Backflow air and pressure analysis in emptying a pipeline containing an entrapped air pocket

ABSTRACT

The prediction of the pressure inside the air pocket in water pipelines has been the topic for a lot of research works. Several aspects in this field have been discussed, such as the filling and the emptying procedures. The emptying process can affect the safety and the efficiency of water systems. Current research presents an analysis of the emptying process using experimental and computational results. The phenomenon is simulated using the two-dimensional computational fluid dynamics (2D CFD) and the one-dimensional mathematical (1D) models. A backflow air analysis is also provided based on CFD simulations. The developed models show good ability in the prediction of the sub-atmospheric pressure and the flow velocity in the system. In most of the cases, the 1D and 2D CFD models show similar performance in the prediction of the pressure and the velocity results. The backflow air development can be accurately explained using the CFD model.     

基于液膜临界速度的井筒积液预测新方法

现阶段用于预测气井积液问题的模型大多针对垂直井筒从Turner所建立的液滴受力模型上发展而来,认为井底积液是由井筒中液滴下落造成的,对井筒倾斜角和液膜逆流的影响没有深入探讨.本文对Unified多相流动模型进行编程,运用多相流动模型对垂直井筒、倾角75°井筒和60°井筒分别进行了数值模拟,分别从井筒内流动压降和液膜速度...     

天然气平焰烧嘴的研究

总结、叙述了DNGP-100天然气平焰烧嘴试验研究结果。研究了此烧嘴的平焰直径和厚度，讨论了回流区的最大直径和长度的变化，试验研究了回流区中心线上的负压分布。     

Axial Mixing for Both Dispersed and Continuous Phases in Pulsed Perforated-Plate Extraction Column by Tracer Co-Injection Method

The effects of operation mode and perforated-plate type on axial mixing of mixer-settler region in both dispersed and continuous phases were studied for a 5 cm I. D. pulsed perforated-plate extraction column of pulser feeder type. The axial mixing coefficient was simultaneously measured to both phases by using the “dynamic tracer co-injection method” proposed by the authors. The characteristics of both phases are observed obviously. Relatively to what it was in the continuous phase, the dispersed phase had short reaching time and long retardation time. The superficial axial mixing coefficient for dispersed phase becomes smaller than for the continuous phase. And experimental results showed that a backflow took place only in dispersed phase at a test section of 50 cm in axial length. The values of backflow ratio are observed from 0.5 to 1.8% of the dispersed net flow.     

钻井液完井液屏蔽环重复形成的实验模拟

深探井及多目的层油气探井在进行中途测试、完井测试和起下钻作业中，不可避免地造成钻井完井液滤饼遭受多次形成与破坏，此时滤饼的性能发生了较大的变化并对测试结果产生重要的影响。选取四川盆地川东北区长兴组和飞仙关组碳酸盐岩储层不同渗透率级别的岩样及改性钻井完井液，利用高温高压动态损害评价仪进行了钻井完井液屏蔽环重复形成与返排的实验模拟。结果表明，多次形成于基块和裂缝岩样中的钻井完井液屏蔽环的暂堵率和强度都得到了提高。重复作业时滤饼不断地向裂缝深部推进并因应力敏感损害而难返排，致使裂缝岩样渗透率恢复率降低明显，相反基块岩样一般仍可得到满意的渗透率恢复率。适当增大返排压差可以提高岩样渗透率的恢复率。     

水泵水轮机泵工况压力脉动和转轮受力特性

为研究水泵水轮机在泵工况下的内部流态变化对压力脉动和转轮叶片受力的影响,采用 SAS－SST 湍流模型对某一模型水泵水轮机的多个非设计工况进行非定常数值模拟,分析了水轮机 内部流态对导叶与转轮之间无叶区、尾水管内的压力脉动和转轮叶片径向受力的影响。结果表明: 在流量为 40% ～80%设计流量时,导叶区内产生旋转失速,转失速涡团初生于固定导叶进口,并随着流量的降低向活动导叶进口发展,且覆盖区域逐渐增大。旋转失速使压力和过流沿周向不均匀分布, 导致压力脉动和转轮径向受力波动大幅上升。在40%设计流量时,失速涡团发展最为充分,无叶区 压力脉动和转轮受力波动的低频分量幅值最高。旋转失速产生的低频脉动可向尾水管传播,形成的低 频压力脉动幅值约为无叶区低频脉动幅值的10%。当流量低于 40%设计流量时,导叶区旋转失速消失,复杂的涡结构形成的压力脉动低频成分没有周期性。此外,转轮进口的流动分离使尾水管内产生复杂的回流涡结构,导致尾水管内形成频谱丰富的压力脉动; 流量降低使转轮进口回流涡结构的湍动 能增加,导致尾水管内压力脉动幅值大幅上升。小流量工况下,转轮进口的涡结构演变是转轮径向力波动的主要影响因素。     

新型混合级联多电平逆变器拓扑及调制策略

为解决现有非对称级联多电平逆变器存在低压单元电流倒灌和输出电平数少的问题,提出一种基于开关电容电路的混合级联多电平逆变器。首先,在两单元非对称级联H桥型逆变器的低压单元中嵌入一个开关电容电路,有效避免了低压单元电流倒灌,且输出电平数得以增加。然后,为减少所提方案应用于三相系统时所需直流电源的数量,提出了用三电平中点箝位型或T型逆变器电路作为高压单元的三相混合级联多电平逆变器拓扑。之后,针对所提逆变器拓扑的特性,提出了含有移相载波和层叠载波的混合调制策略,在满足逆变器输出高质量正弦脉宽调制电压波形的同时,有效减小了开关电容电压纹波和开关器件的切换频率及开关应力。最后,通过实验验证了所提混合级联多电平逆变器拓扑及调制策略的可行性。     

双重移相控制下的双向全桥DC-DC变换器最小电流应力分段优化控制

为了减小双向全桥(dual-active-bridge, DAB)DC-DC变换器的电流应力,提升变换器的效率,提出了一种DAB变换器在双重移相控制下的电流应力分段优化控制策略。该策略首先分析了DAB变换器的结构及其功率特性,推导得到了电流应力与传输功率、移相角之间的关系。然后针对输入输出电压不匹配的情况,对DAB变换器的电流应力进行了分段优化,通过将传输功率分段得到了DAB变换器在不同传输功率范围下的最优移相角。将所提控制策略与传统双重移相控制策略对比分析,发现所提控制策略具有更小的电流应力和回流功率,减少了变换器的导通损耗,提升了变换器的效率。当负载发生突变时,DAB变换器的动态性能得到了大幅提升。最后基于所提控制策略搭建了实验样机,实验结果验证了所提控制策略的正确性和有效性。     

农药生产废水治理

复配农药厂，生产废水量很少、浓度高、水质变化大，针对这些厂的生产废水治理又成为一个新的课题。我们采用水解－兼性氧化－接触氧化法处理某复配农药废水，解决了废水毒性对生物影响、废水可生化性差、小流量难以均匀连续运行等问题，使工艺取得成功。其中调节池停留时间T≥7d，水解时间T≥1h，兼氧池停留时间T＝5h，接触氧化池负荷为：一段1.2kgCOD/m^3.d，二段0.8kgCOD/m^3.d。     

Flow induced surface switching in a bistable nematic device

A surprising feature of liquid crystals is that rapid changes in alignment in these anisotropic liquids can induce significant flow, termed backflow, which in turn influences the alignment. A recent paper has suggested that such backflow may be the mechanism behind the fast switching observed in certain bistable nematic cells on the application of electric fields. In these experiments a weakly chiral nematic with unequal monostable surface anchorings is switched rapidly between a uniform and a -twist configuration, and it is conjectured that the backflow induced initially at the more strongly anchored surface plays a crucial role in the switching process. In this paper continuum theory is employed for nematic liquid crystals to investigate this phenomenon, and confirms that backflow can play an important role in the switching.