摘要
借助数值模拟研究了3种流动性的混凝土在竖直向上泵送时的管内流动行为,分析了混凝土内部压力和流速的分布规律.结果表明:泵送压力主要受润滑层性质的影响,其纵向分布与管壁阻力沿程变化规律有关,截面中心的泵送压力略高于管壁处;混凝土在管内的流动存在“前进—扩散—滞留堆积—汇聚”的行为特征,并据此分析了润滑层的形成和粗骨料的迁移,解释了泵送离析和堵管的原因.
混凝土泵送技术在现代建造中发挥着关键作用,高层建筑和大跨度结构对混凝土泵送提出了更高的要
合理的泵送压力对泵送施工至关重
受限于现有的测量手段,混凝土泵送试验无法获得全面的内部流动信息.而数值模拟可以实时获取混凝土内部的各项参数并实现可视化分析,因此选用合适的模型和方法可以研究混凝土的流动和骨料的迁
本文通过数值模拟方法研究了混凝土竖直向上泵送时的管内流动行为,预测了混凝土的泵送压力,分析了混凝土内的压力分布和流速分布,并进一步分析了粗骨料的迁移运动,揭示了润滑层的动态形成机理.本研究对混凝土泵送性能的预测和提升具有重要的理论意义.
本文采用数值模拟的方法对泵送混凝土内部的压力分布和速度分布进行分析研究,泵送试验数据均引用自文献[
Group | Water | Cement | Sand | Gravel | Additive 1 | Additive 2 |
---|---|---|---|---|---|---|
No.1 | 171 | 323 | 912 | 946 | 0.808 | 0.646 |
No.2 | 179 | 338 | 890 | 936 | 0.845 | 0.676 |
No.3 | 191 | 360 | 872 | 913 | 0.900 | 0.720 |
采用宾汉模型作为混凝土的流变模型,文献[
Group | Material | Density/(kg· | Viscosity/(Pa·s) | Yield stress/Pa |
---|---|---|---|---|
No.1 | Mortar | 2 010.6 | 139.3 | 134.5 |
Concrete | 2 353.5 | 397.0 | 190.0 | |
Gravel | 2 630.0 | 872.8 | 256.4 | |
No.2 | Mortar | 2 186.8 | 78.0 | 114.0 |
Concrete | 2 344.5 | 305.0 | 181.0 | |
Gravel | 2 630.0 | 488.6 | 217.4 | |
No.3 | Mortar | 2 182.1 | 80.3 | 95.9 |
Concrete | 2 337.6 | 297.0 | 149.0 | |
Gravel | 2 630.0 | 503.1 | 182.8 |
使用混凝土水平泵送简易装置进行混凝土的水平泵送试

图1 混凝土泵送流量与泵送压力损失的关系
Fig.1 Relationship between concrete pumping flow rate and pressure loss
考虑到在混凝土泵送过程中粗骨料的迁移,本文使用一种无网格粒子法(I‑MPS,implicit moving particle simulation)来计算颗粒之间的相互作用.该方法将不可压缩的黏性流体离散为有限多个粒子并赋予物理属性.通过纳维-斯托克斯方程(
(1) |
式中:t为时间;u为速度;ρ为流体的密度;P为压力;v为流体的运动黏度,即表观动力黏度μ与密度ρ的比值;g为重力加速度.
本文使用宾汉模型作为混凝土的流变模
, τ >τ0 | (2) |
(3) |
(4) |
式中:τ为剪切应力;τ0为屈服应力;μ0为塑性黏度;为剪切速率;β为固液转化系数.β值越大,正则化的宾汉模型与未正则化的宾汉模型越接近.当β值大于10时,上述2个流变模型的计算差别可忽略不
润滑层与管壁之间的剪切作用即为管壁对泵送混凝土的阻力.尽管学者们对润滑层的组成和厚度尚未达成共
(5) |
式中:τs为管壁阻力;α为滑移黏度系数;VR为混凝土在管壁的流速;Α为滑移屈服应力.
文献[
本研究采用二维数值模型,模拟300 mm混凝土柱在内径为100 mm管内的泵送流动.通常模型的解析度越高,离散流体粒子越小,计算精度越高,但计算效率会大幅降低.为了确定合适的粒子大小(粒径),本节以混凝土No.1为例,控制泵送活塞的推送速度为0.15 m/s,粒子粒径分别设置为1、2、4 mm,计算得到活塞上平均泵送压力分别为9.25、9.29、9.93 kPa.结果表明:当粒径小于等于2 mm时,计算结果达到稳定,即计算结果与粒径无关.因此,本文的数值模型设置离散粒子粒径为2 mm.
本研究根据对混凝土的处理方式建立2种泵送模型:单相均质(SPMP)模型和双相多颗粒(DPMP)模型,如

图2 混凝土管内流动模型
Fig.2 Concrete pipe flow model
根据混凝土在泵管内稳定流动时的受力平衡原
(6) |
(7) |
式中:R为泵管半径;L为泵送混凝土的长度.
在竖直泵送时的泵送压力由混凝土自重和管壁阻力共同产生.由混凝土各自的密度计算其自重压力,使用插值法从
本文分别使用SPMP模型和DPMP模型模拟了3种混凝土的泵送流动行为,其中泵送压力的模拟结果如

图3 泵送压力的理论结果和模拟结果
Fig.3 Theory and simulation results of pumping pressure
由于DPMP模型中粗骨料会引起局部压力波动,SPMP模型的均质流体计算结果更稳定,故本节基于SPMP模型的计算结果进行讨论.视自由表面压力为零,沿泵送方向输出混凝土中轴线上压力P的垂直分布,并根据混凝土密度计算自重压力Pw,结果如

图4 泵送混凝土内中轴线上的压力分布
Fig.4 Pressure profile along central axis in pumped concrete
测量距离活塞不同垂直距离(h)处泵送混凝土内水平截面上的压力分布曲线,如

图5 泵送混凝土内水平截面上的压力分布
Fig.5 Pressure profile on horizontal sections in pumped concrete
3种混凝土在纵向(泵送方向)和径向(垂直于泵送方向)上的速度云图如

图6 竖直泵送混凝土在管内的速度云图
Fig.6 Velocity contours of vertical pumped concrete in pipe
测量距活塞顶面不同高度横截面上的混凝土纵向速度分布,如

图7 竖直泵送混凝土纵向速度分布
Fig.7 Velocity profile along flow direction in vertical pumped concrete
由

图8 竖直泵送混凝土径向流速分布
Fig.8 Velocity profile along radius direction in vertical pumped concrete
以混凝土No.1为例,以活塞为参考系,绘制泵送混凝土在管内的相对流动行为,如

图9 泵送混凝土在管内的相对流动行为
Fig.9 Relative flow behavior of pumped concrete in pipe
基于混凝土的管内循环流动行为,中心部分混凝土裹挟粗骨料向前移动,当到达混凝土前端后向管壁扩散.混凝土中的浆体在管壁形成润滑层并被滞留.润滑层在流动方向的前端不断形成,并沿着管壁逐渐后移、被消耗.润滑层通过自身剪切耗散的方式降低泵送沿程阻力.后部混凝土继续向前移动,形成粗骨料的向前迁移运动,从而导致粗骨料在流动前端堆积.根据DPMP模型的计算结果,统计泵送前端(距活塞25~30 cm区间段)混凝土内粗骨料体积分数的变化,如

图10 泵送前端混凝土内粗骨料体积分数的变化
Fig.10 Volume fraction variation of coarse aggregate in front of pumped concrete
(1)混凝土的泵送压力主要受润滑层性质的影响,而混凝土的流动性主要对管内混凝土的流速分布有影响.降低泵送压力的有效途径是促进润滑层的形成来提高润滑效果.
(2)泵送混凝土内部压力沿流动纵向的分布规律主要受管壁阻力沿泵送方向变化规律的影响.在径向横截面上,中心部分泵送压力分布均匀,管壁处的泵送压力较中心部分有所降低.
(3)泵送混凝土在管内的流动同时包含了纵向流动和径向流动.泵管中轴线周围栓流区混凝土的纵向流速高于管壁处的纵向流速.管内的径向流动在混凝土自由顶面和活塞顶面较为明显.径向流速相较纵向流速很小,一般小于0.15 cm/s.
(4)混凝土在泵管内存在“前进—扩散—滞留堆积—汇聚”的相对流动行为.该流动行为解释了泵送时润滑层在管壁处的动态形成过程.
(5)混凝土的管内相对流动行为会导致粗骨料向流动前端汇聚,容易造成中、高流动性混凝土在流动前端的泵送离析和泵送堵管.
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