Analysis of the coal seam spalling-failure mechanism based on the seepage instability theory.

Coal and gas outburst is a common coal-rock dynamic disaster. Such accidents frequently occur, and the mechanism underlying the occurrence of these outbursts is complex. As a typical failure mode of a gas-filled and pressure-relieved coal body, the spallation mechanism should be investigated to reveal the mechanism of coal and gas outburst and guide outburst-prevention strategies. In this paper, a fluid-solid coupling model for coal seam gas flow is established. This model considers the adsorption characteristics of coal. Numerical calculations are used to simulate the stress field distribution and evolution of gas-filled coal bodies under different boundary conditions. The mechanical mechanism of the spallation occurrence after the pressure relief of coal is explained from the perspective of seepage breaking coal. The control of the flow and stress state of the gas to the spallation failure is analyzed. The mechanical-quantitative conditions for the initial failure of the coal body under seepage and the mechanical-qualitative conditions for the continuous advancement and termination of spallation are studied based on numerical solution results. The numerical calculation results show that the formation of a flow field after pressure relief will apply a drag force (tensile stress) on the porous media of coal. The presence of this force plays a crucial role in promoting the spallation and cracking of coal and, thus, the promotion of spallation. The tensile strength, initial adsorption pressure, and pressure relief rate of the coal body jointly control whether the initial failure can occur and the thickness of the fracture layer cracks. Spallation propulsion is mainly determined by the pressure relief conditions of the undestroyed coal body and pressure changes in the spallation space; the former can be quantitatively obtained by numerical calculations, whereas the latter is related to the thickness of the spalled layer and the degree of the layer-crack structure

Study on Group Movement Characteristics in Stairwell of High-Rise Buildings

In order to cope with limited land availability and the increasing urban population, more high-rise buildings are being built throughout cities, but this has also led to new challenges in the emergency evacuation of people. Therefore, we conducted an evacuation experiment of a group of people in the stairwells of high-rise buildings to explore the movement law of groups in stairwells. The experiment had 18 scenes, including the evacuation movement of an independent group and the crowd evacuation movement of different groups. Analyzing the independent groups’ movement showed that regardless of the type of group, the average speed of the crowd was more than 1.2 m/s, which was longer than that of previous studies. The more constraints in the evacuation process, the smaller the speed difference between males and females. Group cohesion was mostly concentrated at about 1 s. Moreover, group velocity and cohesion were found to have no significant correlation in stairwell movement. Analyzing the crowd evacuation time of eight different scenes showed that it took longer for crowds bearing weight to evacuate than those not bearing weight. According to the spatiotemporal evolution of crowd evacuation in stairwells, the crowd moves intermittently in the evacuation process, especially in the early and middle stages of evacuation, and the influence of group factors on this process was not obvious. Regardless of the kind of scene or stairwell, the number of pedestrians generally first increased and then decreased to a significantly low value, and the number of pedestrians reached about 30 in the scenario of people without any load. Additionally, the speed of the pedestrians entering the stairwell in the back of the group was not faster than that of the pedestrians in the front, which showed that pedestrians rarely engaged in overtaking behavior. The research results of this paper not only enrich evacuation research but also provide support for the design of high-rise stairs

Experimental Study on Fire Suppression of the Outdoor Oil-Immersed Transformer by High-Pressure Water Mist System

Fire accidents due to oil-immersed transformers seriously threaten the safe operation of power systems. In this paper, the similarity principle was used to design a high-pressure water mist fire-extinguishing test platform for a small-scale transformer fire, and the design method achieved a good fire extinguishing effect. The results indicate that a deflagration phenomenon, lasting about 2–4 s, could be observed after activating the high-pressure water mist system; the flame temperature rose rapidly at first, then dropped sharply, and finally cooled to the indoor temperature. The nozzle’s flow rate in this system has a significant impact on the fire extinguishing time. Meanwhile, the adjustment of the upper nozzle height also influenced the fire suppression effectiveness of the system, where a height of 1800 mm achieved the best performance compared to the others. In addition, the ambient wind speed is a very unfavorable factor for transformer fire suppression, where the fire extinguishing efficiency decreases rapidly with the increase in wind speed. Therefore, under low wind speed conditions, the high-pressure water mist system has great advantages in the fire suppression of outdoor oil-immersed transformers, and the above research results can provide a reference for the optimization design of this system

Size dependence of electronic property in CVD-grown single-crystal graphene

The electronic performance of graphene is largely related to its morphology, surface, size and various synthesis conditions, mainly because of the presence of grain boundary. Better understanding on the relationship between the size and electronic property is very important for graphene\u27s applications in potential electronics. Herein, we selectively synthesized single-crystal graphene using a chemical vapor deposition (CVD) method. The obtained CVD-graphene exhibited various sizes, ranging from 20 μm to 120 μm. Our measurements of field effect transistor devices revealed that the charge carrier mobility of CVD-graphene could increase from 17.8 to 720 cm 2 V −1 s −1 with the size decreasing. The better electronic performance in comparable smaller-size graphene was ascribed to less grain boundary compared with the bigger one, which was further confirmed by our observations from scanning tunneling microscope/spectroscopy (STM/STS)