基于旋涡强度法的湍流泡状流混合层流动

用旋涡强度法对PIV测得的湍流泡状流混合层流动进行研究,混合层高低侧流速之比为4∶1,基于速度差和管道水力直径的Reynolds数范围为4400～132000。气泡分别从隔板尾部混合层的中心起始位置与低速侧注入流体,直径为0.5～2 mm。利用旋涡强度法分析流场清楚地显示出排除了剪切作用的涡结构主要集中在混合层中心的锥形区域,随着Reynolds数的增大,旋涡强度的值不断增大而锥形区域不断变窄。在同一Reynolds数下,沿主流方向旋涡强度先增大、后减小。从混合层中心注气时气泡主要分布在隔板下游旋涡强度值较大的区域,而低速侧注入的气泡主要分布在低速侧,但由于涡结构的卷吸作用,低速侧注入的气泡也会进入隔板下游的中心区域。与单相流动相比,低Reynolds数情况下气泡的加入可以加剧流场本身的变化,而高Reynolds数时则对旋涡强度产生的影响较弱。     

鼓泡床内气泡-液体两相湍流代数应力模型的数值模拟

现有的气泡 -液体两相流动的数值模拟中 ,或者不考虑湍流 ,或者仅仅考虑液体湍流 ,但是直接模拟和PIV测量结果都表明气泡由于尾迹的作用有强烈的湍流脉动 .本文首次推导和封闭了同时模拟气泡湍流脉动和液体湍流脉动的二阶矩输运方程两相湍流模型 ,并在此基础上建立了代数应力气泡 -液体两相湍流模型 .用代数应力模型模拟了二维矩形断面鼓泡床内气泡 -液体两相流动 .预报结果给出了气泡和液体两相速度场、两相Reynolds应力及湍动能分布和气泡体积分数分布 .模拟结果与PIV测量结果符合很好 ,表明了模型的合理性 .研究结果表明 ,原先静止的液体在气泡因浮力而产生的上升运动的作用下产生回流流动 ,而气泡则只有上升运动 .气泡速度始终大于液体速度 .在床内气泡湍流脉动确实始终很强烈 .液体则由于气泡的作用以及自身速度梯度产生的双重作用而发生湍流脉动 .气泡的脉动显著地大于液体的脉动 .两相湍流脉动都是各向异性的 ,而且气泡湍流脉动的各向异性比液体的更强烈     

毛细管内多相脉动流动的格子-Boltzmann模拟

基于伪势模型理论,建立毛细管内汽液两相工质脉动流动的等温格子-Boltzmann模型。将利用该模型取得的静态液滴形态以及表面张力、大空间和窄空间内气泡浮升运动时的形态模拟结果与文献的研究结果进行对比,验证了模型的可靠性。对毛细管内泡状流和柱塞流两种流型在边界正弦压力波作用下绝热脉动流动情况进行了数值模拟。通过模拟研究了毛细管内不同压力波振幅下液相Reynolds数和气泡位移幅度周期性变化规律;获得了汽液界面形态在脉动过程中的变化;观察到在边界液相速度方向发生改变时,边界附近区域的Reynolds数振荡现象;分析了重力对脉动运动过程的影响。模拟结果为分析以毛细管为主要构件的脉动热管内汽液两相工质的工作过程提供了一定依据。     

水平管内气液两相泡状流进口段壁面切应力

利用TSI-1268W热膜探头对内径为35 mm的水平管内气液两相泡状流的壁面切应力进行了直接测量,得到了进口段不同截面上沿周向不同位置处的壁面切应力数据,通过分析实验段进口段的壁面切应力沿管长的变化来确定进口段的长度.结果表明,液相中加入气泡后增加了壁面切应力数据的离散程度.在实验参数范围内,测量的壁面切应力进口段的长度在52D～65D之间,并且壁面切应力进口段随着液流Reynolds数和截面含气率的增大而增大.液相中加入气泡后在不同的测量角度壁面切应力的变化呈现不同的特征.     

气泡雾化喷嘴泡状流出口喷雾脉动特征

采用实验和仿真方法,对一特定气泡雾化喷嘴泡状流时混合室内的气液两相混合形态以及喷孔出口喷雾脉动特征进行了研究。研究结果表明,泡状流时气泡尺寸呈近似正态分布,气泡尺寸随液相质量流量和气液质量比增大而减小;喷雾形态和喷孔出口气液流动参数存在较大脉动,喷雾锥角脉动超过20°;气泡数量密度小且气泡直径较大时,喷雾平均锥角相对较小,且喷雾脉动现象比较严重;随着气泡数量密度增加,喷雾平均锥角呈现先快速增大后缓慢增大趋势,而喷雾锥角变异系数先快速增大随后逐渐减小并趋于稳定;复杂的流场结构是喷孔内气泡表观形态发生较大变化以及喷孔出口气液流动参数产生较大脉动的主要原因。     

气泡雾化喷嘴泡状流出口喷雾脉动特征

采用实验和仿真方法,对一特定气泡雾化喷嘴泡状流时混合室内的气液两相混合形态以及喷孔出口喷雾脉动特征进行了研究。研究结果表明,泡状流时气泡尺寸呈近似正态分布,气泡尺寸随液相质量流量和气液质量比增大而减小;喷雾形态和喷孔出口气液流动参数存在较大脉动,喷雾锥角脉动超过20°;气泡数量密度小且气泡直径较大时,喷雾平均锥角相对较小,且喷雾脉动现象比较严重;随着气泡数量密度增加,喷雾平均锥角呈现先快速增大后缓慢增大趋势,而喷雾锥角变异系数先快速增大随后逐渐减小并趋于稳定;复杂的流场结构是喷孔内气泡表观形态发生较大变化以及喷孔出口气液流动参数产生较大脉动的主要原因。     

细小尺度下潜热型功能热流体压降与传热特性

实验研究了相变微胶囊颗粒(囊芯材料为正十六烷,壳材为尿素-甲醛树脂)和去离子水混合制成的潜热型功能热流体流过等热流细小圆管的流动与传热特性,同时以去离子水作为传热工质在相同条件下进行了对比实验。得到了压降随质量流量的变化规律,实验段出、入口温度以及量纲1出口温度随Reynolds数变化规律,量纲1壁面温度沿轴向的分布规律,平均Nusselt数随Reynolds数的变化关系。结果表明,相变微胶囊颗粒的加入会导致流动压降增大,但随着流量增加,流动压降逐渐与单相液体的接近;出口温度及壁面温度要比相同条件下单相液体的低;含有较小相变微胶囊颗粒浓度的潜热型功能热流体的平均Nusselt数是相同条件下单相液体的2.0～4.0倍。     

0.5mm毛细管内气-液Taylor流动换热数值研究

采用移动计算域方法研究0.5mm毛细管内充分发展的气液Taylor流动换热特性,分析了Taylor气泡的形状、压降与换热特性。结果表明,随着入口Reynolds数Re的增大,气泡尾部的不稳定区域增大,液膜厚度逐渐增大,气泡长度变长;随着气泡体积分数ξ_g的增大,气泡形状基本不变而长度逐渐增大。阻力因子f随Re、ξg增大而降低,两相阻力系数高于单相的情况。平均Nusselt数Nu_(tp)随Re增大而增大,增大趋势逐渐降低;随ξg增大而线性降低。Taylor流的Nu_(tp)为单相的1.2~3倍,强化换热效果。     

0.5 mm毛细管内气-液Taylor流动换热数值研究

采用移动计算域方法研究0.5 mm毛细管内充分发展的气液Taylor流动换热特性,分析了Taylor气泡的形状、压降与换热特性。结果表明,随着入口Reynolds数Re的增大,气泡尾部的不稳定区域增大,液膜厚度逐渐增大,气泡长度变长;随着气泡体积分数ξ_g的增大,气泡形状基本不变而长度逐渐增大。阻力因子f随Re、ξ_g增大而降低,两相阻力系数高于单相的情况。平均Nusselt数Nu_tp随Re增大而增大,增大趋势逐渐降低;随ξ_g增大而线性降低。Taylor流的Nu_tp为单相的1.2～3倍,强化换热效果。     

宽矩形硅微通道中流动冷凝的流型

对水力直径90.6 μm、宽深比9.668的矩形硅微通道中的流动冷凝过程进行了可视化研究。研究发现,宽矩形硅微通道中的冷凝,沿程主要有珠状-环状复合流、喷射流和弹状-泡状流等流型。在珠状-环状复合流区,冷凝液膜可覆盖通道竖直侧壁,而在通道长边上,仍然为珠状凝结。喷射流位置随着入口蒸气Reynolds数的增大而延后,通道截面形状对流动冷凝不稳定性也存在很大影响。喷射流之后为弹状-泡状流,弹状气泡沿程逐渐缩短,并在表面张力的作用下收缩成圆球形气泡。冷凝通道的平均传热系数将随着入口蒸气Reynolds数的增大而增大。     

Wavelet autocorrelation identification of the turbulent flow multi-scales for drag reduction process in microbubbly flows

Measurement of turbulent flow was performed in a fully developed channel flow for both single-phase and microbubble injection conditions. A drag reduction was achieved by microbubble injection in the boundary layer. Further understanding of this phenomenon has a significant impact on energy saving. In this experiment, the Reynolds number was 5128 based on the half height of the channel. The particle image velocimetry (PIV) technique was used to obtain two-dimensional full-field velocity components in the streamwise direction near-wall normal plane. Wavelet autocorrelations were applied to the streamwise fluctuating velocity fields. By applying wavelet analysis, many of the shortcomings of Fourier analysis were overcome. The comparison of the wavelet analysis results between single-phase flow and microbubble flow indicated that the microbubbles drag reduction is a multi-scale mode, in which the small-scale fluctuations are suppressed.     

Validation of CFD simulations of a stirred tank using particle image velocimetry data

Methods for validating CFD simulations based on the Reynolds Average Navier-Stokes equation (RANS) against Particle Image Velocimetry (PIV) measurements are investigated and applied to one of the most common problems in the chemical process industry — the prediction of flow field in a stirred vessel. A total of 1024 sequential instantaneous 2D velocity fields along the central axial plane of a stirred vessel with a P-4 axial impeller are obtained through PIV measurement. From the PIV data, the mean velocity, turbulent kinetic energy, Reynolds stresses and dissipation rate fields are extracted. By introducing several tools to quantify the similarities and differences between two-dimensional fields, CFD predictions of the flow field are validated against PIV data. Furthermore, using PIV and LDV data, the effect of boundary conditions on CFD simulation results is examined. The effect of different Reynolds stress closures on the flow prediction is also studied.     

不同翼片扰流特性的PIV对比实验

利用粒子成像测速（PIV）技术对圆管内置梯形翼片后方流场进行了测量,分析了翼片迎流（UFW）和顺流（DFW）两种放置方式对流场的扰动特性。结果表明,迎流翼片形成的涡沿周向延展范围较大,持续性好,涡偶内侧为向壁流;顺流翼片形成的涡沿径向延展范围较大,在较短距离内扰动较为明显,涡偶内侧为背壁流。两种流动结构都能有效提高壁面附近的速度分量,促进主流和壁面附近流体的质量交换。随着Reynolds数增大,纵向涡的稳定性减弱,在Re=3000时,翼片的扰流效果均较好。     

Hydrodynamics features of dispersed bubbles in the ventilated wake flow of a cylinder

An experimental study was conducted to investigate the 2D bubbly flow downstream of a cylinder. Sparsely distributed bubbles were produced using the ventilation method. The carrier flow was measured using the particle image velocimetry (PIV) technique. The shadow imaging technique was used to capture instantaneous bubbly flow images. An image-processing code was compiled to identify bubbles in acquired image, calculate the bubble equivalent diameter and the bubble velocity. The effects of Reynolds number and the flow rate of the injected air were considered. The result indicates that the carrier flow is featured by distinct flow structures and the wake region is suppressed as the upstream velocity increases. Regarding the bubbles trapped in the wake flow, the number of small bubbles increases with the upstream velocity. On the whole, the bubble velocity is slightly lower than that of the carrier flow. The consistency between small bubbles and the carrier flow is high in terms of velocity magnitude, which is justified near the wake edge. The difference between the bubble velocity and the carrier flow velocity is remarkable near the wake centerline. For certain Reynolds number, with the increase in the air flow rate, the bubble equivalent diameter increases and the bubble void fraction is elevated.     

The turbulent behavior of novel free triple-impinging jets with large jet spacing by means of particle image velocimetry☆

A novel reactor that achieves rapid liquid–liquid mixing via free triple-impinging jets(FTIJs) is developed to improve mixing efficiency at unequal flow rates for liquid–liquid reactions. The flow characteristics of FTIJs were investigated using particle image velocimetry(PIV). The instantaneous and mean velocities data at different Reynolds numbers(Re) were analyzed to provide insights into the velocity distributions in FTIJs. The effect of jet spacing on the stagnation points, instantaneous velocity, mean velocity, profiles of the x- and ycomponents of mean velocity, and turbulent kinetic energy(TKE) distributions of FTIJs were investigated at Re = 4100 with a volumetric flow rate ratio of 0.5. The characteristics of the turbulent flows are similar for all jet spacings tested. Two stagnation points are observed, which are independent of jet spacing and are not located in the center of the flow field. However, velocity and TKE distributions are strongly dependent on the jet spacing.Decreasing jet spacing increases the expansion angle and the values of TKE, leading to strong turbulence, improving momentum transfer and mixing efficiency in FTIJs. The present study shows that optimization of the operating parameters is helpful for designing FTIJs.     

Effect of Cylinder Rotation on Flow Characteristics of the Annular Region in Annular Centrifugal Extractor

The flow patterns in the annular region of a 50 mm annular centrifugal extractor (ACE) were studied using phase particle image velocimetry (PIV), by which the distributions of radial velocity, axial velocity, vorticity, turbulent kinetic energy (TKE), and micromixing time of a fluid under different rotating Reynolds numbers were investigated. In the center of the annular region, both the radial and axial velocities of the fluid are close to zero, regardless of the rotating Reynolds number changes. The TKE of the fluid along the radial direction is small at center and large on the edge. The results show that the mixing process mainly occurs at the region near the outer cylinder’s sidewall, and the mixing time in this region is less than that in the internal annular region. Besides, the whole mixing efficiency is proportional to the rotational speed when the speed is below a certain level, and then gradually reaches a plateau when the speed is further increased.     

低速旋转盘腔内液体流动特性

利用粒子图像测速测试平台研究了低速旋转盘腔轴向中截面的液体流动特性,并利用Realizable k-e模型对低速旋转盘腔内液体流场进行三维数值模拟,模拟结果与实验值吻合较好,最大相对误差为16.9%. 利用模型方程研究了导流板数、导流板相对长度、进口雷诺数和旋转雷诺数对盘腔内液体流动的影响. 结果表明,盘腔体积平均相对速度及涡量受进口雷诺数(Re)、导流板数、导流板相对长度影响较大,当进口Re从17000升至53000时,体积平均相对速度及涡量分别增加2.4倍和1.6倍;当导流板数由0提高至4时,体积平均相对速度及涡量增幅分别为62%和30%;当导流板相对长度由0.5提高至0.93时,体积平均相对速度及涡量增幅分别为114%和58%. 盘腔压损主要受进口Re的影响,当进口Re从17000升至53000时,盘腔压损增长8倍,而导流板数及导流板相对长度对压损的影响较小,增幅均小于5%. 为强化旋转盘腔内流体与壁面间传热,减小流体阻力,应优化进口Re、尽量增加导流板数和长度.     

圆管内置梯形翼片的流场特性PIV实验

诱导流体产生纵向涡结构可以有效强化对流换热,因此研究纵向涡发生器的扰流特性对于深化强化传热机理具有重要意义。利用粒子成像测速(PIV)技术,对圆管内置梯形翼片扰流后的流场进行了测量,实验中4个梯形翼片沿周向均布,与管壁夹角135°呈迎流放置,分析了横、纵截面内的流场结构和流动特点。结果显示,管内放置翼片能够诱导形成多纵向涡流动,在下游产生了4个对称的涡偶,每个涡偶对应的两个涡旋转方向相反,形成了内侧向壁流,外侧背壁流的流动结构,增大了垂直于主流方向上的速度分量。实验涉及的Reynolds数范围内,翼片下游横截面内的横向速度和径向速度分别达到主流均速的27%和20%以上;纵向涡沿壁面向下游发展可对流体产生持续扰动,使近壁流体的速度相对于光滑管提升了约1.0～3.6倍;翼片对流体的扰动作用随着Reynolds数的增大而增强,对于圆管内对流换热的强化具有促进作用。     

气升式环流反应器液相流场的研究

运用粒子图像测速仪(PIV)技术测量了一拟二维气升式环流反应器内液相流动状态。避开了气液两相成像带来的图像处理困难。成功测取了时均液相速度、瞬时雷诺应力、剪切应力等在反应器下降段内的分布;考察了反应器上升段进气量和反应器液位高度对液相循环速度的影响;同时,对反应器三个具有代表性的流动部分进行了观测,获得了该反应器内液相流场具有代表性意义的速度分布图。研究结果为认识该类反应器的性能及进行反应器设计、优化提供了有价值的参考信息。