PIV EXPERIMENTAL INVESTIGATION ON THE BEHAVIOR OF PARTICLES IN THE TURBULENT BOUNDARY LAYER1)
GaoTianda*,, SunJiao*,,**, FanYing*,, ChenWenyi*,,2),, XuanRuixiang*, *Department of Process Equipment and Control Engineering, Hebei University of Technology, Tianjin 300130, ChinaResearch Center of Engineering Fluid and Process Enhancement, Hebei University of Technology, Tianjin 300130, China**Department of Mechanics, School of Mechanical Engineering, Tianjin University, Tianjin 300350, China 中图分类号:O357.5$^+$2 文献标识码:A
关键词:两相流;粒子图像测速;湍流边界层;空间多尺度局部平均涡量;顺向涡 Abstract The particle image velocimetry (PIV) is used to conduct experimental research in the solid-liquid two-phase plane turbulent boundary layer. The turbulence statistics such as the average velocity profile, turbulence intensity and Reynolds stress of the particle phase and single-phase clean water are compared to analyze the behavior of the particles in the turbulent boundary layer. The concept of multi-scale spatial locally-averaged vortices is utilized to extract the spatial topologies of the spanwise vortex head and the statistic of the prograde vortex is acquired. From that, the spatial topologies of the fluctuating velocity and streamlines around the prograde vortex at different normal positions can be obtained. The degree of development of the prograde vortex and the surrounding turbulence coherence structure can be compared and analyzed. The results show that compared with the clean water conditions, the buffer layer of the turbulent boundary layer of the particle phase becomes thinner, the logarithmic region moves downward, the turbulence intensity is enhanced, and the Reynolds stress in the logarithmic law region is increased. The fluctuating velocity of the particle phase is different from the clear water condition around the vortex, and the particles can be effectively transferred by the burst process around the spanwise vortex. The particle-laden flow has a large prograde vortex core and develops as the normal position rises. The vortex and the band stretch longer in the flow direction. At the same time, it is found that there is always a retrograde vortex in the lower left of the prograde vortex under both conditions, and the formation of retrograde vortex in the particle-laden flow is weaker than that of single-phase fluids. The number of prograde vortices in both conditions decreases with the increase of the normal position, and finally gradually stabilizes.
Keywords:two-phase flow;PIV;turbulent boundary layer;multi-scale spatial locally-averaged vortices;prograde vortex -->0 PDF (21929KB)元数据多维度评价相关文章收藏文章 本文引用格式导出EndNoteRisBibtex收藏本文--> 高天达, 孙姣, 范赢, 陈文义, 轩瑞祥. 基于PIV技术分析颗粒在湍流边界层中的行为1)[J]. 力学学报, 2019, 51(1): 103-110 https://doi.org/10.6052/0459-1879-18-211 GaoTianda, SunJiao, FanYing, ChenWenyi, XuanRuixiang. PIV EXPERIMENTAL INVESTIGATION ON THE BEHAVIOR OF PARTICLES IN THE TURBULENT BOUNDARY LAYER1)[J]. Chinese Journal of Theoretical and Applied Mechanics, 2019, 51(1): 103-110 https://doi.org/10.6052/0459-1879-18-211 液固两相流常见于日常的生产生活中,如化工单元操作、沙石水力输运和污水处理等.与液固两相流相关的基础问题一直是科研的热点[1 -4],其中颗粒如何在液固两相湍流边界层内运动是研究的重要问题之一.液固两相流中颗粒的运动行为直接受流体的作用,同时又反作用于流体[5- 11].颗粒周围的局部流场受颗粒的影响较大,尤其是颗粒周围的涡结构.涡的无规则运动引起物质、动量、能量的扩散输运,在化工过程的传热传质方面有着十分重要的意义.因此对颗粒与液固两相湍流边界层内涡结构的研究十分重要. Muste等[12]利用DLDV技术对明渠悬浮流动中的流体相与颗粒相进行了测量,发现沉积物颗粒的平均速度低于水的平均速度.Righetti等[13]在平板湍流边界层中进行实验,发现在湍流边界层中近壁相干结构挟带着颗粒做抬升或下扫运动,并且发生了显著的动量交换.Wang等[14-15]对颗粒在垂直平板边界层内的规律进行了研究,发现在边界层内部,颗粒速度大于流体相速度,粒子流的边界层厚度小于流体相的边界层厚度.王汉封等[16]运用PIV技术对充分发展的水平槽道内稀疏湍流两相流中的颗粒行为进行了定量研究.发现槽道下壁面附近,向上运动颗粒的概率大于向下运动的颗粒,两者概率的差异随着法向距离的增大而减小.Sumer等[17-18]对近壁区的颗粒运动进行了测量,发现颗粒做下降运动时的法向速度大于其做上升运动时的法向速度,据此他们推测颗粒是受湍流猝发事件影响的.Li等[19]运用双向耦合的欧拉-拉格朗日点粒子方法研究平板湍流边界层,发现颗粒的流向脉动速度大于流体的流向脉动速度,法向和展向脉动速度则小于流体的脉动速度,且粒子法向和展向脉动速度随粒子斯托克斯数增长而下降.Vinkovic等[20]对稀疏湍流通道内的颗粒行为进行了直接数值模拟,发现被喷射事件包围的颗粒会远离壁面.Marchioli等[21]发现颗粒通过近壁区的扫掠事件在低速条带中聚集,而颗粒通过近壁区喷射事件从壁面区域转移到外区,只有具有明显空间相干性的喷射与扫掠事件才能有效地传递粒子.吴文权和黄远东[22-23]应用离散涡方法计算了两种 {St}的泥沙粒子在圆柱绕流场中的运动,结果表明液固两相流中颗粒运动与旋涡存在有明确的相关性.Dritselis等[24]对垂直湍槽流近壁区域颗粒与流体相互作用机理进行直接数值模拟和拉格朗日粒子追踪,重点研究了颗粒对于近壁区相干结构的影响.发现在单向流和液固两相流中,近壁区中以准流向涡结构为主,无论是颗粒自身运动还是跟随流体运动,都会产生一个反向力矩,从而削弱准流向涡的运动尺度及强度,抑制流场湍动强度.Richter等[25]利用直接数值模拟结合拉格朗日点粒子方法来研究库特流(Couetteflow)中不同雷诺数下颗粒和边界层中发卡涡的相互作用.研究证明,发卡涡由于颗粒相的存在被削弱,并且随着雷诺数的增大削弱程度也在逐渐增大.陈启刚等[26]利用基于二维平面流场的涡结构模式匹配方法对明渠湍流中的涡结构进行了分析,发现涡结构的密度和涡量沿水深逐渐减小,且顺向涡的平均流场具有典型的发夹涡群特征.Hambleton等[27]在对边界层外区的逆向涡进行线性随机估计时发现,顺向涡的左下角常会伴随一个逆向涡.苏健等[28]在利用高时间分辨率粒子图像测速技术(TRPIV)研究超疏水壁面湍流边界层中展向涡的空间分布特征时发现逆向涡对顺向涡的进一步发展起抑制作用.Kaftori等[29-30]实验研究了壁湍流中固体颗粒的运动.证实壁相干结构是影响湍流中固体边界附近颗粒运动以及沉积和夹带的主要因素;发现壁湍流中的颗粒行为与漏斗形旋涡的主要基础结构相一致. 综上所述,前人的研究多集中在液固两相湍流边界层内的速度差异以及喷射、扫掠等相干结构上,对颗粒形成的涡结构研究较少.对涡结构的深入理解对工程中的传质传热,节能减阻有重要的意义.因此本文从涡的角度揭示液固两相湍流边界层内液相与固相之间的关系,突破单点测量的局限性,利用粒子图像测速技术\!(PIV)\!对清水工况和加入颗粒条件下平板湍流边界层进行研究,采用空间多尺度局部平均涡量的概念提取壁湍流展向涡涡头平面结构,通过对比分析颗粒相与单液相的平均速度剖面、湍流度、雷诺应力等统计量及顺向涡结构,获得液固两相湍流边界层中颗粒运动与液相流场之间的关系,为颗粒在液固两相湍流边界层内的运动规律提供参考.
1 实验设备与参数
实验在河北工业大学PIV流体力学实验室中型低速循环水槽中进行,其背景湍流度小于0.8%,实验装置如图1所示.水槽实验段长为2600 mm,宽为500 mm,高为600 mm. 平板长为2200mm,宽为500 mm,厚为15 mm. 平板水平放置在距水槽底部250mm处,并通过调节后缘板将平板表面沿流向的静压力梯度调节至接近于零,平板前端为8:1椭圆修形;为获得充分发展的湍流边界层在距平板前缘100mm位置添加直径5 mm的拌线.实验过程中通过调节变频器使得水槽的自由来 显示原图|下载原图ZIP|生成PPT 图1实验装置示意图... -->Fig. 1Schematic diagram of the experimental facility -->
近壁区的速度场由高低速流体交替构成,低速流体远离壁面的过程称为"喷射",而高速流体冲向壁面的过程称为"扫掠",二者合称湍流的猝发过程,猝发过程对雷诺应力有很大的贡献. 图5为清水对照组和颗粒实验组在$x-y$平面内(法向位置依次是$y^{ +}$=73,$y^{ + }$=77,$y^{ +}$=81)顺向涡周围流向脉动速度空间平均拓扑结构分布.云图很好的展现了展向涡周围流场分布特征:展向涡的下方分布着低速流体($u'<0$的区域),展向涡上方分布着高速流体($u'>0$的区域).从图中脉动速度矢量方向可以 显示原图|下载原图ZIP|生成PPT 图5不同检测中心法向位置展向涡周围流向脉动速度分布云图... -->Fig. 5Contours of the streamwise fluctuating velocity aroundspanwise vortex at different normal position of testing center -->
图7是两种工况条件下检测到的顺向涡数量沿法向分布规律,由图可知,顺向涡的数量随法向距离的增加而减小,且逐渐趋于稳定,这与陈启刚等[26]的结论是一致的. 显示原图|下载原图ZIP|生成PPT 图7顺向涡数目沿法向分布规律... -->Fig. 7Distributions of the number of prograde vortex along\\normal-wall positions -->
4 结论
本文利用粒子图像测速技术分别对清水和颗粒工况下的平板湍流边界层瞬时速度场进行测量,得到了平均速度剖面、湍流度、雷诺应力等统计量,分析了两种工况条件下的发卡涡展向涡头(顺向涡)及逆向涡结构,得到了以下结论: (1)与清水组相比,颗粒组的无量纲速度均小于清水组,湍流度得到加强并且雷诺切应力在对数律区增大. (2)展向涡周围的湍流相干结构能够有效地传递粒子.与清水组相比,颗粒组流向脉动速度绝对值大的区域面积更多,颗粒组发卡涡附近的湍流猝发强度更大. (3)展向涡能够有效地挟带颗粒运动. 通过比较$\alpha$角,颗粒组提取出的涡和条带在流向上拉伸得更长;颗粒组逆向涡的形成较单相流体缓慢.说明流体挟带颗粒需要一定的能量积累,且挟带的颗粒与流体相之间存在滑移. (4)在两种工况下,顺向涡的数量均随着法向位置的升高而减少,最后逐渐趋于稳定. The authors have declared that no competing interests exist.
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