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Influence of gas ventilation pressure on the stability of airways airflow
International Journal of Mining Science and Technology xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Influence of gas ventilation pressure on the stability of airways airflow Zhou Aitao a,b, Wang Kai a,b,⇑, Wu Longgang b, Xiao Yiwen b a b
Beijing Key Laboratory for Precise Mining of Intergrown Energy and Resources, China University of Mining & Technology, Beijing 100083, China School of Resource and Safety Engineering, China University of Mining & Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 18 January 2017 Received in revised form 15 May 2017 Accepted 9 September 2017 Available online xxxx Keywords: Gas accumulation Gas ventilation pressure Airflow stability Underground ventilation
a b s t r a c t Coal mine ventilation is an extremely complicated system that can be affected by many factors. Gas ventilation pressure is one of important factors that can disturb the stabilization of airflow in airways. The formation and characteristics of gas ventilation pressure were further elaborated, and numerical simulations were conducted to verify the role of gas ventilation pressure in the stability of airway airflow. Then a case study of airflow stagnation accident that occurred in the Tangshan Coal Mine was performed. The results show that under the condition of upward ventilation, the direction of gas ventilation pressure in the branch is the same to that of the main fan, airflow of the branches beside the branch may be reversed. The greater the gas ventilation pressure is, the more obvious the reversion is. Moreover, reversion sequence of paralleled branches is related to the airflow velocity and length of the branch. Under the condition of downward ventilation, the airflow in the branch filled with gas may be reversed. Methane in downward ventilation is hard to discharge; therefore, accumulation in downward ventilation is more harmful than that in upward ventilation. Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Coal mine ventilation system plays important roles in the underground mining. It offers a sufficient quantity of air to the underground mine workers to dilute methane and other contaminants, maintain a suitable working environment and prevent accidents [1–3]. During the mining operating, the status of the ventilation system cannot simply be kept constant. Generally, coal mine ventilation is an extremely complicated system. A large number of influencing factors can control or impact the behaviors of the system [4,5]. These include the ventilation network geometry (diagonal network, airway resistance, etc.), the location and operation characteristics of each of the other fans in the system and other external influencing factors [6]. Natural ventilation pressure is an important form of the external disturbance factors and has been observed ever since the beginning of underground mining. Natural ventilation pressure in underground constructions can help to optimize the design of their ventilation systems, reduce their energy consumption and avoid the risk of accumulation of gases or toxic agents [7]. However, natural ventilation pressure can also induce airflow reversal or reduce the airflow rate of some
⇑ Corresponding author at: Beijing Key Laboratory for Precise Mining of Intergrown Energy and Resources, China University of Mining & Technology, Beijing 100083, China. E-mail address: [email protected] (K. Wang).
airways in mine ventilation. Scholars worldwide have conducted extensive research in the field of natural ventilation pressure [8–11]. Natural ventilation pressure is produced by the difference in air density between the intake and return airways. Previous studies have shown that the difference in air density is determined by air temperature and airway elevation difference. Therefore, natural ventilation pressure always occurs in airways with differences in elevation and temperature [12–15]. For instance, when fire occurs in an underground mine, the smoke flows along the airway airflow direction and the airflow temperature in the airway increases as the smoke spreads. Thus natural ventilation pressure is induced by fire if the airway has an elevation difference, which can be named as fire ventilation pressure. However, apart from temperature differences, natural ventilation pressure also can be produced in an airway with an elevation difference by the accumulation of gas, which is called gas ventilation pressure in this paper. Gas ventilation pressure is similar to that of a fire ventilation pressure due to the gas density and elevation differences [16–21]. However, the density difference of a fire ventilation pressure is caused by the air temperature difference between the intake and the return shafts of the mine, whereas the gas ventilation pressure does not consider the impact of temperature changes on the gas density. Because of the density difference between air and methane, the average density of the airflow changes when highconcentration methane flows into the airway. The gas ventilation pressure can be observed as increments in the potential energy
https://doi.org/10.1016/j.ijmst.2017.09.004 2095-2686/Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Zhou A et al. Influence of gas ventilation pressure on the stability of airways airflow. Int J Min Sci Technol (2017), https:// doi.org/10.1016/j.ijmst.2017.09.004
2
A. Zhou et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Table 2 Characteristic curve data of the fan.
Fan pressure (Pa) Airflow rate (m3/s)
First point
Second point
Third point
385 16
248 37
148 52
3. Analysis of gas ventilation pressure under different ventilation ways on mine ventilation network Fig. 1. Simple mine ventilation network graph.
in the airway. Gas ventilation pressure is often neglected by researchers and field staff, but it plays an important role in the stability of airway airflow in underground mines. This phenomenon can lead to airflow stagnation or airflow reversal in normal airways. For instance, an accident involving the cessation of the airflow in an airway had occurred in one inclined airway of the Tangshan coal mine in China. At first, the airflow stagnation accident was regarded being caused by the roof falling, but after a detailed site investigation was conducted, the results clarified that the accident was a result of gas accumulation. This paper aims to explore the mechanism by which the stability of airway airflow in underground mines is induced by gas ventilation pressure, to present measures to prevent airway airflow disasters caused by gas ventilation pressure and to propose advice for mine ventilation design.
The roles of gas ventilation pressure play in mine ventilation network were conducted by the unsteady ventilation network calculation program [22,3]. 3.1. Basic models and initial conditions The designed simple mine ventilation network is shown in Fig. 1. As shown in Fig. 1, branch 2 is filled with methane due to a coal and gas outburst, the methane concentration was 100%. By setting elevation differences of branches, the upward ventilation and the downward ventilation can be simulated. Basic parameters of branches were listed in Table 1, the characteristic curve data of the fan were shown in Table 2. The methane density is 0.717 kg/ m3, air density is 1.225 kg/m3, the calculation time was 2400 s, time interval was 1 s, and space interval was 5 s. 3.2. Influences of gas ventilation pressure on the mine ventilation network under the condition of upward ventilation
2. Formation and characteristics of gas ventilation pressure 2.1. Factors that influence on gas ventilation pressure Gas ventilation pressure is caused by the accumulation of gas in inclined airways, the accumulated gas may induced by coal and outbursts or gas emission for coal seam in the conditions of the ventilation system fails. It can be regarded as one form of natural ventilation pressure. The amount of gas ventilation pressure is mainly affected by the gas concentration and the elevation difference in the airway. Temperature is not considered as a factor when a high concentration gas accumulates in the airway. Because the density of methane is lower than that of air, the density of the mixed gas changes. When the airway also has an elevation difference, gas ventilation pressure is formed. Gas ventilation pressure is calculated as follows:
hM ¼ ðq qa Þ½zð0Þ zðLÞg
ð1Þ
where hM is the gas ventilation pressure of airway, Pa; q the average density of the airway airflow after mixing with gas, kg/m3; qa the density of air, kg/m3; zð0Þ and zðLÞ respectively the elevations of the beginning and end junctions, m.
3.2.1. Gas ventilation pressure and the airflow rate of branches Fig. 2a presents gas ventilation pressure variation with time under the condition of upward ventilation, Fig. 2a shows airflow rate variation with time under the condition of upward ventilation. As shown in Fig. 2, it can be concluded as follows: (1) Under the condition of upward ventilation, the direction of gas ventilation pressure produced by methane accumulation in branch 2 is the same to that of the main fan. Under the influences of gas ventilation pressure, airflow rate of branch 2 increases while airflow rate of branches beside branch 2 (branch 3 and branch 4) decreases gradually. (2) At about t = 12 s airflow rate of branch 4 reduces to zero and about t = 14 s airflow rate of branch 3 reduces to zero. The initial airflow rate of branch 3 and branch 4 are 6.4 m3/s, 4.2 m3/s respectively, the larger the airflow rate, the later the airflow reversals. (3) The total airflow rate (branch 5) of the mine gradually increases. And then it increases from normal volume of 25.8 m3/s to maximum volume of 47.0 m3/s at t = 173 s. Then it decreases.
Table 1 Basic parameters of branches. Branch number
From
To
Resistance (N s2/m8)
Length (m)
Area (m2)
Circumference (m)
Methane concentration (%)
1 2 3 4 5
1 2 2 2 3
2 3 3 3 1
0.125 0.131 0.720 1.631 0.314
1020 980 980 980 620
9.20 7.62 6.14 6.14 9.20
14 12.1 11 11 14
0 100 0 0 0
Elevation difference (m) Upward
Downward
300 120 120 120 180
300 100 100 100 400
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A. Zhou et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
3
Fig. 2. Gas ventilation pressure and airflow rate variation with time under the condition of upward ventilation.
Fig. 3. Variation of methane concentration along the branch axial under the condition of upward ventilation.
Fig. 4. Gas ventilation pressure and airflow rate variation with time under the condition of downward ventilation.
(4) Methane flows into branch 3 and branch 4 induces airflow reversals of the two branches. With the increased methane concentration in branch 3 and branch 4, gas ventilation pressure of two branches increases while that of branch 2 decreases, as shown in Fig.2a. (5) At about t = 107 s, the airflow directions of branch 3 and branch 4 returns to normal and methane gradually flows into return airway (branch 5). (6) After all methane flows out of branch 5, gas ventilation pressure of each branch disappears and the mine ventilation network returns to normal.
(2) Methane concentration distribution of branch 3 is shown in Fig. 3b. With the reversion of airflow in branch 3, partial methane flows into the branch from the ending point. But the reflux length is only 180 m as the airflow returns to normal soon. 3.3. Influences of gas ventilation pressure on the mine ventilation network in the condition of downward ventilation
3.2.2. Methane concentration distribution laws In order to analyze methane concentration distribution laws, branch 2 and branch 3 were selected under the condition of upward ventilation, the results are shown in Fig. 3a and b.
3.3.1. The gas ventilation pressure and the airflow rate of branches Fig. 2a presents gas ventilation pressure variation with time under the condition of downward ventilation, Fig. 2a shows airflow rate variation with time under the condition of downward ventilation. The initial condition is the same to that of upward ventilation. As shown in Fig. 4, it can be concluded as follows:
(1) Methane concentration of branch 2 decreases gradually. As shown in Fig. 3a, for t = 30 s, methane concentration at the 200 m branch 2 is 100%, however, for t = 60 s, methane concentration at the same position of branch 2 is 0%.
(1) Though branch 2 is still filled with methane at the initial time, gas ventilation pressure induced by methane concentration in branch 2 is opposite to the main fan pressure and hinders mine ventilation as. Airflow rate of branch
Please cite this article in press as: Zhou A et al. Influence of gas ventilation pressure on the stability of airways airflow. Int J Min Sci Technol (2017), https:// doi.org/10.1016/j.ijmst.2017.09.004
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A. Zhou et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
Fig. 5. Methane concentration variation with time under the condition of downward ventilation.
Fig. 6. Simplified ventilation system.
decreases in the initial phase and those of side branches (branch 3 and branch 4) increases. At about t = 7 s, airflow of branch 2 is reversed. (2) Total airflow rate of the mine network gradually decreases. And then it decreases from normal volume of 25.8 m3/s to minimum volume of 1.5 m3/s. 3.3.2. Methane concentration distribution laws Methane concentration distribution of branch 2 and branch 3 at different times in the condition of downward ventilation are respectively shown in Fig. 5a and b. Methane concentration in branch 2 gradually decreases, for the 600 m of the branch 2, the methane concentration at 60 s is 100%, and the methane concentration reduced to less than 10% at 300 s, however, compared with the upward ventilation, the rate of descent is significantly lower than the upward ventilation. Meanwhile, some methane flows into branch 3 from its ending point, and methane concentration in branch 3 increases. 4. Case study of airflow stability caused by gas ventilation pressure 4.1. Mine overview Tangshan coal mine is selected for case study, the coal mine is one of the oldest coal mines in China, with 130 years mining history. Production continues in the Yuexu working area on the western border of the coal mine. The simplified ventilation system is shown in Fig. 6. Airway c and airway are two inclined airways,
which are arranged in the coal seam, the composition of the seam is methane, the gas content of the coal seam is 3 m3/t, and the thickness of the coal seam is 3 m. The dips of these two airways are both 30°, and they are connected by airways d and k. The elevation of airway k is 526.7 m and that of d is 451.8 m. The two airways are full-pressure ventilation systems with 6 m3/s airflow rate. The methane concentration of the return airflow is 0.35% at 6 m3/s airflow rates. Ventilation doors are installed in airway k. The simplified ventilation system is shown in Fig. 6. The parameters of the simplified ventilation system were shown in Table 3. And all the airways in the simplified ventilation system with same height and width (the height is 3 m, the width is 3.33 m), the operating point of the main mine fan is 3.4 kPa @ 456 m3/s, the minimum regulatory air velocity required for the situation is 0.5 m/s, the airflow rate in the airways b, c, d, e is 6 m3/s. 4.2. Numerical simulations using an unsteady ventilation program To further analyze the airflow stability induced by gas accumulation, gas ventilation pressure is interpreted using an unsteady ventilation program, and numerical simulations were conducted by the program [23]. The initial conditions are as follows (in Table 3): The methane concentrations in airways c, d and e were 60%, 100% and 60%, respectively, and in the other airways, it was 0%. The density of methane is 0.72 kg/m3, the density of air is 1.29 kg/m3, the fan is installed in airway h, and the fan pressure is 1735 Pa, the airflow rate is 12.752 m3/s. Based on the initial conditions, the solutions for the initial airflow rate for each airway using the unsteady ventilation program are shown in Table 4. They are all within the scope of the field test results. The airflow rate distribution of airway c is illustrated in Fig. 7. As shown in Fig. 7, if the regulator in airway L is opened, methane accumulates in airways c, d and e. A high concentration of methane hinders airflow. The airflow rate in airway c deduced from the program is 2.7 m3/s. This value fluctuates slightly, but
Table 3 Parameters of the numerical simulation. Airway
Length (m)
Area (m2)
Circumference (m)
Resistance (N s2/m8)
Methane concentration (100%)
Elevation difference (m)
a b L c d e f g h
1500 1050 656 110 20 80 244 600 1500
10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
13.0 13.0 13.0 13.0 13.0 13.0 13.0 13.0 13.0
4.3588 2.7605 0.3204 0.6102 0.0581 0.5231 0.4184 1.7435 4.3588
0.0 0.0 0.0 0.6 1.0 0.6 0.0 0.0 0.0
0.0 133.0 110.0 75.0 0.0 75.0 23.0 110.0 0.0
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A. Zhou et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Table 4 Initial airflow rate of airways. Airway 3
Airflow rate (m /s)
A
b
L
c
d
e
f
g
h
12.752
2.717
10.035
2.717
2.717
2.717
2.717
12.752
12.752
Fig. 7. Airflow rate distribution of airway c after gas accumulation.
after a short period, the final airflow rate remains at zero. The simulation results agree well with the case description and field test. 5. Conclusions (1) When methane accumulates in airways, gas ventilation pressure is generated in airways with an elevation difference because the density of methane is lower than that of air. Gas ventilation pressure can be regarded as an increment of natural ventilation pressure. (2) Under the condition of upward ventilation, the direction of gas ventilation pressure is same to that of the main fan and airflow of the branches beside the branch may be reversed. The greater the gas ventilation pressure is, the more obvious the reversion is. Moreover, reversion sequence of paralleled branches is related to the airflow velocity and length of the branch. (3) Under the condition of downward ventilation, the airflow in the branch filled with gas may be reversed. The greater the gas ventilation pressure is, the easier the airflow reversion is. (4) Methane in downward ventilation is hard to discharge; therefore, accumulation in downward ventilation is more harmful than that in upward ventilation.
Acknowledgments This research is financially supported by the National Natural Science Foundation of China (Grant Nos. 51774292, 51474219, 51604278), the State Key Research Development Program of China (Grant Nos. 2016YFC0801402, 2016YFC0600708).
[2] Wallace K, Prosser B, Stinnette JD. The practice of mine ventilation engineering. Int J Min Sci Technol 2015;25(2):165–9. [3] Zhou A, Wang K. Airflow stabilization in airways induced by gas flows following an outburst. J Nat Gas Sci Eng 2016;35:720–5. [4] Cheng J, Yang S. Data mining applications in evaluating mine ventilation system. Saf Sci 2012;50(4):918–22. [5] Kazakov BP, Shalimov AV, Semin MA. Stability of natural ventilation mode after main fan stoppage. Int J Heat Mass Transf 2015;86:288–93. [6] El Nagdy KA. Stability of multiple fans in mine ventilation networks. Int J Min Sci Technol 2013;23(4):569–71. [7] Mazarrón FR, Porras-Amores C, Cañas-Guerrero I. Annual evolution of the natural ventilation in an underground construction: influence of the access tunnel and the ventilation chimney. Tunn Undergr Space Technol 2015;49 (5):188–98. [8] Hansen R. Analysis of methodologies for calculating the heat release rates of mining vehicle fires in underground mines. Fire Saf J 2015;71:194–216. [9] Li C, Li J, Hu L, Hou D. Visualization and simulation model of underground mine fire disaster based on Cellular Automata. Appl Math Model 2015;39 (15):4351–64. [10] Pathak K. Numerical simulations of dynamics of a tunnel fire. In: Proceedings of heat transfer division and fluids engineering division. Charlotte, North Carolina, USA; 2004. p. 363–71. [11] Sasmito AP, Kurnia JC, Birgersson E, Mujumdar AS. Computational evaluation of thermal management strategies in an underground mine. Appl Therm Eng 2015;90:1144–50. [12] Chang X. The transient-state simulation of mine ventilation systems. Michigan: Michigan Technological University; 1987. [13] Zapletal P, Hudecˇek V, Trofimov V. Effect of Natural Pressure Drop in Mine Main Ventilation/Skutki Naturalnego Spadku Cis´nienia W Głównej Sieci Wentylacyjnej Kopalni. Arch Min Sci 2014;59(2):501–8. [14] Zhou X. Optimal control on underground mine fire. Michigan: Michigan Technological University; 1988. [15] Zhu H, Song Z, Hao Y, Feng S. Application of simulink simulation for theoretical investigation of nonlinear variation of airflow in ventilation network. Proc Eng 2012;43:431–6. [16] Cheng J, Wu Y, Xu H, Liu J, Yang Y, Deng H, et al. Comprehensive and integrated mine ventilation consultation model – CIMVCM. Tunn Undergr Space Technol 2015;45:166–80. [17] Haoran Z, Sanmiquel Pera L, Zhao Y, Vintro Sanchez C. Researches and applications on geostatistical simulation and laboratory modeling of mine ventilation network and gas drainage zone. Process Saf Environ Prot 2015;94:55–64. [18] Kurnia JC, Sasmito AP, Mujumdar AS. Simulation of a novel intermittent ventilation system for underground mines. Tunn Undergr Space Technol 2014;42(5):206–15. [19] Nyaaba W, Frimpong S, El-Nagdy KA. Optimisation of mine ventilation networks using the Lagrangian algorithm for equality constraints. Int J Min Reclam Environ 2014;29(3):201–12. [20] Toraño J, Torno S, Menéndez M, Gent M. Auxiliary ventilation in mining roadways driven with roadheaders: validated CFD modelling of dust behaviour. Tunn Undergr Space Technol 2011;26(1):201–10. [21] Toraño J, Torno S, Menendez M, Gent M, Velasco J. Models of methane behaviour in auxiliary ventilation of underground coal mining. Int J Coal Geol 2009;80(1):35–43. [22] Wang K, Wu Z, Zhou A, Jiang Y, Feng S. Experimental study of high concentrations of coal mine methane behavior in downward ventilated tilted roadways. J Wind Eng Ind Aerodyn 2016;158:69–80. [23] Wang K, Zhou A, Li S. Computer simulation of dynamic influence of outburst gas flow on mine ventilation network. Disast Adv 2012;6(9):31–8.
References [1] Kursunoglu N, Onder M. Selection of an appropriate fan for an underground coal mine using the Analytic Hierarchy Process. Tunn Undergr Space Technol 2015;48:101–9.
Please cite this article in press as: Zhou A et al. Influence of gas ventilation pressure on the stability of airways airflow. Int J Min Sci Technol (2017), https:// doi.org/10.1016/j.ijmst.2017.09.004
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Influence of gas ventilation pressure on the stability of airways airflow Zhou Aitao a,b, Wang Kai a,b,⇑, Wu Longgang b, Xiao Yiwen b a b
Beijing Key Laboratory for Precise Mining of Intergrown Energy and Resources, China University of Mining & Technology, Beijing 100083, China School of Resource and Safety Engineering, China University of Mining & Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 18 January 2017 Received in revised form 15 May 2017 Accepted 9 September 2017 Available online xxxx Keywords: Gas accumulation Gas ventilation pressure Airflow stability Underground ventilation
a b s t r a c t Coal mine ventilation is an extremely complicated system that can be affected by many factors. Gas ventilation pressure is one of important factors that can disturb the stabilization of airflow in airways. The formation and characteristics of gas ventilation pressure were further elaborated, and numerical simulations were conducted to verify the role of gas ventilation pressure in the stability of airway airflow. Then a case study of airflow stagnation accident that occurred in the Tangshan Coal Mine was performed. The results show that under the condition of upward ventilation, the direction of gas ventilation pressure in the branch is the same to that of the main fan, airflow of the branches beside the branch may be reversed. The greater the gas ventilation pressure is, the more obvious the reversion is. Moreover, reversion sequence of paralleled branches is related to the airflow velocity and length of the branch. Under the condition of downward ventilation, the airflow in the branch filled with gas may be reversed. Methane in downward ventilation is hard to discharge; therefore, accumulation in downward ventilation is more harmful than that in upward ventilation. Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Coal mine ventilation system plays important roles in the underground mining. It offers a sufficient quantity of air to the underground mine workers to dilute methane and other contaminants, maintain a suitable working environment and prevent accidents [1–3]. During the mining operating, the status of the ventilation system cannot simply be kept constant. Generally, coal mine ventilation is an extremely complicated system. A large number of influencing factors can control or impact the behaviors of the system [4,5]. These include the ventilation network geometry (diagonal network, airway resistance, etc.), the location and operation characteristics of each of the other fans in the system and other external influencing factors [6]. Natural ventilation pressure is an important form of the external disturbance factors and has been observed ever since the beginning of underground mining. Natural ventilation pressure in underground constructions can help to optimize the design of their ventilation systems, reduce their energy consumption and avoid the risk of accumulation of gases or toxic agents [7]. However, natural ventilation pressure can also induce airflow reversal or reduce the airflow rate of some
⇑ Corresponding author at: Beijing Key Laboratory for Precise Mining of Intergrown Energy and Resources, China University of Mining & Technology, Beijing 100083, China. E-mail address: [email protected] (K. Wang).
airways in mine ventilation. Scholars worldwide have conducted extensive research in the field of natural ventilation pressure [8–11]. Natural ventilation pressure is produced by the difference in air density between the intake and return airways. Previous studies have shown that the difference in air density is determined by air temperature and airway elevation difference. Therefore, natural ventilation pressure always occurs in airways with differences in elevation and temperature [12–15]. For instance, when fire occurs in an underground mine, the smoke flows along the airway airflow direction and the airflow temperature in the airway increases as the smoke spreads. Thus natural ventilation pressure is induced by fire if the airway has an elevation difference, which can be named as fire ventilation pressure. However, apart from temperature differences, natural ventilation pressure also can be produced in an airway with an elevation difference by the accumulation of gas, which is called gas ventilation pressure in this paper. Gas ventilation pressure is similar to that of a fire ventilation pressure due to the gas density and elevation differences [16–21]. However, the density difference of a fire ventilation pressure is caused by the air temperature difference between the intake and the return shafts of the mine, whereas the gas ventilation pressure does not consider the impact of temperature changes on the gas density. Because of the density difference between air and methane, the average density of the airflow changes when highconcentration methane flows into the airway. The gas ventilation pressure can be observed as increments in the potential energy
https://doi.org/10.1016/j.ijmst.2017.09.004 2095-2686/Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Zhou A et al. Influence of gas ventilation pressure on the stability of airways airflow. Int J Min Sci Technol (2017), https:// doi.org/10.1016/j.ijmst.2017.09.004
2
A. Zhou et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Table 2 Characteristic curve data of the fan.
Fan pressure (Pa) Airflow rate (m3/s)
First point
Second point
Third point
385 16
248 37
148 52
3. Analysis of gas ventilation pressure under different ventilation ways on mine ventilation network Fig. 1. Simple mine ventilation network graph.
in the airway. Gas ventilation pressure is often neglected by researchers and field staff, but it plays an important role in the stability of airway airflow in underground mines. This phenomenon can lead to airflow stagnation or airflow reversal in normal airways. For instance, an accident involving the cessation of the airflow in an airway had occurred in one inclined airway of the Tangshan coal mine in China. At first, the airflow stagnation accident was regarded being caused by the roof falling, but after a detailed site investigation was conducted, the results clarified that the accident was a result of gas accumulation. This paper aims to explore the mechanism by which the stability of airway airflow in underground mines is induced by gas ventilation pressure, to present measures to prevent airway airflow disasters caused by gas ventilation pressure and to propose advice for mine ventilation design.
The roles of gas ventilation pressure play in mine ventilation network were conducted by the unsteady ventilation network calculation program [22,3]. 3.1. Basic models and initial conditions The designed simple mine ventilation network is shown in Fig. 1. As shown in Fig. 1, branch 2 is filled with methane due to a coal and gas outburst, the methane concentration was 100%. By setting elevation differences of branches, the upward ventilation and the downward ventilation can be simulated. Basic parameters of branches were listed in Table 1, the characteristic curve data of the fan were shown in Table 2. The methane density is 0.717 kg/ m3, air density is 1.225 kg/m3, the calculation time was 2400 s, time interval was 1 s, and space interval was 5 s. 3.2. Influences of gas ventilation pressure on the mine ventilation network under the condition of upward ventilation
2. Formation and characteristics of gas ventilation pressure 2.1. Factors that influence on gas ventilation pressure Gas ventilation pressure is caused by the accumulation of gas in inclined airways, the accumulated gas may induced by coal and outbursts or gas emission for coal seam in the conditions of the ventilation system fails. It can be regarded as one form of natural ventilation pressure. The amount of gas ventilation pressure is mainly affected by the gas concentration and the elevation difference in the airway. Temperature is not considered as a factor when a high concentration gas accumulates in the airway. Because the density of methane is lower than that of air, the density of the mixed gas changes. When the airway also has an elevation difference, gas ventilation pressure is formed. Gas ventilation pressure is calculated as follows:
hM ¼ ðq qa Þ½zð0Þ zðLÞg
ð1Þ
where hM is the gas ventilation pressure of airway, Pa; q the average density of the airway airflow after mixing with gas, kg/m3; qa the density of air, kg/m3; zð0Þ and zðLÞ respectively the elevations of the beginning and end junctions, m.
3.2.1. Gas ventilation pressure and the airflow rate of branches Fig. 2a presents gas ventilation pressure variation with time under the condition of upward ventilation, Fig. 2a shows airflow rate variation with time under the condition of upward ventilation. As shown in Fig. 2, it can be concluded as follows: (1) Under the condition of upward ventilation, the direction of gas ventilation pressure produced by methane accumulation in branch 2 is the same to that of the main fan. Under the influences of gas ventilation pressure, airflow rate of branch 2 increases while airflow rate of branches beside branch 2 (branch 3 and branch 4) decreases gradually. (2) At about t = 12 s airflow rate of branch 4 reduces to zero and about t = 14 s airflow rate of branch 3 reduces to zero. The initial airflow rate of branch 3 and branch 4 are 6.4 m3/s, 4.2 m3/s respectively, the larger the airflow rate, the later the airflow reversals. (3) The total airflow rate (branch 5) of the mine gradually increases. And then it increases from normal volume of 25.8 m3/s to maximum volume of 47.0 m3/s at t = 173 s. Then it decreases.
Table 1 Basic parameters of branches. Branch number
From
To
Resistance (N s2/m8)
Length (m)
Area (m2)
Circumference (m)
Methane concentration (%)
1 2 3 4 5
1 2 2 2 3
2 3 3 3 1
0.125 0.131 0.720 1.631 0.314
1020 980 980 980 620
9.20 7.62 6.14 6.14 9.20
14 12.1 11 11 14
0 100 0 0 0
Elevation difference (m) Upward
Downward
300 120 120 120 180
300 100 100 100 400
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A. Zhou et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
3
Fig. 2. Gas ventilation pressure and airflow rate variation with time under the condition of upward ventilation.
Fig. 3. Variation of methane concentration along the branch axial under the condition of upward ventilation.
Fig. 4. Gas ventilation pressure and airflow rate variation with time under the condition of downward ventilation.
(4) Methane flows into branch 3 and branch 4 induces airflow reversals of the two branches. With the increased methane concentration in branch 3 and branch 4, gas ventilation pressure of two branches increases while that of branch 2 decreases, as shown in Fig.2a. (5) At about t = 107 s, the airflow directions of branch 3 and branch 4 returns to normal and methane gradually flows into return airway (branch 5). (6) After all methane flows out of branch 5, gas ventilation pressure of each branch disappears and the mine ventilation network returns to normal.
(2) Methane concentration distribution of branch 3 is shown in Fig. 3b. With the reversion of airflow in branch 3, partial methane flows into the branch from the ending point. But the reflux length is only 180 m as the airflow returns to normal soon. 3.3. Influences of gas ventilation pressure on the mine ventilation network in the condition of downward ventilation
3.2.2. Methane concentration distribution laws In order to analyze methane concentration distribution laws, branch 2 and branch 3 were selected under the condition of upward ventilation, the results are shown in Fig. 3a and b.
3.3.1. The gas ventilation pressure and the airflow rate of branches Fig. 2a presents gas ventilation pressure variation with time under the condition of downward ventilation, Fig. 2a shows airflow rate variation with time under the condition of downward ventilation. The initial condition is the same to that of upward ventilation. As shown in Fig. 4, it can be concluded as follows:
(1) Methane concentration of branch 2 decreases gradually. As shown in Fig. 3a, for t = 30 s, methane concentration at the 200 m branch 2 is 100%, however, for t = 60 s, methane concentration at the same position of branch 2 is 0%.
(1) Though branch 2 is still filled with methane at the initial time, gas ventilation pressure induced by methane concentration in branch 2 is opposite to the main fan pressure and hinders mine ventilation as. Airflow rate of branch
Please cite this article in press as: Zhou A et al. Influence of gas ventilation pressure on the stability of airways airflow. Int J Min Sci Technol (2017), https:// doi.org/10.1016/j.ijmst.2017.09.004
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A. Zhou et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx
Fig. 5. Methane concentration variation with time under the condition of downward ventilation.
Fig. 6. Simplified ventilation system.
decreases in the initial phase and those of side branches (branch 3 and branch 4) increases. At about t = 7 s, airflow of branch 2 is reversed. (2) Total airflow rate of the mine network gradually decreases. And then it decreases from normal volume of 25.8 m3/s to minimum volume of 1.5 m3/s. 3.3.2. Methane concentration distribution laws Methane concentration distribution of branch 2 and branch 3 at different times in the condition of downward ventilation are respectively shown in Fig. 5a and b. Methane concentration in branch 2 gradually decreases, for the 600 m of the branch 2, the methane concentration at 60 s is 100%, and the methane concentration reduced to less than 10% at 300 s, however, compared with the upward ventilation, the rate of descent is significantly lower than the upward ventilation. Meanwhile, some methane flows into branch 3 from its ending point, and methane concentration in branch 3 increases. 4. Case study of airflow stability caused by gas ventilation pressure 4.1. Mine overview Tangshan coal mine is selected for case study, the coal mine is one of the oldest coal mines in China, with 130 years mining history. Production continues in the Yuexu working area on the western border of the coal mine. The simplified ventilation system is shown in Fig. 6. Airway c and airway are two inclined airways,
which are arranged in the coal seam, the composition of the seam is methane, the gas content of the coal seam is 3 m3/t, and the thickness of the coal seam is 3 m. The dips of these two airways are both 30°, and they are connected by airways d and k. The elevation of airway k is 526.7 m and that of d is 451.8 m. The two airways are full-pressure ventilation systems with 6 m3/s airflow rate. The methane concentration of the return airflow is 0.35% at 6 m3/s airflow rates. Ventilation doors are installed in airway k. The simplified ventilation system is shown in Fig. 6. The parameters of the simplified ventilation system were shown in Table 3. And all the airways in the simplified ventilation system with same height and width (the height is 3 m, the width is 3.33 m), the operating point of the main mine fan is 3.4 kPa @ 456 m3/s, the minimum regulatory air velocity required for the situation is 0.5 m/s, the airflow rate in the airways b, c, d, e is 6 m3/s. 4.2. Numerical simulations using an unsteady ventilation program To further analyze the airflow stability induced by gas accumulation, gas ventilation pressure is interpreted using an unsteady ventilation program, and numerical simulations were conducted by the program [23]. The initial conditions are as follows (in Table 3): The methane concentrations in airways c, d and e were 60%, 100% and 60%, respectively, and in the other airways, it was 0%. The density of methane is 0.72 kg/m3, the density of air is 1.29 kg/m3, the fan is installed in airway h, and the fan pressure is 1735 Pa, the airflow rate is 12.752 m3/s. Based on the initial conditions, the solutions for the initial airflow rate for each airway using the unsteady ventilation program are shown in Table 4. They are all within the scope of the field test results. The airflow rate distribution of airway c is illustrated in Fig. 7. As shown in Fig. 7, if the regulator in airway L is opened, methane accumulates in airways c, d and e. A high concentration of methane hinders airflow. The airflow rate in airway c deduced from the program is 2.7 m3/s. This value fluctuates slightly, but
Table 3 Parameters of the numerical simulation. Airway
Length (m)
Area (m2)
Circumference (m)
Resistance (N s2/m8)
Methane concentration (100%)
Elevation difference (m)
a b L c d e f g h
1500 1050 656 110 20 80 244 600 1500
10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
13.0 13.0 13.0 13.0 13.0 13.0 13.0 13.0 13.0
4.3588 2.7605 0.3204 0.6102 0.0581 0.5231 0.4184 1.7435 4.3588
0.0 0.0 0.0 0.6 1.0 0.6 0.0 0.0 0.0
0.0 133.0 110.0 75.0 0.0 75.0 23.0 110.0 0.0
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A. Zhou et al. / International Journal of Mining Science and Technology xxx (2017) xxx–xxx Table 4 Initial airflow rate of airways. Airway 3
Airflow rate (m /s)
A
b
L
c
d
e
f
g
h
12.752
2.717
10.035
2.717
2.717
2.717
2.717
12.752
12.752
Fig. 7. Airflow rate distribution of airway c after gas accumulation.
after a short period, the final airflow rate remains at zero. The simulation results agree well with the case description and field test. 5. Conclusions (1) When methane accumulates in airways, gas ventilation pressure is generated in airways with an elevation difference because the density of methane is lower than that of air. Gas ventilation pressure can be regarded as an increment of natural ventilation pressure. (2) Under the condition of upward ventilation, the direction of gas ventilation pressure is same to that of the main fan and airflow of the branches beside the branch may be reversed. The greater the gas ventilation pressure is, the more obvious the reversion is. Moreover, reversion sequence of paralleled branches is related to the airflow velocity and length of the branch. (3) Under the condition of downward ventilation, the airflow in the branch filled with gas may be reversed. The greater the gas ventilation pressure is, the easier the airflow reversion is. (4) Methane in downward ventilation is hard to discharge; therefore, accumulation in downward ventilation is more harmful than that in upward ventilation.
Acknowledgments This research is financially supported by the National Natural Science Foundation of China (Grant Nos. 51774292, 51474219, 51604278), the State Key Research Development Program of China (Grant Nos. 2016YFC0801402, 2016YFC0600708).
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Please cite this article in press as: Zhou A et al. Influence of gas ventilation pressure on the stability of airways airflow. Int J Min Sci Technol (2017), https:// doi.org/10.1016/j.ijmst.2017.09.004