基于CFD-DEM方法的离心泵内盐析液固两相流动研究

发布时间：2018-12-17 09:15

【摘要】：盐析是工业输运盐溶液过程中常见的现象。由盐析造成的壁面结盐,过流断面减小等问题给相关的工业生产带来了巨大的能源浪费和经济损失。离心泵作为输运盐溶液的主要动力机械之一,研究其内部盐析两相流场对防结盐有重要意义。针对泵内伴有盐析的两相流动,传统的数值模拟利用CFD中的多相流模型,将晶体颗粒视为拟流体,但这种方法不能合理地体现颗粒-颗粒以及颗粒-壁面间的行为。因此,本文在盐析动力学理论以及实验观测的基础上,首次运用DEM方法研究离心泵内盐析颗粒的运动,对其液固两相流动进行了CFD-DEM耦合计算,准确预测了离心泵内盐析过程中的颗粒碰撞、聚并以及破碎行为。以此为基础,本文以颗粒与壁面间碰撞次数为主要优化指标,采用了正交试验设计,对离心泵叶轮的几何参数进行了优化。得到的结果可为流体机械的防结盐设计提供依据。本文研究的主要工作和创造性成果如下:1、总结了离心泵内液固两相流动和盐析两相流动的研究现状,发现原有的多相流模型不能合理体现颗粒-颗粒、颗粒-壁面和颗粒-液体的相互作用,无法满足本文的研究需要。结合DEM处理离散相的优势,应用CFD-DEM耦合计算离心泵内盐析两相流场。2、根据晶体动力学理论,分析了盐析流场中晶体聚并和破碎的主导机制。流体的运动导致盐析晶体颗粒之间相互碰撞,此时,由于弱相互作用力(范德华力等)的存在,会发生颗粒粘附,形成聚并体;当碰撞冲击力大于碰撞晶体颗粒的相互作用力时,则发生破碎现象,由此构建了DEM颗粒模型。3、使用了体网格生成技术,连续相的湍流求解使用了可实现的k-epsilon湍流模型,同时采用混合处理的方式(All y+Wall Treatment)处理壁面边界,得到了较低的壁面y+值。4、通过分析CFD-DEM耦合计算的结果,得到了颗粒的运动轨迹、分布状况、聚并、碰撞、液固两相互作用以及离心泵外特性等规律。5、以降低盐析颗粒黏附于叶轮流道壁面概率,减缓壁面结盐为优化原则。选定颗粒与叶片碰撞的次数为主要优化指标,确定了叶轮的五个优化因素进行正交试验设计。对9个试验方案进行CFD-DEM耦合模拟,找到了影响颗粒与叶片碰撞次数和扬程指标的因素顺序,得到了叶轮的最优结构参数。6、分析发现影响晶体颗粒与叶片压力面发生碰撞次数的主要因素为叶片压力面附近的低速流场。低速区域尺寸越小,更规则,可使颗粒更加顺畅地离开叶轮,缩短颗粒在流道中的滞留时间,减少盐析晶体颗粒与壁面接触次数,降低壁面结盐的概率。
[Abstract]:Salting out is a common phenomenon in the industrial transportation of salt solution. The problems of salting on the wall caused by salting out and the reduction of cross section bring huge energy waste and economic loss to the related industrial production. As one of the main power machinery for transporting salt solution, it is important to study the internal salting-out two-phase flow field of centrifugal pump. For the two-phase flow with salting out in the pump, the traditional numerical simulation uses the multi-phase flow model in CFD to treat the crystal particle as a pseudo fluid, but this method can not reasonably reflect the behavior between particle and wall. Therefore, on the basis of the theory of salting-out dynamics and the experimental observation, the movement of salting-out particles in centrifugal pump is studied by DEM method for the first time, and the liquid-solid two-phase flow is calculated by CFD-DEM coupling calculation. The particle collision, coalescence and breakage behavior in the process of salting out in centrifugal pump are accurately predicted. Based on the above results, the geometric parameters of impeller of centrifugal pump are optimized by orthogonal design, taking the number of collisions between particles and walls as the main optimization index. The results can provide the basis for the design of anti-salting of fluid machinery. The main work and creative achievements of this paper are as follows: 1. The research status of liquid-solid two-phase flow and salt-out two-phase flow in centrifugal pump is summarized. It is found that the original multi-phase flow model can not reasonably reflect the particle-particle. Particle-wall and particle-liquid interactions can not meet the needs of this study. Combined with the advantage of DEM in treating discrete phase, the two-phase flow field of salting out in centrifugal pump was calculated by CFD-DEM coupling. 2. According to the theory of crystal dynamics, the dominant mechanism of crystal aggregation and fragmentation in salting-out flow field was analyzed. The movement of fluid leads to the collision of crystal particles in salting-out. At this time, due to the existence of weak interaction force (van der Waals force, etc.), particles will adhere to each other and form a coalescence. When the impact force is greater than the interaction force of the colliding crystal particles, the phenomenon of fragmentation occurs, and the DEM particle model is constructed. 3. The volume mesh generation technique is used, and the k-epsilon turbulence model is used to solve the continuous phase turbulence. At the same time, (All y Wall Treatment) is used to treat the wall boundary, and the lower value y. 4 of the wall is obtained. By analyzing the results of CFD-DEM coupling calculation, the motion trajectory, distribution, coalescence, collision of particles are obtained. The rule of liquid-solid interaction and the characteristics of centrifugal pump. 5. The optimization principle is to reduce the probability of salting-out particles sticking to the wall of impeller passage and to slow down the formation of salt on the wall. The number of collisions between particles and blades was chosen as the main optimization index, and five optimization factors of impeller were determined for orthogonal design. Through CFD-DEM coupling simulation of nine test schemes, the order of factors influencing the impact times and lift index of impeller was found, and the optimum structural parameters of impeller were obtained. It is found that the main factor affecting the number of collisions between crystal particles and the blade pressure surface is the low velocity flow field near the blade pressure surface. The smaller the size of the low-speed region is, the more regular the particles are, which can make the particles leave the impeller more smoothly, shorten the retention time of the particles in the channel, reduce the number of contact between the crystal particles and the wall surface, and reduce the probability of salt deposition on the wall.
【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TH311

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