Shock Waves Generated in the Presence of Barriers in Gas Explosions

This paper examines conditions that generates shock waves in a gas explosion through experiment. The result shows that the presence of barrier and thin film, which is similar to air door and stopping in underground coal mines, is very important for the generation of shock waves in a gas explosion. When there are physical barriers, the speed of transmission would be very fast and shock waves will result in the process; when the film is destroyed in gas explosion, shock waves will also appear which would result in an increase in explosion power. Therefore, in order to weaken the gas explosion and prevent the generation of shock waves, the number of barriers should be reduced, and the air door and stopping in tunnels should be strengthened in coal mines. The result of research is very important to prevent gas explosion and decrease the power of such explosions

Electrical response and pore structure evolution affected by cyclical plasma breakdown

Artificial improvement of coal seam permeability is the key to solve the low gas drainage efficiency and prevent gas dynamic disaster. Plasma based on physical discharge is one of the effective means of coal seam antireflection. However, previous studies focus on the characterization of pore-fissure structure of single breakdown, ignore the polarization effect of plasma on coal, and lack of in-depth research on the electrical properties and the evolution characteristics of pore-fissure structure under the action of cyclical plasma of loaded coal. In this paper, the cyclical plasma breakdown experiments of coal samples were carried out. The voltage and current waveforms in the process of coal breakdown were monitored thorough combining with high-voltage attenuation rod and the Roche coil, and the variation of electrical parameters such as pre-breakdown period, plasma breakdown period, number of wave peaks, peak voltage, peak current and energy conversion efficiency were analyzed. The evolution of small pores, medium pores, large pores and micro cracks under the conditions of 1, 5, 10, 15 and 20 times of breakdown was tested by NMR. The fractal dimension of seepage pore was also discussed through geometric fractal theory. The results show that the pre-breakdown period is at the level of thousands of microseconds only for the first time, and then it will drop to tens of microseconds, while the plasma breakdown period shows a “ladder” growth with the increase of breakdown times. The energy conversion efficiency is maintained at 28.7%−55.9%, showing a trend of rapid increase at first and then stable, indicating that the polarization effect of plasma on the electrical properties of coal is limited. The growth rate of medium and large pores is the most obvious, and some micro fracture structures will achieve a “0” breakthrough, indicating that plasma will significantly improve the seepage of gas. The fractal dimension shows a decreasing trend after breakdown, which proves that the originally isolated pore structure will be connected by the fracture from the perspective of pore fracture space dimension. The similarity between the change trend of porosity and energy conversion efficiency shows that plasma has an impact on the electrical properties of coal, and the change of coal electrical properties will also have an impact on the distribution of plasma channels

Evolution of coal petrophysical properties under microwave irradiation stimulation for different water saturation conditions

Coalbed methane (CBM) reservoirs generally have very low permeability and requires stimulation to make gas extraction economical. Hydraulic fracturing has been widely applied to enhance CBM productivity, but this technology has a number of key limitations, including fractures only propagate along existing joints, large amount of water usage, and potential environmental impact. Microwave irradiation technology can likely overcome the above limitations. In this study, the effect of microwave irradiation on the petrophysical properties of an unconstrained bituminous coal was comprehensively investigated through a suite of integrated diagnostic techniques including Nuclear Magnetic Resonance and X-ray Computed Tomography. A series of experiments were conducted both on centrifuged samples and on samples with different water contents ranging from 1 to 15%. The mineral removal and moisture evaporation due to the microwave selective heating lead to the enlargement, opening, and interconnection of coal pores. The NMR-determined porosity increases linearly with the microwave power while grows exponentially with respect to water contents. When the water content is above 6%, the porosity increases by around 98~211%. The fracture volume and coal permeability increase while the P-wave velocity decreases with increasing water contents. Microwave irradiation is effective in enlarging mesopores and macropores and in enhancing the pore connectivity. The significant enhancement of coal permeability and pore fracture structure indicates that the microwave irradiation is effective in improving gas productivity thus has the potential to become a new CBM reservoir simulation technology

Influence of coupled effect among flaw parameters on strength characteristic of precracked specimen: Application of response surface methodology and fractal method

Hydraulic slotting is an effective method for enhanced coalbed methane (ECBM) recovery, and it has been widely employed in China. Although there have been many studies of this technique, the influence of slot parameters on the strength characteristic of the coal, which is an important factor that affects the permeability enhancement effect, has rarely been studied. Thus, only limited information is available regarding the pressure relief and permeability enhancement mechanisms of this technique. In the current study, the influence of flaw parameters on the compressive strength of a precracked sample under biaxial compression is discussed. The results indicate that an increase in the flaw length and width has a negative effect on the compressive strength, whereas an increase in the flaw inclination angle has a positive effect on the compressive strength. The results of the response surface methodology (RSM) indicate that the interactions among the flaw parameters have a significant influence on the compressive strength. The propagation patterns of cracks are quantified by the fractal dimension, which is used to explore the mechanism of compressive strength variation with changes in the flaw parameters. The study results indicate that the variation in the flaw parameters changes the propagation pattern of cracks, resulting in different compressive strengths. In addition, an opposite variation trend of the compressive strength and fractal dimension with flaw parameters is also observed. The research results are expected to guide the field application of hydraulic slotting

Bullous pemphigoid complicated with macroamylasemia: a case report

High amylase level is a typical sign of acute pancreatitis. However， it can also be found in other diseases. In the article， a case of persistent elevation of amylase level for over 1 year was reported. A 66-year-old male patient suffered from bullous pemphigoid complicated with diabetes mellitus for 3 years. After bullous pemphigoid treatment 8 months ago， he presented with elevated amylase level ranging from 556 to 1106 U/L， serum lipase level between 62 U/L and 73 U/L and urine amylase level between 553 U/L and 1162 U/L， respectively. The renal amylase clearance rate /creatinine clearance ratio was 0.8% （<1%）. The patient had no gastrointestinal discomforts. Abdominal color Doppler ultrasound （color ultrasound） and enhanced CT scan revealed that the pancreas was normal. Color ultrasound showed no abnormality in the thyroid and salivary gland. The possibility of salivary gland disease was excluded by stomatologist consultation. The patient was diagnosed with macroamylasemia. The patient reported no discomfort during the 8-month follow-up. The serum amylase level was gradually decreased but still higher than the normal range. This case suggested macroamylasemia should be considered when a patient present with persistent elevation of amylase level， without dynamic changes， gastrointestinal symptoms or elevated serum lipase level

Novel integrated techniques of drilling-slotting-separation-sealing for enhanced coal bed methane recovery in underground coal mines

Coal bed Methane (CBM), a primary component of natural gas, is a relatively clean source of energy. Nevertheless, the impact of considerable coal mine methane emission on climate change in China has gained an increasing attention as coal production has powered the country's economic development. It is well-known that coal bed methane is a typical greenhouse gas, the greenhouse effect index of which is 30 times larger than that of carbon dioxide. Besides, gas disasters such as gas explosive and outburst, etc. pose a great threat to the safety of miners. Therefore, measures must be taken to capture coal mine methane before mining. This helps to enhance safety during mining and extract an environmentally friendly gas as well. However, as a majority of coal seams in China have low-permeability, it is difficult to achieve efficient methane drainage. Enhancing coal permeability is a good choice for high-efficiency drainage of coal mine methane. In this paper, a modified coal-methane co-exploitation model was established and a combination of drilling–slotting-separation–sealing was proposed to enhance coal permeability and CBM recovery. Firstly, rapid drilling assisted by water-jet and significant permeability enhancement via pressure relief were investigated, guiding the fracture network formation around borehole for high efficient gas flow. Secondly, based on the principle of swirl separation, the coal–water–gas separation instrument was developed to eliminate the risk of gas accumulation during slotting and reduce the gas emission from the ventilation air. Thirdly, to improve the performance of sealing material, we developed a novel cement-based composite sealing material based on the microcapsule technique. Additionally, a novel sealing–isolation combination technique was also proposed. Results of field test indicate that gas concentration in slotted boreholes is 1.05–1.91 times higher than that in conventional boreholes. Thus, the proposed novel integrated techniques achieve the goal of high-efficiency coal bed methane recovery

p-Kirchhoff type problem with a general critical nonlinearity

In this article, we consider the p-Kirchhoff type problem $$\Big(1+\lambda\int_{\mathbb{R}^N}|\nabla u|^p +\lambda b\int_{\mathbb{R}^N}|u|^p\Big)(-\Delta_p u+b|u|^{p-2}u) =f(u), x\in\mathbb{R}^N,$$ where $\lambda>0$, the nonlinearity f can reach critical growth. Without the Ambrosetti-Robinowitz condition or the monotonicity condition on f, we prove the existence of positive solutions for the p-Kirchhoff type problem. In addition, we also study the asymptotic behavior of the solutions with respect to the parameter $\lambda\to0$