湍流泡状流混合层流动的PIV测量

在单相实验的基础上,使用PIV对不同位置注入气泡的泡状流湍流混合层流动进行实验研究。混合层高低侧流速比为4∶1,基于两股流体速度差和管道水力直径的Reynolds数范围为4400～158400。与单相相比,气液两相混合层流动脉动速度的测量结果表明,在低Reynolds数时气泡的加入会增强混合层的速度脉动,但随着Reynolds数的增大气泡的加入反而会减弱速度脉动。雷诺应力集中在隔板下游一个较窄的区域内,并随Reynolds数的增大而增大。从流动横截面上雷诺应力的分布可以看出,气泡的加入减小了混合层内的平均雷诺应力,并且与单相相比泡状混合层流动在同一横截面上的雷诺应力分布曲线会产生波动。     

双喷嘴水平对置撞击流混合器内湍流流动及混沌特性

应用激光多普勒测速系统, 对双喷嘴水平对置撞击流混合器内的速度场进行测量, 并且分别采用湍流理论和混沌理论对所测得的瞬时速度场进行分析, 研究其瞬时速度场内湍流特性参数(速度脉动均方根、湍流强度和湍动能)以及混沌吸引子的特征参数(关联维、Kolmogorov熵和最大Lyapunov), 得出该参数随喷嘴间距变化和进口雷诺数变化的分布情况, 并且得到有利于提高混合器内微观混合效果的最优工况。通过混沌分析得到双喷嘴水平对置撞击流混合器的瞬时速度场具有混沌特征和分形特性。研究结果表明:流场内的湍流参数和混沌参数均与进口雷诺数呈正相关关系, 但是两参数却随着喷嘴间距的增加, 呈先增加后减小的变化趋势, 从而可以得到在实验考察范围内L=3d为最合适的喷嘴间距。     

考虑多组分颗粒运动各向异性的鼓泡流化床生物质气化数值模拟

考虑鼓泡流化床生物质气化过程多组分颗粒运动特点,建立多组分颗粒速度脉动二阶矩模型,结合化学反应动力学方法描述鼓泡流化床内生物质气化过程。模拟的气体组分结果与采用原始颗粒动理学模型的模拟结果进行了比较,并给出了两种粒径碳颗粒的浓度与温度瞬时分布。分析了两种碳颗粒的速度时均径向分布及速度脉动二阶矩时均径向分布,两种碳颗粒的速度分布一致,说明不同粒径碳颗粒混合充分。粒径较大的碳颗粒速度脉动二阶矩在轴向与径向上均较大,粒径的增加使得颗粒速度脉动增强。模拟统计了计算域内两种颗粒的速度脉动各向异性随颗粒浓度变化关系,各向异性随颗粒浓度的增加逐渐减弱,粒径较大的碳颗粒在计算域内的各向异性平均效果不如粒径较小的碳颗粒明显。     

导向管喷动床内单相流场及声波对流场影响的数值模拟

为阐明超细粉在声场导向管喷动流化床内的流化机理,并为进一步优化和完善床层结构及操作条件提供基础,采用标准k-ε湍流模型计算了导向管喷动流化床内的单相气体流场,考察了进口流化气速和射流气速对气体流动规律的影响,以及声场对导向管喷动流化床内气体轴向速度分布及其脉动均方根的影响。结果表明:在高速射流条件下,导向管喷动流化床内气体呈内循环流动,气体循环流量随流化气速度的增加而减小,但随射流气速度的增加而增加;外加声场使环隙区和喷泉区的气体流动更加均匀,显著增加环隙区和喷泉区气流的湍动程度,且湍动程度随声压级的增大而显著增大,随声波频率的升高而小幅度降低。     

导向叶片对导叶式旋流器内流场的影响

借助Fluent软件,采用雷诺应力模型对导叶式水力旋流器进行了数值模拟,得到了内部流场的轴向速度、切相速度和径向速度的分布规律,对比不同结构参数的导向叶片对旋流器内速度场的影响。通过分析,导叶式旋流器的叶片有最适宜的角度,它可减小湍流脉动,增加旋流器分离的稳定性。     

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。     

旋流滤清器内湍动规律的研究

分析了滤清器和旋流分离器的技术现状,介绍了自行设计的旋流滤清器的结构和原理,应用LDA(Laser Doppler Anemometer)的测试数据研究了旋流滤清器中的湍动规律,得出瞬时速度的时间序列和脉动速度、湍动度随半径及脉动速度、湍动度随雷诺数的变化曲线等;分析了湍动规律对过滤的影响,明确了在靠近过滤介质处的湍动能和切应力大是不形成滤饼的因素,从而减缓过滤速度的衰减。     

浸没式撞击流反应器流场涡特性的数值研究

利用大涡模拟（LES）方法研究了撞击流反应器内流场涡特性,分析撞击区域流体流动特征。改变进口速度、喷嘴间距,讨论流场速度、涡量和平面涡能量分布规律,并分析了流场流型、涡演化过程和涡核形式。在反应器内靠近撞击驻点的涡尺寸小、脉动性高,随着撞击距离的增加,流体速度逐渐减小,涡影响范围变大。平均涡量和平均涡能量随进口速度的增加,先增加后减小。结合Q判据分析了反应器内涡的演化过程和流体流型。根据径向射流涡的演变过程,得到径向射流两侧涡演化的周期,在0.15~0.20 s之间。撞击区的涡结构主要为马蹄涡和肋状涡,在出口位置存在涡环。研究结果为深入分析撞击流反应器流体运动规律和优化反应器提供了理论参考。     

浸没式撞击流反应器流场涡特性的数值研究

利用大涡模拟（LES）方法研究了撞击流反应器内流场涡特性,分析撞击区域流体流动特征。改变进口速度、喷嘴间距,讨论流场速度、涡量和平面涡能量分布规律,并分析了流场流型、涡演化过程和涡核形式。在反应器内靠近撞击驻点的涡尺寸小、脉动性高,随着撞击距离的增加,流体速度逐渐减小,涡影响范围变大。平均涡量和平均涡能量随进口速度的增加,先增加后减小。结合Q判据分析了反应器内涡的演化过程和流体流型。根据径向射流涡的演变过程,得到径向射流两侧涡演化的周期,在0.15~0.20 s之间。撞击区的涡结构主要为马蹄涡和肋状涡,在出口位置存在涡环。研究结果为深入分析撞击流反应器流体运动规律和优化反应器提供了理论参考。     

波形壁湍流复涡黏模型时间解析PIV测量研究

采用高时间分辨率粒子图像测速技术(TRPIV)在水槽中对不同雷诺数下的波形壁面湍流空间流场进行测量,得到瞬时湍流边界层速度场空间分布的时间序列,分析了雷诺应力分量和平均速度变形率分量的空间分布,发现其在近壁区受波形壁面影响比较大,周期性比较明显,而远离壁面时受壁面条件的影响较小。利用空间互相关技术分析了雷诺应力分量与平均速度变形率分量之间的流向相位关系,发现二者在流向空间上存在相位差,且相位差的分布由壁面向外区从0先逐渐减小,在0.4—0.5波形壁波长的法向位置处达到最小值,然后再逐渐增大,且在一定的范围内雷诺数的变化对相位差的影响并不大。验证了湍流雷诺应力复涡黏性模型的合理性,为建立更加准确预测非平衡湍流的复涡黏性模型提供了实验依据。     

CC板通道入口效应对传热特性和阻力特性的影响

选取低Reynolds数k-ε湍流模型,采用数值模拟的方法计算了不同Reynolds数下入口效应和CC板通道单元体传热特性、阻力特性的关系。数值模拟的结果表明：入口效应的存在使得CC板通道前3个单元体的湍流强度显著变大,在第4个单元体之后入口效应对传热特性的影响可以忽略;随着Reynolds数的增加,由于CC板通道入口处湍流强度增加的程度逐渐变小,入口效应对传热效果的强化能力下降;随着Reynolds数的增加,入口段和充分发展段的摩擦阻力系数均有所下降;此外,由于入口效应的存在,CC板通道入口段的摩擦阻力系数比充分发展段高20%左右。     

蜗壳式旋风分离器内的湍流特性(Ⅰ)分离空间

采用激光多普勒测速系统测试了蜗壳式旋风分离器内的流场,分析了最主要的分离空间内湍流特性.内旋流区、排尘口附近、近壁处以及上下行流交界点和内外旋流交界点等处的湍流度变化非常剧烈,而且数值上也大得多.比较轴向和切向湍流度得知表明分离空间的湍流是各向异性的.Reynolds应力值在内旋流区大于外旋流区,而且在排尘口附近、排气管下口附近都有陡增,湍流黏性系数也反映了这个特点.     

Modelling of turbulent flow and mass transfer with wall function and low-reynolds number closures

A recent laser doppler anemometer study, by other researchers, of turbulent flow through a sudden expansion has been simulated using a kinetic energy of turbulence-dissipation rate of turbulence (k - ?) model. The near wall region was modelled in two different ways; using wall functions (WF), and a low-Reynolds number (LRN) formulation. The reattachment length under particular flow conditions was obtained. Radial profiles for the mean axial and radial velocity, turbulence kinetic energy and Reynolds shear stress were determined for six locations downstream of the sudden expansion. Both models performed reasonably well in predicting the flow with the LRN approach performing slightly better than the WF approach near to the wall. The application of the LRN approach to the calculation of local mass transfer rates in wall bounded complex turbulent flows is demonstrated.     

基于湍流模型的S-CO2干气密封流场与稳态性能分析

为探究湍流效应对S-CO₂干气密封性能的影响规律,以螺旋槽干气密封为研究对象,引用考虑离心惯性力效应的湍流Reynolds方程,选择Ng-Pan湍流系数表达式,采用物性软件REFPROP对CO₂真实物性进行计算。之后,根据普适能量方程,通过引入包含湍流效应、离心惯性力效应的平均速度,建立了可压缩流体简化能量方程。通过对湍流Reynolds方程与简化能量方程进行耦合求解,分析讨论了不同工况参数与平均膜厚下湍流效应对密封性能的影响。研究表明：湍流效应使得气膜流场内压力与温度分布发生显著变化,流场计算时不可忽略;在不同进口压力、进口温度下,湍流下的开启力和泄漏率显示出与层流一致的变化趋势;在不同平均膜厚下,考虑湍流效应后的开启力呈现出与层流不同的变化规律,而泄漏率表现出与层流相同的变化趋势;在不同进口压力、进口温度、平均膜厚下,湍流下的开启力和泄漏率均比层流下的低,且在两种流态下的这种差异随着进口压力、进口温度、平均膜厚的增大而逐渐增大;在不同转速下,开启力和泄漏率在湍流下分别表现出与层流不同的变化趋势。这些结果为进一步研究湍流效应对S-CO₂干气密封的影响提供了支撑。     

Cleanabilty study of a Scraped Surface Heat Exchanger

Scraped Surface Heat Exchangers (Contherm SSHE model 6 × 3) are widely used industrially. In this kind of heat exchanger, the inlet and the outlet are difficult to clean due to their particular geometry and the presence of seals. A specific study was conducted on the inlet bowl of a SSHE, whose design has been optimized by the manufacturer in order to minimize risk of deposition by eliminating hydrodynamic dead zones. For this purpose, measurements of wall shear stress were made by an electrochemical method, for different hydrodynamic conditions. On the other hand, cleanability measurements were also performed. The bowl geometry tested presented no dead zones. However, the available space for flow significantly reduces the Reynolds number and turbulence intensity. As a consequence, three areas of increasing contamination appeared throughout the bowl due to the low mean and fluctuating shear stress values. The use of a pulsating flow increases these fluctuations, and thereby reduces the residual contamination.     

层流中脉动气流横掠平板强化传热

为研究脉动气流下等热流密度平面的换热特性,搭建了脉动气流强化传热实验台架,进行了不同雷诺数（Re=433～1733）的脉动气流下高温共烧陶瓷发热片组的换热实验研究,脉动气流的脉动频率f固定为30 Hz,脉动振幅prms固定为165 Pa。结果表明,在合适的脉动参数下（f=30 Hz, prms=165 Pa）,脉动气流有效地强化了等热流密度平面的换热性能,本文获得的强化换热效率介于9.7%和10.8%之间,随着雷诺数的增加轻微地线性增加。另外,结果揭示了在层流流动中,不管在稳定气流下还是在脉动气流下,等热流密度平面的换热性能都随着雷诺数的增加而线性地增加,但脉动气流下线性拟合的结果的斜率较大,为稳定气流下的斜率的1.26倍。最后,结果显示了在稳定气流中加入脉动分量能迅速增大换热面下游的温度水平,预示着脉动气流在强化传热的同时,也增强了气流内部的热传递。     

Heat transfer and pressure drop performance of alumina–water nanofluid in a flat vertical tube of a radiator

This work presents experimental investigation on the effects of nanofluid inlet temperature (40–90°C), Reynolds number (12,000–30,000), particle concentration (0–1 vol.%), and air velocity (0.25–0.55?m/s) on thermal and flow characteristics of water-based alumina nanofluids in a flat vertical tube of a radiator. The specific heat capacity, viscosity, density, and thermal conductivity were measured experimentally. The heat transfer coefficient enhanced (up to 31%) with an increase in fluid inlet temperature, particle volume concentration, Reynolds number as well as air inlet velocity. The pressure drop increased with an increase in the particle volume concentration and Reynolds number, while it decreased slightly with an increase in the fluid inlet temperature. The friction factor and pumping power increased with particle concentration. The friction factor decreased, while the pumping power increased with sn increase in fluid flow rate.     

两喷嘴对置撞击流径向射流流动特征

对喷嘴间距与喷嘴直径比为0.5～100范围内两喷嘴对置撞击流径向射流的湍流脉动特征、速度分布和扩展率等进行了实验研究和数值模拟。研究结果表明,撞击流径向射流明显较自由射流湍动强烈;从驻点开始径向射流速度逐渐增大到最大值后开始衰减,射流呈现自相似性;随着喷嘴间距增大,撞击流径向射流的扩展率呈现增大的趋势,大约为自由圆射流的1.5～3倍。采用CFD软件对撞击流径向射流的速度分布特征进行了数值模拟,与实验结果相比,两方程湍流模型预报的撞击流径向射流的扩展率明显偏小,雷诺应力模型的预报精度有较大改进。     

Fine particle turbulence modulation

Streamwise turbulence intensities of fine particulate suspensions were studied in a 26 mm N.B. horizontal pipe loop. Colloidal silica spheres were prepared in 10^?4M and 1M KNO₃ solutions to control the degree of aggregate formation in the suspension. Using an ultrasonic Doppler velocity profiling sensor, the turbulence intensities of the fine particle suspensions were compared with those of a particle‐free flow over a range of Reynolds numbers. At low electrolyte concentration, the silica particles remain dispersed, with the turbulence intensity of the suspension flow comparable with that of the particle‐free flow. At high electrolyte concentration, increased particle‐particle interaction leads to the formation of particle aggregates which support turbulence augmentation over a critical Reynolds number range. The range of Reynolds numbers over which this turbulence enhancement is observed is limited by both fluid dynamic effects at low Reynolds numbers (Re ≈ 5500) and aggregate breakup at high Reynolds numbers (Re ≈ 8000). © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Heat transfer and pressure drop in single rows of tubes in cross-flow

Results of experimental investigations on the influence of turbulence intensity and pitch-to-diameter ratio on heat transfer and pressure drop in single rows of plain tubes are presented. Vertically arranged tubes with 16 different pitch-to-diameter ratios between 1.26 and 5.73 were heated by saturated steam, condensing inside, and cooled outside by air in cross-flow. The turbulence intensity in the entrance cross-section was enhanced by means of different biplanar grids installed in the test section of a wind tunnel upstream of the single rows. The mean streamwise turbulence intensity behind the grids, measured with a constant temperature hot-wire anemometer, varied between 0.8% and 38.8%. Reynolds numbers ranged from 4 × 10³ to 2 × 10⁵. For single rows, the measurements show that the Nusselt number for a given turbulence intensity increases with increasing Reynolds number. In the investigated range of Reynolds numbers, the drag coefficient is only slightly influenced by the inlet turbulence intensity. The use of turbulence grids thus leads to higher efficiencies of heat exchangers.