Explosion risk of gas and coal dust exists in more than 60% coal mines and 87.4% state-owned coal mines in China. Hence, the characteristics and prevention of gas explosion need further research[1- 2].

In developed country, mines for experiments of gas and coal dust explosion were established to investigate the flame propagation characteristic and pressure trend in gas and coal dust explosion. In 1982, a total length of 896 m (710 m for experiment) lane, which section was similar to the real coal mine in China, was built by Chongqing Branch of China Coal Mine Research General Institute. It is the first experimental lane and lays the foundation of preventing gas and coal dust explosion in China. Some experimental researches were conducted with that lane[3-5]. Recently, the small size tube was utilized in gas and coal dust experiment. New techniques such as high speed photograph and schlieren were adopted in the experiment of gas explosion and the results were demonstrated more directly and accurately. In the small size tube, the influence of obstacles in the explosion shock wave and flame propagation attracted researchers attention[6] and it is important to study the flow field distribution around the refuge chamber as an obstacle in lane[7]. In the explosion, the evolution of flame and shock wave links to the degree of the accident damage. Compared with experiment, simulation can be of lower cost, more efficient and more convenient in parameters analysis. The propagation of gas explosion shock wave[8-14] and the impact resistance of refuge chamber[15-18] were analyzed and discussed via experiment and simulation. However, the load in some previous studies may not agree with the fact and the characteristics of gas deflagration need to be investigated further.

与他相反，刘雁衡教外国文学史，却常常跑到中国文学上来。在说到某部外国经典时，总是抗衡似的，举出中国相应的一部著作，与洋鬼子理论一番，切磋一番。这使得学生广为不满，女生甚至给他起了个雅号：大唐进士。幸好，刘雁衡在报纸副刊发表的新诗，都是呼唤光明与民主的，这才抵消了学生的不满心理。

Self-sustained deflagration can accelerate constantly and transfer to detonation with a proper boundary condition, which should be considered in the accident prevention. In detonation, leading shock wave ignites the mixture while the chemical energy released from ignition, and it can sustain the detonation wave propagating in supersonic[2]. Characteristics of CH4 explosion were discussed and discovered by some Chinese researchers[19-22]. Gas can be seen as a hazard leading explosion in coal mine while more than 95% of gas is CH4 so that it can be chosen instead of gas in study. Because CH4 is instable, which is of low sensitive and high detonation limit pressure, it is difficult to obtain accurate experimental results. There are hardly discussion and conclusion about premixed CH4-O2 mixture. In this paper, flow field load distribution for coal mine lane and pressure load for each part of the refuge chamber are analyzed and discussed via experiments in a small size tube and simulations in the real size.

1 Simulation for Flow Field Load of Refuge Chamber in the Coal Mine Lane

The finite element model for premixed CH4+2O2 mixture explosion in a lane was established and AUTODYN was utilized to calculate the parameters of explosion wave propagating in the lane. The load against the surface of the refuge chamber was due to an instantaneous explosion source in the simulation. Pressure of CH4+2O2 mixture explosion could 20 times higher than the initial pressure by experience, and the accurate numerical results are determined by the initial pressure and type of mixture. If the explosion pressure peak was 0.6 MPa, 0.6×25 MPa should be the initial value in the simulation. The corresponding explosion wave and initial pressure were calculated with the initial pressure 21.6 kPa.

1.1 Flow fieldload model

In Fig.15a-Fig.15f, the maximum displacement is at the middle cabin and middle and upper part of the front face, which is 15.12 mm maximumly. Refuge chamber maximum deformation is lower than 20 mm, and no local fracture or crack exist.

A fluid-structure interaction method was utilized to calculate and analyze the structural dynamic response of the refuge chamber under the pressure load which was calculated in the coal dust explosion simulation. ANSYS/LS-DYNA was used to analyze the response of the refuge chamber structure under the shock wave changing with the time.

Fig.1 Simplified physical arched lane model

In the model,the length of explosion segment was 28 m, while the length of shock wave propagation segment was 100 m and the one of refuge chamber was 22 m. The explosion source and air area were built of solid element, the outlet was determined as outflow and other boundaries were rigid walls. Mesh size was 100 mm. As Fig.2, flow field detection points were set at front and rearface (detection point 2 and 27), the lateral middle position of each section (detection point 3, 4, 5, 6, 7, 20, 21, 22, 23, 24, 25, 26), the top surface of each section (detection point 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19) and it is 1 m in front of the refuge chamber (detection point 1).

Fig.2 Meshing and flow field pressure detection points distribution of the chamber

1.2 Results and discussion

Fig.3 Pressure-time history of the chamber in the lane

The pressure-time history of the chamber is shown in Fig.3. The flow field pressure is shown in Fig.3a. The detection points at the front and rear face are shown in Fig.3b and Fig.3c. There are 12 sections in the refuge chamber. In Fig.3d, curves c(1)-c(12) demenstrate the pressure on the lateral of each section, while curves d(13)-d(24) are the pressures on the top surface. In Fig.4 and Fig.5, the pressure of the shock wave rises suddenly at the front surface, then propagates along the chamber to the rear surface in a short time (10-1 s magnitude). The process of impact on the refuge chamber comes to end.

Fig.4 Corresponding pressure-time history of detection points on lateral surface of the chamber

Fig.5 Corresponding pressure-time history of detection points on upside of the chamber

根据压汞分形原理，进行直线拟合后分形维数D=3-K(表2)，不同类型储层分形维数取数范围见表3。由于笔者所选样品经计算处理后，数据点表现为末端平缓的曲线(图2)。因而采用了分段直线拟合的方式，分段点对应的孔径值分别为10 000、1 000、100 nm，与前述孔径划分对应，发现本次选取的灰岩样品中随着孔径的增大，表现出直线斜率变化更为明显，拟合度也变高的趋势。又由于本文所取灰岩样品微米中孔占据比例较大，所以主要选取d>10 000 nm的孔隙分形维数值进行分析，通过观察计算结果可以发现，5个样品微米中孔分形维数处于2.666 9～2.840 3，相关系数在0.94以上，具有较强的相关性。

Fig.6 Peak pressure change of each segment

2 CH4+2O2 Mixture Detonation in Small Size Tube

Fig.7 Design of the rectangular tube

The shock wave time delay in different parts cannot be ignored in the simulation according to the waveform and characteristics of explosion shock wave mentioned before. In the lane, the calculated flow field pressure load was determined as the pressure load on the refuge chamber. Single domain size was less than one cabin surface size and the maximum load in the domain was considered as the pressure load.

Fig.8 Installation of the rectangular tube

The length×width×height of KJYF-96/12 refuge chamber is 10 662 mm×2 072 mm×1 800 mm, skin and flange is 34 mm, respectively. In the simulation, the model was established as the real size. Solid beams and columns were set as solid elements, hollow beams and columns as shell elements, shell structure as shell elements. Structural mesh was used mostly to discretize the refuge chamber. On the each side of solid beams and columns section were more than 2 rows. The mesh size at shell elements was 5-20 times larger than shell structures. Key area and small components were in the real size. The domain was determined by the boundary if the boundary existed around the refuge chamber was 0.5 times less than the structural maximal size. The maximum size in the shell element was 50 mm, in the solid element as 25 mm, and in the small component as 10-15 mm. Shell element, solid element, and rigid element were 548 975 totally (Fig.12). Component and materials of the refuge chamber were shown in Tab.1 and parameters of materials in Tab.2. Fixed or simply-supported connection was chosen due to the real connection method and the connection points, the parts and method should be identical with the refuge chamber in reality.

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Fig.9 Velocity of CH4+2O2 mixture detonation under different initial pressures

Δt1=77.6 μs, Δt2=75.2 μs, Δt3=78.8 μs,Δt4=76.8 μs, and Δt5=76.4 μs were measured on the oscilloscope and the pressure-time curves are in Fig.10 and Fig.11. Under the initial pressures 5 kPa and 10 kPa, the peak pressures are 1 MPa and 2 MPa, respectively. The shock wave form in the small tube experiment illustrates that the pressure-time curve will change with the time. The differences about the peak pressure and propagation time cannot be ignored.

Fig.10 Pressure-time curve under initial pressure of 5 kPa

Fig.11 Pressure-time curve under initial pressure of 10 kPa

3 Impact Simulation on the Refuge Chamber

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3.1 Model parameter

The velocity of CH4+2O2 mixture detonation under different initial pressures is shown in Fig.9. The results can prove that the detonation is stable in the experiment.

3.2 Results and discussion

A small size rectangular experimental tube was built (Fig.7) to study the explosion shock wave in the lane. The rectangular tube consists of two plates and an aluminium alloy plate which was slotted on each side to fit the rubber seal ring. In Fig.8, two pressure transducers were fitted on the aluminium plate and the distance is ΔL=20.4 cm, while Δt was the peak distance of two waveforms on the oscilloscope. CH4+2O2 mixture was pre-mixture for more than 24 h to ensure an intensive mixing.

Fig.12 Mesh generation of KJYF-96/12 refuge chamber

Tab.1 Structures and materials of KJYF-96/12 refuge chamber

StructuresMaterialThickness/mmNoteMaincabindoorQ345-B18FlangeatmaincabindoorQ345-B30EmergencyexitQ345-B30FlangeatemergencyexitQ345-B22FlangeatjointofcabinQ345-B35SkinQ345-B12StiffenerincabinQ345-BWeldedTsteel

Tab.2 Material characteristic parameters of KJYF-96/12 refuge chamber

ParameterQ345-BDensity/(g·cm-3)7.85Modulusofelasticity/GPa206Poissonratio0.3Yieldstress/MPa345Tensilestrength/MPa510-660

The detection points diagram is shown in Fig.13a, the field pressure load was calculated and the pressure-time history on all parts are shown in Fig.13b-Fig.13e.

Fig.13 Detection points and pressure-time history on all parts of KJYF-96/12 refuge chamber

Fig.14 Stress distribution for KJYF-96/12 refuge chamber

In Fig.14a-Fig.14f, a stress distribution is demonstrated from low to high. With the explosion wave propagating, stress change can be observed at the front surface of the cabin. The maximum stress is 339.3 MPa, at the connection of the door frame and the connection of door handle, which is lower than the yield strength.

The arched lane with the height as 2.6 m and the width as 3.2 m was built as Fig.1. Considering the influence on the explosion waveform, the refuge chamber was at the geometric center in the horizontal direction.

The explosion shock wave form can change with the propagating distance and time. The time of shock wave propagating at each segment is different. These can affect the structural response of refuge chamber under the explosion. In fact, the explosion shock wave propagates constantly so that it is difficult to record waveform in time and calculate the structural response. In the simulation, pressure results are higher and attenuation coefficient is smaller than one of experiments, for which the gravity of gas, heat loss and roughness at tube wall were not considered. The peak pressure change of each segment is in Fig.6.

Fig.15 Displacement distribution for KJYF-96/12 refuge chamber

4 Conclusions

① The explosion initial pressure was calculated with “Chemical Equilibrium Analysis”. The lane with a refuge chamber model was built in real size and the flow field pressure load and other pressure loads on the cabin were calculated.

② The pressure-time curve in the small size tube agrees with the waveform change in the simulation, which validates the model of CH4+2O2 mixture and the explosion load.

③ The inpressure-time curve and the explosion wave form changes with the distance and time. The peak pressure and propagation time are the main different points which can have influences on the simulation. Flow field load history was analyzed and more accurate pressure load on the refuge chamber were calculated, which is important on the method and practical applications for preventing coal mine gas explosion.

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吉卜林的人格构成是复杂的。因为在印度生长，回到英国后的吉卜林并没有成为英国亲朋好友之间受欢迎的对象。后来吉卜林在自传里把这段时间称为“凄凉之屋里的恶意折磨”。这与他的“不良”习气以及肤色有关。“他不同于大多数英国人的一个特征是皮肤深黑”，“长大以后在他的英国同伴之中，他也常常成为不受欢迎的对象”。在英国接受教育的日子里，吉卜林远离父母，孤苦无助，甚至受到虐待。

对于邓强提出的上诉意见，经查，广东省纪委专案组对邓强立案并采取“两规”调查措施之前，省纪委已经掌握其受贿犯罪线索，且邓强在接受调查问话时，开始并没有交代自己的犯罪事实，过了几天才交代，故不构成自首。另外，省纪委事前已掌握肇庆非法采砂及销售有关问题线索，邓强反映的情况对侦破有关案件没有起到实质性作用，因此，邓强的行为也不构成立功。