摘要
当流体流过一非流线型物体时,在一定条件下漩涡会形成涡街,两相流涡街的形成与脱落除了与雷诺数有关,还与含气率有很大联系。两相流研究的一个基本课题是判断流动形态及其相互转变,另一个基本课题,是关于分散相在连续相中的运动规律及其对传递和反应过程的影响。
随着计算机技术的快速发展,数值仿真也得到了相应的发展,成为分析和解决问题的强有力和用途广泛的工具。作为数值仿真的一个分支,计算流体动力学(CFD)也在最近的几十年时间里获得了长足的发展,广泛的应用于流体及多相流参数的测量,而且具有较高的可靠性和可信度。
在参阅了大量的国内外有关气液两相流、流场实验技术、数值计算技术等相关文件的基础上,选用梯形柱为漩涡发生体,构造了流场的三维结构。
本论文的研究目的在于,在已有的研究成果的基础上,通过采用气液两相流的实验研究和数值仿真相接合的方法,探索流场中气泡、液体两相湍流瞬态结构,产生机理及其相互作用。主要完成了以下工作:
1、从理论上分析了涡街的脱落频率,和影响脱落频率的主要因素。
2、根据经典理论,以雷诺平均N-S方程为控制方程,大涡模型为湍流模型,建立气液两相流的三维流场模型。
3、进行了纯水实验,并将根据实验数据与仿真结果进行比对,仿真结果与实验平均相对误差小于5%,最小误差达到2.4%,表明模型对流场的模拟可信。
4、运用验证后模型,对低含气率下两相流进行仿真,并将仿真结果与实验结果验证,该模型最大相对误差小于8%;平均相对误差小于5%,表明对两相流流场有较好的预测效果。
5、对比单相与两相流仿真结果,研究漩涡发生体后流体横向速度与湍流,发现有气泡时液体的湍流脉动大于无气泡的相应值,说明了气泡加强了液体湍流,且气泡的湍流脉动大于液体的湍流脉动.
6、改进了传统的仿真流程,在残差收敛数量级和运算精度不变的情况下降低了运算时间。
When the fluids flow through a non-streamlined object, vortex will form a vortex street under certain conditions. The formation and shedding of the vortex street in two-phase flow is related to both the Reynolds number and the gas fraction. One basic issue about Two-phase flow is to determine the flow patterns and their transformations into each other, another one is about the motion pattern of dispersed phase in the continuous phase and its impact on the delivery and reaction process.
With the rapid developments of computer technology, numerical simulation technique is developing accordingly and has become a powerful widely used tool to analyze and solve problems. As a branch of numerical simulation, computational fluid dynamics (CFD) has been developing a lot in recent decades, and is widely used to measure flow parameters of liquid and Multiphase Flow with a pretty high creditability and reliability.
After researching on documents about gas-liquid two-phase flow, the flow field technique and numerical calculation technique ,this paper selects the trapezoidal column as Obstacles to construct a three-dimensional structure of the flow field.
Based on the existing research results ,this paper uses the combination of the experimental research and numerical simulation technique to explore the transient structure of turbulence, generation mechanism and interactions between the bubbles and liquid phase. The following work are completed.
1. The shedding frequency of vortex street,and the impact of its components were Analyzed theoretically.
2. According to classical theory, the NS equations was chosen to be the control equation and large eddy model to be the turbulence model the three-dimensional flow model of the gas-liquid two-phase flow were Established.
3. The pure water test was Conducted, the experimental data was compared with the simulation one,and a conclusion was reached that the flow field simulation model is credibility basing on the fact that the relative error is lower than 5% while the minimum error is 2.4%.
4. The two-phase flow simulation with low gas fraction was done using the models which have been verified. Comparing the simulation result with the experimental result, a better prediction was reached basing on the fact that the maximum relative error is less than 8%,. the average relative error is less than 5%.
5. After comparisons of simulation results between single phase and two-phase flow, the Horizontal fluid velocity and turbulence behind the vortex eddy were studied. a fact was discovered that turbulent fluctuation of fluid with bubbles was bigger than that of fluid without bubbles. That means the bubbles enhanced the turbulent fluctuation of fluid and the bubbles’turbulent fluctuation are greater than that of fluid.
6. The traditional simulation process was improved and the calculation time was decreased as the magnitude of the residual and the calculation precisions remained the same.
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