相同集聚因子时纤维悬浮液的流动特征及机理

纤维悬浮液在管路中从层流发展为湍流的过程中会表现出复杂的流动状态。为了解这一现象的内在原因,以相同集聚因子时4种不同尺寸的尼龙纤维悬浮液为实验对象,利用超声多普勒测速仪测量它们在圆管内流动的速度场。根据流场分布确定了纤维柱塞尺寸,并利用流动阻力损失计算柱塞内部纤维网络结构的屈服应力。基于柱塞内纤维平均体积分数的变化规律以及柱塞边缘雷诺应力与屈服应力的比值分析讨论了尼龙纤维悬浮液流动特征的变化机理。研究结果表明,柱塞尺寸随着雷诺数的增大而不断减小。在流动阻力的压缩作用下,柱塞内部纤维网络变得致密,其平均体积分数随着雷诺数的增大逐渐增加,屈服应力不断上升。由于平均雷诺应力增加速率要高于屈服应力,当柱塞半径与管道半径之比小于0.2时,湍流强度明显高于纤维网络的结构强度。柱塞内部受到湍流侵蚀,柱塞内纤维平均体积分数及其屈服应力开始下降,柱塞中纤维网络逐渐被破坏直至崩溃。相同集聚因子时,形状系数较小的纤维悬浮液内部纤维网络结构强度更高,因此具有较大半径的柱塞和较高的屈服应力。同时在相同雷诺数时,形状系数较小的尼龙纤维悬浮液柱塞边缘平均雷诺应力较小,表明较大半径的柱塞会对水环的湍流产生较强的抑制作用。

Flow characteristics and mechanism of fiber suspension with the same crowding number

In the process from laminar flow to turbulent flow in a pipe,the fiber suspension presents a complex flow state.In order to understand the internal causes of this phenomenon,the ultrasonic Doppler velocimeter was employed to measure the velocity profiles of the fiber suspensions with 4 different sizes of nylon fibers respectively flow in the pipe under same crowding number.According to the distributions of the local velocity,strain-rate and turbulence intensity,the plug sizes were decided,and the yield stresses of the plugs were calculated combined with the pressure losses.The average volume percentage concentrations in the plugs were derived from the result of the static rheological experiment for the nylon fiber suspension.Based on the variation of the average volume percentage concentration in the plug and the ratio of Reynolds stress at the plug edge to yield stress,the mechanism of the flow characteristics of nylon fiber suspension was discussed.The results showed that,all plug sizes reduced with the increasing Reynolds number.The compression effect arising from the flow resistance led to a densification of the fiber network in plug,indicating that the average volume percentage concentration and the yield stress increased steadily.When the ratio of the plug radius to the pipe radius was less than 0.2,the turbulence intensity was obviously higher than the fiber network s strength,and the average volume percentage concentration and yield stress of the plug began to decrease.This was due to the average Reynolds stress increased faster than the yield stress,the fiber network in the plug was gradually eroded by the turbulence until it was destroyed totally.The fiber suspension with smaller aspect ratio had larger plug size and higher yield stress under the same Reynolds number.This indicated that the fiber suspension with smaller aspect ratio had higher fiber network s strength at the same crowding number.The average Reynolds stress at the plug edge was also smaller.That meant that the larger plug exhibited more inhibiting effect on the water annulus s turbulence at the same time.