通流截面积的突变对添加剂水溶液管流减阻效果的影响及其机理

The additive drag reducing technology has been successfully applied to the regional water-transporting systems in European and American countries and Japan. It should be noted that so far most drag-reducing studies have mainly focused on a fully developed turbulent pipe flow. But, a water-transporting pipeline network is necessarily with valves and locally sudden alternations in pipe diameter,which certainly will affect the drag-reducing efficiency and flow structure in an additive drag-reducing pipe flow with making flow developing, because its drag-reducing behavior is dependent on shear stresses. In the present project,in an additive drag-reducing pipe folw with ring-shaped roughness elements at the middle and an additive drag-reducing pipe flow with a locally sudden alternation in pipe inner diameter, the influence of sudden changes in flow area on the additive drag-reducing behavior on the downstream from the location where the flow area is sudeenly altered is firstly revealed. The time-space evolvement of depressing of additives on the disturbed flow regions and turbulent burst events induced by coupling the disturbance with the pipe innre-wall is examined by visualizing the disturbed flow field with dye. The developing characteristics of flow structures along with the pipe axis is also clarified and it is revealed how the transition to turbulent flow is delayed by using a laser Doppler velocimetry. Moreover,it is investigated how generation and growth of turbulent puffs and slugs in the initial stage of the transitional flow is suppressed by additives, through the pressure time-evolution acquired by two pressure taps. The project has important scientific and economic value for further revealing directly the additive drag-reducing mechanism and designing an additive drag-reducing water-circulating pipeline system with higher drag reduction.

添加剂减阻技术已在欧美日的水循环输送系统得到了实际应用，取得了显著的经济效益。此领域的研究大多仅限于管内充分发展紊流的减阻，而水循环管网中的阀门、突扩及突缩等通流截面积突变形成的扰动会使很多区段的流动处于发展中，进而必使减阻特性依存于剪切强弱的减阻流的减阻率及流动结构发生改变。本项目在充分发展的减阻管流中安放不同内径的矩形横截面圆环或改变上下游管径，物理模拟实际水输送管网中的不同通流截面积突变的流动条件。在通过下游沿程压差测试揭示通流截面积突变对下游减阻率影响规律的基础上，采用流场染色可视化技术察明减阻剂对扰动流团抑制的时空演化特征、对扰动和管壁耦合作用下紊流猝发的抑制规律；通过LDV流场测试探究,受扰动的下游减阻流由主流向时均速度、主流向及横向速度脉动强度以及雷诺剪切应力的断面分布所表征的流动结构的发展特性及层流向紊流延迟转捩的过程；通过两静压孔的静压时域波形，揭示下游所形成的紊流泡囊或紊流塞的数量生成强度和随流动的生长速度被抑制的规律。本项目对进一步直接阐明表面活性剂减阻的动力学机理及合理设计表面活性剂减阻水循环系统具有重要的理论和经济意意义。

有关表面活性剂减阻的研究大多集中在直管水流中，本课题集中在水循环管网中通流横截面突变对表面活性剂减阻效果的影响及其水动力学机理的研究. . 对于直径比为1:1.5和1:2的100ppm和200ppm表面活性剂减阻突扩流，当细管雷诺数低于达到最大减阻率的临界值时，局部阻力系数较纯水流有10%量级的减少，在大于临界雷诺数时，局部阻力系数开始随雷诺数增加，当上游减阻效果完全丧失后，局部阻力系数可达纯水的1.5倍，而且于下游再次形成新的压力梯度充分发展的减阻流需要约160倍下游粗管管径的距离。表面活性剂减阻突扩流的再附长度，在上游管雷诺数低于临界值时，约为15倍台阶高度，而纯水仅有7倍，主流向速度脉动强度的断面分布在下游180倍粗管管径位置仍不能完全稳定。对于通过1.5:1和2:1的突缩减阻流，在低雷诺数域，局部阻力系数随雷诺数改变且略低于纯水流，在大于临界雷诺数的高雷诺数区域，大于纯水流。对于渐扩的减阻流的减阻率，在位置雷诺数低于某个临界值时，随下游距离而增加，大于临界值时，减阻率随下游距离逐渐减小，直至减阻效果完全丧失。由横截面突变而诱导的胶束减阻发展流的减阻行为，主要受胶束结构的形成和松弛时间特性的支配。. 通过两个压力传感器同步采集的壁面压力时域信号，揭示了由管内入口段的环塞诱导的紊流Slug的生成、生长速度对雷诺数的依存特性。对于纯水流，只要环塞台阶高度足够大，可使层流维持在雷诺数高达12000的管流在雷诺数为2200时，流态便开始向紊流转捩。雷诺数大于3600，紊流Slug开始出现，且尾部和头部的生长速度随雷诺数增加而增大，最大可达周围层流域断面平均速度的30-50%。对于层流向紊流流态转捩的临界雷诺数为37500的减阻流，雷诺数小于11500时，环塞所诱导的紊流Paff都将随流衰减，紊流Slug出现的最小雷诺数为13450，尾部和头部的生长速度仅为周围流体断面平均速度的10-30%的量级。