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Procedia Engineering ProcediaProcedia Engineering 00 (2011) Engineering 26000–000 (2011) 1851 – 1856 www.elsevier.com/locate/procedia
First International Symposium on Mine Safety Science and Engineering
Experimental study on inhibiting the gas and coal dust explosion by water mist in tube with obstacle Li Zhen-fenga,Cao Shao-longb,An Anb,Hu Pengb, a* a
School of Safety Science and Engineering,Henan Polytechnic University, Jiaozuo 454003,China b Zhengzhou Coal Administrative bureau, Zhengzhou 450015,China
Abstract The ordinary water mist has well affected on suppression and extinguishment flame, but the ability of physical extinguishment is influenced by the surrounding environment. The roadway in coal mine is very complex. So the related experiments need be considered. The light-sized experiment of the water mist is built for taking mine environment analog simulation with the pipeline. It is suppressed by medium pressure water mist the experiment of gas and coal dust explosion propagation in the alone laneway. Obstacle is laid on the different positions, whose blockage ratio is 30%. Then we compare the effects of suppression on gas flame with the different location. The experimental results indicate that the obstacle is placed in the front, middle and end of mist area. The propagation velocity of gas flame is different. The gas content and blockage ratio is set low in experiment. It can achieve better inhibition effect that the obstacle is placed in the middle or end of water mist area. Thus, if the obstacle can't be avoided in water mist system, the designer must consider the position of obstacle. It can enhance the suppression effect of water mist.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of China Academy of Safety Science and Technology, China University of Mining and Technology(Beijing), McGill University and University of Wollongong. Keywords: water mist; obstacle; flame propagation velocity
Introduction With the development of Chinese economy, the output of coal has increased year by year. There are lots of accidents with coal production at the same time. For example: coal and gas outburst gas explosion, coal dust explosion, mine fire, water inrush, roof caving and so on. From the statistic of accident, it is higher proportion of the gas explosion. So the reason of gas explosion is need to be analyzed. The
* Corresponding author. Tel.:13839126396; 454000. E-mail address:
[email protected]
1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.11.2376
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corresponding scheme can be work out to prevent and control disaster. The gas explosion needs to be controlled effectively. It has great significance for coal mine safety production [1, 2]. With the advancement of science and technology, water mist is a new fire suppression technology to attract people attention. Because the water is a extinguishment substance which have environmental protection and low-cost character. It has a great advantage than other fire extinguishing agent. A considerable amount of research has been studied by many scholars at home and abroad [3] . For example: Liu Xuanya and others have researched the gas explosion which is inhibited by water spray [4]. Chen Xiaokun and others also have studied gas explosion. It is controlled by water mist which contain additive [5] . Mining flameproof and suppressing device is an important equipment to control gas explosion. At the time of the gas explosion, the spread of the explosion can be prevented by the preset devices, and also the fire range is limited. It has come true that gas explosion is controlled by water mist in coal mine. But the circumstance of coal mine is very complex. So the related experiments need be considered. When pipeline is installed in coal mine, it will protrude some pipes in binding site. Tramcar is left over in the roadway. These are equivalent obstacle which has low blockage ratio (the size of baffle area in proportion to the size of tunnel cross section). On the base of the existing research, the gas explosion flame can be accelerated by the obstacle [6, 7]. If the coal dust is included in the gas explosion, the explosive reaction will become much greater [8]. Qin Youhua and others have study the Influence of Obstacles and Coal on the Flame Propagation [9]. Xie Bo and others study that flame acceleration and overpressure influenced by baffle-obstacle [10]. Lin Boquan and others study that flame transmission and explosion wave in gas explosion is influenced by obstacle [11]. Through previous studies, flame propagation velocity is also affected with the boundary condition of the confined spaces and form of the barrier [12, 13]. Wang Zhiqing and others through the experiment discovers that gas explosion is affected with sharp of obstacle [14]. Based on the results of experiments, Starke and others find the flame propagation velocity can be accelerated in cylindrical enclosures with obstacles [15]. 1. Experiment Air intake and extraction are installed in the 3×3×3 limited space. When the door is closed, the space is filled with air through the intake. And the toxic gas is exhausted by extraction. An experiment system of suppression premixed gas by water spraying is set up. According to the combination, the system can be divided into five categories. As shown in Fig.1. Water mist fire protection system Gas explosion test pipeline
Gas explosion ignition system Dynamic data acquisition system
High-speed photography system
Fig.1 The experimental system
Low concentration methane and coal dust are introduced in premixed gas. The flame and explosive wave of mixed gas explosion is inhibited by water mist with obstacle. The experimental data invol ve
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flame propagation speed、pipeline temperature and image flame characteristic. 1.1 Experiment equipments and methods The experimental pipeline is made of two segments. The one is the tube of ignition; the other is the tube of inhibition. The inner diameter is 168 mm. The end of the ignition is closed and fixed, and another side is closed with vent plastic film which is made from polythene. The nozzle of water mist is made in our laboratory. By LS-200 laser particle size analyzer measuring, the nozzle type is showed in table 1. Ⅱ Nozzle is selected, and the pressure is set into 1.8MPa. It is placed in the tube of inhibition. Table 1. The nozzles types and testing parameters
Sprayer type
SMD(µm)
Ⅰ Ⅱ Ⅲ
Flow ( L/min) 1.2(MPa)
1.4 (MPa)
1.6 (MPa)
1.8(MPa)
57.47
2.55
2.813
2.967
3.271
106
3.840
3.873
3.917
3.973
190.97
2.464
2.627
2.762
2.933
The picture of gas explosive flame is caught by the Lavision’camera systems-High Speed Camera which is produced in Germany Nikon Company. The shoot length is about 1000mm. For giving real coal mine situation; the coal dust is added in mixed gas. And the methane concentration is set 7%. This article analysis that the effort of water mist which mixed gas flame is suppressed in tube is influenced with obstacle position. So the different blockage ratio is not considered in experiment. The blockage ratio is set into 30% in every time. The experiment system also contain distribution machine、 spark plug ignition device、gas detector and so on. As shown in Fig.2 Fog area
water compression pump
coal dust
vent plastic film
sprayer
spark plug
obstacle
gas detector observation window of toughened glass
PC
R es tar P o wer
eed High sp
CH4
air
photo
gh Hi Sp eed Co r olle ntr
high speed camera
USB I/O p ort
P o wer
Camera controller
Fig.2. Schematic of the experimental system
1.2 Experimental procedure As shown in Fig 2, obstacle is placed in different position. It has been given the name: A, B and C. at the beginning of experiment, the coal dust which quality is 0.4g is put into ignition tube; an air blower is used to roll up the coal dust. And then the methane is put into. The gas is closed until the concentration
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become 7%. Spark plug is ignited to ensure enough energy which is not less than 375MJ [16]. The frame of High Speed Camera is set 1000fps. The premixed flame propagation and flame picture are collected. 2The experiment result and analysis 2.1 The mixed gas flame transmission speed The principle of flame transmission speed can be simplified as formula 1. The average speed in any time can be calculated.
v = ∆l (ti − ti −1 )
(1)
Type 1: v is the average speed of flame vanguard. m/s; Δl is the distance of flame vanguard between ti and ti-1, m. t i-the time of i, s. ti-1-the time of i-1, s.
Fig. 3. (a) Obstacle in the C position; (b) Obstacle in the B position(c) Obstacle in the A position
Obstacle is placed in position C. the flame propagation velocity of the mixed gas has been accelerated a lot across obstacle. Though it influenced by water mist, in most cases it is limited. Because the speed is increase very fast, the mixed gas also passes mist area with high velocity. Obstacle is placed in position B. the flame propagation velocity of the mixed gas is inhibited in the first half of the water mist. It also increases across obstacle, and then it is inhibited in the second half. The final result is that mixed gas pass mist area with slower velocity than A. Obstacle is placed in position A. Before flame reach the barrier, the flame propagation velocity of the mixed gas is suppressed to a very low velocity. These results show that the flame propagation velocity is influenced by obstacle position in water mist. 2.2 The pictures of flame passing obstacle in water mist
Fig. 4. (a) Obstacle in the C position; (b) Obstacle in the B position(c) Obstacle in the A position
Analyzing image: The obstacle is placed in position C. It is greater that the flame area and picture brightness are caught by high speed camera. It means higher energy for flame. When the flame spread to the obstacle, the turbulence is shaped around the obstacles. It leads to amplify surface area of combustion
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flame. The more methane and coal dust are absorbed in turbulence to attend explosive reaction. So the flame propagation velocity is accelerated, which in turn enhance the turbulence. When obstacle is positioned B and A. it is small that the picture of energy is caught by high speed camera. In picture c, the flame is suppressed by water mist before obstacle. There have two reasons to explain. (1) The droplet of water mist is granule shape in the air. Based on the sprayer type, there is different density in different position. Generally, the largest density lies vertically below the sprayer. When the turbulence is shaped around the obstacle, the droplet in the surrounding space also is absorbed in it. Water mist is evaporated, and a lot of heat is taken away. If the obstacle is placed in core region, there will be more droplet than other region. (2) Although the flame propagation velocity is accelerated in obstacle, it is inhibited by water mist in a long distance. The speed becomes slow. Experiments prove this relationship. If the position of obstacle is changed, the flame of the mixed gas will have been differently affected in water mist. 3Conclusions
The flame propagation velocity of the mixed gas will be accelerated crossing the obstacle in water mist. So the obstacle will be cleared up in mist area. When the existence of the obstacle is inevitable in mist area, the nozzle number and pressure need to be increased. Chemical additives could be added in water. These measures enhance inhibiting effect of water mist. If the obstacle is placed in the front of mist area, the water mist has a little effect on inhibiting flame propagation. If the obstacle is positioned at the middle and back of mist area, the water mist has a good effect on inhibiting flame propagation.
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