微距粒子图像测速法测量球床多孔介质单孔流场实验

Experiment on Flow Field of a Pore in the Pebble Bed by Using Macro Particle Image Velocimetn Technology

PIV技术是20世纪80年代发展起来的一种瞬态、多点、无接触的流体力学测速方法。它能够在不干扰流场的情况下,实现对流场瞬时和全局测试。试验在现有PIV中添加微距镜头,对多孔介质某一孔隙的流动特征进行检测和分析研究。实验选用的多孔介质是由直径20 mm的水晶玻璃球随机堆积而成,孔隙率为0.592 3,迂曲度为1.33,选用的流体为65%苯甲醇和35%无水乙醇混合液,其折射率须与多孔介质完全匹配。经过微距PIV实验,得到多孔介质某一孔隙内的流场矢量图。分析发现:低雷诺数时,流体流动受外界干扰随机扰动,方向出现波动性,随着雷诺数增加,惯性力增强,干扰和波动性被抑制,流动变得稳定规则,流线为直线;当雷诺数达到154.1时,开始出现局部流线弯曲,因而可以认为此时多孔介质进入流动转捩期,当雷诺数超过214.9时,流动进入完全湍流,全场流动转为不规则流动,同一雷诺数下不同区域流动大小方向不断发生变化,不同雷诺数下同一区域的流场也不相同。同时还发现,不稳定流动可能源于某些未被惯性抑制的随机流动。当雷诺数超过482.2后,横向流动或者旁路流使湍流流动愈益复杂,不同位置交替出现流体的聚团和剥离消散。这些为多孔介质内部流动转捩机制提供了深入认识和新的发现。

During this test,macro lens is added to current PIV equipment to discover and analyze the flow characteristics of one pore in the porous medium. The porous medium used in this experiment is filled randomly with balls of crystal glass which diameter is 20 mm,porosity is 0. 45 and tortuosity is 1. 33. The fluid used in this experiment is the mixture consisting of 65% benzyl alcohol and 35% absolute ethanol,which refractivity is same with refractivity of porous medium. Porous medium flow field of pores were obtained from this macro-PIV experiment. After analysis,it shows that in low Reynolds number,flow direction is random disturbance by the external cause. With the increase of Reynolds number,inertial force increased so that disturbance and waviness are suppressed,fluid flow becomes stable and regular,and streamline is linear. When Reynolds number is 154. 1,parts of streamline begin to curve,therefore this situation can be regarded as flow transition period. When Reynolds number is more than 214. 9,it becomes fully turbulent flow and full-flow field turns into random flow,the direction of flow in different regions is changing continuously at the same Reynolds number and the direction of flow in the same region is also changing continuously at the different Reynolds number. At the same time,unstable flow could be driven by random flow not to be suppressed by inertia. When Reynolds number surpass 482. 2,cross flow or bypass stream makes turbulence more complex,the stream accumulates and dissipates at different regions. All of these provide depth understanding and new discovery for transition mechanism of internal flow in porous medium.