Pillar Width Research Articles

Nucleate boiling enhancement by structured surfaces with distributed wettability-modified regions: A lattice Boltzmann study

In this paper, we conceive a novel pillar-structured surface for enhancing nucleate boiling heat transfer, namely a pillar-structured surface with distributed wettability-modified regions on the top of each pillar. A three-dimensional thermal multiphase lattice Boltzmann model with liquid–vapor phase change is employed to investigate the boiling performance on the pillar-structured surface with distributed wettability-modified regions and the associated mechanism of nucleate boiling heat transfer enhancement. According to the distribution of the wettability-modified regions, the bubble dynamics on the newly conceived pillar-structured surface can be classified into three regimes, i.e., regimes I, II, and III, among which the regime II shows relatively better boiling performance than the other two regimes due to the synergistic effects of surface structure and mixed wettability. It is found that in the regime II the bubbles nucleated at the wettability-modified regions do not coalesce with each other, which therefore elongates the length of the triple-phase contact lines on the pillar top in comparison with the pillar-structured surface with a unified wettability-modified region. Meanwhile, in the regime II the bubbles on the pillar top receive a strong bubble-wake effect supplied by the bubbles generated at the bottom substrate, which shortens the bubble growth cycle and promotes the departure of the bubbles on the pillar top, and also reduces the area of dry spots on the pillar top. The influences of the pillar width and the width of the wettability-modified regions are also studied. It is shown that the best boiling performance is always achieved in the cases that fall into the regime II.

Research on the Reasonable Coal Pillar Width and Surrounding Rock Supporting Optimization of Gob-Side Entry under Inclined Seam Condition

In order to study the optimal coal pillar width and surrounding rock control mechanism of gob-side entry under inclined seam condition, the 130205 return air entry adjacent to 130203 gob in Yangchangwan No. 1 well is taken as a typical engineering background. By means of engineering background analysis, theoretical analysis based on inside and outside stress field, numerical simulation by FLAC3D software, and in situ industrial test and relevant monitoring methods, the optimal coal pillar width and surrounding rock control technology are obtained. The results show that the influence range of inside stress field is about 12.2∼12.8 m based on theoretical calculation result; under the influence of 10 m coal column, the overall deformation of the roadway is relatively small and within the reasonable range of engineering construction, so the width of the coal pillar along the return air roadway is set to 10 m which is more reasonable; the cross-section characteristics of special-shaped roadway lead to asymmetric stress distribution and fragmentation of surrounding rock, and then the asymmetric surrounding rock control technology under the coupling effect of roof prestressed anchor + high-strength single anchor cable + truss anchor cable support is proposed. The monitoring results of this support method are effective for the maintenance of gob-side entry, and the study conclusions provide new guidance for the surrounding rock control mechanism of gob-side entry under inclined seam conditions.

Experimental research and application of automatically formed roadway without advance tunneling

Longwall mining is a widely used coal mining method in underground engineering, in which a considerable roadway tunnelling is required. This makes it difficult for the roadway surrounding rock control. In order to realize the safety of coal mining and also economic impact, a new mining method called automatically formed roadway (AFR) without advance tunnelling is proposed. The S1201-II working face of the Ningtiaota Coal Mine is used as an engineering background to explore this method. We conduct numerical comparisons between the AFR and the traditional mining methods. The mining pressure distribution and evolution of the two mining methods under the influence factors of different pillar width, in-situ stress level, roof cutting height, and angle are systematically analyzed. Furthermore, a geo-mechanical model test system for the new mining method is independently developed. In order to clarify the control mechanism of the AFR surrounding rock, the model test is performed. Corresponding engineering suggestions are then proposed according to the results of the numerical simulations and geo-mechanics model test. The in-situ application results show that the new mining method satisfies the surrounding rock control requirement.

Numerical Investigation on Shape Optimization of Small-Spacing Twin-Well for Salt Cavern Gas Storage in Ultra-Deep Formation

Small-spacing twin-well (SSTW) salt caverns have an extensive application prospect in thin or bedded rock salt formations due to their good performance, while they are rarely used in ultra-deep formations. The target strata depth of Pingdingshan salt mine is over 1700 m, and it is planned to apply an SSTW cavern to construct the underground gas storage (UGS). A 3D geomechanical model considering the viscoelastic plasticity of the rock mass is introduced into Flac3D to numerically study the influence of internal gas pressure, cavern upper shape and well spacing on the stability of an SSTW salt cavern for Pingdingshan UGS. A set of assessment indices is summarized for the stability of gas storage. The results show that the minimum internal gas pressure is no less than 14 MPa, and the cavern should not be operated under constant low gas pressure for a long time. The cavern with an upper height of 70 m is recommended for Pingdingshan gas storage based on the safety evaluation and maximum volume. The well spacing has a limited influence on the stability of the salt cavern in view of the volume shrinkage and safety factor. Among the values of 10 m, 20 m and 30 m, the well spacing of 20 m is recommended for Pingdingshan gas storage. In addition, when the cavern groups are constructed, the pillar width on the short axis should be larger than that on the long axis due to its greater deformation in this direction. This study provides a design reference for the construction of salt cavern gas storage in ultra-deep formations with the technology of SSTW.

Effect of mining parameters on surface deformation and coal pillar stability under customized shortwall mining of deep extra-thick coal seams

Underestimation of customized shortwall mining parameters of deep extra-thick coal seams (namely, mining height, mining width, and pillar width) may result in coal pillar instability, surface subsidence, and mining structure failure, while their overestimation would cause a waste of coal resources. This study proposes a new method for optimizing these parameters, whose main purpose is to achieve the maximum coal recovery rate while satisfying surface deformation and coal pillar stability requirements. A case study of an extra-thick coal seam in Eastern China with one km-overburden depth and a thickness of 10 m was analyzed. First, a new method for predicting surface deformation under deep customized shortwall mining based on surface deformation data from fully caving mining was proposed. The variation pattern of the extreme values of surface deformation parameters (including surface subsidence, tilt, curvature, and horizontal strain) versus the area-based recovery rate and mining width was analyzed. After that, the evolution law of the surface deformation with the changing mining height and area-based recovery rate was simulated using the FLAC3D software. The maximum recovery rate and the corresponding mining height, the area-based recovery rate were preliminarily determined, while the surface deformation fell within the allowable range. Simulations of evolutions of the plastic zone width, elastic core ratio, and vertical stress of coal pillars with varying coal pillar width under the maximum recovery rate were performed. The optimal mining scheme had a mining height of 10, a mining width of 100, a coal pillar width of 122 m. It ensured the coal pillar’s long-term stability with the surface deformation within the allowable range and achieved the maximum coal recovery rate of 45%. These findings are considered instrumental in the customized shortwall mining of extra-thick coal seams under buildings, in particular, under Eastern China mining and geological conditions.

Mechanical performance evolution and size determination of strip coal pillars with an account of thermo-mechanical coupling in underground coal gasification

In this study, underground coal gasification (UCG) technology with the strip mining method was recommended to avoid the instability of surrounding rock in the UCG cavity and possible groundwater pollution, in which mechanical performance and reasonable size of the strip pillar play the crucial role. The respective innovative calculation method considering the thermo-mechanical coupling effect was proposed and applied to size designs of pillars in the UCG projects of the Shanjiaoshu Mine and Mazino coal deposit. The proposed method feasibility was experimentally validated and yielded the following predictions of the coal pillar mechanical performance evolution during the UCG process. Heat-affected zones in the coal pillar and immediate roof gradually increased with the following decline. High temperatures above 250 °C affected the coal and rock masses only at a distance not exceeding 2 m from the surface, but their effect continued even after the UCG termination. High temperatures reduced the load-bearing capacity in the area near the coal pillar surface and induced long-term thermal stresses in coal pillars, which alternatively played a dominant role in affecting the ultimate strength of the coal pillar at different stages. The plastic zone width xp in the pillar is varied and will remain unchanged once it reaches its peak value. The width of strip pillars Wp is greatly influenced by high temperatures; it exhibits a W-shaped variation pattern during the UCG process and slowly drops after gasification. The strip pillar width of UCG can be determined using the maximum value of function Wp(t) considering a certain safety factor. While the wide pillar's total strength and size are slightly affected by the temperature, its width can be obtained from Wp(0) with a safety factor. This study can be used for predicting the mechanical performance evolution and size determination of pillars in the UCG process.

Determining the critical blasting load of cracked strip bauxite pillars

Cracked pillars have limited capabilities for the bearing of dynamic and static loads, consequently threatening the stability of entire goafs. In the current study, we establish a structural mechanical model for cracked strip pillars and determine a mathematical formula for the total energy based on an underground bauxite mine in Shanxi, China. Furthermore, the instability criterion is employed to derive an analytical method for the estimation of the critical blasting load qcr for cracked strip bauxite pillars. The relationship between qcr and pillar height (H), pillar width (b), pillar length (L), and the crack loosen zone width (d) is quantitatively evaluated. Results demonstrate that when L and d are constant, qcr decreases as H increases. Moreover, qcr is observed to increase with b for constant H and d, with a continuous reduction in curvature fluctuations. Although qcr increases with L for fixed H and b, the increase amplitude is small, indicating the limited influence of L. For constant L and b, qcr decreases as d increases, and when the value of d exceeds that of the inflection point, the rate of change is steeper. The proposed analytical method is validated using numerical results and a case study. Our work provides theoretical support for the safe production of bauxite mining.

Study on Optimization of Shallow Section Coal Pillar Width of Gently Inclined Medium-Thickness Coal Seam

This paper takes a coal mine gently inclined medium-thickness coal seam as the object. Firstly, the principal stress characteristics and displacement characteristics of roadway surrounding rocks after roadway driving along goaf under section columns with different breadthes were analyzed by simulation method, and the roadway stability was evaluated to determine the reasonable breadth. The results show that when the breadth of column is 4 m, the Mohr stress circle of the pillar intersects the strength envelope and the roof and side displacement of roadway are significant; when the breadth is 6 m, the Mohr stress circle on both flanks of the pillar intersects at the strength envelope and the roof and side displacement of roadway are significant; when the breadth is 8-12 m, the Mohr stress circle of surrounding rock is separated from the strength envelope and the surrounding rock displacement meets the production needs. Therefore, it is in a stable state. 8-12 m is the reasonable width range of coal pillar. Secondly, through the actual project verification, the 8 m wide coal pillar is the most reasonable for the shallow part of the gently inclined medium-thickness coal seam.

Analytical Design and Experimental Verification of Lateral Superjunction Based on Global Region Normalization Method

The global region normalization (GRN) method is proposed to optimize the lateral superjunction (SJ), in this article. The GRN features normalizations in the global doping range of the SJ from zero to the maximum doping concentration N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> that was theoretically determined by the pillar width W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SJ</sub> . Then the optimization covers all the possible design points of the SJ. For the SJ with given W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SJ</sub> and pillar length L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SJ</sub> , the specific ON-resistance R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on,sp</sub> and breakdown voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> are normalized in the global region of the doping concentration N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SJ</sub> . With the GRN method, the global normalization function f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">η</sub> is deduced to reflect the quantitative variation trend of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on,sp</sub> - V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> and a design formula of the optimized N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SJ</sub> is presented. Furthermore, lateral SJ devices were fabricated according to the formula. The measured R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on,sp</sub> values of the SJ devices are reduced by almost 35% when compared with the theoretical limit of the triple RESURF device in a voltage range of 100-400 V. The experiments also reveal that the measured net R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on,sp</sub> of the SJ region approaches the R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on.sp</sub> ∝ V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.03</sup> relationship.

Evolution Law of Floor Fracture Zone above a Confined Aquifer Using Backfill Replacement Mining Technology

Disturbances owing to coal mining result in the movement and failure of floor strata. Mining-induced fractures within the floor may propagate to the confined aquifer, thereby causing water inrush disasters. In this study, we propose using strip mining and backfill replacement mining above the confined aquifer to investigate the failure depth of the floor. The problem is simplified as a distributed force model on a half-plane body. First, the stress disturbance of the floor during strip mining is calculated and the results are combined with the von Mises yield criterion. Then, the destruction of the floor after replacing the remaining coal pillars is explored. The results show that the widths of the strip mining face and coal pillars play an important role in affecting the failure depth of the floor: the greater the width, the larger the failure depth. Based on the parametric study results, the mining face and retention coal pillar width of 20 m is sufficient to prevent the occurrence of water inrush accidents. After the replacement of the remaining coal pillars, the failure area of the floor rock mass increases, but the maximum failure depth remains unchanged. Finally, we employed field measurement techniques at the Bucun coal mine to monitor the shear and vertical strains of the floor. The data collected was compared with the predicted results obtained from the proposed theoretical model. Good agreement was found between the monitoring and calculation results, which demonstrate the effectiveness of the proposed method.

Control of Gob-Side Roadway with Large Mining Height in Inclined Thick Coal Seam: A Case Study

The gob-side roadway of 130205, a large-mining-height working face in the Yangchangwan coal mine, was investigated in terms of the mine pressure law and support technology for large mining heights and narrow coal pillars for mining roadways. The research included field investigations, theoretical analysis, numerical simulation, field tests, and other methods. This paper analyzes the form of movement for overlying rock structure in a gob-side entry with a large mining height and summarizes the stress state and deformation failure characteristics of the surrounding rock. The failure mechanism of the surrounding rock of the gob-side roadway and controllable engineering factors causing deformation were analyzed. FLAC3D numerical simulation software was used to explore the influence law of coal pillar width, working face mining height, and mining intensity on the stability of the surrounding rock of the gob-side roadway. Ensuring the integrity of the coal pillar, improving the coordination of the system, and using asymmetric support structures as the core support concept are proposed. A reasonably designed support scheme for the gob-side roadway of the working face for 130205 was conducted, and a desirable engineering effect was obtained through field practice verification.

Effect of the Hybrid Hydrophobic-Hydrophilic Nanostructured Surface on Explosive Boiling

The influence of different wettability on explosive boiling exhibits a significant distinction, where the hydrophobic surface is beneficial for bubble nucleation and the hydrophilic surface enhances the critical heat flux. Therefore, to receive a more suitable surface for the explosive boiling, in this paper a hybrid hydrophobic–hydrophilic nanostructured surface was built by the method of molecular dynamics simulation. The onset temperatures of explosive boiling with various coating thickness, pillar width, and film thicknesses were investigated. The simulation results show that the hybrid nanostructure can decrease the onset temperature compared to the pure hydrophilic surface. It is attributed to the effect of hydrophobic coating, which promotes the formation of bubbles and causes a quicker liquid film break. Furthermore, with the increase of the hydrophobic coating thickness, the onset temperature of explosive boiling decreases. This is because the process of heat transfer between the liquid film and the hybrid nanostructured surface is inevitably enhanced. In addition, the onset temperature of explosive boiling on the hybrid wetting surface decreases with the increase of pillar width and liquid film thickness.

Effect of pillared surfaces with different shape parameters on droplet wettability via Lattice Boltzmann method

In this study, to explore the influences of different pillar shape parameters on the wettability of a water droplet, a two-dimensional pseudopotential model at a large density ratio was established to predict the wettability of a pillared wall microstructure via lattice Boltzmann method. The improved force term and tunable surface tension term were implemented in the model. The formation of vortex was studied during droplet contacts the surface. The influences of different pillar heights, widths, and spacings on the wettability and wettability conversion of droplets were investigated. The results reveal that the vortexes will be generated during droplet contacts the surface. The pillars can hold up droplets, thereby greatly increasing the droplet contact angle. With the increase of the pillar height, the droplet gradually shifts from filling the convex gap to being held up by the air in the gap. The pillar width mainly determines contact angle. Moreover, the contact angle decreases with the increase of the pillar spacing. However, when the pillar spacing is large enough, the droplet can enter the gap closest to the droplet. As roughness increases, the water droplet is converted from Wenzel state to Cassie state.

Droplet rolling angle model of micro-nanostructure superhydrophobic coating surface.

The droplet rolling angle is one of the important indicators to measure the coating's hydrophobic performance, but the specific factors affecting the droplet rolling angle on the micro-nanostructured superhydrophobic coating surface are not yet known. Based on the rolling mechanism of droplets on rough surfaces, and from the perspective of coating microscopic energy conservation, this paper points out that the micron-scale morphology and the nanoscale morphology can comprehensively affect the droplet rolling angle. From the above perspective, a mathematical model of the droplet rolling angle on the micro-nanostructure superhydrophobic coating surface was established. The model shows that the droplet rolling angle is positively correlated with the ratio of nano-sized pillar width to spacing, the ratio of micron-sized papilla radius to spacing, and the liquid-gas interfacial tension, and is negatively correlated to the droplet intrinsic contact angle, droplet volume and droplet density. The droplet rolling angle calculated by the presented model is in good agreement with the experimentally tested results. This model can provide good accuracy in predicting the droplet rolling angle on the micro-nanostructured superhydrophobic coating surface.

Numerical Study on Soft Coal Pillar Stability in an Island Longwall Panel

Roadway support and management of longwall panels in an island soft coal panel are always difficult work. In a test mine, stress distribution, deformation characteristic, and plastic zone distribution around the roadway and coal pillars in the development and mining periods were investigated with respect to the widths of different coal pillars using theoretical and simulation methods. The most reasonable width of coal pillars was comprehensively determined, and the field test was conducted successfully. The results show that a reasonable width of coal pillars is 7.0–8.2 m using the analytical method. The distribution of vertical stress in the coal pillars showed an asymmetric “double-hump” shape, in which the range of abutment pressure was about 26.0–43.0 m, and the roadway should be laid away from stress concentration. When the coal pillar width is 5.0–7.0 m, deformation of the roadway is half that with 8.0–10.0 m coal pillar in the development and mining period. The plastic zone in the surrounding rock firstly decreases and increases with increasing coal pillar width; the smallest range occurs with a coal pillar width of 5.0 m. Finally, a reasonable width for coal pillars in an island panel was determined to be 5.0 m. Industrial practice indicated that a coal pillar width of 5.0 m efficiently controlled deformation of the surrounding rock, which was an important basis for choosing the width of coal pillars around gob-side entries in island longwall panels with similar geological conditions.

The pressure relief protection effect of different strip widths, dip angles and pillar widths of an underside protective seam.

To design underside protective seam strip layout. Similarity model experiments, numerical simulations and theoretical calculations are used to quantitatively study the pressure relief protection effect of different strip widths, dip angles and coal pillar widths of a thin underside protective seam under deeply buried conditions. The optimal strip width range is obtained according to the change law of strain during the mining process of the underside protective seam in a similar model experiment. The change law of the expansion of the protected coal seam is obtained and the fitting surfaces among the dip angle and strip width of the coal seam with the protection distance and pressure relief angle along the strike and dip of the protected coal seam are established according to the numerical simulation results of underside protective seam mining. It is concluded that the best pressure relief effect can be achieved when the dip angle is 16.7° and the strip width is 70 m. According to the stability threshold of coal pillars considered in strip mining theory, the coal pillar width is calculated to be 50 m. Similarity model experiments and numerical simulations of protected coal seam mining verify the pressure relief effect of the designed protective seam strip width and pillar width. A calculation method of the protective seam strip width, position and pillar width required by the specific width of the protected seam is proposed.

Feasibility of Coupling PS System with Building Protection in an Ultrasoft Strata Colliery

To guarantee the stability of a building complex above a planned mining district with ultrasoft strata, strip mining technology (SMT) was applied to control the displacement and deformation caused by underground exploitation. This study attempts to design a reasonable pillar width to establish a stable pillar-support (PS) system composed of ground buildings with coal pillars underneath. Based on the stratigraphic structure of ultrasoft strata and in situ measurement data of mining subsidence monitoring, this study takes an ultrasoft strata colliery in western Henan province, central China, as an example to examine the technical and economical feasibility of the proposed PSsyst under two mining scenarios. The major results indicated that the initial design of pillar width would be 120 m under scenario 1, with expected damage of only 450 mm maximum subsidence predicted by probability integration method (PIM); while under scenario 2, the cost of compensation for buildings’ mining-induced damage would increase to CNY 61.31 million with an expected output of 7.629 million tons of raw coal. Moreover, the protection rate of the residential area in the proposed postmining area of scenario 1 can reach as much as 6.91% comparing to the fully mechanized coal winning technology in scenario 2. Overall, the proposed PSsyst will bring good benefits both economically and environmentally and should be worth promoting as a reference for similar geological and mining conditions in the future.

Stability Analysis on the Long-Term Operation of the Horizontal Salt Rock Underground Storage

The construction of the vertical cavern in the salt dome deposit can meet the requirements of both storage capacity and tightness. However, if the vertical cavern is still used as the design shape of the salt rock underground storage in the layered salt rock deposit, the high design capacity cannot be guaranteed while the tightness is satisfied. In this case, the use of a large-span horizontal cavern as the design shape of the salt rock storage can not only effectively increase the design capacity of the storage, but also solve the problems such as the stability and tightness of the storage during the operation period by improving the structural form and working mode. Based on this, the ellipsoid-shaped horizontal salt rock underground storage is taken as an example, and a single-cavern horizontal salt rock underground storage model with different diameter-to-height ratios is established by using FLAC3D software. The change law of vertical and horizontal displacements, volume loss rate, and plastic zone distribution of salt rock storage changing with the diameter-to-height ratio are studied, and the optimal diameter-to-height ratio is determined. And then the long-term operation process of the double-cavern horizontal salt rock underground storage under the optimal diameter-to-height ratio is simulated, and the optimal pillar width is obtained.

Investigation of pool boiling heat transfer on hydrophilic-hydrophobic mixed surface with micro-pillars using LBM

Surface modification technology by controlling the surface wettability or applying micro/nano-structures to enhance the boiling heat transfer performance has attracted a great deal of interest in recent years. Abundant experiments were performed to investigate the boiling processes on the modified surfaces, and lots of experimental data as well as reliable conclusions were obtained. With the advantages of saving costs and time, the numerical method has been a new reliable way to investigate the bubble dynamics and heat transfer during pool boiling processes on the modified heated surfaces. Pseudopotential LB model is capable of simulating the entire boiling processes including the bubble nucleation, and this model has been successfully applied to simulate the pool boiling processes on both the smooth surfaces and the hydrophilic-hydrophobic mixed surfaces with micro-pillars. However, the numerical simulations of the boiling processes on the mixed surfaces were still rare, and the influence of geometrical parameters of pillar structures of the mixed surface on boiling heat transfer performance was still unclear yet. Moreover, some of the conclusions in existing literature were inconsistent with those in others. Thus it's necessary to carry out more numerical and experimental researches to solve these problems. In this study, the MRT pseudopotential model coupled with phase-change model was applied to simulate the pool boiling processes on the hydrophilic-hydrophobic mixed surface textured with micro-pillars. Under different wall superheats, the bubble dynamics and heat transfer during boiling processes on the mixed surface with micro-pillars were compared to those on the smooth hydrophilic surface and the hydrophilic-hydrophobic mixed surface without micro-pillars. The heat transfer enhancement mechanism of the mixed surfaces was revealed and the influence of geometrical parameters of pillars, including pillar width and pillar number, on bubble dynamics and heat transfer performance during pool boiling processes was investigated in detail.

On the issue of scientific substantiation of the method for forecasting the hazardous properties of coal seams

The study of filtration permeability of the barrier pillar between the isolated fire section and the worked longwall ventilation drift was completed using the mathematical model developed by the authors for coupled processes of rock deformation and gas filtration. The problem is solved involving a finite element method implemented in the author's programme. When solving, the parameters of width of the barrier pillar and thickness of the mined coal seam were varied. The article presents the results of calculating the values of geomechanical parameters, permeability coefficients and gas pressure in the filtration area. It is shown that with an increase in thickness of the coal seam, intensity of the crack formation process increases in the barrier pillar. With small pillar width, the coal seam along its entire length is in a disturbed state, as well as the rocks of its roof and soil. The filtration areas around the worked longwall ventilation drift and the isolated conveyor drift are connected both along the coal seam and along the host rocks. This means that at a certain value of the pressure drop, filtration of fire gases into the mine workings of the worked excavation section is possible. With a greater width of the pillar, between disturbed rocks around the mine workings of the worked section and isolated fire one, there is a barrier made of undisturbed rocks. That impermeable barrier prevents gas filtration from one mine workings to another. Its width decreases with an increase in the thickness of the coal seam. Based on the results of calculating gas pressure, it is shown that methane from the coal seam moves into atmosphere of the mine workings, where gas pressure has lower values. At the same time, for the accepted conditions, with a pillar width of 20 m and a seam thickness of 0.4 m, gas is filtered from the mine workings of isolated fire section into the adjacent excavation section along the disturbed rocks of the seam roof. In the rest of the cases, the areas of low pressure around the mine workings are separated by zones of higher pressure, exceeding pressure of fire gases in the isolated section. Filtration of fire gases into atmosphere of the worked excavation section is impossible under such conditions.