工质对微石英管内部流动特征的影响

为采用“两管法”以消除进出口压力降及入口效应对流动的影响,分别以蒸馏水、CCl_4及N_2为工质,测量其流过内径d分别为19．60、44．63、91．65及140．86μm微石英管的流量与进出口压降,得到微管内部充分发展流的摩擦阻力系数f与Re的关系．实验结果表明,除d=19．60μm的管外,其他管在Re较小时,微管内部的f值与Hagen-Poiseuille解几乎一致;当Re升高到1300时,f已开始偏离Hagen-Piseuille解．对于d=19．60μm的管,在Re＜500,工质为蒸馏水时,f比Hagen-Poiseuille解高10%～15%;工质为CCl_4时,f仍然约等于Hagen- Poiseulle解;工质为N_2时,f比Hagen-Poiseulle解低8%～10%．     

Friction resistance for gas flow in smooth microtubes

A new tube-cutting method was used to measure the pressure and Mach number distribution along a microtube of 108.3 μm. Experiments were also performed concerning the average Fanning friction factors of five kinds of microtubes whose diameters range from 80.0 to 166.6 μm. It is found that the pressure distribution in a microtube becomes nonlinear at a high Mach number and the product of measured average Fanning friction factors 75, and Reynolds number Re is higher than 16. Numerical results show that the gas compressibility leads to a variation of the velocity profile from parabolic, and results in a large velocity gradient at the tube inner wall surface. The transition from laminar to turbulence in microtubes also occurs at Re ≈ 2 300, and the phenomenon of early transition is not observed in the experiments.     

微管内部流动粘性耗散的实验研究与数值模拟

为深入了解微管内部流动及换热机理,对液体在微管内部流动引起的粘性耗散问题进行了理论分析、数值计算和实验验证.以蒸馏水为工质,流过内径为25μm及50μm微光滑石英管,采用非接触式温度测量方法——红外热成像测量液体粘性耗散导致的微管壁面的温度场变化,获得精确的微管外壁温度分布.同时利用SIMPLEC计算方法对其内部流动耗散进行了数值模拟,得到了微管内部流动雷诺数Re和流体温升的关系.实验结果与数值模拟结果相吻合,表明忽略粘性耗散的影响会最终影响微管摩擦系数和Re数的表观实验结果.     

微管道内壁面轴向热流实验研究

以蒸馏水流过内径分别为168μm和399μm的不锈钢微管,研究了微管壁面温度分布和壁面轴向导热特征.依据微面热成像技术,用红外热像仪获得了恒定雷诺数和不同加热功率下微管壁面温度分布图.用一个简化的数学模型表示了壁面轴向导热和流体对流传热关系.实验和理论分析表明微管内壁面轴向导热量微小,与液体对流换热相比可以忽略.     

Flow frictional characteristics of microencapsulated phase change material suspensions flowing through rectangular minichannels

1 Introduction MEPCM, i.e. microencapsulated phase change material, is formed as shown in Fig. 1 by packing phase change material (PCM) into a microcapsule with a solid but flexible shell; the size of the MEPCM particles ranges from 1 to 100 μm[1,2]. The shell separates the core PCM (solid or liquid) from outside substances and tolerates volume changes in the core due to phase change. Widespread applications of MEPCM include: enhancing performance of fluid convective heat transfer, te…     

Characteristics of pressure drop for single-phase and two-phase flow across sudden contraction in microtubes

Single-phase and gas-liquid two-phase pressure drops caused by a sudden contraction in microtubes were experimentally investigated at room temperature and atmospheric pressure, using nitrogen and water. The experimental results on pressure drop with a novel measurement method, the tiny gaps on the tubes, were used to characterize the sudden contraction pressure drop for tube diameters from 850 to 330 μm. The ranges of the gas and liquid superficial velocity were 2.55–322.08 and 0.98–9.78 m/s in the smaller tube respectively. In single-phase flow experiments, the contraction loss coefficients were larger than the experimental results from conventional tubes in the laminar flow. While in the turbulent flow, the contraction loss coefficients were slightly smaller than those from conventional tubes and predicted well by K _c=0.5×(1−σ²)^0.75. In two-phase flow experiments, the slip flow model with a velocity slip ratio S=(ρ_L/ρ_G)^1/3 showed a good prediction that reveals the occurrence of velocity slip. An empirical correlation for two-phase flow pressure drops caused by the sudden contraction was developed based on the proposed contraction loss coefficients correlation for single-phase flow and Martinelli factor. Supported by the National Key Basic Research Program (Grant No. G2000026300) and Beijing Open Fund     

矩形微通道中流动阻力特性的实验研究

以蒸馏水作为工质,实验研究了水力直径为107.76~199.07m矩形紫铜微通道内的流动特性.实验中的紫铜微通道是由晶体生长法直接生长密封而成,有别于以往相关实验中刻蚀或微机加工所形成的微通道,避免了传统微通道加工方法所引入的误差,从而确保了微观尺寸的测量精度.实验测量了Reynolds数在70到1400之间摩擦阻力系数.实验结果表明:当Re数小于300时,在实验误差内,摩擦系数的值近似等于经典理论计算值;随着Re数的增大,的值大于理论值,这可能是由微通道内部壁面粗糙度效应所导致的.     

微粗糙管内部流动与对流换热的实验研究

以蒸馏水为工质,流过内径分别为168μm和399μm、内部粗糙度分别为8％和3％的不锈钢管．通过直接电加热,采用红外成像仪测量了各种恒定加热功率及不同雷诺数(Re)下的钢管壁面的温度场,同时测量了微管进出口压力降和流量,由此计算了在不同Re下的努谢尔特数(Nu)和管内摩擦阻力系数(f)．实验结果表明, 在低Re下,微管内部f的值与Hagen-Poiseuille解基本一致,但微钢管内部的Nu要略高于Hausen的解;当Re 达到800左右时,内径为168μm的管内部f的值已开始偏离Hagen-Poiseuille解,Nu急速上升并形成拐点,这是由于流动发生转捩导致的．     

螺旋回热器的流体阻力研究

本文在相似理论指导下,用水作介质对螺旋回热器的流体阻力进行了实验研究,并在较大的迪恩准数范围内,获得了螺旋回热器的摩擦系数与迪恩准数的经验方程式。所得结果对于计算螺旋回热器中的流体阻力,全面评价螺旋回热器的经济性能等都具有重大意义。     

基于Micro-PIV的毛细管内流场的可视化研究

采用Micro-PIV和FLUENT软件,对水包油乳液在内径为0. 7 mm的圆截面毛细管内的流场结构进行了实验和模拟研究,得到了毛细管内的速度矢量场分布。研究结果显示,实验结果与模拟结果较接近,毛细管内从层流到紊流的转捩发生在Re为2 000左右,与宏观尺度管内流动的临界雷诺数接近,并未发现流动转捩提前的现象。     

非牛顿流体边界层减阻流动研究

研究了四常数Maxwell-Oldroyd模型非牛顿流体边界层流动,给出了边界层内速度分布数值解,以及边界层厚度、摩阻系数沿程变化关系。为探讨高分子稀溶液管内流动减阻提供了理论依据。     

Numerical simulation of particle deposition in obstructive human airways

To investigate airflow pattern and its impact on particle deposition, finite-volume based computational fluid dynamics (CFD) simulations were conducted in the diseased triple-bifurcation airways. Computations were carried out for twenty Reynolds numbers ranging from 100 to 2 000 in the step of 100. Particles in the size range of 1–10 μm were conducted. Two particle deposition mechanisms (gravitational sedimentation and inertial impaction) were considered. The results indicate that there are strong relationship between airflow structures and particle deposition patterns. Deposition efficiency is different for different particles in the whole range of the respiratory rates. Particles in different sizes can deposit at different sites. Smaller particles can be uniformly deposited at the inside wall of the considered model. Larger particles can be mainly deposited in the proximal bifurcations. Deposition fraction varies a lot for different inlet Reynolds numbers. For lower Reynolds numbers, deposition fraction is relatively small and varies a little with varying the diameters. For higher Reynolds numbers, there is a most efficient diameter for each Reynolds number to target the aerosols at the specific site.     

窄通道内层流-紊流转捩区流动与传热特性分析

为了研究壁面加热对窄通道内层流-紊流转捩点的影响,通过测量转捩区的阻力与传热特性,间接分析了转捩起始点的影响因素,并结合流迹显示实验对流动传热实验结论进行了验证.研究表明:壁面加热导致层流-紊流转捩延迟,粘性变化对转捩过程的影响非常小,加热导致径向速度剖面发生变化是影响转捩的主要因素;此外,转捩区的壁面温度沿轴向不是线性变化,层流突变导致局部努赛尔数的轴向变化存在一个拐点;层流突变点处的局部努赛尔数及局部雷诺数主要受入口雷诺数的影响.     

圆管层流脉动流动的数值模拟

为研究圆管层流脉动流动特性,建立了二维非稳态轴对称模型,对壁温恒定、入口速度周期性变化的流体进行数值模拟.分析了速度振幅和频率对层流流动的影响,得到了频率和振幅对压力、温度、壁面摩擦系数影响的规律.研究表明:脉动流动时,压力、温度、壁面摩擦系数围绕相同Re下稳态流动时的数值波动;压力与速度之间存在相位差;压力、壁面摩擦系数的波动与速度振幅和频率成正比;温度的波动与速度振幅成正比,与频率呈反比;当流体脉动的频率和振幅足够大时,会在近壁面处出现回流,并且出现回流的时间会随着频率和振幅的增大而增长.     

混合流态下短径向滑动轴承热流体润滑特性研究

以高速短径向滑动轴承为研究对象,研究混合流态下采用不同温黏模型时轴承静态特性的变化。考虑油膜中同时存在层流和紊流两种流态,基于两种温黏模型,联立求解雷诺方程、能量方程,分析油膜中流态变化,得到压力分布、承载力、摩擦力和最大温度等特性参数,并将某工况下混合流态时油膜特性与等黏度模型和单一层流流态时的结果进行比较。结果表明:基于不同温黏模型时油膜内流态分布明显不同;与等黏度模型相比,变黏度模型承载力和温升明显下降;与单一层流流态相比,混合流态下油膜承载力和摩擦力均较大。     

Convective heat transfer and flow characteristics of Cu-water nanofluid

An experimental system is built to investigate convective heat transfer and flow characteristics of the nanofluid in a tube. Both the convective heat transfer coefficient and friction factor of Cu-water nanofluid for the laminar and turbulent flow are measured. The effects of such factors as the volume fraction of suspended nanoparticles and the Reynolds number on the heat transfer and flow characteristics are discussed in detail. The experimental results show that the suspended nanoparticles remarkably increase the convective heat transfer coefficient of the base fluid and show that the friction factor of the sample nanofluid with the low volume fraction of nanoparticles is almost not changed. Compared with the base fluid, for example, the convective heat transfer coefficient is increased about 60% for the nanofluid with 2.0 vol% Cu nanoparticles at the same Reynolds number. Considering the factors affecting the convective heat transfer coefficient of the nanofluid, a new convective heat transfer correl     

摇摆条件下窄通道内单相水流态转捩特性

矩形窄通道广泛应用于紧凑式换热器中,其内单相流动流态转捩特性会受到摇摆运动的影响.结合流迹显示技术,对摇摆条件下矩形窄通道内单相水从层流向湍流转变临界点附近的流动特性进行了实验研究.结果表明稳态条件下临界Re为2 550左右.摇摆条件下当Re小于2 400时,流态显示为稳定的层流,Re大于3 500时,流态为湍流,摇摆对稳定的层流区和湍流区流态没有影响;Re为2400～3 500之间时流动处于过渡区,摇摆对其流态转变有一定影响.在临界点附近摇摆振幅越大,流态转捩越容易发生;摇摆周期对流态转捩影响很小.摇摆运动改变系统重位压降及产生附加压降,从而对流态转捩的影响不可忽视.     

小尺度流道换热器流动特性实验研究

在单相强迫对流情况下,对不同当量直径,流道截面形状分别为矩形或三角形的小尺度流道换热器,以水和乙二醇溶液为介质,进行实验研究,并与常规尺度流道换热器的流动特性进行比较,结果表明：小尺度流道内流动阻力系数低于常规尺度流道,矩形流道换热器阻力特性明显优于三角形流道换热器,流动阻力系数f随当量直径的增大而增大,阻力系数与介质Pr数无关,小尺度流道内流体流动的临界雷诺数Rec在700-1200之间。     

城市污水的流动特性理论研究

为了能使城市污水在污水源热泵方向进一步推广应用,以原生污水源热泵系统中的流动工质——城市污水为研究对象,采用理论推导与实验相结合的方法对城市污水在圆管中层流和紊流的流动特性进行分析与研究.结果表明:城市污水在流变性上体现为剪切稀化流体特性,可以通过临界广义雷诺数判别流态.在此基础上,通过理论推导得到了层流和紊流状态下的压降计算公式,并且通过阻力特性实验进行了验证.研究结果对原生污水源热泵系统中水泵的选择有一定的指导意义.     

流体横掠圆柱数值模拟流态转换临界雷诺数的研究

采用层流模型和RNG k-ε紊流模型,对亚临界雷诺数条件(Re=300~300 000)下,流体横掠圆柱体工况进行了数值模拟,得出了此工况下扰流流态转捩的规律。与经典试验数据进行对比验证,结果表明,阻力系数Cd和斯特劳哈数St分别可在来流雷诺数小于22 000和100 000时采用层流模型计算,误差分别小于16.4%和7.3%。