Subsection amplification cyclic deterioration mechanism of creep damaged coal
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摘要:
受开挖影响,深部煤岩在高应力长时作用下产生明显的蠕变损伤,在工作面采动作用下已蠕变损伤的煤体突变失稳诱发冲击地压灾害,给深部煤柱安全回采造成了严重威胁。基于长时蠕变−循环载荷−卸围压试验,研究了初始蠕变损伤与循环载荷叠加作用下煤岩劣化特性的演化规律,分析了循环载荷下不同应力区间煤岩的强度特征及累积损伤特性,探究了循环载荷次数对煤岩蠕变损伤效应的组合作用机制,揭示了蠕变损伤效应下煤岩变形破坏过程中能量的转化机理。结果表明:煤岩在循环载荷低应力区间内受蠕变预损伤效应影响较小,随应力水平区间的上升,煤岩的长时损伤越大且非线性劣化越明显,失稳后的破裂程度越剧烈;当蠕变应力处于弹性阶段内,蠕变可使经历周期载荷煤样在强化到劣化之间存在时长不超过16 h,在高应力区间内劣化作用才得到显著呈现;循环加卸载后期,加卸载变形模量随循环次数剧烈波动,不可逆形变稳定增加,预示煤样处于即将失稳破坏的“临界点”。基于损伤力学理论创建了蠕变与循环载荷叠加作用损伤演化模型,结合试验结果发现长时蠕变后的煤样因内部劣化程度较高,储存的能量较少,应力释放有所缓和,突变失稳现象不明显;煤样储能能力的大小在蠕变损伤时长方面同样存在“临界点”,较长的蠕变损伤时长可使试样存储的可释放弹性能减小,在一定程度上延缓了弹性能的释放,减小了发生动力灾害破坏的范围。研究成果将对减少遗煤长时蠕变诱冲灾害,提高采空区遗煤回采效率,推动矿区生态文明建设具有重要的工程意义。
Abstract:Under the excavation effects of deep coal and rock, obvious creep damage is produced under the action of high stress for a long time and the sudden destabilization of the creep-damaged coal body induces rock burst disaster under the mining action in working surface, which poses a serious threat to the safety of deep coal pillar mining. Through long-term creep-cyclic loading-unloading confining pressure test, the evolution law of deterioration characteristics of coal rock under the superposition of initial creep damage and cyclic load is studied. The strength characteristics and cumulative damage characteristics of coal in different stress intervals under cyclic load are analyzed. The coupling mechanism of cyclic load times on creep damage effect of coal is explored, and the energy conversion mechanism in the process of deformation and failure of coal under creep damage effect is revealed. The research results show that: coal samples are less affected by creep pre-damage effect within the low stress range of cyclic loading, with the increase of the stress level range, the larger the long-term damage of coal samples, the more obvious the nonlinear deterioration, and the more severe the degree of rupture after sudden destabilization; When the creep stress is in the elastic stage, the time between strengthening and deterioration is generally less than 16 h, and in the high stress range, the deterioration effect is significantly; At the later stage of cyclic loading and unloading, the loading and unloading deformation modulus fluctuates sharply with the number of cycles, and the irreversible deformation increases steadily, indicating that the coal samples is at the “critical point” of sudden destabilization. Based on the theory of damage mechanics, a damage evolution model of creep and cyclic loading is established. Combined with the test results, it is found that the internal deterioration of coal samples after long-term creep is high, the stored energy and stress release are less, and the sudden instability is less evident; The energy storage capacity of coal samples also has a “critical point” in terms of creep damage duration. Due to the longer creep damage duration, the releasable elastic energy stored in the specimen is reduced, which retards the release of elastic energy to a certain extent, reducing the scope of the occurrence of dynamic disaster damage. The research results will be of great engineering significance to reduce the long-term creep induced disaster of residual coal, improve the recovery efficiency of residual coal in goaf, and promote the construction of ecological civilization in mining area.
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Keywords:
- creep /
- cyclic loading /
- unloading confining pressure /
- damage of coal /
- nonlinear deterioration
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0. 引 言
浮选是细粒物料分选中应用最广、效果最好的选矿方法。浮选机按吸气和搅拌混合方式的不同可以分为机械搅拌式和无机械搅拌式2类[1-3],其中,射流混合装置在无机械搅拌式浮选机中运用广泛,如旋流微泡浮选柱的微泡发生器,喷射式浮选机的充气搅拌装置等[4-6];射流混合装置对药剂和气泡具有良好的分散混合作用,且可沿矿浆输送管道沿程布置,具有将动能转化为压力势能和调浆时间长的特点,在整个射流卷吸过程中,空气反复溶解、析出微泡,药剂黏附在气泡液膜表面,发生强烈的耦合作用 [7-8];射流混合装置的结构参数、工作参数及引射特征对多相流体的分散混合具有重要影响,其应用在煤泥浮选领域又具有界面调控特征[9-11]。
在煤泥浮选过程中,煤泥、药剂和气泡的充分分散混合及相互作用,对煤泥颗粒界面亲、疏水性调控有着重要的影响,不同工作及结构参数下射流混合流场中微粒的引射和混合规律差异明显。邱白晶等[12]采用FLUENT软件数值模拟计算了不同喷嘴截面积和喉嘴距下射流调浆混药装置的混药性能,确定了最佳结构参数的数值范围;周良富等[13]研究了射流装置关键结构参数对调浆混药效果的影响;HLOBEŇ等[14]设计了4种不同结构的射流调浆混药装置,采用数值模拟方法对流场进行了分析,得出引射管在喉管后部引射性能最佳;徐幼林等[15]通过流体力学分析,建立了密度比、压强比、流量比和面积比与射流混药性能的理论关系;何培杰等[16]采用红外光谱仪对流体与药液的混合过程进行检测,分析了混药装置的性能;KOLLER等[17]研究了射流发生器内的气泡生成与能量输入之间的关系,并分析了其产生气泡的能力和气泡中间粒径分布范围,得出概率密度分布曲线;MANIZHEH等[18]研究了逆流、旋流、射流3种浮选流场,发现射流流场可有效促进微粒的分散混合,更有利于细粒级颗粒的分选回收。综上,射流混合装置的参数对引射性能、微粒运动及作用机制产生重要的影响,但应用于煤泥浮选领域的射流混合装置多变参数有待进一步优化,从微观角度来看,射流流场对煤泥、引射流体的作用尚不明确,有待深入研究。
1. 射流混合装置在煤泥浮选中的应用
射流混合装置在煤泥浮选领域应用广泛[19-20]。图1为一种射流和搅拌协同作用的柱式浮选装置,结合射流卷吸混合、叶轮搅拌分散和柱式浮选逆流碰撞于一体的多维搅拌装置,可充分发挥射流流场具有的多相混合、微泡析出,叶轮搅拌流场具有的大区域分散及循环,柱式浮选流场具有的低紊流逆流碰撞等机制特征;其中射流混合装置直接影响煤泥、药剂和气泡等微粒的有效分散、碰撞粘附及界面作用[21-23],是该类型浮选设备高效运转的关键。
射流混合装置的工作原理如图2所示。工作流体(矿浆)由喷嘴高速射流,流束产生的絮动扩散作用卷吸引射流体(药剂及空气),在喉管内实现两股液体动量、质量的交换;射流流束沿着行进方向可分为初始段和射流主段。在射流初始段,存在射流核心区,射流速度基本保持喷嘴出口流速;随着引射流体的介入,射流边界逐渐变宽,流速降低,在转折截面之后,进入射流主段;射流主段的起点一般在轴向距离20Dz(Dz为喷嘴直径)的位置,轴线上的射流速度开始衰弱。
射流混合装置应用在浮选领域,主要考核其引射性能指标,一般采用无量纲参数流量比q表征。
$$ q=\frac{{Q}_{\text{y}}}{{Q}_{\text{g}}}=\frac{{Q}_{\text{c}}-{Q}_{\text{g}}}{{Q}_{\text{g}}} $$ 式中,Qg为工作流体的流量,m3/s;Qy为引射流体的流量,m3/s;Qc为混合流体的流量,m3/s。
2. 试 验
2.1 试验系统
射流混合系统如图3所示。其中,螺杆泵为射流装置的工作流体提供动力;压力表和流量计监测工作参数,为控制阀和调节阀的动作提供依据。射流装置根据试验要求设计多种结构样式。气泡粒径测试系统如图4所示,螺杆泵输送的水经射流装置喷嘴高速喷出,引入空气形成气泡,为了让气泡处于稳定状态,在此过程中添加起泡剂,使用引出管把含气液体引入到激光粒度分析仪,引出管上设置有分流管确保含气液体均匀稳定地进入激光粒度分析仪。
2.2 试验内容
试验内容包括2部分:
1)物理模型参数优化:设计制作不同结构参数的试验装置,测试面积比Ar$ ({{A}}_{\text{r}}=\left({\dfrac{{{D}}_{\text{h}}}{{{D}}_{\text{z}}}}\right)^{\text{2}} $,Dh为喉管直径)、喉嘴距Le (喷嘴与喉管入口的距离)、引射管开启度γ、喉管长度L、喷嘴出口速度V对引射性能的影响,优化物理模型多变参数。
2)射流流场对物料的作用机制:通过混合成型物料在搅拌作用流场中粒度解析情况,测试射流流场对煤粒表面细泥罩盖的剥离能力;运用高速摄像仪捕捉射流流束对引射气泡的卷吸破碎情况;采用激光粒度分析仪,测试气泡的粒径大小和分布范围。
2.3 试验方法
2.3.1 物理模型参数优化方法
影响射流混合装置引射性能的参数较多,采取分步寻找各参数最优值的方法,依次优化面积比Ar、喉嘴距Le、引射管开启度γ、喉管长度L和喷嘴出口速度V的最优值范围,前序参数的优化值,应用在后续参数的优化方案中。射流装置的固定参数为:入料管直径为30 mm;引射管对称布置,直径为10 mm;喷嘴直径Dz=10 mm,喉管长度L=90 mm(喉管长度优化方案中喉管长度为可变参数);工作流体为水,流量为2.03 m3/h,喷嘴出口速度V为7.20 m/s;引射流体为水。可变参数取值方案见表1。
表 1 可变参数取值方案Table 1. Variable parameter value scheme方 案 可变参数 变化范围 其他参数 面积比Ar优化 喉管直径Dh /mm Dh=10+4n(n=0,1,2,3,4,5)
(Ar=1 ~ 9)Le=7 mm 喉嘴距Le优化 喉嘴距
Le /mmLe=5n(n=1,2,3,5,6,7,8)
(Le=0.28Dh~2.50Dh)Dh=18 mm
(Ar=3.24)引射管开启度γ优化 引射管开启度γ/% γ=n2(n=2,3,···,9,10) Le=10 mm
Dh=18 mm喉管长度L优化 喉管长度L/mm L$=\left\{\begin{array}{c}\text{5}{n}\text{,}({n}=\text{0,1}\text{,}\text{2,3}\text{,}\text{4})\\ \text{10}{n}({n}=\text{5,6},\cdots,\text{12})\end{array}\right.$ Le=10 mm
Dh=18 mm喷嘴出口速度V优化 喷嘴出口速度V/(m·s−1) 5、7.2、11.30、14.70、20 Le=7 mm
Dh=12 mm2.3.2 射流流场对煤粒表面细泥罩盖的剥离能力试验方法
根据物理模型参数优化值,调节射流混合系统。喷嘴直径Dz=10 mm;喉管直径Dh=18 mm;面积比Ar = 3.24;喉嘴距Le=10 mm;喉管长度L=90 mm,入料量1.41 m3/h,喷嘴出口速度V=5 m/s。
将粒度为0.5~0.25 mm的低灰精煤和小于0.045 mm的高岭石按照4∶1均匀搅拌混合,恒温70 ℃烘干,烘干后制成块状人工混合矿;取1 g人工混合矿,平稳地加入到100 mL水中,静置1 min后,倒入SALD-7101激光粒度分析仪中,其自带搅拌装置以500 r/min转速搅拌,分别在1、1.5、2、2.5、3、3.5、4、4.5、5、5.5 min测试悬浮液的粒度分布;取一定量的人工混合矿,配制成质量浓度10 g/L的矿浆,静置1 min后,开启射流混合系统,在试验槽内随机取样进行激光粒度分布测试和EDS能谱测试。EDS能谱仪利用不同元素的X射线光子特征能量不同进行元素成分分析,试验中采用型号为XFlash6130的EDS能谱仪对物料进行测试。
2.3.3 射流流束对引射气泡的卷吸破碎能力试验方法
物理模型参数同上。调节螺杆泵出口管道的调节阀,喷嘴出口流量分别为2.03、1.01 和0.50 m3/h,对应的喷嘴出口速度分别为7.20、3.6 和1.8 m/s;引射管设置在固定位置,匀速向引射管注射空气,确保引射管出口端的气泡大小一致,使用高速摄像仪在1 000帧/s的摄像条件下捕捉卷吸气泡的粒径迁移过程。
2.3.4 气泡粒径迁移规律试验方法
物理模型参数同上。喷嘴出口距离槽底100 mm,入料量2.03 m3/h,喷嘴出口速度为7.20 m/s;起泡剂选择甲基异丁基甲醇,用量为0.012 4 mL/L(按煤泥浮选单位矿浆用药10 g/m3计算);吸气量为变参数,通过调节气体流量计进口的开启度,分别控制吸气量为500、400、300、200和100 L/h。
3. 结果与分析
3.1 射流装置结构参数对引射能力的影响
面积比和喉嘴距对引射能力的影响结果如图5所示,由图5可知,当面积比Ar从1增大到9,引射流体流量比q先增大后减小,其中面积比Ar=3.24时,流量比q最大;当喉嘴距Le从0.28Dh增大到2.50Dh,引射流体的流量先增大后减小,但总体波动范围较小,其中当喉嘴距Le=0.56Dh时,流量比q最大。
引射流体为水、气时引射管开启度对引射能力的影响曲线分别如图6、图7所示。由图6和图7可知,在相同的工作参数下,引射流体的流量与引射管开启度成正比关系;引射管开启度为16 %时,引射流体的引射速度最大。当射流速度为7.20 m/s时,水的引射速度上限为4.10 m/s,气的引射速度上限为8.20 m/s。
喉管长度对引射能力的影响结果如图8所示。由图8可知,当喉管长度L从0 增加到90 mm时,引射流体的流量逐渐增加,此后随着喉管长度逐渐增加到140 mm,引射流体的流量保持不变,说明喉管长度L=9Dz是喉管能否起到稳压作用的临界点;综合考虑引射能力和沿程损失,喉管的长度应保持在9Dz。
喷嘴出口速度对引射能力的影响曲线如图9所示。由图9可知,引射流量随着喷嘴出口速度的增加先快速后缓慢增加,引射流体(水)的速度达到上限,流量基本保持一致;从引射能力的角度考虑,喷嘴出口速度应设置在15 m/s。
3.2 射流流场对物料的作用机制
人工混合煤样(0.5~0.25 mm制成)粒度解离情况如图10所示。由图10可知,射流流场具有较强的物料解离及分散作用,人工混合矿经射流装置单次射流混合作用后,悬浮液中细颗粒物料累计产率明显增加,作用效果等同于激光粒度分析仪自带搅拌装置以500 r/min转速搅拌4~4.5 min。
采用EDS能谱仪测试射流作用前后物料所含元素的变化,直观判断射流流场对煤粒表面细泥的剥离效果。人工混合煤样射流前后EDS能谱图如图11所示。由图11可知,射流清洗前人工混合煤样的EDS能谱图中有C、O、Al和Si等元素,其中C元素峰值最高,O、Al和Si也具有明显的峰值;射流清洗后,人工混合煤样的EDS能谱图中Al和Si元素峰值不明显,说明经射流清洗后煤粒表面罩盖的高岭石被有效剥离。
不同射流速度时气泡卷吸粉碎的动态捕捉图如图12所示。由图12可知,当射流速度小于1.8 m/s时,气泡会变形、撕裂但很难被粉碎;当射流速度达到3.6 m/s时,气泡被粉碎,但气泡粒径较大;当射流速度达到7.2 m/s时,气泡会瞬间被粉碎成粒径小、数量多的气泡群,由此可知射流速度的最佳工作机制应大于7.2 m/s,即喷嘴出口速度V >7.2 m/s。
不同粒径气泡的累计产率如图13所示。由图13可知,引射吸气量大小对气泡粒径的分布影响较小,主要原因是射流流场中所有引射空气均受到工作流体的粉碎作用且粉碎程度相对均匀;90%以上气泡粒径均小于45 μm,说明射流卷吸作用吸气量大且有利于产生微泡。
4. 结 论
1)射流混合装置多变参数对引射性能的影响规律:随着面积比Ar和喉嘴距Le的增大,引射流体的流量比q先增大后减小,其中面积比Ar=3.24、喉嘴距Le=0.56Dh时引射性能最佳;引射流体的流量与引射管开启度γ成正比关系;引射速度受限于射流速度,当射流速度达到15 m/s,引射速度达到上限;喉管长度L对喉管的稳压作用至关重要,喉管长度L=9Dz是喉管能否起到稳压作用的临界点。
2)射流流场具有较强的物料解离及分散作用。单次射流混合作用对物料解离及分散效果等同于激光粒度分析仪自带搅拌装置以500 r/min转速搅拌4~4.5 min;射流后样品EDS能谱图中Al和Si元素含量大幅降低,说明射流流场对煤泥表面的黏土矿物有强烈清洗剥离作用。
3)射流流束对引射气泡粒径迁移的影响规律。当喷嘴出口速度V >7.2 m/s时,引射气泡会瞬间被粉碎成粒径小、数量多的气泡群,气泡的粒径分布均匀,90%以上气泡均小于45 μm,射流作用有利于产生微泡。
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表 1 煤样冲击倾向性测定结果
Table 1 Determination results of impact tendency of coal samples
单轴抗压
强度/MPa冲击能量
指数弹性能量
指数动态破坏
时间/ms综合判定
结果10.68 2.26 17.2 2428 弱 表 2 煤样不同蠕变时长下各应力区间内循环次数与破坏情况
Table 2 Cycle times and failure of coal samples in different stress range under different creep time
蠕变时长/h 循环次数 是否卸围压破坏 0.2σ′~0.4σ′ 0.4σ′~0.6σ′ 0.6σ′~0.8σ′ 0 20 10 5 是 8 20 10 5 是 16 20 10 4(破坏) 24 20(破坏) 表 3 不同蠕变时长下煤样蠕变与循环加载耗散能、损伤变量及可逆增大系数值
Table 3 Dissipative energy, damage variable and reversible increase coefficient of creep and cyclic loading of coal samples under different creep durations
蠕变
时间/h试件
编号蠕变耗
散能/
(J·m−3)蠕变耗散能
平均值/
(J·m−3)蠕变损伤
变量Dc蠕变损伤
变量Dc
平均值循环耗
散能/
(J·m−3)循环耗散能
平均值/
(J·m−3)蠕变损伤
变量Dc蠕变损伤
变量Dc
平均值可逆应变
增大系数$ {\varphi _{\mathrm{c}}} $可逆应变
增大系数$ {\varphi _{\mathrm{c}}} $平均值16 M16-1 15.36 16.58 0.21 0.23 70.88 71.23 0.69 0.69 1.48 1.36 M16-2 18.29 0.25 69.86 0.68 1.28 M16-3 16.09 0.22 72.94 0.71 1.32 24 M24-1 29.99 28.28 0.41 0.39 52.39 53.08 0.51 0.52 1.20 1.25 M24-2 26.33 0.36 56.50 0.55 1.25 M24-3 28.53 0.39 50.34 0.49 1.31 -
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