Fracture features of brittle coal under uniaxial and cyclic compression loads

Under the efects of complex geological and stress environments,burst hazards continue to be a major challenge for underground space utilization and deep resources exploration as its occurrence can lead to personnel causalities,equipment damage and structural collapse.Considering the stress path experienced by in-situ coal body,cyclic loading appears in quite various forms for instance shearer cutting,overlying strata breakage,hydro-fracturing and blasting,during tunnel,mining and underground space utilizing process.The stability of the underground coal body subject to periodic loading/unloading stress is extremely important for maintain the function of designed engineering structure for waste storage,safe mining,roadway development,gas recovery,carbon sequestration and so on.The mechanical properties of hard rock subject to cyclic fatigue loads has been intensively investigated by many researchers as the rock burst induced by supercritical loads has long been a safety risk and engineering problems for civil and tunneling engineering under deep overburden.More recently,the mechanical properties of coal samples under cyclic fatigue loads is investigated from the aspect of hysteresis,energy dissipation and irreversible damage as the burst hazards of brittle coal is rising in many countries.However,the crack propagation and fracture pattern of brittle coal need more research to understand the micro mechanism of burst incubation subject to cyclic fatigue loads as brittle coal can store more elastic strain energy and rapidly release the energy when its ultimate strength once reached.This research studied the internal crack status corresponding to diferent cyclic fatigue loading stage of brittle coal samples.The AE monitoring was applied during the uniaxial and cyclic loading process of brittle coal samples to record the crack intensity of samples at diferent loading stages.The damage evolution curve corresponding to loading status was then determined.The fracture pattern of coal samples determined by micro-CT scan was observed and discussed.It has been found by this paper that brittle coal of uniaxial compression tests demonstrated sudden failure caused by major splitting fracture while that of cyclic fatigue tests experienced progressive failure with mixture fracture network.

Maximum useful energy rate and efficiency of a recuperative Brayton cogeneration plant

A thermodynamic model was developed to analyze the performance of cogeneration plant based on irreversible recuperative Brayton cycle. A parameter, dimensionless total useful energy rate (DTUER), was used as the criterion for performance optimization of cogeneration plant. The effects of cycle parameters, internal irreversibilities, and recuperator efficiency on maximum DTUER and on the efficiency at maximum DTUER were numerically investigated. The relation between DTUER and cogeneration efficiency was also analyzed. The results show that there exists an optimal compressor pressure ratio which maximizes the DTUER. It is also found that there exists an optimal power-to-heat ratio which results in a dual-maximum DTUER.

Optimization design of deep lean-ore mining and safety evaluation

In order to settle the mining optimization design and safety problem of the above 1 150 m pillar of No.1 ore-body in No.Ⅱ Mining Jinchuan, the lean-ore above 1 250 m, the 1 150 m horizontal pillar and the ore-body below 1 100 m regarded as research objects based on the original design project, and nine calculation schemes on different mining sequence and different fill body strength were put forward based on cement-sand ratio of 1:4, 1: 12 and 1: 24.Calculation parameters were got by the back analysis method of field monitoring data, and the FLAC^(2D) program was applied to compute for these schemes,stress and displacement of ground settlement, shaft and stope roof were analyzed, and some conclusions were got.Results show that the intensity of filling body and the mining technique have very important effect on controlling settlement and stability of surrounding rock;Developing of lean ore have some influences to the 16th return air filling shaft, especially for 1 500～1 400 m of the shaft;The best project is the first project.This research supply some technique references and safety appraisals for the mining of lean-ore of No.Ⅱ Mining Jinchuan.

Effects of thermal treatment on physical and mechanical characteristics of coal rock

To study the physical and mechanical properties of coal rock after treatment at different temperatures under impact loading, dynamic compression experiments were conducted by using a split Hopkinson pressure bar(SHPB). The stress–strain curves of specimens under impact loading were obtained, and then four indexes affected by temperature were analyzed in the experiment: the longitudinal wave velocity, elastic modulus, peak stress and peak strain. Among these indexes, the elastic modulus was utilized to express the specimens' damage characteristics. The results show that the stress–strain curves under impact loading lack the stage of micro-fissure closure and the slope of the elastic deformation stage is higher than that under static loading. Due to the dynamic loading effect, the peak stress increases while peak strain decreases. The dynamic mechanical properties of coal rock show obvious temperature effects. The longitudinal wave velocity, elastic modulus and peak stress all decrease to different extents with increasing temperature, while the peak strain increases continuously. During the whole heating process, the thermal damage value continues to increase linearly, which indicates that the internal structure of coal rock is gradually damaged by high temperature.

Crack propagation mechanism of compression-shear rock under static-dynamic loading and seepage water pressure

To reveal the water inrush mechanics of underground deep rock mass subjected to dynamic disturbance such as blasting, compression-shear rock crack initiation rule and the evolution of crack tip stress intensity factor are analyzed under static-dynamic loading and seepage water pressure on the basis of theoretical deduction and experimental research. It is shown that the major influence factors of the crack tip stress intensity factor are seepage pressure, dynamic load, static stress and crack angle. The existence of seepage water pressure aggravates propagation of branch cracks. With the seepage pressure increasing, the branch crack experiences unstable extension from stable propagation. The dynamic load in the direction of maximum main stress increases type I crack tip stress intensity factor and its influence on type II crack intensity factor is related with crack angle and material property. Crack initiation angle changes with the dynamic load. The initial crack initiation angle of type I dynamic crack fracture is 70.5°. The compression-shear crack initial strength is related to seepage pressure, confining pressure, and dynamic load. Experimental results verify that the initial crack strength increases with the confining pressure increasing, and decreases with the seepage pressure increasing.

Failure mechanism of Mesozoic soft rock roadway in Shajihai coal mine and its surrounding rock control

In view of the buckling failure caused by large deformation of Mesozoic soft rock roadway in Shajihai mining area, such as serious roof fall, rib spalling, floor heave, etc., based on the detail site investigation,theoretical analysis, mineral composition test, microstructure test, water-physical property test and field experiments were carried out. And we revealed the compound failure mechanism of Mesozoic soft rock roadway in Shajihai mining area, namely the molecule expansion-shear slip of weak structural plane-construction disturbance. On this basis, the coupling support technology whose core is constant resistance with large deformation bolt was proposed. The feature of this supporting technology is that a new type of structural composite material was used, which makes the supporting system not only has the ideal deformation characteristics, but also has high supporting resistance. Thus the fully release of plastic energy within surrounding rock and reasonable control of the thickness of the plastic ring were realized. Then the differential deformation between the surrounding rock and support was eliminated by the secondary coupling support of bolt–mesh–cable, and the bolt with high strength was applied in the base angle to control floor. Eventually the collaborative bearing system of surrounding rock–support was formed. Through field tests the validity and rationality of support was also verified.

Floor stress evolution laws and its effect on stability of floor roadway

According to the distribution of abutment stress in a stope,this research established the mechanical model of mining abutment pressure transmission in floor base on the theory of semi-infnite plate body in elasticity.This study takes the 762 working face of Haizi Coal Mine as a case in point,and analyzed the dynamic evolution law of seam floor stress during the mining process.With an organic combination of the mining floor stress and surrounding rock stress,the study obtained the change laws of the maximum principle stress and the minimum one for the floor roadway surrounding rock when mining the upper working face.Considering the non-constant pressure force state and the cracks revolution mechanisms of floor roadway surrounding rock,the research built the mechanical model of roadway stress.Simulation results verify the reliability of the above conclusions.Moreover,this model could provide the theoretical basis and technical support for controlling floor roadway surrounding rock.

Deformation mechanism of high-stress and broken-expansion surrounding rock and supporting optimization based on the gray correlation theory

Aiming at the large deformation and support problems of high-stress and broken-expansion surrounding rock, and taking 1 000 m level roadway of Mine II in Jinchuan as the research object, an investigation on the deformation and damage of roadway surrounding rock and an analysis of its mechanism were carried out. The gray correlation theory was used in support scheme optimization design. First, causes and mechanism of deformation of the 1 000 m horizontal transport channel were analyzed through field investigation, laboratory test and data processing methods. We arguued that poor engineering geological conditions and deep pressure increases were the main factors, and the deformation mechanism was mainly the ground deformation pressure. Second, the gray correlation theory was used to construct supporting optimization decision method in the deep roadway. This method more comprehensively considers various factors, including construction, costs, and supporting material functions. The combined support with pre-stressed anchor cables, shotcrete layer, bolt and metal net was put forward according to the actual roadway engineering characteristics. Finally, 4 support schemes were put forward for new roadways. The gray relational theory was applied to optimizing the supporting method, undertaking technical and economic comparison to obtain the correlation degree, and accordingly the schemes were evaluated. It was concluded as follows: the best was the flexible retaining scheme using the steel strand anchor; the second best was the one using plate anchors on the top and rigid common screw steel bolt on the two sides; the third was; the rigid common screw steel bolt in full section of roadway; and the worst is the planished steel rigid support. The optimized scheme was applied to the 1000 m level of new excavation roadway. The results show that the roadway surrounding rock can reach a stable state after 5 to 6 months monitoring, with a convergence rate less than 1 mm/d.

Thermodynamic optimization and fluid selection of organic Rankine cycle driven by a latent heat source

Organic Rankine cycle(ORC) is applicable for the heat-work conversion. Whereas, there also exist a lot issues that influence the efficiency and the cost of the system. In this work, eleven pure working fluids(as categorized into alkanes, and fluorinated alkanes) are investigated based on the first and second law of thermodynamics. The major objective is to obtain the most suitable working fluid for the latent heat source. The results show that the working fluid is an important factor of the system performance. The heat absorption of the working fluid in the evaporator is inversely proportional to the evaporating temperature, but the thermal and exergetic efficiencies are just the opposite. RC318 has the highest net power output and the lowest outlet temperature of the heat source, but its global warming potential(GWP) value is too high. The cyclohexane shows the highest thermal efficiency among the fluids investigated. Moreover, the figure of merit(FOM) of the isobutane is higher than that of other working fluids. Overall, the cyclohexane shows that the optimal comprehensive performance is more feasible for medium grade heat source in engineering applications.

Control technology and coordination deformation mechanism of rise entry group with high ground stress

Based on engineering practices of Wuyang Coal Mine, we carried out X-ray diffract researches on No. 3 coal; and the rocks of its roof and floor by XRD meter, and simulated the interactive effect of the surrounding rock deformation by FLAC2D5.0 numerical simulation software under the condition of different tunneling method of multimine roadway in parallel. The internal structures of the surrounding rocks of 76 belt roadway were monitored by borehole observation instruments; and then, we analyzed the reason of failure and deformation of surrounding rocks of several rise entry, and proposed the technical measures for controlling interactive effect of several rise entry surrounding rock deformation at last. For the thickness seam rise roadway, two conclusions were drawn: one is that the co-deformation among roadway groups mainly reflect on that both shear failure and deformation in coal pillar among roadways have decreased the width of pillar core region and clamping action of coal pillar to roof strata, increased the actual span of roof strata, intensified the flexural failure of roof strata and prized the bed separation of roof deep rock strata. The other conclusion is that the factors controlling the interactive deformation among roadways is obvious when appropriate re-adjustment in construction sequence of the tunneling of multimine parallel roadways because the construction sequence among roadways also has great effects on deformation of the surrounding rock in roadway.

Numerical Simulation Study of Goaf Methane Drainage and Spontaneous Combustion Coupling

In order to study the coupling problem between methane drainage and spontaneous combustion of residual coal in the collapsed zone after mining ignitable coal seams with high methane,we have analyzed the effects of differ-ent methane drainage modes on spontaneous combustion of residual coal through numerical simulation. The results show that deep and large flux methane drainage modes increases the air leakage from work faces to the goaf and formed new spontaneous combustion zones induced by drainage near vents,which increases the risk of self-ignition of coal—reducing the self-ignition period and enlarging the scale of self-ignition. The spontaneous upstream combustion oxidation of the main fire zone can be suppressed when both drainage and nitrogen injection were adopted. Our research results provide an effective technical measure and theoretical basis to determine the best methane drainage scheme and drainage parameters.

Application of deep borehole blasting on fully mechanized hard top-coal pre-splitting and gas extraction in the special thick seam

In order to solve the problems of top-coal inadequate destruction and large amounts of gas emission in mining extra thick and hard coal seam,this study investigated the pre-splitting for deep borehole blasting and gas pre-draining technologies on top coal.The mechanism of the technologies was systematically expounded based on hard top-coal cracks development obtained by numerical simulation and theoretical analysis.The results show that explosive blasting in the hard rock results in a large number of cracks and large displacement in the rock mass due to the effect of explosion stress.Meanwhile,the thick top-coal caves,and desorbing gas flows along the cracks improve gas extraction.Finally,the pre-splitting for deep borehole blasting and gas pre-draining technologies was applied in No.3802 working face of Shui Liandong Coal Mine,which increases monthly output in the face to 67.34 kt and the drained gas concentration to 86.2%.The drained gas average concentration from each borehole reaches 40%,and the effect is remarkable.

Relationship between water inrush from coal seam floors and main roof weighting

A water-resistant key strata model of a goaf floor prior to main roof weighting was developed to explore the relationship between water inrush from the floor and main roof weighting. The stress distribution,broken characteristics, and the risk area for water inrush of the water-resistant key strata were analysed using elastic thin plate theory. The formula of the maximum water pressure tolerated by the waterresistant key strata was deduced. The effects of the caved load of the goaf, the goaf size prior to main roof weighting, the advancing distance of the workface or weighting step, and the thickness of the waterresistant key strata on the breaking and instability of the water-resistant key strata were analysed.The results indicate that the water inrush from the floor can be predicted and prevented by controlling the initial or periodic weighting step with measures such as artificial forced caving, thus achieving safe mining conditions above confined aquifers. The findings provide an important theoretical basis for determining water inrush from the floor when mining above confined aquifers.

Experimental study and numerical simulation of spread law for fire on tunnel

In order to research spread law and distribution law of temperature nearby fire sources on roadway in mine, according to combustion theory and other basic, the theory model of temperature attenuation was determined under unsteady heat-exchange between wind and roadway wall. The full-size roadway fire simulation experiments were carried out in Chongqing Research Institute of China Coal Technology & Engineering Group Corporation. The development processes of mine fire and flow pattern of high temperature gas were analyzed. Experimental roadway is seen as physical model, and through using CFD software, the processes of mine fire have been simulated on computer. The results show that, after fire occurs, if the wind speed is less than the minimum speed which can prevent smoke from rolling back, then the smaller wind speed can cause smoke to roll back easily. Hot plume will lead to secondary disasters in upwind side. Because of roadway wall, hot plume released from roadway fire zone has caused the occurrence of the ceiling jet, and the hot plume has been forced down. Whereas, owing to the higher temperature, buoyancy effect is more obvious. Therefore, smoke rises gradually along the roadway in the flow process, and the hierarchical interface appears wavy.Oxygen-enriched combustion and fuel-enriched combustion are the two kinds of combustion states of fire. The oxygen content of downwind side of fire is maintained at around 15% for oxygen-enriched combustion, and the oxygen content of downwind side of fire is maintained at around 2% for fuel-enriched combustion. Furthermore, fuel-enriched combustion can lead to secondary disasters easily.

Field investigations of high stress soft surrounding rocks and deformation control

Field investigations of high stress soft rock deformations show that the high stress soft rock roadway can slide with large deformation. Severe extrusion and floor heave can also be subsequently observed. The supported roadway can be locally damaged or completely fail, where the floor has a large deformation and/or is seriously damaged. The factors inducing large deformation of surrounding rocks in deep roadway are rock strengths, structure face cutting types, stress states, stress release, support patterns, and construction methods. Based on the deformation characteristics of high stress soft rock roadway, a comprehensive support scheme is proposed. The overall support technology of＂step-by-step and joint, hierarchical reinforcement＂for roadway is presented, and the anchor cable and bolt parameters to check the design methods are also given. Finally, the proposed comprehensive support method＂bolt t metal mesh t U-steel arch t shortcrete t grouting and cable＂ is used in the extension section of east main haulage roadway at ？850 m level of Qujiang coal mine. The 173-day monitoring results show that the average convergence of sidewalls reaches 208 mm, and the average relative convergence of roof and floor reaches 448 mm, suggesting that this kind of support technology for controlling large deformation of high stress soft surrounding rock roadway is effective.

Experimental and numerical analysis of secondary disasters induced by oxygen rich combustion within a tunnel

Various physical parameters, including gas concentrations (O2, CO, CH4, and H2) and temperatures at dif- ferent air velocities, were determined for full scale wood fires in the Chongqing Coal Research Institute fire test tunnel. Both experimental measurements and numerical simulations are discussed. The numer- ical analysis was performed with the computational fluid dynamics software package ''FLUENT''. The results show that the experimental data agree with the simulation results. The results verify that Roberts' theory of burning is correct. They also prove that the air velocity is the key factor that determines the type of combustion. Also, it is shown that secondary disasters are unlikely for oxygen rich combustion with a limited fire load.

The propagation law and analysis of gas explosion in U type duct

In order to explore the propagation law of gas explosion in U type laneways,the propagation law of flame and shock wave in U type duct were experimentally and theoret- ically investigated.It is shown that the shock wave takes on the complicated stress state and the flame takes on complicated change rules in the U type duct.The propagation process of gas explosion in bend duct is the mutual action of explosion wave,flame and complicate flow,the destruction in bend surface is especially serious.In the mine exploita- tion and laneway design,the bend laneway should be avoided,especially continuous bend laneway.

A thermodynamics comparison of subcritical and transcritical organic Rankine cycle system for power generation

A comparison on subcritical and transcritical organic Rankine cycle(ORC) system with a heat source of 110 °C geothermal water was presented. The net power output, thermal and exergy efficiencies and the products of the heat transfer coefficient(U) and the total heat exchange area(A)(UA values) were calculated for parametric optimization. Nine candidate working fluids were investigated and compared. Under the given conditions, transcritical systems have higher net power outputs than subcritical ones. The highest net power output of transcritical systems is 18.63 k W obtained by R218, and that of subcritical systems is 13.57 k W obtained by R600 a. Moreover, with the increase of evaporating pressure, the thermal and exergy efficiencies of transcritical systems increase at first and then decrease, but the efficiencies of subcritical ones increase. As a result, the efficiencies of transcritical systems cannot always outperform those of the subcritical ones. However, the subcritical systems have lower minimum UA values and lower expansion ratios than the transcritical ones at the maximum net power output. In addition, the transcritical cycles have higher expansion ratios than the subcritical ones at their maximum net power output.