Stroboscopic two-dimensional ultrasonic velocity profiling for measuring flow transition in Taylor Couette systems

Flow characterization in a Taylor Couette system was made by investigating the radial velocity component with Ultrasonic Doppler Velocimetry based flow mapping. With the technique presented in this work, it is possible to measure the radial velocity components for variable axial position in a Couette cell within Taylor vortex flow (TVF), wavy vortex flow (WVF), modulated vortex flow (MVF) as well as spiral vortex domains in a conical shaped gap. The resulting maps for the different flow states show the location of vortices in the annular gap between the inner and outer cylinder. Cylindrical and conical concentrically rotating inner bodies were applied and respective flow patterns were analyzed. The method uses a stroboscopic triggering to synchronize flow measurements and rotational motion. The oscillation frequency f of unsteady motion in WVF, MVF, and spirals can be obtained from the power spectrum of velocity. The UVP transducer was preferably positioned in radial direction, perpendicular to the surface of the inner rotating body for measuring the radial velocity component. At the same time, the transducer was moved with constant velocity vertically along the outer cylinder height.     

Couette-Taylor-Poiseuille流的数值模拟

对中等半径比同心旋转圆柱间Couette-Taylor-Poiseu ille流进行了数值计算,并与已有的实验数据进行比较以获得流场的更多信息。结果表明,数值计算与实验结果吻合较好,依次再现了层流涡、波动涡、非波动螺旋涡以及波动螺旋涡;轴流可以起到稳定流场的作用,轴流存在时,流场转捩的临界泰勒数Ta值会变大,涡胞变小,涡心不再位于轴间隙的中间,从左向右的轴流比较明显,交替指向内轴和外轴,并缠绕在涡胞的周围;除去平均轴流速度后,速度矢量场显示出不同的涡形,形状与相同Ta时的涡胞基本相同;在不同的泰勒数Ta和雷诺数Re下,涡心的轴向传播速度约为平均轴向流速的1.17倍,相传播速度约为内筒转速的0.42倍。     

管内盐析两相流动的PDPA试验研究

管内伴有盐析两相流动的研究是了解盐析机理的重要途径。运用相位多普勒粒子分析仪(Phase-Doppler particle analyzer, PDPA)对层流状态下管内Na2SO4溶液的盐析流动规律进行了测量,并详细分析盐析粒子成核速度和溶液流动参数之间的相互关系。试验采用激光单点测量方法对管内充分发展的层流溶液不同截面进行非接触式两相流测量,测量结果表明,随着溶液过饱和度、流动速度及降温速度的变化,管内Na2SO4晶体的盐析速率均会发生不同的变化;盐析粒子的出现对管内层流场产生一定的扰动,扰动引起的湍流脉动提高传热效率从而又加快盐析粒子的形成;所运用的试验手段及分析方法将为研究盐析两相流、掌握动态盐析机理及输送盐溶液装置的优化设计提供一定的参考。     

斜截椭圆柱式涡流发生器强化传热的大涡模拟

对流体在放置斜截椭圆柱式涡流发生器矩形槽道内的流动与传热特性进行大涡模拟,得出流场中速度、温度与压力参数的瞬态变化特性,再现温度场、压力场及诱导旋涡的变化过程,并对流动结构及涡流发生器强化传热的机理进行分析。为验证大涡模拟计算结果的准确性,在相同条件下对未布置涡流发生器的空槽道分别采用湍流模型和大涡模拟进行对比计算,两者的计算结果符合较好。计算结果表明：流场中布置的涡流发生器可以诱导漩涡,而由其所诱导的流向涡对强化传热起主要作用。与相同条件下未布置涡流发生器的情况相比,局部对流换热系数可提高64%～105%,平均对流换热系数则可提高17%～36%;涡流发生器附近位置的对流换热系数提高幅度最大,传热面附近流体的流动状况及流动结构与传热密切相关。     

Axial slit wall effect on the flow instability and heat transfer in rotating concentric cylinders

The slit wall effect on the flow instability and heat transfer characteristics in Taylor-Couette flow was numerically studied by changing the rotating Reynolds number and applying the negative temperature gradient. The concentric cylinders with slit wall are seen in many rotating machineries. Six different models with the slit number 0, 6, 9, 12, 15 and 18 were investigated in this study. The results show the axial slit wall enhances the Taylor vortex flow and suppresses the azimuthal variation of wavy Taylor vortex flow. When negative temperature gradient exists, the results show that the heat transfer augmentation appears from laminar Taylor vortex to turbulent Taylor flow regime. The heat transfer enhancement become stronger as increasing the Reynolds number and slit number. The larger slit number model also accelerates the flow transition regardless of the negative temperature gradient or isothermal condition.     

Quantitative investigation of the transition process in Taylor-Couette flow

The transition process from circular Couette flow to Taylor vortex flow regime was experimentally investigated by measuring the instantaneous velocity vector fields at the annular gap flow region between two concentric cylinders. The proper orthogonal decomposition method, vorticity calculation, and frequency analysis were applied in order to analyze the instantaneous velocity fields to identify the flow characteristics during the transition process. From the results, the kinetic energy and corresponding reconstructed velocity fields were able to detect the onset of the transition process and the alternation of the flow structure. The intermittency and oscillation of the vortex flows during the transition process were also revealed from the analysis of the instantaneous velocity fields. The results can be a measure of identifying the critical Reynolds number of the Taylor-Couette flow from a velocity measurement method.     

Thermal influence of the Couette flow in a hydrostatic spindle on the machining precision

Hydrostatic spindles are increasingly used in precision machine tools. Thermal error is the key factor affecting the machining accuracy of the spindle, and research has focused on spindle thermal errors through examination of the influence of the temperature distribution, thermal deformation and spindle mode. However, seldom has any research investigated the thermal effects of the associated Couette flow. To study the heat transfer mechanism in spindle systems, the criterion of the heat transfer direction according to the temperature distribution of the Couette flow at different temperatures is deduced. The method is able to deal accurately with the significant phenomena occurring at every place where thermal energy flowed in such a spindle system. The variation of the motion error induced by thermal effects on a machine work-table during machining is predicated by establishing the thermo-mechanical error model of the hydrostatic spindle for a high precision machine tool. The flow state and thermal behavior of a hydrostatic spindle is analyzed with the evaluated heat power and the coefficients of the convective heat transfer over outer surface of the spindle are calculated, and the thermal influence on the oil film stiffness is evaluated. Thermal drift of the spindle nose is measured with an inductance micrometer, the thermal deformation data 1.35 μm after running for 4 h is consistent with the value predicted by the finite element analysis's simulated result 1.28 μm, and this demonstrates that the simulation method is feasible. The thermal effects on the processing accuracy from the flow characteristics of the fluid inside the spindle are analyzed for the first time.     

组合涡发生器在螺旋板式换热器上的强化传热研究

在螺旋板式换热器螺旋通道内设置三角翼和椭圆柱组合涡发生器,利用流体计算软件Fluent进行三维数值模拟。研究了Re为4000～7000内组合涡发生器对通道平均Nu和平均阻力系数f的影响,并应用场协同原理进行了分析。与只加装椭圆柱涡发生器的螺旋板式换热器进行对比,结果表明,纵向涡发生器产生的二次流能改善螺旋通道内的速度场与温度场的协同性,起到强化换热作用。在正三角形排列方式下,组合涡发生器通道的平均Nu比椭圆柱涡发生器的平均Nu增大8.7%,阻力因子f减小23.7%,强化换热的效果较好。     

车用小通道平行流换热器传热流动试验与分析

小通道平行流换热器是燃料电池汽车的主要散热部件。吸收了电堆废热的冷却液(50%乙二醇溶液),流过小通道换热管,由换热器外侧空气冷却。在进液温度、进风温度、冷却液流量以及风速变化的试验工况下,测试了换热器的传热流动性能。引入量纲一参数k,评估了各工况参数对换热量、阻力影响的强弱。接着,分析液侧努谢尔数Nu和摩阻系数f随雷诺数Re的变化趋势,结果显示:在小通道内(当量直径D=2.685 mm),冷却液从层流到湍流的转浪点Re_c=1 750,介于微尺度与常规尺度的临界值之间。在此基础上,通过多元回归法,拟合得到层流和湍流的液侧换热系数,摩阻系数的关联式,以及空气侧阻力f_a公式。Nu和f的计算值与试验值误差分别在[-7.06%,5.93%]和[-3.95%,4.11%]内,f_a的误差在[-2.22%,3.62%]内。基于这些关联式,建立数学模型,可在广泛多变的运行条件下,对换热器的运行性能进行理论预测和评估。     

Burnett simulations of gas flow and heat transfer in microchannels

In micro- and nanoscale gas flows, the flow falls into the transition flow regime. There are not enough molecule collisions and the gas deviates from the equilibrium. The Navier-Stokes equations fail to describe the gas flow in this regime. The direct simulation Monte Carlo method converges slowly and requires lots of computational time. As a result, the high-order Burnett equations are used to study the gas flow and heat transfer characteristics in micro- and nanoscale gas flows in this paper. The Burnett equations are first reviewed, and the augmented Burnett equations with high-order slip boundary conditions are then used to model the gas flow and heat transfer in Couette and Poiseuille flows in the transition regime.     

CFD analysis of fin tube heat exchanger with a pair of delta winglet vortex generators

Among tubular heat exchangers, fin-tube types are the most widely used in refrigeration and air-conditioning equipment. Efforts to enhance the performance of these heat exchangers included variations in the fin shape from a plain fin to a slit and louver type. In the context of heat transfer augmentation, the performance of vortex generators has also been investigated. Delta winglet vortex generators have recently attracted research interest, partly due to experimental data showing that their addition to fin-tube heat exchangers considerably reduces pressure loss at heat transfer capacity of nearly the same level. The efficiency of the delta winglet vortex generators widely varies depending on their size and shape, as well as the locations where they are implemented. In this paper, the flow field around delta winglet vortex generators in a common flow up arrangement was analyzed in terms of flow characteristics and heat transfer using computational fluid dynamics methods. Flow mixing due to vortices and delayed separation due to acceleration influence the overall fin performance. The fin with delta winglet vortex generators exhibited a pressure loss lower than that of a plain fin, and the heat transfer performance was enhanced at high air velocity or Reynolds number.     

垂直管中定常螺旋流涡量特性的PIV试验研究

为了研究螺旋流场中的涡量特征分布,提高流体传质效率,利用激光粒子测速系统(PIV)对垂直管中不同介质的螺旋流场进行了测量,并利用Tecplot对螺旋流的涡量进行了显示和局部涡量场提取,研究了不同切向速度下,不同介质的螺旋流涡量场分布特点和其局部涡量特征。结果显示,螺旋流的涡量具有贴壁特征,随着切向速度的增加,螺旋流的涡量强度增大,正涡和负涡交换的频率增大,有利于提高流体传质效率。     

Combined Streamline Upwind Petrov Galerkin method and segregated finite element algorithm for conjugate heat transfer problems

A combined Streamline Upwind Petrov-Galerkin method (SUPG) and segregated finite element algorithm for solving conjugate heat transfer problems where heat conduction in a solid is coupled with heat convection in viscous fluid flow is presented. The Streamline Upwind Petrov-Galerkin method is used for the analysis of viscous thermal flow in the fluid region, while the analysis of heat conduction in solid region is performed by the Galerkin method. The method uses the three-node triangular element with equal-order interpolation functions for all the variables of the velocity components, the pressure and the temperature. The main advantage of the presented method is to consistently couple heat transfer along the fluid-solid interface. Four test cases, which are the conjugate Couette flow problem in parallel plate channel, the counter-flow in heat exchanger, the conjugate natural convection in a square cavity with a conducting wall, and the conjugate natural convection and conduction from heated cylinder in square cavity, are selected to evaluate efficiency of the presented method.     

Heat/mass transfer characteristics in the near-tip region on a turbine blade surface under combustor-level high inlet turbulence

Heat/mass transfer characteristics on the near-tip blade surface under combustor-level high inlet turbulence have been investigated within a high-turning turbine rotor passage by using the naphthalene sublimation technique. The inlet turbulence intensity and length scale are 14.7% and 80 mm, respectively. The tip gap-to-chord ratio is changed to beh/c = 0.74, 1.47, and 2.94 percents. Increasingh/c results not only in higher heat/mass transfer in the pressure-side tip region but also in more convective transport on the pressure surface even far away from the tip edge. Severe heat/mass transfer is always observed in the suction-side tip-leakage flow region which can be divided into two distinct high transport regions. There is a local maximum of heat/mass transfer along the trailing-edge centerline. This arises from the interaction of a tip-leakage vortex with a trailing-edge vortex shedding. Comparisons of the present data forh/c = 2.94 percents with the previous low turbulence one show that there is a large discrepancy of heat/mass transfer in the pressure-side near-tip area, which diminishes with departing from the tip edge. The suction-side heat/mass transfer in the tip-leakage flow region is less influenced by the high inlet turbulence than that at the mid-span. The leading-edge heat/mass transfer under the high inlet turbulence is always higher than that in the low turbulence case, while there is no big difference in the trailing-edge heat/mass transfer between the two cases.     

三维方管层流冲击射流流动与传热的数值研究

通过求解三维稳态不可压缩N-S方程和能量方程,对半封闭层流方管冲击射流的流动与传热特性进行了数值研究。根据计算结果分析了射流中四个偏心速度峰值形成的原因,在层流范围内考察了射流雷诺数和冲击高度对流场结构和传热性能的影响。计算结果表明,冲击射流的传热特性受流场结构的控制,冲击面附近水平断面上四个偏心速度峰值的形成,导致在相应断面上形成温度分布的四个偏心最小值,以及在冲击面上形成局部Nu数的四个偏心峰值。     

Qualitative and quantitative measurements of horseshoe vortex formation in the junction of horizontal and vertical plates

Horseshoe vortex formation around a vertical flat plate mounted in the water channel is experimentally investigated. In order to demonstrate the development of the horseshoe vortex system in the junction of the horizontal and vertical plates a Particle Image Velocimetry (PIV) and dye visualization techniques were used. Instantaneous and time-averaged velocity vector fields, corresponding streamline topology, vorticity values were analyzed. Boundary layer separation, developing vortices, secondary or counter-rotating vortices, merging of developing and counter-rotating vortices which results in a primary horseshoe vortex system and corner vortex were demonstrated qualitatively and quantitatively.     

水平管内以叶轮起旋的螺旋流的数值模拟

采用RNG k-ε模型对水平管内以叶轮起旋的螺旋流的流动特性进行了数值模拟研究,通过试验验证了数学模型的准确性和可靠性。研究结果表明:叶片旋流器产生的旋流场,切向速度分布具有近似Rankine组合涡结构的特点,径向速度最大值达轴向速度的0.5倍。雷诺数对旋流强度影响较大,Re数越大,旋流强度越大,且旋流强度的衰减速率越小。强旋流使轴向逆压力梯度足够大而引发轴向回流,从而产生了中心回流区域;能够增加反应介质的滞留时间,提高气液两相的传质和传热效率,促进水合物浆体快速生成,保障管道安全输送。     

旋转管式膜分离器流型的研究

采用三相异步电动机的输入电磁转速与三相异步电动机的实际转速之差的变化来表征两旋转圆筒间流场特性的参数。主要研究了清水介质时无轴向流和有轴向流条件下流型的划分,并将无轴向流时的流型分成了6种：Couette层流,层流泰勒涡,层流波涡,层流调制波涡、湍流泰勒涡和湍流;有轴向流时的流型分成了4种：Couene层流、层流泰勒涡、湍流泰勒涡和完全湍流。并且验证了流型转变的滞后特性。     

Improving basic relationships of pipe hydraulics

This communication presents the approaches set up for processing spinner flowmeter well logs in vertical wells with a single fluid phase, which is the most widely used in assessing wells productivity. These focus on improving the pipe hydraulics relationships so that the different fluid inputs throughout the well can be quantified. Since vertical flow inside wells varies with depth between laminar flows (very low Reynolds number, i.e. Re < 10³) and turbulent (Re > 4·10³) the aim has been to reduce the uncertainty in the transition interval. Starting from bibliographical data and/or well-known formulas for laminar and for turbulent flow, several continuous relationships have been developed for any regime: 1) an expression for the radial distribution of velocity inside the pipeline (velocity profile) was developed. 2) A relationship between the average velocity and the velocity at the axis (velocity factor) was created. 3) A third equation was generated to obtain the friction factor in smooth pipes (and starting from this, a new explicit equation for rough pipes). The purpose has been to have a set of empirical expressions of easy and continuous application for any regime, as an alternative to the use of computer simulations.     

Heat transfer correlations for laminar flows within a mechanical seal chamber

Numerical investigation of conjugate heat transfer associated with laminar flow within the chamber of a mechanical seal is presented. It involves simultaneous solution of the Navier-Stokes and energy equations. The computational model takes into account the temperature distribution within the rotating and stationary rings. A series of simulation results are presented for predicting the performance of a mechanical seal assuming that the flow in the seal chamber remains laminar. Expressions are developed for predicting the convective heat transfer coefficient on the outer surfaces of the seal rings exposed to the process fluid in the seal chamber.