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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
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/10/8 9:03:01  

摘要:随着海洋天然气开发深度的增加,混输管线的天然气水合物堵塞问题越来越严重,水合物堵塞阻断天然气流通使管道局部压力变大对管道设备造成破坏,防控水合物堵塞逐渐成为保障天然气管道安全运营的重要问题。防止水合物堵塞管道的传统方法主要有通过加热降压来改变水合物生成条件,或者注入热力学抑制剂来抑制水合物生成。这些方法不但成本高而且对坏境也会造成污染,现在的研究更多不是关注水合物抑制而是允许水合物生成保证水合物安全流动来达到风险控制的目的。在不少的研究中提到螺旋流,管道螺旋流能够加强气体与壁面之间的剪切力,很大程度上阻止了固体颗粒粘附在管壁上,而且还提高了颗粒输送的携带能力。

论文首先提出了水合物的未来使用发展前景,接着梳理了天然气长输管道中水合物预测、防堵技术以及螺旋流动的研究现状。然后通过建立管道及扭带的物理模型,基于控制方程、湍流方程、离散相模型建立数学模型及计算方法,并对数值计算进行可行性的验证。最后以天然气水合物颗粒为研究对象,通过数值模拟的方法对管道螺旋流中天然气水合物颗粒的沉积和传热特性进行重点研究。

研究了以短扭带起旋的水合物管道螺旋流动与传热规律。主要分析了天然气管道内的速度分布、湍流强度分布、颗粒沉积特性、传热特性。研究发现随着短扭带扭率越小,颗粒的切向速度越大。水合物颗粒粒径越大,水合物颗粒容易沉积在管底。通过有无扭带管道中颗粒的沉积位置对比,发现在放置扭率为6.2扭带的管道中颗粒携带距离比平直光管大概提高了2.1倍。在传热方面,扭带的扭率越小它的努塞尔数越大,Nu最大可以提高3-4倍。颗粒的衰减与扭带扭率和颗粒浓度有关,随着扭率的增大,颗粒旋流数在减小,浓度越大旋流数越小。

研究了以长扭带起旋的水合物管道螺旋流动与传热规律。主要分析了管道中速度分布、压降分布、传热特性、颗粒沉降特性。在放入长扭带的管道中,在扭带两边会形成两个明显的涡流,在两个涡中心速度最大。管道压降随着雷诺数的增加而变大,在颗粒雷诺数不变的情况下,扭带的扭率越大,产生的螺旋流强度越弱,管道的压力降也就越小。相比于短扭带,因为长扭带充满了整个管道,所以流体在管道中一直保持螺旋流动努塞尔数衰减的幅度比较小。颗粒雷诺数越大越不容易沉积,

颗粒浓度越大,颗粒沉积位置越靠近管口。长扭带的携带效果是最好的,短扭带次之,长扭带的携带距离大概是无扭带的3-4倍,为水合物安全输送提供理论指导

With the development of offshore naturalgas,the problem of gas hydrate blockage in mixed pipelines is becoming more andmore serious.The blockage of gas hydrate causes the local pressure of pipelinesto increase and causes damage to pipeline equipment. Preventing and controllinggas hydrate blockage has gradually become an important issue to ensure the safeoperation of natural gas pipelines. The traditional methods to prevent hydratefrom blocking pipelines are mainly to change hydrate formation conditions byheating and reducing pressure, or to inject thermodynamic inhibitors to inhibithydrate formation. These methods not only cost a lot, but also pollute theenvironment. The current research is not focused on hydrate suppression, butalso on allowing hydrate formation to ensure the safe flow of hydrate toachieve the purpose of risk control. In many studies, helical flow ismentioned. Pipeline helical flow can strengthen the shear force between gas andwall, prevent solid particles from adhering to the pipe wall to a large extent,and improve the carrying capacity of particle transportation.

Firstly, the development prospect ofhydrate in the future is put forward.Then,the research status of hydrateprediction, blockage prevention technology and helical flow in long distancenatural gas pipeline is summarized. Then the physical model of pipe and twistedbelt is established, and the mathematical model and calculation method areestablished based on the governing equation, turbulence equation and discretephase model, and the feasibility of numerical calculation is verified. Finally,taking gas hydrate particles as the research object, the deposition and heattransfer characteristics of gas hydrate particles in pipeline spiral flow arestudied by numerical simulation.

The helical flow and heat transfer ofhydrate pipes with short twisted bands are studied. The velocity distribution,turbulence intensity distribution, particle deposition characteristics and heattransfer characteristics in natural gas pipelines are analyzed. It is foundthat the tangential velocity of particles increases with the decrease oftorsion ratio of short twisted tape. The larger the particle size of hydrate,the easier the hydrate particles to deposit at the bottom of the pipe. Bycomparing the deposition positions of particles in twisted tape pipes with orwithout twisted tape, it is found that the carrying distance of particles inthe pipes with 6.2 twisted tape is about 2.1 times higher than that in the straightpipes. In terms of heat transfer, the smaller the twist rate of the twistedtape is, the larger the Nusselt number is, and the maximum Nu can be increasedby 3-4 times. The attenuation of particles is related to the twist rate oftwist band and particle concentration. With the increase of twist rate, theswirl number of particles decreases, and the larger the concentration, thesmaller the swirl number.

The helical flow and heat transfer ofhydrate pipes with long twisted strips are studied. The velocity distribution,pressure drop distribution, heat transfer characteristics and particlesettlement characteristics in the pipeline are analyzed. In the pipeline withlong twisted tape, two distinct eddies will be formed on both sides of thetwisted tape, with the maximum velocity in the two vortices. The pressure dropof pipeline increases with the increase of Reynolds number. When Reynoldsnumber of particles remains unchanged, the greater the torsion ratio of twistedtape, the weaker the spiral flow intensity and the smaller the pressure drop ofpipeline. Compared with the short twisted tape, because the long twisted tapeis full of the whole pipeline, the Nusselt number attenuation of the helicalflow of the fluid in the pipeline is relatively small. The larger the Reynoldsnumber, the more difficult it is to deposit. The higher the particleconcentration, the closer the particle deposition position is to the orifice.

关键词:天然气水合物;螺旋流;沉降规律;传热规律;数值模拟

hydrate;spiral flow;settlement law; heat transfer law;numerical simulation

上一篇:螺旋两相流与水合物浆体流动数值模拟研究     下一篇:改性微孔分子筛MCM-22择形催化合成对二甲苯的研究
 
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