水平直管内冰浆流动过程冰晶粒径演化研究

本文采用实验研究与数值模拟相结合的方法,研究了冰浆在水平直管内流动过程中冰晶粒径的分布及其演化规律。通过CFD-PBM耦合模型模拟研究冰浆在不同过冷度、流速、含冰率下冰晶粒径分布及其演化规律,并通过实验对数值模拟结果进行验证。结果表明:在含冰率为15%的冰浆管道入口处,冰晶粒径数量密度分布均近似高斯分布;随着冰浆流速和含冰率的增加,沿流动方向中心轴的冰晶平均粒径增大,截面冰晶粒径分布越不均匀;流速越小、含冰率越大,冰晶粒径的长度数量密度分布曲线的峰值升高,峰值所对应冰晶粒径的平均值减小。     

定热流状态下湍流冰浆传热特性的数值模拟

冰浆是一种有效的空调蓄冷和输送冷量的介质,有着广阔的应用前景,而冰浆的流动特性和传热性能又是将冰浆介质应用于实际工程的最为重要的基础资料。在对冰浆流体的换热情况做出一些假设的前提下,利用等效比热描述冰晶粒子融化吸收潜热,建立了管内湍流冰浆的传热模型。运用有限差分的方法,对所得的离散方程进行了求解,从而得到了定热流状态下圆管内湍流冰浆流体的传热特性。计算数据表明,随着含冰率和流速的增大,对流换热系数增大,换热性能得到加强。     

冰浆生成器中抑制冰粘附的研究

冰浆中含有大量微小冰晶粒子,因此表现出有极好的流动、传热和储能特性。冰浆生成过程中,冰晶微粒易粘附在换热壁面,降低传热效率,增加流动阻力,甚至引起冰堵,影响冰浆的持续生成。分析了冰粘附特性,综述冰浆技术中冰移除技术的研究进展,建立冰浆生成实验台,采用综合控制制冰溶液和载冷剂流速的方法抑制冰堵的发生。     

冰浆在水平直管中流动特性研究

冰浆在实际中的应用大多是先通过管道输送的方式到达需要冷却的地方进行热交换,研究输送过程中的流动换热特性对改善其实际应用效果具有重要意义。流动压降主要受到含冰率、冰粒直径、黏度、流速以及管径、管长和管道形状的影响。水平直管是最简单的管道形式之一,目前国内外学者研究方法主要是结合试验方法和数值模拟对已有的理论模型进行修正,尚未有统一的理论模型。本文采用数值模拟的方法,在双欧拉流体理论模型和相变换热基础上,定性分析流动过程中的压降受管径、含冰率、流速和管长的影响,并且在考虑冰晶和水两相相变的情况下,得到直管横截面上冰晶的分布情况。     

冰浆流体流动与换热研究综述

介绍了冰浆流体的特点、应用以及固液两相流体等效比热的概念及其在冰浆流体中应用的困难;概述目前国内外冰浆流体的传热性能、流变模型和流态以及粘性和流动阻力的研究成果,指出由于固液两相密度的不同导致了在不同流速下冰水分层的现象;同时对冰浆流体的的研究进行了综述,最后提出了关于冰浆流体的进一步研究建议.     

管道内冰浆流动压降特性模拟和实验研究

本文采用以颗粒相动力学为基础的Euler-Euler模型,利用FLUENT研究冰浆在水平直管、90°弯管和T型管中的流动压降特性(计算过程不考虑相变)。模拟结果显示,在管道入口处压降较明显,90°弯管拐弯后内侧压力小于外侧,T型管在分流直角拐角处出现压力最高点和最低点。冰浆流动实验发现,冰浆单位压降随流速和含冰率的增加而增大,且3种管型中直管压降最小,T型管压降最大。对比实验与模拟结果,单位压降随流速的变化趋势一致,且误差在20%以内,但在大流速时,模拟值大于实验值。     

尿素溶液制备冰浆的流动特性研究

在实际应用中,通常采用管道输送的方式将冰浆运送至待冷却区域进行热交换,因此,研究冰浆的流动特性具有重要意义。本文基于冰浆制备及流动特性测试实验台,以质量分数为5%的尿素溶液制备的冰浆为研究对象,通过可视化观测冰浆中冰粒的尺寸及分布并测量冰浆的运动黏度,分析了管径、含冰率(IPF)、流动压降以及管道摩擦阻力系数与雷诺数(Re)之间的关系,同时分析了不同管径水平不锈钢管内冰浆摩擦阻力系数的实验值(λ)与将冰浆看做牛顿流体计算得到的理论值(λ0)的比值偏离1的程度与IPF和Re的关系。研究结果表明:λ/λ0随IPF的增加而增大,随Re的增加而降低,即在高Re情况下冰浆更接近于牛顿流体,而高IPF的冰浆却与牛顿流体偏差较大。利用幂律模型分析冰浆的流动特性,发现流动特征指数n′随着IPF的增大而降低,在管径为6 mm管内,当IPF从6%增至26%,n′从1.006降至0.611;而稠度系数K′与IPF呈现正相关,在管径为8 mm管内,当IPF从6%增至26%,K′从0.015增至0.274。当IPF介于5%~30%范围内,n′随着管径的增大略有减小,K′随着管径的增大而逐渐增大。通过研究修正雷诺数和范宁摩擦阻力系数的关系,发现随着IPF的增加,管径为4、6和8 mm管道内的转折修正雷诺数ReMR的范围分别为2500~3200、1600~2300和1500~1900。     

竖直管内导流叶片形成的气液旋流场的数值模拟研究

利用CFD FLUENT对竖直管内导流叶片形成的气液旋流场进行数值模拟研究分析。发现气液混合流体经过导流叶片后会形成环状流,且气液分离效果沿着管线逐步增加,存在气液分离最佳位置。在管道中心和管壁处的切向速度最小,随着流动发展,切向速度变小。轴向速度沿着管线先变大后变小,管道中心处出现最大值。管内压力场呈现中心低外围高,且沿着流动方向,外围和中心压差逐渐减小,压力曲线趋于平缓。     

单声道超声水表测量特性分段校正方法的研究

当管道内流体处于不同雷诺数测量条件时,超声水表的线平均流速v_L与面平均流速v_S之间存在着显著的非线性。根据管道内被测流体介质流动分布状态不同,提出了一种分段流量测量特性校正新方法,在其临界处设立校正分界点,当层流时,采用常系数校正;湍流与过渡流时,分别采用拟合直线方程校正。经实验验证,该方法是可行的。     

不同流速对LTE态下氢等离子体特性的研究

采用二维流体力学模型研究了多级直流弧放电装置中流速对处于局部热平衡状态下氢等离子体特性的影响.分析了氢等离子体中心轴线处电场和压强的分布情况;各粒子密度在通道中的分布状态;通道出口处等离子体温度以及电导率的分布情况.模拟结果表明,随着流速的增大,中心轴线处电场和压强均增大;通道中氢等离子体的各粒子密度变化很小;通道出口处等离子体温度以及电导率在出口处沿径向的分布影响不大.     

Investigation on flow characteristics of ice slurry in horizontal 90° elbow pipe by a CFD-PBM coupled model

Elbow pipes are important components for ice slurry pipeline transport. However, the flow characteristics of ice slurry in elbow are far from fully being understood, especially the influence of ice particle kinetics on ice particle size distribution (PSD). This study is intended to provide a better understanding of the behavior of ice slurry flow in elbow pipe. A CFD-PBM coupled model is employed to investigate the flow characteristics of ice slurry in horizontal 90° elbow pipe. The quadrature method of moments is utilized to solve the population balance equations. Based on the revised model, the flow characteristics of ice slurry in the horizontal 90° elbow pipe are investigated. The simulation results show that in the range of calculations, the pressure drop of elbow pipe is increased with the increase of velocity and ice packing fraction (IPF). An adverse pressure gradient is formed due to the change in flow direction. The emergence of secondary flow is caused by the centrifugal force. It makes the ice particles gather on the outer wall of the elbow section. The ice diameter increases along the flow direction due to the aggregation. The evolution of particle size distribution (PSD) is not significant. However, aggregation and stratification cannot be ignored in the process of long distance transport of ice slurry. The results are of significance for guiding the safety design and operation of ice slurry transportation.     

Measurements of Particle Velocity and Concentration for Dilute Swirling Gas-Solid Flow in a Vertical Pipe

Experimental studies concerning the characterization of a dilute swirling gas-solid flow were carried out in a vertical pipe with a height of 12 m and an inner diameter of 80 mm. Polyethylene pellets, with mean diameter of 3.2 mm, were used as test particles. The initial swirl number varied from 0.0 to 0.94, the mean gas velocity varied from 9 to 25 m/s, and the solid-gas ratio varied from 0.2 to 0.7. In this study, the particle velocity and concentration profiles were measured by the photographic image technique for both nonswirling (axial) and swirling gas-solid flows. It was found that the particle velocity of the swirling flow is lower than that of the axial flow in the range of high gas velocity; however, high particle velocity in the former flow can be obtained in the range of low gas velocity. The particle velocity profiles, on the other hand, were found to be nearly uniform in both the swirling and axial flows. The particle concentration profiles in the swirling flow exhibited symmetric distributions with respect to the pipe axis, and a higher particle concentration appeared in the vicinity of the wall located in the acceleration region.

gas-solid two-phase flow particle concentration particle velocity pipeline swirling flow     

An experimental study on a horizontal-vertical pneumatic conveying system using oscillatory flow

To reduce the power consumption of a horizontal-vertical pneumatic conveying system, an oscillator is mounted with a 45° oblique plane through the pipe axis in this study. This experimental study focuses on the effect of oscillatory flow using the oscillator on the horizontal-vertical pneumatic conveying system in terms of the overall pressure drop of the system, power consumption, local pressure drop, and particle velocity. Compared with conventional pneumatic conveying (axial-flow), the pressure drop and power consumption can be reduced using the oscillatory flow in a lower air velocity range. Meanwhile, the particle axial velocity of the oscillatory flow is higher than that of the axial-flow near the bottom of pipe. This outcome indicates that the accelerating effect of oscillatory flow is obvious near the bottom of the pipe, and the particle vertical velocity of the oscillatory flow is positive, whereas the particle vertical velocity of the axial-flow is almost negative. This result shows that the particles of the oscillatory flow are suspended sufficiently, but the particles of the axial-flow have a tendency of deposition. Furthermore, the fluctuation intensity of the particle velocity of the oscillatory flow is higher than that of the axial-flow, especially near the bottom of the pipe.     

Experimental investigation on inhomogeneity of ice packing factor in ice slurry flow

We investigated inhomogeneity in ice slurry, which can pose problems such as sudden increase in pressure drop and blockages in flow. Although in many previous studies fully dispersed ice slurry flow is assumed homogeneous, inhomogeneity appears as a variation in physical properties in the actual measurements. From experiments, it was found that the fluctuation in pressure drop is relatively small at the beginning, but becomes larger after long-term periods. Additionally, the ice particle size included in the ice slurry also affects this fluctuation. We supposed the fluctuation is caused by the variation in IPF, because it appears only in laminar flow. This implies ice slurry is not homogeneous after long flow periods. From the present study, the variation of IPF was estimated to be 2.6 wt% at maximum. Although the physical mechanism of the variation in IPF has not been clarified, we suggest that it is a consequence of the accumulation/adhesion of ice particles in the pipe.     

Concentration distribution and viscosity of ice-slurry in heterogeneous flow

Mathematical modeling of two phase flows, especially liquid–solid flows is very complex. Especially when a distribution of the solid phase in a carrier liquid is not homogenous but heterogeneous or even when a moving or stationary bed occurs. In this case, the rheological characteristics of suspension are changing and affect transport characteristics. Therefore, the slurry flow may present a Newtonian and non-Newtonian fluid as well, depending on the operation characteristics. In this paper the fully suspended ice-slurry flow in a horizontal pipe is analysed. The model allows us to avoid the definition on what kind of fluid ice-slurry is present. For the taken ice-particle diameter, the ice-concentration profiles depending on various average velocities and pipe diameters are shown. The viscosity of the ice-slurry is presented, depending on average concentration, velocity, pipe diameter and ice-particle size. The results of the analysis have shown that the ice slurrys can be treated as Newtonian-fluid at higher average velocities, and lower average concentrations as well. As the ice concentration increases and velocity decreases the viscosity depends not only on the ice concentration but also on the average velocity and the pipe diameter. The ice-slurry behaves then as a non-Newtonian fluid. The results show also the area where the safe operation of an ice-slurry-district-cooling system can be performed.     

Effects of storage on flow and heat transfer characteristics of ice slurry

The effect of storage on flow and heat transfer characteristics of ice slurry was investigated experimentally. After ice slurry had been stored in the storage tank, variations in ice particle size were measured using a microscope, and diameter distribution and average diameter determined. The ice packing factor, Reynolds number and storage time were varied as experimental parameters. The pressure drop and heat transfer coefficient were measured when the ice slurry flowed in the horizontal tube. For laminar flow, the ratios of pipe friction and heat transfer coefficient decreased with storage time. For more than 12 h storage time, the ice slurry could not flow in the tube. The adhesion between ice particles seemed to cause a blockage in the tube. On the other hand, for turbulent flow, the pipe friction and ice slurry heat transfer coefficients were similar to that of the ethanol solution, and the storage effect was insignificant.     

Ultrasonic determination of the particle concentration in model suspensions and ice slurry

The development of ice slurry for refrigeration systems and the enhancement of its efficiency depend on an accurate control of the ice concentration. We present here an ultrasonic method capable to measure precisely the particle concentration in ice slurry. To calibrate the ultrasonic measurement, we first determine the sound velocity and attenuation in two model suspensions (glass beads/polyethylene glycol and polyethylene beads/vaseline oil) for different particle volume fractions. The experimental results show a good agreement with the predictions of the two-component models in the long-wavelength limit. Additionally, the sound attenuation reveals a clear signature of the aggregate formation in the nearly iso-dense suspension. We next conduct the measurement of the sound velocity in the polypropylene glycol ice slurry where the ice concentration changes with temperature. The ice concentrations extracted from our sound velocity measurements are well consistent with the values determined from the binary phase diagram.