388 / 2018-12-18 21:01:12

Study on the flow direction of liquid slug-vapor plug in a pulsating heat pipe (脉动热管管内液塞-汽弹转向机制研究)

Pulsating heat pipe; Meniscus; Liquid slug; Vapor plug (脉动热管；弯液面；液塞；汽弹)

全体主题 > 节能、联产、热泵

Evolution of flow patterns inside a pulsating heat pipe (PHP) is closely related to the complicated non-equilibrium thermo-dynamic process between the condensation section and the evaporation section. The ideal operation mode of the internal working fluid is one-way circulation. However, the previous visualization experiment shows that the one-way circulation is difficult to form, and random oscillations usually occur, as shown in Fig. 1. There are few studies on the mechanism of one-way circulation and random oscillation of liquid slug-vapor plug, which is the basis of PHP modeling and mechanism analysis.
In this paper, we developed a one-dimensional force balance model considering a liquid slug and its two adjacent vapor plugs as a unit, as shown in Fig. 2. The force analysis of the liquid slug includes the vapor pressure at both ends of the slug, the friction between the wall and the liquid slug, the gravity and the capillary resistance generated by the curvature of the interface at the different advancing and receding positions. By considering the force effect of the extended evaporation meniscus formed near the vapor-liquid-solid three-phase contact line, we have built the correlations between the slug velocity and the geometry of the pulsating heat pipe, the thickness of the thin liquid film and the physical properties of the working fluid. In addition, a pulsating heat pipe made of the glass tube was used for visualization experiments, and a high-speed camera and image processing methods were applied to obtain the geometric and kinetic parameters of the slugs-plugs. Through the theoretical and experimental results, the mechanism of the uncertainty of the flow directions of liquid slug-vapor plug in the pulsating heat pipe is further revealed.
脉动热管管内流型的演变与冷凝段与蒸发段之间复杂的非平衡热力学过程密切相关。理想的管内工质稳定运行方式是单向循环，但可视化实验表明单向循环难以形成，通常会出现随机脉动情况，如图1所示。目前对液塞-汽弹单向循环与随机振荡的机理研究较少，而此正是脉动热管建模和机理分析的基础。
本文采用一维力平衡法，取一个液塞及其相邻的两个汽弹作为分析单元，如图2所示。对液塞进行受力分析，包括液塞两端的汽弹压力、壁面与液塞的摩擦力、液塞重力与液塞前进和后退位置不同界面曲率产生的毛细阻力。考虑汽液固三相接触线附近延展蒸发弯液面形成的力效应，计算得到单液塞的速度与脉动热管几何尺寸、薄液膜厚度和工质物性参数的关联式。此外，采用玻璃管制作的脉动热管进行了可视化实验，应用高速摄像机及图像处理方法获取液塞汽弹的几何及运动参数。通过理论与实验结合，进一步揭示脉动热管管内液塞-汽弹转向的机制。