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The smooth motion profile with less oscillation ensures the processing quality of machined parts. Meanwhile, keeping feedrate under its limit as high as possible can reduce the processing time and save cost. In this paper, a novel feedrate scheduling algorithm with acceleration lookahead is presented. Through the sin2 acceleration profile, the continuity of jerk can be guaranteed. Based on the 4-phase profile, the look-ahead function is also extended for acceleration, which avoids the frequent changing of acceleration when scheduling short segments. On a designed benchmark, the effectiveness of the proposed algorithm is verified and compared with the state-of-the-art. The algorithm is implemented on a laser patterning machine with the galvanometric scanners as its feed drive system. The results confirm the effectiveness of the presented scheduling method. [ABSTRACT FROM AUTHOR]

Water blocking damage is one of the most serious formation damages that restricts the development of low-permeability coalbed methane (CBM) reservoirs. Characterized by the conversion of the electromagnetic energy into heat, microwave heating may be a promising method to remove water blocking damage. In this study, contact angle tests and permeability tests were performed to investigate the effects of microwave heating on water blocking damage in CBM reservoirs. The results indicate that microwave heating can lead to hydrophobization of coal. The permeability of coal first decreases after water intrusion and then increases after microwave heating. The permeability sensitivity of the moist coal to microwave heating is much greater than that of the dry coal. Furthermore, the increase in the water content of coal promotes the microwave recovery effect of permeability. Theoretical analysis shows that the petrophysical evolution of coal during microwave heating involves gas desorption, dehydration, pore damage, fracturing and decomposition. Outcome of this study implies that microwave heating can be used as a supplemental measure of hydraulic fracturing to enhance CBM recovery by removing the water blocking damage. [ABSTRACT FROM AUTHOR]

Abstract As a clean energy resource, coalbed methane (CBM) has drawn worldwide attention. However, the CBM reservoir has strong adsorption capacity and low permeability and thus requires stimulation. Realized by the conversion of the electromagnetic energy into the thermal energy, microwave heating may be a sound stimulation method. In this study, the effect of microwave heating on the petrophysical characteristics of coal was investigated by combining modelling, experiment and analysis. The numerical model coupled the electromagnetic and heat transfer physics by the thermal-dependent coal permittivity. The results indicate that the increase in the microwave power contributes not only to rapid heating but also to thermal heterogeneity. The Nuclear Magnetic Resonance (NMR) tests show that microwave heating can induce the enlargement, opening and interconnection of pores and fractures in coal. Furthermore, the NMR multifractal theory was proposed for quantitative pore structure characterization. The calculated adsorption fractal dimension increases with the temperature, while the seepage fractal dimension shows a positive linear correlation with the thermal heterogeneity. Outcomes of this study provide valuable insight into microwave-assisted CBM recovery. Graphical abstract Unlabelled Image Highlights • The model coupled the electromagnetic and heat transfer physics. • High microwave power contributes to rapid heating and thermal heterogeneity. • Microwave induces the enlargement, opening and interconnection of pores. • The calculated adsorption fractal dimension increases with the temperature. • The seepage fractal dimension increases linearly with the thermal heterogeneity. [ABSTRACT FROM AUTHOR]

Abstract Microwave heating has gained widespread popularity in coal processing. Numerical simulation is a promising tool to visualize and quantize microwave-coal interactions. However, coal is always assumed as a solid continuum in the current models, which cannot accurately predict the thermodynamic behavior. In this study, a fully coupled electromagnetic, heat transfer and multiphase porous media model was developed to investigate microwave heating of coal. Results show that microwave absorption by coal induces significant redistribution of the electromagnetic field in the cavity, forming high- and low-energy regions. The nonuniform electromagnetic distribution and microwave selective heating generate hot and cold spots, whereas heat convection tends to homogenize the thermal field. The temperature rise during microwave heating is characterized by "fast-slow-fast". In addition, water evaporation and surface heat convection can exert profound impacts on the thermal evolution of coal. When water evaporation is included in the model, the temperature increases nonlinearly because the dielectric property of coal is constantly changing during microwave heating. Outcomes of this study can be used to identify the heating behaviors of coal during microwave processing and thereby help to optimize the microwave applicators. Highlights • Coupled electromagnetic, heat transfer and multiphase porous media model was built. • Introduction of coal can significantly redistribute the electromagnetic field. • Water evaporation and heat convection have great impacts on thermal evolution. • When water evaporation is considered, the temperature increases nonlinearly. [ABSTRACT FROM AUTHOR]

• The electric spark owing to the piezoelectric effect would ignite the premixed gas-air. • An instantaneous high voltage formed when the roof sandstone was broken. • Electric spark generated by roof rupture broke down and ionised the premixed gas-air. The spatial characteristics of goaf are conducive to causing gas explosions. Understanding the source of ignition is helpful in preventing and controlling this hazard. A high number of accident cases show that it is difficult for spontaneous combustion of coal or rock friction collision to become ignition sources. Therefore, this paper proposes that electric sparks generated by the piezoelectric effect of a coal seam roof ignites goaf gas to cause disasters. Through theoretical analysis and experimental research, it was determined that the piezoelectric effect of the roof was generated by quartz, which is the main component of roof sandstone. An instantaneous high voltage formed when the roof sandstone was broken, related not only to the quartz content but also the compressive strength of the rock. In the process of roof sandstone rupture, instantaneous discharge formed owing to the piezoelectric effect, which could break through the air. Considering the Babao coal mine gas explosion accident, the high quartz content of the roof rock reached a certain compressive strength and experienced simultaneous stress mutation, exhibiting instantaneous roof rupture and generating an electric spark. The premixed gas-air broke down and ionised, ignited the gas and led to the gas explosion. [ABSTRACT FROM AUTHOR]