不同压力差下微通道尺寸和表面粗糙度对摩擦系数的影响

该文数值模拟液体在圆形和梯形截面微通道内的流动,分析了层流和湍流下液体在微圆管内的流动状态。着重研究不同压力差、微通道尺寸和表面粗糙度下,液体在微通道内的流动摩擦系数,并通过摩擦系数随雷诺数的变化曲线推断微通道流动转捩的雷诺数范围。研究表明:微通道中流动的摩擦系数随雷诺数的增大逐渐减小;通道截面的当量直径会改变过渡状态存在的雷诺数范围;粗糙度会影响湍流状态下流动的摩擦系数,相同雷诺数下,粗糙度越大,摩擦系数越大。     

圆管及网格填料回热器非定常交变流动阻力特性的研究

首先简要介绍了国内外有关周期振荡／往复流动的研究进展情况。然后,从最基本的流体力学方程出发,探讨了描述振荡流动从层流到湍流转变的无量纲准则数。研究了这内处于充分发展层流的周期性往复流动时的流动阻力特性以及动力学特性,最后,对广泛使用于回热式热机（发动机和制冷机）的回热器的流动阻力特性及动力学特性进行了分析研究,在综合前人各种理论及实验成果的基础上,提出了作者的看法,以上研究对深入认识周期往复流动的     

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。     

斯特林发动机内振荡流流动特性研究综述

斯特林发动机的关键部件存在着平均速度为零的周期性非定常流（振荡流）。研究振荡流的流动机理，降低流动损失是提高发动机循环效率的有效手段。本文主要阐述了振荡流流动特性，包括相似参数的选取、层流振荡流、湍流振荡流和流动转捩的研究方法。     

蜂窝板换热器内部流动传热特性研究

建立了蜂窝板换热器湍流流动的物理数学模型,并应用数值分析方法模拟了蜂窝板换热器的三维流动传热过程;分析了不同雷诺数下通道内流动阻力和换热性能及其随雷诺数的变化规律,并与相同当量直径的平行平板通道的流动换热性能进行了对比.结果表明,蜂窝板换热器在换热系数提高的同时流动阻力也增大了,在雷诺数Re=3000~15000的范围内,其传热努塞尔数比平行平板增大了0.93~2.12倍,阻力系数增大了2.24~2.35倍.最后从场协同理论的角度分析了蜂窝板强化传热的机理.     

蜂窝板换热器内部流动传热特性研究

建立了蜂窝板换热器湍流流动的物理数学模型,并应用数值分析方法模拟了蜂窝板换热器的三维流动传热过程;分析了不同雷诺数下通道内流动阻力和换热性能及其随雷诺数的变化规律,并与相同当量直径的平行平板通道的流动换热性能进行了对比。结果表明,蜂窝板换热器在换热系数提高的同时流动阻力也增大了,在雷诺数Re=3000~15000的范围内,其传热努塞尔数比平行平板增大了0.93~2.12倍,阻力系数增大了2.24~2.35倍。最后从场协同理论的角度分析了蜂窝板强化传热的机理。     

气体涡轮流量计流场仿真及计量误差分析

利用控制体积法的SIMPLEC算法对气体涡轮流量计的内流场进行了数值模拟,给出了内流场信息,分析了内部几何结构对压力和速度分布的影响,及其与流量系数的关系.结果表明在湍流状态时的仪表系数K为常数,累计流量和瞬时流量的误差较小;而在层流以及转捩状态时,仪表系数总是在变化,累计流量和瞬时流量的误差较大.建议通过结构优化,促使层流向湍流状态的快速转捩,并保证叶轮动平衡,从而加大涡轮流量计的量程范围.该研究结果对涡轮流量计的结构优化设计具有一定的指导意义.     

微流道换热器内阻力特性的实验研究

本文是制冷换热器小型化研究的一部分，采用等壁温法，对水在正方形，正三角形微流道内受热单相流动的阻力特性进行了实验研究，通过对实验数据的分析发现，层流的摩擦阻力系数随微流道壁面的温度的升高而下降，且低于经典层流阻力系数公式的预测值，流态由层流向紊流过渡也比常规尺度流道提前，且临界雷诺数随微流道壁面温度的升高而减小。     

翼形表面流动激励载荷特征试验研究

为了研究水下航行体翼形表面流动激励载荷特征,采用柔性基底微型传感器阵列,建立风洞中翼形结构表面流动激励载荷测试方法。分析了翼形表面层流、转捩、湍流发展过程中激励力试验特征,试验结果表明:翼形表面经过层流、转捩、湍流发展过程,随着来流速度的增大,翼形表面转捩区位置逐渐前移直至完全湍流。提出了转捩点预报修正方法,修正后预报值与试验值偏差在 10 % 以内,为水下航行体翼形流动激励力相关研究提供试验基础。     

影响叶片表面换热主要因素的分析

采用实验和数值计算方法研究独立参数出口马赫数、出口雷诺数和来流湍流度对叶片表面换热系数的影响,结果表明:马赫数的分布情况是决定叶片表面换热系数分布的最主要的准则数,出口雷诺数是影响叶片表面整体的换热水平、流动状态发生改变位置和边界层发生分离位置的因素,来流湍流度和壁温与燃气温度比的高低仅仅影响叶片整体的换热水平.     

Convective heat transfer coefficients for near-horizontal two-phase flow of nitrogen and hydrogen at low mass and heat flux

Correlations for convective heat transfer coefficients are reported for two-phase flow of nitrogen and hydrogen under low mass and heat flux conditions. The range of flowrates, heat flux and tube diameter are representative of thermodynamic vent systems (TVSs) planned for propellant tank pressure control in spacecraft operating over long durations in microgravity environments. Experiments were conducted in normal gravity with a 1.5° upflow configuration. The Nusselt number exhibits peak values near transition from laminar to turbulent flow based on the vapor Reynolds number. This transition closely coincides with a flow pattern transition from plug to slug flow. The Nusselt number was correlated using components of the Martinelli parameter and a liquid-only Froude number. Separate correlating equations were fitted to the laminar liquid/laminar vapor and laminar liquid/turbulent vapor flow data. The correlations give root-mean-squared (rms) prediction errors within 15%.     

Formation and development of turbulence during movement of a disperse system in a round tube

The experimental results are presented for a study of the turbulence intensity, coefficient of intermittence, and average velocity profile during the transition of the laminar flow of an aqueous dispersion of bentonite clay to turbulent flow. A calculation of the critical value of the Reynolds number is offered.     

A criterion of similarity and transition to turbulence for electric arc flows

A method is suggested for determining the similarity criterion and the criterion of transition of an electric arc flow in a plasmatron channel from a laminar regime to a turbulent one (equivalent Reynolds number) that does not require using any reference temperature. Application of this method to the available experimental data on the transition to turbulence demonstrates its correctness and efficiency. The critical value of the proposed similarity criterion corresponding to the boundary of the transition from a laminar regime to a turbulent one has been revealed. A marginal curve separating the regions of the laminar and turbulent regimes of the plasma flow has been plotted in the space of the operating plasmatron parameters. A phenomenon of a double change of the electric arc flow regime with electric current rise upon constant plasma-forming gas flow rate has been discovered.     

直径30cm圆柱的气动力参数和绕流特性研究

为研究光滑圆柱的气动力系数和绕流特性,在均匀流中进行不同风速下的测压风洞试验,试验获得了阻力系数、升力系数、表面风压分布、风压相关性系数、斯托罗哈数等随雷诺数的变化特征,并将试验结果与以往结果进行比较。研究表明:升力系数的脉动值大于阻力系数的脉动值,说明涡脱造成的横风向激励比顺风向紊流激励剧烈;雷诺数位于临界区域时,圆柱表面风压分布呈现出对称-不对称-对称的变化过程,反映了由层流分离转化为湍流分离的全过程;在雷诺数为352000时呈现一侧为层流分离、另一侧为湍流分离的临界流态,风压呈现出左右不对称的单边泡形式;获得层流分离和湍流分离时的表面风压相关性分布特征,层流分离时圆柱同一侧的风压测点均呈较强的正相关,而湍流分离时在分离点前的区域相关性较强,分离点之后的区域相关性较弱;层流分离的升力系数谱有显著的峰值,表明尾流是规则的漩涡脱落,而湍流分离的升力系数谱没有明显峰值,表明尾流是随机的漩涡脱落。     

Experimental and numerical investigation on forced convection heat transfer and pressure drop in helically coiled pipes using TiO2/water nanofluid

In the present article, forced convection heat transfer and pressure drop in helically coiled pipes using TiO₂/water nanofluid as working fluid were investigated experimentally and numerically. The aim is to investigate and provide additional insight about the effects of physical and geometrical properties on heat transfer augmentation and pressure drop in helically coiled tubes. The experiments were conducted in the range of Reynolds number from 3000 to 18,000 and in the nanoparticle concentrations of 0.1, 0.2, and 0.5% for five different curvature ratios. In numerical simulations the thermophysical properties of the working fluid were assumed to be a function of nanofluid temperature and concentration. For turbulent regime the standard k − ε model was used to simulate the turbulent flow characteristics. The numerical results were in good agreement with the experimental data. The results showed that utilization of nanofluid instead of distilled water leads to an enhancement in the Nusselt number up to 30%. Also, four formulas were introduced to obtain the average Nusselt number and friction factor in helically coiled tubes under constant wall temperature condition for both laminar and turbulent flow regimes.     

Laminar and Turbulent Transition of Fine-Grained Slurries

An experimental investigation of flow behavior of fine-grained highly concentrated slurries, i.e., water mixtures of kaolin and fly ash from a fluidic-type combustion chamber produced during the process of desulphurization, in horizontal straight pipes is presented. A pipe loop with hydraulically smooth stainless steel pipes was used to measure the slurry flow parameters. Kaolin slurry has time-independent, yield pseudo-plastic response for volume concentrations higher than about 6%. In contrast, fluidic fly ash-gypsum water mixture is time dependent and showed substantial decrease of flow resistance due to the effect of shearing during the initial period of pumping. An intensive shearing of concentrated fluidic fly ash-gypsum slurry results in a substantial reduction of the hydraulic gradient in the laminar region and in a marked shift of the laminar/turbulent transition point towards a lower velocity value. After shearing in a turbulent regime a reduction in the hydraulic gradient at the transition point reached about 50% of its original value. The transition from laminar to turbulent regime results in an abrupt increase of flow resistance. The flow patterns become fundamentally different for the two regimes. It was found that pressure fluctuation could well indicate the laminar/turbulent transition. The transition from laminar to turbulent flow is very important for accurate and efficient design and operation of dense slurry pipelining. The optimum operational condition is slightly above the laminar/turbulent transition point, where flow conditions are often very attractive from an economic point of view.     

Laminar and Turbulent Transition of Fine-Grained Slurries

An experimental investigation of flow behavior of fine-grained highly concentrated slurries, i.e., water mixtures of kaolin and fly ash from a fluidic-type combustion chamber produced during the process of desulphurization, in horizontal straight pipes is presented. A pipe loop with hydraulically smooth stainless steel pipes was used to measure the slurry flow parameters. Kaolin slurry has time-independent, yield pseudo-plastic response for volume concentrations higher than about 6%. In contrast, fluidic fly ash-gypsum water mixture is time dependent and showed substantial decrease of flow resistance due to the effect of shearing during the initial period of pumping. An intensive shearing of concentrated fluidic fly ash-gypsum slurry results in a substantial reduction of the hydraulic gradient in the laminar region and in a marked shift of the laminar/turbulent transition point towards a lower velocity value. After shearing in a turbulent regime a reduction in the hydraulic gradient at the transition point reached about 50% of its original value. The transition from laminar to turbulent regime results in an abrupt increase of flow resistance. The flow patterns become fundamentally different for the two regimes. It was found that pressure fluctuation could well indicate the laminar/turbulent transition. The transition from laminar to turbulent flow is very important for accurate and efficient design and operation of dense slurry pipelining. The optimum operational condition is slightly above the laminar/turbulent transition point, where flow conditions are often very attractive from an economic point of view.     

Experimental and theoretical progress in pipe flow transition

There have been many investigations of the stability of Hagen-Poiseuille flow in the 125 years since Osborne Reynolds' famous experiments on the transition to turbulence in a pipe, and yet the pipe problem remains the focus of attention of much research. Here, we discuss recent results from experimental and numerical investigations obtained in this new century. Progress has been made on three fundamental issues: the threshold amplitude of disturbances required to trigger a transition to turbulence from the laminar state; the threshold Reynolds number flow below which a disturbance decays from turbulence to the laminar state, with quantitative agreement between experimental and numerical results; and understanding the relevance of recently discovered families of unstable travelling wave solutions to transitional and turbulent pipe flow.     

An experimental study of subsonic microjets escaping from a flat nozzle

We have experimentally studied subsonic laminar gas jets escaping from a flat nozzle with dimensions of 83.3 × 3600 μm. Reynolds numbers calculated for the given nozzle height and average flow velocity at the nozzle output edge ranged within 58–154. The working gas was air at room temperature. Distributions of the gas velocity and its pulsation along the jet axis were determined. It is established that the obtained characteristics of laminar subsonic microjets are fundamentally different from those of macroscopic turbulent jets. Based on the results of velocity pulsation measurements, it is shown that a laminar–turbulent flow transition is present and its position is determined.