SK型静态混合器湍流速度脉动特性

以SK型静态混合器为研究对象,运用激光多普勒测速仪对管内的速度脉动进行测量。结果表明:湍流时在第1个元件内的轴向速度脉动较小,进入第2个混合元件后轴向脉动均方根可达平均流速的0.5—0.7倍且基本达到稳态;每个元件的入口有1个均方根可达到0.8—1.0倍平均流速的轴向速度脉动尖峰;流体离开混合元件后,各个方向上的速度脉动都迅速衰减,经过1个混合元件长度后脉动速度均方根衰减到平均流速的0.3倍左右,流过4—5个混合元件长度后基本衰减到混合器入口处的速度脉动水平。     

旋风管内气相的湍流运动特性

利用二维激光多普勒测速仪(LDV)测量了旋风管内湍流运动的时均速度、脉动速度、能谱函数、微分尺度及湍流耗散等参数，对湍流特性参数分布进行了详细的描述，并初步分析了旋风管内湍流运动特性对颗粒分离的影响.     

运动气泡近界面湍流场的激光测量及其湍流结构特性

运用二维激光多普勒测速仪,测定了运动气泡近界面的动力学特性,其中包括时均特性及湍流场结构.实验表明,尾涡区的时均流速呈非均匀分布,湍流场由双结构组成,即由完全随机的湍流涡及规律性的相干涡构成,这种双结构现象在尾涡的中心及边缘部又有所变化,其相关函数、谱密度函数及统计尺度亦随不同的部位而变化.泡前区受尾涡区周期性Karman涡脱落的影响,呈现相同频率的周期波动.这种特性存在于切向及法向速度中.     

搅拌槽内三维流场的LDV测量

利用三维激光多普勒测速系统(3D-LDV)对搅拌槽内流场进行了湍流测量和流场分析,同时对测量系统和测量原理进行了介绍,给出了在偏心搅拌和中心搅拌时搅拌槽内的时均速度场分布和湍流度等实验结果.这些结果为搅拌槽的设计和放大提供了很有价值的实验依据.     

旋流板上流场的LDA实验研究

在φ３００ｍｍ塔内，实现了用二维激光多普勒测速仪对旋流板上气体速度场的测定，获得了不同高度剖面的切向、轴向及径向速度分布的数据，探明了板上气流场的基本特点；还采用气－固流模拟湿板操作，测定了模拟液滴的流场。实验结果证实了旋流板的传统设计思想。     

搅拌器内湍流场的CFD模拟研究

采用CFD技术研究了DT和PTU型桨叶搅拌器内的湍流场,结果表明:对于PTU和DT桨叶,理论预测速度场分布规律与LDA测试结果吻合较好,理论预测漩涡的空间位置坐标与实测值完全一致。预测速度值与实测值在桨叶叶尖附近相对误差比较大,在离旋转中心轴线2R/3处竖直线上, 对于DT桨叶搅拌器,CFD预测的速度值和LDA测试值吻合比较好,最大相对误差在11%左右。而对于PTU桨叶,在同样的空间位置上,CFD预测值与测试结果的变化趋势相吻合,速度峰值附近的误差较大,相对误差高达30%左右。     

SMV型静态混合器流速场的实验测定

通过使用激光多普勒测速仪测量 SMV 型静态混合器的流速场,可以得到不同混合元件数下的时均速度场、湍流强度场。静态混合器流体流型基本上符合 SMV 混合元件的层状结构,不同混合元件数下时均速度场、湍流强度场比较相似。湍流强度随着混合元件数增加而增大,其中在混合元件由1个增加到2个时,湍流强度增幅达到0.61至0.74倍。     

射流泵湍流场的数值模拟与实验研究

采用k-ε湍流模型和非等间距加密网格,对射流泵流场进行了数值模拟和分析,并对相应的流场进行了实验研究.结果表明,流场轴向速度剖面在扩散管段具有较好的自相似性,而在喉管段则不然;这种速度剖面变化的转折点与喉管的长度有关;流场的湍动能分别在喷嘴出口与扩散管入口处产生峰值,并且前者远大于后者,可见射流泵流场中,湍流主要发生在喉管入口处,湍动能的不平衡将导致额外的能量损失.本研究结果对工程应用有指导意义.     

气体涡旋燃烧器三维湍流场的数值模拟

以气体湍流运动的微分方程组及 ｋε湍流模型为基础,采用 Ｓ Ｉ Ｍ Ｐ Ｌ Ｅ 算法数值模拟研究了气体涡旋燃烧器中的三维旋转湍流场。旋转方向的迭代采用了 Ｃ Ｔ Ｄ Ｍ Ａ 方法,计算收敛速度明显加快。预测结果与日本古火田朋彦的 Ｌ Ｄ Ｖ 冷模实测值吻合较好。     

90°弯管内湍流流动的数值模拟

进行了90°弯管内湍流流动的数值模拟实验。结果表明,切向速度在开始旋转阶段内侧的速度增大、压力减小,外侧速度降低、压力增大;当转过60°截面后,外侧的速度增大、压力减小,内侧速度降低、压力增大。弯管内流体旋转产生的离心力导致压力分布的变化,使得弯管内流体产生垂直于主流切向速度的轴向速度和径向速度,形成了二次流。90°弯管流场是主流切向速度与二次流的叠加,呈现出复杂的三维流动特性。     

Liquid re-circulation in turbulent vertical pipe flow behind a cylindrical bluff body and a ventilated cavity attached to a sparger

The turbulent flow field (Re=60024) in the wake of a cylindrical bluff body in a 0.105 m internal diameter pipe with an area blockage ratio of 82% in turbulent single-phase flow was studied using laser Doppler velocimetry (LDV). The results for the time-averaged velocity showed a toroidal vortex below the bluff body. The axial location below the bluff body where both the time-averaged radial and axial velocity components were zero (eye of the vortex) was found at approximately 0.72D. The end of the re-circulation region as defined by a stagnation point on the centreline of the pipe was found at an axial location below the bluff body of approximately 1.3D. These two locations did not change when altering the liquid superficial velocity confirming that the geometry (i.e., size) of the toroidal vortex is not dependent on the superficial liquid velocity or the speed of the vortex.Similar measurements using LDV were taken in the wake of a ventilated cavity in a vertical 0.105 m internal diameter pipe, with an area blockage ratio of 80%. The flow beneath the cavity was turbulent two-phase bubbly flow and the liquid-only flow ahead of the cavity was turbulent (Re=45618). The cavity was attached to a (central) sparger, which is a scale-up of the design used by Bacon (1995). The average gas void fraction in the wake of the cavity was 7%. The results for the time-averaged velocity confirmed the formation of a toroidal vortex remarkably similar to the vortex formed below the bluff body. The eye of the vortex and the end of the re-circulation region were found at an axial location below the ventilated cavity of 0.78 and 1.35D, respectively, i.e., almost identical to the results for the bluff body.The LDV results of the cylindrical bluff body and the ventilated cavity were compared with the fully predictive model of the velocity distribution in the vortex proposed by Thorpe et al. (2001) and good agreement was found in both cases. The model also agreed well with the data of van Hout et al. (2002) for a Taylor bubble rising in stagnant liquid in a 0.025 m internal diameter pipe. The CFX simulations of Thorpe et al. (2001) carried out for a 0.050 m internal diameter pipe, agreed well with the experimental data of the cylindrical bluff body, the ventilated cavity and the data obtained by van Hout et al. (2002) when correlating the results in the appropriate dimensionless form. Our analysis showed that the maximum axial re-circulation velocity in the centre of the vortex ring was directly proportional to the mean velocity in the annulus at the base of the cylindrical bluff body, the ventilated cavity or the Taylor bubble. The proportionality constant for all cases was found to be approximately 0.38 confirming the value proposed by Thorpe et al. (2001).     

Numerical modelling of swirl flow induced by a three-lobed helical pipe

Increasingly in the process industries, the introduction of swirl flow upstream of bends is being used in particle-laden flow to reduce wear. This paper describes a computational fluid dynamics model of the swirl flow that is induced in an airflow passing through a horizontally mounted three-lobed helical pipe. It then goes on to discuss the results of the model verification and validation studies performed. The numerical model was used to further investigate the flows observed within and downstream of both a straight circular pipe and a three-lobed helical pipe that were previously studied during experiments conducted on a laboratory rig for a range of Reynolds numbers. A comparative analysis of the experimental and model predicted data concluded that the simulation results replicated well the experimental results for the control pipe. For the helical pipe, axial velocities were in reasonable agreement with experimental measurements across the range of Reynolds numbers studied. However, tangential velocities were under-predicted by an average of 35% in comparison to the experimental results. As a result, swirl intensity values were also under-predicted, but the values of the computed swirl decay rates were in agreement with those observed during the experiments.     

Experimental research on drag reduction by polymer additives in a turbulent pipe flow

In order to investigate the effects of injection position on drag reduction as well as further the effects of polymer additives on turbulent structures, LDA measurements of turbulent pipe flows were conducted. The results show that the amount of drag reduction grows with the increase of the Reynolds number, and injecting the polymer at the centre of pipe is more effective than at the wall. Due to the addition of polymer solution, the axial, radial r.m.s. velocity fluctuations and Reynolds stress decrease over the entire pipe cross‐section, the time auto‐correlation coefficients of axial and radial velocity fluctuation at the centre of pipe decay more slowly, the number of spectrum peaks is decreased, and the peak shifts towards lower wave numbers. The results also reveal that, due to the addition of polymer solution, the large‐scale vortices are enhanced and small‐scale vortices are suppressed.     

涡流空气分级机环形区流场的实验研究

为优化涡流空气分级机内的流场,提高分级机性能,研究了涡流空气分级机蜗壳内加水平隔板和不加水平隔板2种结构对分级精度的影响。2种结构分级机环形区流场的激光多普勒测速计测量结果表明:在涡流空气分级机蜗壳内加隔板后,减小了环形区气流的轴向速度,沿转笼轴线方向,切向速度分布趋于均匀一致,流场稳定。重质CaCO3物料实验表明:在所试验的风速和转速范围内,采用较高转速分级,加隔板分级精度降低。环形区内适当的湍流能够提高物料的分散性,对提高分级精度有利。     

Numerical prediction of wall mass transfer rates in turbulent flow through a 90° two-dimensional bend

The present paper reports a numerical study of mass transfer in a curved channel flow using a low Reynolds number k–ε model modified to include the influence of curvature on the turbulent transport. Predictions were obtained of flow through a 90° 2-D bend for Reynolds numbers of Re = 2 × 10⁴, 10⁵ and 10⁶, curvature ratio's of R_c/d = 1, 1.5 and 3, and Schmidt numbers of Sc = 500, 1 500 and 5 000. Based on the numerial simulations, the value of the mass transfer coefficient, k, was observed to generally decrease and increase along the inner and outer walls, respectively, compared to the value in a straight duct. The behaviour appears to be sensitive to both the curvature ratio and Reynolds number, being most pronounced for the case of tight bends and low Reynolds numbers.     

PDF method for population balance in turbulent reactive flow

Turbulent reactive flows with particle formation, such as soot formation and precipitation, are characterized by complex interactions between turbulence, scalar transport, particle formation and particle transport and inter-particle events such as coagulation. The effect of formation, growth and coagulation on the particle size distribution (PSD) must be modelled by the population balance equation (PBE). While the PBE has been studied extensively in homogeneous systems and, recently, in simple flows, its coupling with turbulent reactive flows poses a wealth of new questions. Processes such as nucleation, growth and coagulation are described by kinetic laws that link them to the local concentrations of the reactive scalars, which are random in a turbulent flow. This accounts for additional mechanisms that induce randomness and fluctuations to the particle concentration and PSD. Furthermore, conventional RANS closure of the coagulation term PDE (which describes the evolution of the PSD) leads to unknown correlations. In this work a new pdf approach is developed, based on the transport of the joint pdf of reactive scalars and particle number densities at different sizes, which overcomes the additional closure problems. It is also shown how the pdf method can be solved numerically via Monte-Carlo methods, and this is demonstrated via two applications in a partially stirred reactor: precipitation via nucleation-growth and coagulation. In each case the pdf method is compared with models that neglect correlations at various levels, and it is demonstrated that the interaction of turbulence with particle formation mechanisms accounts for significant deviations in the PSD.     

Thermophoretic particle deposition efficiency in turbulent tube flow

This study investigated the thermophoretic particle deposition efficiency numerically. The critical trajectory was used to calculate thermophoretic particle deposition in turbulent tube flow. The numerical results obtained in turbulent flow regime in this study were validated by particle deposition efficiency measurements with monodisperse particles (particle diameter ranges from 0.038 to 0.498 μm) in a tube (1.18 m long, 0.43 cm i.d., stainless-steel tube). The theoretical predictions are found to fit the experimental data of Tsai et al. [Tsai, C. J., J. S. Lin, S. G. Aggarwal, and D. R. Chen, “Thermophoretic Deposition of Particles in Laminar and Turbulent Tube Flows,” Aerosol Sci. Technol., 38, 131 (2004)] very well in turbulent flows. In addition, an empirical expression has been developed to predict the thermophoretic deposition efficiency in turbulent tube flow.     

Velocity measurements and flow field characteristic analyses in a turbo air classifier

Turbo air classifier is one of the most widely used powder classification equipment. The rotor cage as a rotary component can create a forced centrifugal field, so it is a key part for turbo air classifier. In order to investigate the effect of structural variations of the rotor cage on flow field characteristics, three dimensional velocity measurements of the annular region in a turbo air classifier equipped with two different rotor cage bottom plates (A and B type) are performed by laser Doppler velocimeter (LDV). It is found that the different bottom plates have different axial and tangential velocity distributions in the annular region. However, the structural variations of the rotor cage have hardly any effect on the radial velocity. Based on the classification principle, the relation between the classification performance and the flow field characteristics is investigated in great detail.The results of the flow field measurements were tested by the classification experiments carried out with cement raw meal and ground calcium carbonate. The results demonstrate that B type bottom plate can realize the production of narrow particle size distributions, so it is more favorable for classification than A type bottom plate. Classification experiment results are in good agreement with the results of the flow field measurements.