| Coal spontaneous combustion occurred in the the large space goaf is one of the major natural disasters during the mine production. It is a serious environment, health, and safety hazard throughout the world, which burns down masses of coal resources and triggers gas explosion, resulting in massive casualties and significant economic loss. The concealed fire in mine goaf exhibits a huge burned area and extends in a spatial development, rendering the traditional slurry, inert gas, retardant, etc, hardly to work restricted by the limited diffusion area or gravity of extinguish materials. The large-flow expansion foam has proved to be an efficient means of controlling the goaf fire. However, the unstable agent addition, the poor foam generator and the large pressure loss of foaming system, as well as the deficiency of the foam technique implementation in the mine field, severely limit its widespread application in coal mines. To overcome these shortcomings, the key technology and equipment of the large-flow preparation of extinguishing foam is proposed in the present paper. The main results and conclusions are obtained and summarized as follows.The principle and method of agent quantitative addition using the jet cavitation were put forward. The gas-liquid interface wave was observed and its movement rule was analyzed in detail. The mechanism for realizing the quantitative addition was expounded. The suction performance influenced by the inlet pressure, flow ratio and the cavitation region scale were tested. Experimental results show that the negative pressure in the suction region is about-94--95 kPa, close to but a little higher than the saturated vapor pressure. The maximum cavitation suction quantity is 0.086 kg/s and the maximum cavitation flow ratio is 19.5%. The downstream pressure, critical pressure ratio and the cavitation region length all decrease with the cavitation flow ratio increasing. The bubble diameter decreases with the cavitation flow ratio increasing at the throat cavity, while it keeps unchanged at the suction cavity, which creates a favorable condition for the stable liquid suction. The bubble diameter in the suction cavity is larger than that in the suction cavity. The simulated result show that the mixture density, pressure and velocity all make sudden changes in the vapor-liquid transition face. The vapor region shrinks until vanishes with the outlet pressure increasing. The vapor volume fraction decreases from 58.6% to 0. The velocity vector is from the scution inlet to the jet device interior, ensuring a stable absorption amount, which proves that achieving quantitative liquid suction using the jet cavitation is scientific and veracity.A new design of sprial jet foam generator was developed. It consists of a jet, a sprial nozzle, a diffuser and two composite concave nets. The foaming performance of the new foam generator was investigated and evaluated using a self-built experimental testing system for monitoring working liquid pressure, flow rate, outlet pressure, foam production quantity and foam expansion ratio. Experimental results show that increasing the foam liquid pressure corresponds to a slight linear increase in the intake gas pressure and a significant decline in the gas flow rate. The foaming effect is poor at a low liquid flow rate. As the liquid flow rate increases, the foam production quantity and foam expansion ratio both increase. The optimum working flow rate is 3.5-4.5 m3/h. The further increase in the foam production quantity is unobvious and the foam expansion ratio decreases instead when the liquid flow rate contiunes to increase. The intake air pressure increases significantly with the outlet pressure increasing especially at a larger foam liquid flow. The foam production quantity takes little change at a low outlet pressure, while it shows a sharp decline when the outlet pressure exceeds the critical outlet pressure. The simulated result show that the jet mixing device increases the gas-liquid mixability in the foam generator and homogenizes the velocity distribution in the outlet section, which attains better at a longer jet length. It completes an operating condition for the foam generator at a higer liquid pressure and low gas pressure supply. A higer turbulent kinetic energy arises in the upper gas inlet when the jet outlet lies in the front of gas inlet. The adverse condition is improved when the jet outlet is moved in the back of gas inlet, from which the turbulent kinetic energy is dramatically reduced.The flow characteristic of expansion foam in the porous goaf was revealed. A visualization goaf platform was built through designing the foam preparation and infusion system. The foam accumulation, diffusion, stability, plugging, inhibition and temperature reduction in the top goaf were studied. Test results indicate that the foam diffuses quickly at the initial infusion stage and presents a trapezoid frame in the goaf. Then the diffusion velocity shows a little decline at the horizontal direction due to the increase of foam flow resistance. The terminal diffusion velocity is 0.0084-0.0087 m/s in the goaf. The foam flowing through the fracture aisle is fast along with a large tensile deformation. The high foam expansion ratio corresponds to a fast foam attenuation and then a short foam half-life. The plugging time of foam in the porous goaf is about 10 min, with the plugging pressure of 6 kPa. The foam accumulates quickly at the top of goaf until extinguishes the open fire and the coal fire center temperature reduces from 632.6 ℃ to 38.5 ℃ during only about 20 s. Foam exhibits a better inhibition property on the coal than the conventional slurry. The numerical simulation results of foam flow in the full-scale porous goaf beased on ANSYS FLUENT show that the foam flow pressure and diffusion range increase with raising the infusion time. The infusion pressure rises during the infusion time 2-20 h. The foam gushes from the working face after infusing foam 20-24 h, while less than 10 m has been extended into the suffocative zone. The maximum pressure migrates from the axle line towards the interior goaf. The foam diffusion range is greater along the working face than that expands in both sides. The stack velocity of foam in the vertical direction is slow at the initial infusion stage, then it shows a fluctuation within a narrow range, which tends to be stable in the end.The foam technology used for extinguishing the goaf fire was developed based on the agent quantitative addition and sprial jet foam generator. Through definiting the formation cause and developmental feature of the large space mine fire, the specific technique implementation and extinguishing fire scheme are scheduled to prevent the goaf fire in the open pit mine and underground coal mines, along with the evaluation indicator of extinguishment effect. To evaluate the extinguishing performance using foam, field trials were carried out in the goafs of 4# coal, 1245 flat plate, Donglutian Open Pit Mine and No. 3103 working face, Zouzhuang Coal Mine. The field practice proved that the water consumption of foam technology was less, with a small proportion of foaming agent(0.5%-1%). The foam expansion ratio reached 70-100. The temperature reduction was remarkable and the fire was hard to rekindle. CO concentration in the goaf was significantly reduced. The foaming system has the advantages of convenient operation, strong practicality and high reliability, which successfully solves the prevention challenge of concealed mine fire. The large-scale mine fire is rapidly controlled, realizing an ultimate aim of fast fire extinguishment and creating a signigicant economic, environmental and social benefit for coal mines. |
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