External overpressure of vented hydrogen-air explosion in the tube

External overpressure of vented hydrogen-air explosion in the tube

international journal of hydrogen energy xxx (xxxx) xxx Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/l...

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international journal of hydrogen energy xxx (xxxx) xxx

Available online at www.sciencedirect.com

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External overpressure of vented hydrogen-air explosion in the tube Yong Cao*, Yongxu Wang, Xianzhao Song, Huadao Xing, Bin Li, Lifeng Xie** School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, Jiangsu, China

highlights  Lower L/D leads to the significantly higher internal overpressure.  External overpressure was proportional to internal peak overpressure.  Peak overpressure measured at PT2 steady increases with hydrogen concentration.  It is found a better agreement between prediction and experimental results.

article info

abstract

Article history:

Vented explosion experiments involving hydrogen-air mixtures are performed in a 2 m-

Received 9 July 2019

long cylindrical tube under the influences of the hydrogen concentration and vent burst

Received in revised form

pressure. Photos of the external flame shot by a high-speed camera show that the jet flame

3 October 2019

was expelled outside the vessel, and the relation between the flame propagation and

Accepted 11 October 2019

external overpressure is summarized. The internal peak overpressure increases and then

Available online xxx

decreases with increasing hydrogen concentration. In contrast, the external peak overpressure exhibits the opposite correlation in comparison with the internal peak over-

Keywords:

pressure. The variations in the pressure peaks of the internal pressure curves are also

Hydrogen

discussed. When the hydrogen concentration is lower than 40 vol %, the second pressure

Vented explosion

peak plays a more dominant role than the other pressure peaks. However, when the

Peak overpressure

hydrogen concentration is higher than 40 vol %, the third pressure peak becomes more

External explosion

dominant.

Tube

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction As a result of facility and design deficiencies, accidental deflagration and hydrogen explosions can easily occur in an enclosure. The flame and overpressure are two key factors that play important roles in producing disastrous

consequences in this scenario [1e3]. As a convenient method for preventing accidental explosions, vented explosions have been extensively investigated by experimental and numerical methods with vented containers of different shapes [4e12]. Solberg [13] showed that a suddenly enhanced flame front develops a Taylor instability during the venting process when the vent is opening. Chippett [14] generated vented

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Cao), [email protected] (L. Xie). https://doi.org/10.1016/j.ijhydene.2019.10.086 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Cao Y et al., External overpressure of vented hydrogen-air explosion in the tube, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.086

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deflagration models in consideration of flame acceleration and an increased burn rate caused by hydrodynamic instabilities. A large number of theoretical works have attempted to predict the reduced overpressure in simply vented vessels, and guidelines were given for the design of vented areas with regard to gas explosions based on empirical or semi-empirical correlations. Bradley and Mitcheson [15,16] proposed a vented explosion model by summing the available experimental data for guiding the design of vented areas and presented recommended values for the maximum pressure rise plotted against A , where A is the product of the vent area S0 and coefficient of discharge divided by the total spherical area and S0 is the ratio of the gas flow velocity ahead of the flame front to the acoustic velocity. However, the abovementioned curve requires that the maximum explosion overpressure Pred not exceed Pstat. In addition, a low-pressure venting model was developed by Yao [17]; this model considers that the gas mixture is ignited at the centre of the vented spherical vessel and takes into account the effects of turbulence and the flame stretch factor c. A dimensionless venting parameter was determined as a, and two curves related to c and a were drawn to predict the explosion overpressure. The reduced pressure Pred and vent opening overpressure Pstat were assumed to be no larger than 0.69 bar and 0.2 bar, respectively. Molkov [18] developed an earlier model in which the variable discharge coefficient can be determined independently during an experiment. New parameters were defined as the Bradley number Br and the turbulent Bradley number Brt, and cm, where c represents the stretch factor of the flame front generated by the turbulent flow at the open vent and m is the generalized discharge coefficient, was obtained from experiments performed in enclosures with different shapes and dimensions. Moreover, the authors mentioned above proposed a universal correlation to predict the explosion overpressure Pred, and the correlation showed good fit with experimental data. Usually, this criterion and formula are applicable to enclosures with an L/D ratio ranging from 1 to 5, but such guidelines are invalid for the range of parameters in an elongated enclosure. Relatively little research has focused on vented explosions in large L/D vessels. The development of pressure in the vented explosion in a vessel with a large L/D is quite different from that in a vessel with a small L/D due to the flame acceleration phenomenon in elongated enclosures. More experimental and simulation studies are needed to determine the mechanisms of different pressure histories. In addition, gas explosions vented through ducts and ductless configurations have been numerically simulated by varying the duct size and ignition position with computational fluid dynamics (CFD) models based on the unsteady Reynoldsaveraged Navier-Stokes (URANS) approach [19]. Vermorel [20] investigated the propagation of premixed flames in vented gas explosions using the large eddy simulation (LES) technique. Particular attention was paid to the effect of the turbulent combustion model on the overpressure and flame speed through a comparison with other algebraic models for the sub-grid scale. For such a small scale, the simulation results were consistent with experimental results. However, when

the experimental conditions switched to a medium or large scale, the simulations were not suitable without a coefficient change. Previous studies have considered many internal explosion processes, but relatively few experiments have been performed to investigate the relationship between an external explosion outside a vessel and an internal explosion. Harrison and Eyre [21] analysed the occurrence of an external explosion and how external explosions influence the internal pressure dynamics. Chao [22] proposed a simple model to estimate the peak overpressure in a vented hydrogen-air explosion and found that the peak overpressure is dominated by the interaction among the maximum flame area, the burning rate and an external deflagration. Fakandu [23] compared the influences of single vents and multiple vents on the flame speed and overpressure caused by an external explosion. Catin [24] studied how the outlet speed of a flame and the fuel reactivity influence the external overpressure peaks generated by an external explosion; the external overpressure peaks reached a maximum value with increasing flame outlet speed, after which the external overpressure remained nearly constant. Furthermore, the impact of the ignition location on the occurrence of an external explosion and the relationship between the internal and external overpressures were investigated by Cao [25]. A simulation study was performed to investigate the deflagration of hydrogen in a vented enclosure by Tolias [26]. The study focused on the overpressure generated by an external explosion and suggested that CFD models have the potential to predict vented explosions, although a more extensive model is still required. Molkov [27] compared simulated and experimental pressure histories and the dynamics of flame propagation inside and outside an enclosure based on the LES technique and experimental data; it was revealed that an external pressure rise is caused by the highly turbulent deflagration outside the enclosure and that the internal burning rate does not increase at the same time. However, currently available CFD models may not be able to precisely predict the overpressure of a vented deflagration when the mixture is ignited near the vent, as shown in the example of Bauwens [28]. Palmer [29] proposed a simple correlation for the external pressure in a vented explosion; the experimental research was conducted in a duct with an open end to study the flame dynamics outside the duct [30]. Similarly, Forcier [31] had reviewed several correlations for the external overpressure in a vented explosion; a spherically symmetric blast wave model was explored as a means to calculate the external overpressure, and the results have an accuracy comparable to the experimental data in the case with a lower blast pressure. While the importance of external explosions on the rise in vented pressure has reached extensive agreement, the mechanism of this pressure increase triggered by an external explosion still needs to be determined with a more in-depth investigation. In this paper, the internal and external overpressures in a tube during a vented hydrogen explosion were investigated in detail.

Please cite this article as: Cao Y et al., External overpressure of vented hydrogen-air explosion in the tube, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.086

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Experimental setup Apparatus All experiments were performed in a 2 m-long cylindrical tube with an L/D of 16 at atmospheric pressure and ambient initial temperature (see Fig. 1). The inner diameter of the cylindrical tube is 7 cm with a vent cover of the same size. The pressuretime traces were obtained by four pressure transducers (from PT1 to PT4), two of which (PT1 and PT2) were located on the two ends of the tube. The remaining two transducers were located on the two measuring instruments; the distances between PT3 and PT4 from the vent were 0.25 m and 1.0 m, respectively. The measurement range of the pressure transducer is 0e1.379 MPa, and the voltage sensitivity is 3514 mV/MPa. A data acquisition instrument was used to record the signals with a recording rate of 10,000 samples per unit time. A highspeed camera was used to capture the external flame with a frame rate (3.5 m from the tube) of 5000 fps.

Procedures Each test was performed in an evacuated tube first. The hydrogen-air mixture was achieved by the partial pressure method, and the tube was filled separately with hydrogen of different concentrations (c ¼ 15 vol %, 20 vol %, 30 vol %, 40 vol % and 50 vol %) and air with a cardboard cover of two thicknesses (d ¼ 0.28 mm and 0.24 þ 0.24 mm). The gas circulation system was run for 10 min to keep the mixture uniform. The high-speed camera and the pressure transducer were triggered simultaneously when the mixture was ignited with an ignition energy of 200 mJ. Pressure signals measured by the pressure transducers were collected by a data recorder at the same time.

Results and discussion Overpressures with different concentrations The internal pressure histories with different concentrations are shown in Figs. 2 and 3. The peak overpressure increases and then decreases with increasing hydrogen concentration.

3

The peak overpressure reaches a maximum of 247 kPa when the hydrogen concentration is 40 vol %. Compared with the experimental results for a vented hydrogen explosion in a vessel with a low L/D, whose peak pressures close to the vent cover are always lower than the peak overpressures at remote locations, the flame in this work experiences a continuously accelerated process that causes higher peak overpressures at remote locations. Former studies concluded the dynamics of pressure development in the vented elongated vessel [32e35]. Alexiou [32] summarized the formation mechanism of five pressure peaks in a side-vented explosion in an elongated vessel. The pressure curves measured by Ajrash [33] presented two or three pressure peaks in a vented tube with side vents. Schiavetti [34] also discussed the correlation between flame behaviours and pressure peaks. The pressure curves measured at PT2 show three peaks (P1 to P3) during the whole process. When the hydrogen concentration is 40 vol %, the third pressure peak plays a more dominant role than the other pressure peaks. However, when the hydrogen concentration is 20 vol %, the second pressure peak is more dominant. In this experiment, in view of the position of the transducer, the first pressure peak is related to the opening of the vent cover. The second and third pressure peaks may be associated with the flame acceleration of the downstream gases ahead of the flame and the external explosion or acoustic effect, respectively. This is probably because the external explosion is the strongest due to the relatively high unburned mixture concentration when the hydrogen concentration is 40 vol %, and thus, the third pressure peak plays a more dominant role than the first and second pressure peaks. Alexiou [32] studied the effect of the L/D ratio on the overpressure during vented gas explosions. The initial overpressure is associated with a large flame surface area due to the high speed of the elongated flame in large L/D vessels. After the flame reaches the wall of the tube, a large proportion of flame quenches at the wall; then, the production rate of the burnt gas volume drops rapidly, and the heat loss increases rapidly, causing the first decrease in internal pressure. In contrast, the external peak overpressure exhibits a different correlation with the mixture concentration from the internal peak overpressure, as shown in Fig. 3. The external peak overpressure always increases with increasing hydrogen concentration. The external peak overpressure reaches a minimum of 15 kPa when the hydrogen concentration is 10 vol %.

Fig. 1 e Experiment setup. Please cite this article as: Cao Y et al., External overpressure of vented hydrogen-air explosion in the tube, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.086

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Fig. 2 e Pressure histories with different concentration.

Fig. 3 e Pressure histories with different concentrations.

Fig. 4 e Internal overpressure with different L/D.

The peak overpressure at PT1 is different from the peak overpressure at PT2. The peak overpressure measured at PT2 steadily increases with the hydrogen concentration. This indicates that the explosion process shows severe differences among different regions. Ferrara [36] presented the correlation between the internal and external pressure histories and demonstrated both that flame propagation influences the extent of an external explosion directly and that the external peak overpressure is proportional to the internal peak overpressure. The same relationship was also found by a number of vented experiments in spherical vessels [25,37,38]. The experiment in this study reaches the same conclusion as previous studies, as shown in Figs. 3 and 10. The external overpressure is proportional to the internal peak overpressure. With an increase in the hydrogen concentration, the experimental results always indicated that the internal overpressure reaches a maximum when the equivalence ratio of hydrogen exceeds 1 due to the limitation of the oxygen concentration. However, the external overpressure will continue to increase with the hydrogen concentration because fresh air

Fig. 5 e External overpressure with different L/D.

Please cite this article as: Cao Y et al., External overpressure of vented hydrogen-air explosion in the tube, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.086

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Fig. 6 e High speed photographs of external flame with different hydrogen concentrations.

Fig. 7 e External flame speed with concentrations.

Fig. 8 e The relationship between external overpressure and flame speed.

Please cite this article as: Cao Y et al., External overpressure of vented hydrogen-air explosion in the tube, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.086

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Fig. 9 e Comparison between predicted external overpressure and experimental results.

Fig. 11 e Comparison between predicted external overpressure and experimental results.

the time at which the flame reaches the walls increases, and the overpressure will decrease with an increase in the L/D ratio at this time.

External flame propagation

Fig. 10 e Internal overpressure vs. external overpressure.

can react with the unburned mixtures in the open space outside the chamber. The internal overpressures of the tube with L/D ¼ 16 in this work and the external overpressures of the vessels with L/ D ¼ 1 from the results of previous experiments [25] are compared in Figs. 4 and 5. The internal overpressure of low L/ D vessels is significantly higher than the internal overpressure of the tube with L/D ¼ 16 when the hydrogen equivalence ratio exceeds 1. However, the overpressures of the two L/D ratios are very close when the hydrogen concentration is under 20%. Phylaktou and Andrews [39] reported gas explosions with different L/D values in elongated vessels. As the L/D ratio decreases, the explosion time decreases and the time for heat to be lost from the system is reduced, resulting in an increasing maximum pressure. When the L/D ratio increases,

Fig. 7 presents the instantaneous speed of the external flame with different hydrogen concentrations. High-speed photographs of the external flame are shown in Fig. 6. The photographs in Fig. 6 show the characteristics of the external flame with different hydrogen concentrations, which are different from the characteristics of the external flame in small L/D vessels. In this experiment, the jet flame appeared when the vent was open. As the hydrogen concentration increased, the external flame burned more vigorously. It can be speculated that the intensity of the external explosion also increases with the hydrogen concentration. When the flame reached the maximum distance, the flame gradually started to decay and became shorter. For the external flame, the tendency of the initial external flame acceleration and the variation in the speed when the concentration was 40% were similar to the other two results. Additionally, an increase in the concentration resulted in an increase in the propagation time of the external flame in this concentration range. Harrison and Eyre [21] concluded the relationship between the flow velocity u at the vent and the maximum external pressure Pem. Fig. 8 presents the relationship between the external flame speed and external overpressures in this experiment. The results show that the predicted overpressure has a smaller error under the condition of rear ignition than under the condition central ignition. As shown in Fig. 9, an approximate linear relationship was found between the external overpressure and predicted external overpressure, and this relationship appears to provide a better prediction than the result in Fig. 11. However, the flow velocity is generated by the pressure difference on both sides of the vent as a result of the complex interaction between the internal

Please cite this article as: Cao Y et al., External overpressure of vented hydrogen-air explosion in the tube, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.086

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and external explosions. It is therefore difficult to predict the flame velocity based on venting guidelines. Pem ¼ 0.021u1.28

(1)

Predictions of the external overpressure Hattwig measured the external pressures of vented gas explosions and included the following correlation in Eckhoff’s textbook [40]. P ¼ (Pred$C1$C2)/d

 r¼

L ðE,Su ,tÞ2 2

7

13

Su ðtÞ , Af ðtÞ ¼ 4pESu r2 ,XðtÞ

(7)

(8)

The correlation between the experimental results and predictions is shown in Fig. 11. A better agreement than this line cannot be found. The relevant parameters will be determined experimentally [44,45]. The parameters should be changed by considering experimental data to accurately predict the external overpressures for vented explosions under different conditions.

(2)

Conclusions where P is the blast overpressure at a specific distance d (m), Pred is the reduced overpressure of the vented explosion, and C1 can be determined by logC1 ¼ 20.26/Av þ0.49, where Av is the vent area (m) and C2 ¼ 1 m. Palmer and Tonkin [29] developed correlations initially presented by Butlin and Tonkin for vented explosion blast pressures based on methane explosion experiments in a large enclosure. They concluded a simple relationship between the distance from the vent and the external pressure: P ¼ k/(d1þ d0)

(3)

where P is the external pressure, k is the constant related to the vent area Av and the combustible gas, d is the distance from the vessel (9 m < d < 18 m), and d0 is the characteristic dimension of the vessel. Wirkner-Bott [41] proposed the following empirical equation related to the vent size Av, vessel volume V and reduced overpressure Pred for the maximum external explosion pressure Pem: 0.18 Pem ¼ 0.2$A0.1 Pred v V

(4)

The results of the experiment in this paper also indicate a linear relationship between the internal overpressures and external overpressures in Fig. 10. However, a point with considerable deviation still exists. Van Wingerden [42] also compared the reduced overpressure and external overpressure and demonstrated that the decrease in the external overpressure is an acoustic phenomenon. The peak external overpressure in a gas explosion is given using the acoustic source theory [43]: P¼

 b d q Su ðtÞ , Af ðtÞ 4pa20 r dt

(5)

where P is the external overpressure, a0 is the velocity of b is an effective dimensionless heat addition, r is the sound, q radius when the flame is propagating away from the vent, Su is the effective normal burning velocity and Af (t) is the effective area of the flame front. Some of these parameters can be calculated as Equations (6)e(8) [44,45]. b ¼ g0 ðru  rb Þrb ¼ g0 ðE  1Þ q

(6)

Vented explosion experiments for hydrogen-air mixtures were performed in a 2 m-long cylindrical tube under the influences of the hydrogen concentration and vent burst pressure. The internal and external pressure histories were discussed in detail. The internal peak overpressure increases and then decreases with increasing hydrogen concentration. The peak overpressure reaches a maximum of 247 kPa when the hydrogen concentration is 40 vol %. In contrast, external peak overpressure exhibits the opposite correlation in comparison with the internal peak overpressure; the external peak overpressure always increases with increasing hydrogen concentration. The peak overpressure at PT1 is different from the peak overpressure at PT2. The peak overpressure measured at PT2 steadily increases with the hydrogen concentration. It can be concluded that the external pressure is proportional to the internal peak overpressure. Compared to the overpressures of L/D ¼ 1 vessels from previous experiments, the internal overpressures of vessels with a lower L/D are significantly higher than the internal overpressures of L/ D ¼ 16 vessels when the hydrogen equivalence ratio exceeds 1. However, the overpressures of the two L/D ratios are very close when the hydrogen concentration is under 20%.

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Please cite this article as: Cao Y et al., External overpressure of vented hydrogen-air explosion in the tube, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.086