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螺旋两相流与水合物浆体流动数值模拟研究
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/10/8 9:01:15  

摘要:油气开采和运输过程中在高压低温条件下可能发生水合物堵塞管道的风险。将螺旋流应用到水合物防治领域是一种新型的拓展水合物安全流动边界的方法。一方面螺旋流可以强化气液间的传质与传热,使水合物在可控区间内生成,另一方面其提供的切向速度使颗粒均匀分布在管壁四周悬浮输送。颗粒的自旋抑制了颗粒间的液桥力使其不易团聚,颗粒与管壁的切向力使其不易粘附堆积在管壁上。因此利用螺旋流动是拓展安全流动边界的有效手段,可显著降低水合物堵管的风险。

本文首先对有关气液两相螺旋流型、水合物浆体流动特性及水合物防治现状进行了梳理。在此基础上建立了以长、短扭带为起旋装置的几何物理模型。为了简化模型,将整个混输过程分为两部分。第一部分是水合物生成前气液两相螺旋流的研究,第二部分是水合物生成后的水合物浆体流动输送特性研究。

为了研究以扭带起旋的水平管气液两相螺旋流流型转换规律,采用RSM模型和VOF方法与Level Set方法耦合的CLSVOF模型对其进行了数值模拟。结果得到四种不同的流型,分别是螺旋层状流、螺旋泡状流、螺旋团状流和螺旋轴状流;并研究了气液相折算速度对流型的影响,结果表明两相滑移速度产生的曳力对气相存在的形式有很大影响。同时由于螺旋流的衰减,管道不同位置处的流型也会发生变化,最后绘制了气液两相螺旋流流型图并与mandhane流型图进行了比较,发现泡状流和轴状流的转换边界提前,且在螺旋流较强段并未出现塞状流。

为了研究以扭带起旋的水平管内水合物浆体的流动特性,采用RSM模型和DPM模型对低浓度下CO2水合物浆体流动进行了数值模拟。模拟主要对螺旋流体系下的速度场、湍流强度、传热规律、壁面剪切力、螺旋流衰减规律及水合物沉积特性等进行了研究。研究结果表明以短扭带起旋时螺旋流能提高壁面传热效率,与普通平直流相比,传热效率提高50%~100%,此外传热效率还与Re和颗粒浓度有关;流体在扭带作用下速度开始呈对称双峰形式运动,最后峰值在管道中心处合并;涡首先在扭带边缘两侧产生,然后向扭带边缘中间部分移动,在形成较为稳定涡后又向两端靠近边壁处移动达到稳定,最后涡核中心几乎与速度中心重合。水合物颗粒的旋转方向与扭带扭转方向相同,且扭率Y越小Re越大,对称涡线发生合并的位置越远。初始旋流数的大小主要与扭率Y有关,而与Re无关。在无扭带的管道下游螺旋流呈指数规律衰减,其衰减速率主要与Re有关,而与扭率Y无关,并得出螺旋流衰减指数β与Re的关系式;与光管相比,螺旋流能够明显提高水合物的输送距离,以短扭带起旋时在螺旋流较强段水合物颗粒均匀分布在管壁四周,扭率越小水合物输送效率越高;以长扭带起旋时旋螺旋流强度几乎不衰减,流线呈周期性变化。水合物颗粒主要集中在扭带边缘压力面处呈对称分布,且随着距离的增加水合物螺旋运动越明显。虽然长扭带的携带效率提高,但是压能损失是短扭带的两倍以上是无扭带情况下的三倍以上,且随着流速的增加压降损失差距在不断增大。长扭带起旋存在一个临界雷诺数在2000~4000之间,当流速低于该临界雷诺数时全程起旋方式也存在堵管风险。

In the process of oil and gas exploitationand transportation, the risk of gas hydrate blocking pipeline may occur underhigh pressure and low temperature conditions. The application of spiral flow tohydrate control is a new method to extend the safe flow boundary of hydrate.Firstly, the spiral flow can enhance the mass transfer and heat transferbetween gas and liquid, so that hydrate can be created in a controllable range.On the other hand, the tangential velocity provided by the spiral flow enablesthe parti-cles to be evenly distributed around the pipe wall to be suspendedand transported. The spin of particles inhibits the liquid bridging forcebetween the particles and makes it difficult to agglomerate, while thetangential force between the particles and the wall makes it difficult toaccumulate. There-fore, the spiral flow can be utilized to expand the safe flowboundary to reduce the risk of hydrate pipe blockage.

In this paper, the gas-liquid two-phasespiral flow pattern, hydrate slurry flow characteristics and hydrate preventionand treatment are re-viewed. On this basis, the physical geometric model of thelong and short twisted belt as the starting device is set up. In order tosimplify the model, the whole diversified transportation process is dividedinto two parts. The first part is the study of gas-liquid two-phase spiral flowbefore hydrate formation, and the second part is the study of flow andtransportation characteristics of hydrate slurry after hydrate formation.

RSM model and CLSVOF model were used tostudy the flow pattern transformation of gas-liquid two-phase spiral flow in ahorizontal pipe with a twisted belt. Results four different flow patterns wereobtained, which were spiral stratified flow, spiral bubble flow, spiral slugflow and spiral annular flow. The results show that the drag force produced bythe two-phase slip velocity has a great influence on the existence of the gasphase. At the same time, due to the attenuation of the spiral flow, the flowpattern at different positions of the pipeline will also change. Finally, theflow pattern diagram of gas-liquid two-phase spiral flow was drawn and comparedwith the mandhane flow pattern diagram. It was found that the transitionboundary of bubbly flow and axial flow was advanced, and there was no plug flowin the strong section of the spiral flow.

In order to study the flow characteristicsof hydrate slurry in hori-zontal pipe with twisted belt, RSM model and DPMmodel were used to simulate the flow of CO2 hydrate slurry at lowconcentration.The velocity field, turbulence intensity, the heat transfer law,the wall shear force, spiral flow attenuation law and hydrate depositioncharacteristics were studied in the simulation. The results show that use theshort twisted belt to cause spiral flow can enhance the heat transferefficiency of the wall surface, and the heat transfer efficiency can beimproved by 50%~100% compared with the ordinary straight flow. In addition, theheat transfer efficiency is also related to Reynolds number and particleconcentration. Under the action of the twisted belt, the velocity of the fluidbegan to move in the form of symmetrical double peaks, and finally the peaksemerged at the center of the pipe. Vortexes are first generated on both sidesof the twisted belt edge and then move towards the middle part of the twistedbelt edge. After rel-atively stable vortexes are formed they move towards bothends near the boundary wall to achieve stability. Finally, the core of thevortex almost coincides with the velocity center. The rotation direction ofhydrate parti-cles is the same as that of the twisted belt, and the smallertorsion rate Y is or the greater Re is the farther away from the symmetricvortex line merging position will be. The size of the initial swirl number ismainly related to twist rate of Y, which has nothing to do with Re.

关键词:水合物;螺旋流;流型;衰减规律;数值模拟

hydrate;spiral flow;flow pattern;attenuation law;numerical simulation

上一篇:石墨烯复合材料的性能研究及其在石油化工污染物处理上的应用     下一篇:管道螺旋流中天然气水合物颗粒的沉降和传热数值模拟研究
 
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