closed with a plastic sheet characterized by an opening pressure of approximately 2.4 kPa.
Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Inside the CVE test facility different kind of obstacles were placed (see Fig. 1(b)). The hydrogen concentration under investigation ranged from 7% vol. to 13% vol. During the release of hydrogen inside the enclosure the concentration was homogenized through the use of a fan. The fan was turned off at the end of the release conveniently earlier with respect to the ignition in order to prevent initial turbulence inside the vented volume. Despite the use of the fan, hydrogen concentration showed a stratification behaviour inside the facility, with lower values at the bottom and higher concentration under the ceiling. The difference of measured concentrations between the lower and the upper sampling points was about 1.5% vol. in every test. The results presented in this paper are the average of the measured values sampled at 5 different heights between the floor and the ceiling. For all the tests analysed in this paper the ignition was set in the middle of the CVE's wall opposite the vent at 1 m high from the floor. The trigger was connected to a remote driven circuit and designed to prevent accidental sparks. Pressure transducers were placed in the centre of the wall opposite the vent and in the centre of the wall opposite the glass wall. Pressure readings were recorded at frequency of 5 kHz.
Results and discussion Results in terms of maximum overpressures measured inside the vented volume are listed in Table 1 for 92 selected tests. Fig. 2 shows the maximum peak overpressure attained in the CVE test facility as a function of average hydrogen concentration for all the tests included in this paper. As shown in Fig. 2, the peak overpressure does not exceed the vent opening pressure for concentrations lower than 10% vol.
3
For all the tests performed, except the ones with configuration n.8 (see Fig. 1), the maximum peak pressure developed inside the enclosure exhibits non-monotonic behaviour in the range of concentrations under investigation. The maximum is reached in the range between 11% vol. to 11.5% vol. of hydrogen concentration, then decreases for concentrations higher than 11.5% vol. Beyond 12% vol. H2, peak pressure increases again monotonically with hydrogen concentration. A similar result was described by Kumar [10] during tests of vented hydrogen deflagration in a rectangular 120 m3 facility. Kumar found that with far vent ignition the peak pressure increased at first with increasing hydrogen concentration, reached a peak at 9% vol., and then decreased. Beyond 10% vol. H2, peak pressure increases again monotonically with hydrogen concentration. A higher maximum peak pressure was found in most cases with obstacles as compared to the case with the chamber empty (blue (in the web version) squares in Fig. 2). The cases where the maximum pressure attained in the empty chamber was higher or comparable to the ones obtained with obstacles were with H2 concentration ranging from 11% to 11.5% which is the range of concentrations at which the non-monotonic maximum peak pressure behaviour was recorded. The overpressure time history graph of each test performed showed the characteristics of the gas explosion theory summarized in the introduction. Fig. 3 shows the unfiltered pressure time history of six different tests performed at increasing hydrogen concentrations. These tests were selected as an example, in order to discuss the main physical effects responsible for the pressure build-up in the vented volume. In all tests, the first peak (P1) corresponds to the deployment of the vent's plastic sheet. Before the vent opens, the
Fig. 1 e (a) Photo of the CVE test facility and (b) internal obstacle configurations. Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Table 1 e List of the experimental tests. #
Test ID
Obstacle config.
H2 conc. [%vol.]
Maximum overpressure [kPa]a
#
Test ID
Obstacle config.
H2 average conc. [%vol.]
Maximum overpressure [kPa]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46
SM02 SM03 SM07 SM08 SM09 SM10 SM11 SM12 SM13 SM14 SM15 SM16 SM17 SM18 SM19 SM20 SM21 SM22 SM23 SM24 SM25 SM26 SM27 SM28 SM29 SM30 SM34 SM35 SM36 SM37 SM38 SM39 SM43 SM44 SM45 SM46 SM47 SM48 SM49 SM50 SM51 SM52 SM53 SM54 SM55 SM56
1 1 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 2 2 2 2 2 2 2 2 3 4 4 4 4 4 5 5 5 5 6 6 6 6 8 8 8 8 2 2
7.4 10.4 7.5 7.4 8.4 8.5 8.8 9.4 8.2 8.4 9.3 10.4 11.2 11.2 12.2 12.3 12.8 12.8 10.7 10.5 11.9 11.9 11.5 11.6 12.4 12.6 7.5 9.5 10.5 11.5 12.6 9.5 9.4 10.4 11.3 12.5 9.5 10.6 11.5 12.2 9.5 11 11.4 12.3 10.3 11
Vent Vent Vent Vent Vent Vent Vent Vent Vent Vent Vent 5 19.4 19.8 11.1 12.2 12.9 9.8 11.7 5.5 12.5 6.9 10.4 6.7 6.9 7.3 Vent Vent 7.7 10.2 9.0 Vent Vent 5.6 12.0 12.7 Vent 12.0 20.5 19.3 Vent 15.4 20.7 33.7 6.4 23.2
47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92
SM57 SM58 SM59 SM60 ISP25 ISP26 RED1 RED2 RED3 RED4 RED5 RED6 RED7 RED8 RED9 RED10 RED11 RED12 RED13 RED14 RED15 RED16 RED17 RED18 RED19 RED20 RED21 RED22 RED29 RED33 RED36 RED37 RED38 RED39 RED40 RED41 RED42 RED43 RED44 RED45 RED46 RED47 RED48 RED49 RED50 RED51
2 4 4 3 4 4 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 1 1 1 1 1 1 1 1
12.4 10.5 12.5 12.1 11.4 12.4 10.4 10.5 10.5 11.6 11.3 11.2 11 10.5 10 10.5 10.1 10.5 11.1 11.1 9.7 9.6 8.6 10.6 10 10 11.1 11.6 12 12 11 10.6 11.2 11.4 10.4 11.4 11.4 11.4 10.8 11.3 11.4 12.3 10.8 11 12.8 10.2
12 8 12.6 17 12.2 8.6 Vent 3.2 3 6 5.4 12.5 12.6 8 Vent 3.4 Vent Vent 12 11.8 Vent Vent Vent 5 Vent Vent 8.5 5.7 6.3 6.3 12.4 Vent 12.6 15 3.5 12.1 12.5 11.3 6 16.3 15 4 4 10 6 2.6
a
Vent opening pressure is about 2.4 kPa but can slightly vary in every tests and as a general rule is slightly lower for tests at higher concentration.
increase of pressure is caused by the production of hot combustion products within the room; this production depends on the laminar flame velocity and hence on hydrogen concentration. The time of vent deployment decreases with increasing hydrogen concentration, reaching an asymptote at 0.245 s after the ignition at concentrations from approximately 12% and higher (see Fig. 4). In tests characterized by very lean hydrogen concentrations, i.e. less than 9% vol., a second minor peak is present just after the first peak (see Fig. 3(a)). Under these conditions, when the vent is deployed the flame front has already travelled a
considerable distance from the point where the mixture is ignited, since the event occurs several seconds after the ignition. Hence, the turbulent flow field generated by the deployment of the vent suddenly affects the flame front, causing a fast increase of pressure as the flame approaches the vent area. The pressure time history recorded from the transducer opposite the vent (see Fig. 3(a)) shows a pressure drop after the flame front reaches the vent area (P2), at the same time the pressure recorded from the transducer opposite the glass wall continued to increase. In fact at this moment, the combustion products occupy a sort of cone which extends from the
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Fig. 2 e Maximum overpressure's peak attained inside the CVE test facility vs. average hydrogen concentration for all the test performed.
opposite wall (were the sensor is located) to the vent area and the unburned mixture occupies only the sides and the corners of the vented volume, so the flow of the burned mixture towards the vent produces the slight decrease of pressure at the wall opposite the vent area. Also, the slight difference of readings between the two pressure transducers, some kPa, has been observed at the end of the explosion in every test. For the tests performed at higher concentrations (Fig. 3(b)e(f)), whose pressure time history are showed in Fig. 3 and where pressure oscillations are involved, only one pressure reading is plotted to provide a more clear view. Tests with very lean hydrogen concentration (i.e. under 9% vol., for example see Fig. 3(a)) show a third peak (P3) which was described in the introduction. This can be explained as follows: after the flame front reaches the vent, the flame area continues to increase for a period of time in the rest of the chamber due to the low burning velocity of the mixture in direction perpendicular to the vent The reduction of the flame front area towards the end of the explosion, as it reaches the walls and burns in the isolated pockets of unburned mixture, generates the third peak (P3). In other words the third peak P3 is caused by the decrease of the combustion products generation. For tests at higher concentrations (Fig. 3(b)e(f)), this peak is substituted by the oscillatory pressure peak (P4). In tests were the vent opens when the flame front is not close to the vent area, Helmholtz oscillations were observed after the vent deployment. These oscillations eventually decay as the flame continues to expand. After the vent opens, a flow field is introduced into the vented volume which generates turbulence that in turn interacts with the flame front as soon as it approaches the vent
opening. The flame radius keeps increasing and eventually the flame stretches, driven by the turbulent flow of unburned gases carried out through the vent area (see Fig. 5). In tests were H2 concentration exceeds 11% vol., the rate of combustion products generated becomes soon larger than the outflow rate, and, consequently, the internal pressure starts to rise dumping in the early stage Helmholtz oscillations generated by the vent deployment (see Fig. 3(e) and (f)). The pressure rises until the flame front reaches the vent, then due to the difference of density of the vented gases, or to an external explosion, a second pressure peak is generated (P2). Concerning the second peak, in tests involving lean hydrogen mixtures as the ones performed in the present experimental tests, in the opinion of the author, the possibility that this may be generated by the external explosion is unlikely, since the turbulent flow of unburned gases exiting the vent should enhance the mixing with external air reducing the already lean H2 concentration outside the vent. Moreover, when the flame front approaches the vent area, pressure oscillations are triggered inside the vented volume. According to Harris formula, the volumetric flow rate through the vent area has a sudden increase when the burned gases start to flow out of the enclosure. This discontinuity may then be responsible of generating the physical response of the chamber as well as affecting the burning behaviour of the expanding flame front making it more susceptible to interactions with acoustic oscillations generated inside the vented volume. In fact, before reaching the vent area, the flammable envelope is accelerated by the production of combustion products. When the flame reaches the vent area and the combustion products start to flow out of
Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Fig. 3 e Pressure time history observed in some of the tests.
the vent their productions does not contribute to the acceleration of the flame front as they were doing before, so the flame velocity may drop. The conditions may become more close to the one of a static flame which most likely may interact with acoustic waves, as extensively studied in the literature. The measured oscillations have a variable frequency, which tends to increase as the deflagration progresses, since the vented volume is progressively filled with combustion products having higher temperature and lower density
compared to the unburned mixture. These oscillations, generated by the physical response of the enclosure [14,15], provoke a resonant coupling with the combustion inside the vented volume, giving rise to very high combustion rates and creating a significant net overpressure within the enclosure. The coupling of the two phenomena seems to generate stronger effects when the flame approaches the walls of the enclosure, in fact the amplitude of the oscillation was observed to increase by two orders of magnitude as the flame approaches the walls.
Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Fig. 4 e Time of vent deployment after the ignition.
Fig. 5 e Photogram of the video recorded during a test.
In respect to the observed non-monotonic behaviour of the maximum overpressure, the following scenario is suggested: pressure oscillations are triggered as the flame reaches the vent area and the density of flowing gases through it, suffers a discontinuity, at the first stage acoustic waves are only superimposed to the flame front and the interaction is weak. When the flame front approaches the walls the interaction increases and so does the amplitude of the pressure oscillations generating a positive overpressure. If the flame front reaches the vent area generating the second peak (P2) and triggering the oscillations when the flame front in radial direction is far from the walls, then the oscillations than the interaction between the flame front and the acoustic waves becomes strong enough to affect the pressure after a delay of time, generating a distinctive peak (P4) that occurs when the
effect of the overpressure generated by the second peak is already decreased (see Fig. 3(b)e(d), (f)). Instead, when the flame front reaches the vent while in radial direction is close to the walls, the oscillatory peak may completely overlap with the second peak generating a higher pressure (see Fig. 3(e)). In tests performed at H2 concentrations between 11% and 11.5% vol. the two peaks were recorded very close to each other, hence, producing a higher overpressure inside the facility. In tests were the H2 concentration is higher than 12% vol., the flame front expands faster in radial direction and reaches the walls before the oscillations are triggered. In this case, more than one oscillatory peak may be observed, each one representing an unburned pocket of gas reaching the walls. Hence, a possible explanation to the non-monotonic behaviour observed in the tests is the superimposition of the two peaks generated by the flame reaching the vent area (P2) and by the resonant coupling between the flame and the acoustic waves (P4) respectively. As reported in the literature by some authors, the presence of the obstacles may reduce the effect of the interaction between the flame front and the oscillations produced during the deflagration. In fact, the maximum peak pressure was found to be higher for tests performed in the empty room with respect to the ones performed with obstacles, particularly at concentrations were the two peaks P2 and P4 overlap. This behaviour was not supported for all obstacle configurations. For example, in configuration 3 and 6 the maximum overpressure generated at H2 concentration around 11% vol. was comparable with the one generated inside the empty room. It should be underlined that for these tests the pressure build up attributed to the second peak is higher than the one obtained in the empty room or in other obstacle configurations, probably due to the fact that the flame is forced to travel towards the vent passing through the higher part of the facility were the concentration is higher (see Fig. 7). The acoustic flame
Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Fig. 6 e Maximum overpressure's peak attained inside the CVE test facility vs. average hydrogen concentration for 4 selected setups.
interaction generates a lower net overpressure if compared with the empty room case, even though the superimposition of the two effects gives similar results. The reduction of the effect of the interaction between the flame front and the oscillations was particularly effective in configuration 7 (see Fig. 6) were the obstacle had a surface facing the walls perpendicular to the vent. In the configuration # 8, the flame front undergoes a stronger acceleration approaching the vent due to the
obstacles configuration, with this acceleration being responsible for the higher second peak (P2) than the one attained in other configurations for the same H2 concentration. In this configuration, the non-monotonic behaviour was not confirmed by the results. In fact, the main contribution to the pressure generation is the acceleration of the flame front towards the vent at higher H2 concentration, overcoming the contribution of the overlap of the two peaks, which was observed during the tests (see Fig. 8).
Fig. 7 e Comparison of overpressure's peak attained inside the empty CVE test facility and in configuration 3. Please cite this article in press as: Schiavetti M, Carcassi M, Maximum overpressure vs. H2 concentration non-monotonic behavior in vented deflagration. Experimental results, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.180
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Fig. 8 e Comparison of overpressure's peak attained inside the CVE test facility in configuration 8.
Conclusions Experiments were performed at Scalbatraio laboratory, DICI Department of University of Pisa, in a 25 m3 vented combustion test facility that hosted internally different obstacle configurations, to study the combustion behaviour of hydrogen-air mixtures in vented environments. The test facility vent dimension was set to a value of 1,004 m2, while the ignition location was in the middle of the wall opposite the vent for all experiments. The study included hydrogen concentrations from 8.5% to 12.5%. A general description of the pressure-time history obtained during the tests was given following the four peak theory. Peak overpressure developed by far-vent ignition exhibits non-monotonic behaviour in the range of concentrations under investigation. It reached a maximum at 11% vol. of hydrogen concentration and then decreased. At higher than 11.5% vol. H2 concentrations, peak pressure increases again monotonically with hydrogen concentration. The same nonmonotonic behaviour was described by Kumar [10] during tests of vented deflagration in a rectangular 120 m3 facility, but the peak was reached at 9% vol. instead of 11% vol. In respect of the observed non-monotonic behaviour, the following scenario is derived: pressure oscillations are triggered as the flame reaches the vent area and the flow through it suffers a discontinuity. At the first stage, acoustic waves are only superimposed to the flame front and the interaction is weak. When the flame front approaches the walls, the interaction increases and so does the amplitude of the pressure oscillations which generate a positive overpressure. If the flame front reaches the vent area, thus producing the second peak and triggering the acoustic response of the chamber, at the time when the flammable envelope in radial direction is
still far from the walls, then the interaction with the flame front starts blandly and becomes high enough to affect the net overpressure, after a time delay, generating a distinctive peak. The oscillatory peak occurs when the effect of the overpressure generated by the second peak has already decreased. If the flame front reaches the vent when the flammable envelope is close to the wall then the interaction between the acoustic waves and the flame front immediately produce a net overpressure inside the enclosure and the two peaks overlap, being the superimposition dependent on H2 concentration, geometry of the enclosure, obstacle configuration and all the other variables that affect the time history of a deflagration. The non-monotonic behaviour was not observed in configuration 8. In this case, the turbulent flow field generated by the vent deployment provokes a stronger acceleration of the flame front. In fact, in this configuration the main contribution to the pressure generation is the turbulent combustion generated by the acceleration of the flame front towards the vent, which increases with H2 concentration, overcoming the contribution of the acoustic peak whose overlap was observed in the range from 11% to 11.5% vol. of H2 concentrations.
references
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