粘弹性气液两相流减阻机理研究

Viscoelastic drag reduction has remarkable energy-saving effect on single-phase turbulent flow systems. It also has broad prospects for the applications in gas-liquid two-phase flow systems in power engineering, petroleum engineering and so on. In this project, we will combine the novel diffuse-interface model of Newtonian fluid and the conventional constitutive model of viscoelastic fluid, and establish a mathematical model coupling the interactions of the two phases and the turbulent flow of the viscoelastic liquid. Numerical simulation method using unstructured adaptive mesh technique will be studied to automatically refine the meshes around the gas-liquid interfaces with irregular distribution. The numerical stability will be enhanced for the direct numerical simulation of the viscoelastic gas-liquid two-phase turbulent flows. The problem of large computational burdens of two-phase flow simulations in small scales will be solved. The combination of the particle image velocimetry technology and the proper orthogonal decomposition method will be extended to the experimental characterization of turbulent structures for viscoelastic gas-liquid two-phase flows with small and few bubbles. The mathematical model and numerical methods will be validated and improved through the analyses of the experimental results, and then the drag reduction characteristics in more conditions that can hardly be measured in the experiment will be studied through the numerical approach. Via the multiple and combined studies, the flow pattern variations of the viscoelastic gas-liquid two-phase flow and burst law of the viscoelastic liquid turbulence will be revealed. Thus, the detailed drag reduction mechanism via the variations of the flow patterns and turbulence structures by viscoelasticity will be discussed. The outcomes can be a good reference in theories and methods for the engineering applications of viscoelastic gas-liquid two-phase flows.

粘弹性减阻在单相湍流系统中的节能降耗效果显著，在动力工程和石油工程等所涉及的气液两相流领域也有广阔的应用前景。本项目拟将新兴的牛顿流体扩散界面模型和传统的单相粘弹性湍流本构模型结合，建立耦合气液两相相互作用和液相粘弹性湍流的数学模型；研究非结构化自适应网格技术自动加密不规则相界面附近网格的高效数值模拟算法，增强粘弹性气液两相湍流直接数值模拟的稳定性，解决气液两相流模拟在小尺寸下计算量过大的问题；将粒子图像测速技术和特征正交分解方法结合的思路拓展到微小气泡尺寸和空泡份额条件下的粘弹性气液两相流湍流结构的实测表征。由实验结果验证和完善数学模型和数值方法，进而由数值模拟研究难以实测的减阻流动特征。通过上述多角度互为补充的研究，揭示粘弹性气液两相流流型变化规律和液相粘弹性湍流在气相作用下的猝发机制，研究粘弹性改变流型和湍流结构从而实现减阻的细微作用机理，为相关工程应用提供理论指导和方法支撑。

本项目研究气液两相流中加入减阻剂以后的流动特征变化及减阻机理基础理论问题，研究成果对发展高效的多相流输送技术与方法、节能降耗、环境保护都有重要意义。通过理论分析、数值模拟和实验研究从减阻剂与气液相界面相互作用微观角度和减阻溶液与微气泡共存时的宏观流动特性两个层面，对气液两相湍流减阻机理开展了研究。考虑液相黏弹性非牛顿流体湍流运动特点，将描述牛顿流体两相流的相界面模型扩展到气液两相流黏弹性减阻领域，提出了反映气液两相流特点和液相黏弹性流体湍流运动特点的气液两相流黏弹性减阻数学模型；研究了气液相界面VOSET方法与黏弹性本构方程耦合计算时的数值稳定性，通过用光滑的Heaviside函数对黏弹性应变进行插值，有效克服了由气液相界面导致的数值计算的不稳定性，并且实现了用一个统一的控制方程完成两相流动求解，解决了气液两相流中特有的在采用高阶有界格式离散Giesekus本构方程对流项后仍出现的越界现象，大幅提高了数值计算的稳定性，为研究气液两相减阻湍流特征扫清了数值模拟方法障碍；通过直接数值模拟发现加剂后整体脉动强度被抑制、雷诺数降低、时均速度以及整场液相平均速度有较大幅度增大，证实了往液相中添加减阻剂可抑制湍流脉动实现气液两相湍流减阻，并证明了气液两相减阻流动也有类似于单相减阻流动的减阻极值问题。通过PIV实验发现微气泡的注入减小了壁面附近处液相的雷诺应力，提高了壁面附近处液相的平均速度，同时也轻微的减小了壁面附近处液相的速度脉动强度和雷诺剪应力，而且微气泡在壁面附近的聚集抑制了展向涡结构的发展和扩散。展向涡的抑制，正是速度脉动强度减小的直接原因。