基于高速图像采集的管束间气液两相绕流旋涡特性研究

采用高速图像采集方法,研究了不同节距比时以旋转正方形排列的错列管束间气液两相流旋涡的演化特性,通过动态图像重建了旋涡生成、卷吸及聚合过程,分析了旋涡演化机理,并运用时间序列图像相关性对比分析了旋涡脱落的周期特征.结果表明:随着节距比的增大,斯特罗哈数增大;随着含气率的增大,斯特罗哈数减小;当含气率大于0.14时,不存在周期性旋涡脱落现象;随着雷诺数的增大,旋涡脱落频率增加.     

两相流垂直下降横掠"T"形柱体的斯特罗哈数

试验研究了两种规格梯形柱体，在垂直下降气液两相流中，发生气液两相涡衡时，气液两相特斯罗哈数的变化规律。在测得大量数据的基础上，得出了发生气液两相涡街时，气液两相斯特罗哈数的通用关系式。研究表明，气液两相斯特罗哈数在两相工况下为一变数，与来流截面含气率、涡街发生体形状与特征尺寸和来流方向等因素有关。应用此关系式，根据测得的两相涡街频率可将涡街发生体作为测量两相流流量与组分的测量元件。     

垂直向上气液两相流中T形柱体两相斯托拉赫数的研究

试验研究了两种规格的Ｔ形柱体在垂直上升气液两相流中发生气液两相涡街时,气液两相斯托拉赫数的变化规律。在测得大量数据的基础上,得出了此时气液两相斯托拉赫数的通用关系式。研究表明,气液两相斯托拉赫数在两相工况下为一变数,与来流截面含气率、涡街发生体形状和特征尺寸、来流方向等因素有关。根据测得的两相涡街频率应用此关系式,可将涡街发生体作为测量两相流流量与组合的测量元件。     

气液两相流横掠错列圆柱形成旋涡脱落诱发管束振动的试验研究

对相同直径的 3排错列圆柱表面的旋涡脱落工况进行了试验研究。测试圆柱采用Φ30mm的有机玻璃圆柱。试验时两相隙缝流Re数范围为 2 .0× 10 4～ 6 .0× 10 4,含气率的范围在 0～ 0 .3之间。通过试验分析获得了不同排列形式的错列管束中两相流斯特拉赫数 ;试验发现两相流斯特拉赫数Sttp随含气率的增加而减小 ;提出了旋转正三角形排列的错列管束的斯特拉赫数曲线 ;指出可以近似应用单相流时的Weaver曲线作为两相流中旋转正方形排列和正三角形排列管束旋涡脱落诱发管束振动的判别标准     

气液两相流垂直冲刷错列管束漩涡脱落特性的实验研究

采用高速摄像仪拍摄的方法,对垂直上升的气液两相流体在矩形管道内冲刷节距比为1.0、1.5和2.0三种旋转正方形排列的错列管束漩涡脱落特性进行了研究.给出了在一个漩涡的形成与发展过程中,对周围气泡进行卷吸并在柱后形成气核的全过程;通过对漩涡脱落周期的统计,得出了在本实验范围内随含气率的增大漩涡脱落频率逐渐增大,漩涡脱落,St逐渐减小的结论;当含气率α=0.147时周期性漩涡脱落现象消失.     

水平PDMS微通道内气-液弹状流特性观察

对以空气、甘油为工作流体在水平聚二甲基硅氧烷(polydimethylsiloxane,PDMS)微通道内产生的弹状流进行了可视化实验研究.实验结果表明,微通道中弹状流的气柱速度随着体积含气率增大而阶梯状减小.气柱长度在体积含气率小于0.06时几乎不发生变化;当体积含气率大于0.06时,随体积含气率的增大而呈线性增大.气柱间距随体积含气率的变化与气柱长度相反.气柱和微通道壁面之间存在稳定的薄液膜,当地薄液膜压力稳定,气柱流经当地时所造成的压力波动很小;当地静压随体积含气率的增大而减小.     

考虑气体压缩性的液力透平内气液两相流场分析

为研究在气液两相条件下液力透平内部的流动规律,选择比转速为55.7的单级单吸离心泵反转作液力透平(Pumps as Turbines,PAT),在考虑气体可压缩的基础上对该模型在不同流量、不同含气率下进行数值计算。分析含气率对液力透平外特性和液力透平各过流部件内流场的影响规律,总结不同工况下液力透平内气液两相流动规律。研究发现:随着含气率的增大,液力透平的效率和功率逐渐减小、扬程逐渐增加,气体的存在对液力透平效率影响较大;液力透平叶片进口有明显的旋涡,随着流量和含气率的增大,混合介质的相对速度均增加;含气率从液力透平进口到出口逐渐增大,叶片背面的含气率要比工作面大,过流部件内的气体分布不对称,随着含气率的变大,气体分布的均匀性变差。     

壳侧气液两相流沿水平方向横掠水平管束截面含气率的预测

本文在实验研究的基础上,结合前人的研究结果,将预测管内平均截面含气率的理论模型经适当修正后,提出了预测带的流板的管壳式换热器壳侧气液两相流沿水平方向横掠水平管束截面含气率的理论模型。结果表明,该模型能很好地预测壳侧顺排和叉排水平管束中不同物纱两相流体的平均截面含气率。     

水平管内油气水三相分层流截面含气率的研究

以油气水三相混合物为工质,对水平管内分层流动的平均截面含气率进行了理论与实验研究。通过对分层流动的简化动力学分析,得到了截面含气率的理论模型,计算值与实验值符合良好。结果表明：影响分层流截面含气率的因素不仅包括折算气速和折算液速,还包括油水混合物的含油率。     

竖直矩形窄缝通道内气液两相流的实验研究

为研究竖直矩形窄缝通道内气液两相垂直向上流动的现象,搭建了气液混合流动的实验台,采用高速照相机对竖直矩形窄缝通道内的气液两相流进行拍摄,通过Matlab软件实现了图像识别以及相近气泡的区分。结果表明:拟合得到的平均截面含气率α与体积含气率β的关系为α=0.88β;对Chen模型的系数C修正后,摩擦压降计算值与实验值具有较高的匹配度。     

Vortex shedding and heat transfer dependence on effective Reynolds number for mixed convection around a cylinder in cross flow

The influence of surface heating of a circular cylinder on the wake structure and heat transfer in the range of Reynolds number (Re) for which parallel vortex shedding occurs, is investigated numerically for different values of the buoyancy parameter, Gr. The role of buoyancy induced baroclinic vorticity on the wake formation is addressed in the present study. The variation of Strouhal number and Nusselt number with the ’effective Reynolds number’, is analyzed for different values of cylinder to free stream temperature ratio. Both Strouhal number and the rate of heat transfer increases monotonically with the increase of the effective Reynolds number. The validity of the correlations, which have been established by several authors, between the effective Reynolds number and Strouhal/ Nusselt number for forced convection, is examined in the mixed convection regime. The curves between the effective Reynolds number and the computed data for Strouhal number and Nusselt number do not collapse for the range of temperature ratio considered here. The flow field is found to be asymmetric and the cylinder experiences a negative lift. The drag coefficient increases steadily with the rise of surface temperature.     

Numerical Investigation of Convective Heat Transfer in Unsteady Laminar Flow Over a Square Cylinder in a Channel

Unsteady laminar flow past a heated square cylinder mounted inside a plane channel was investigated numerically. The blockage ratio was chosen as 1/8 and the Reynolds number based on the mean flow velocity and chord length of the square cylinder was selected as less than 200, for which the two-dimensional behavior of the flow is assured. The time-averaged Nusselt number as well as some integral parameters such as drag coefficient, recirculation length, and Strouhal number were obtained and compared with literature. Results show a nearly linear increase in recirculation length and decrease in drag coefficient with increasing Reynolds number for the steady flow regime. There is an increase in the total Nusselt number and drag coefficient with a Reynolds number for unsteady flow regime, where vortex shedding is observed from the cylinder. A correlation was obtained for the variation of the total Nusselt number with the Reynolds number.     

Effect of Thermal Buoyancy on the Two-Dimensional Upward Flow and Heat Transfer Around a Square Cylinder

The effect of aiding/opposing buoyancy on the two-dimensional upward flow and heat transfer around a heated/cooled cylinder of square cross section is studied in this work. The finite-volume-based commercial computational fluid dynamics (CFD) software FLUENT is used for the numerical simulation. The influence of aiding/opposing buoyancy is studied for Reynolds and Richardson numbers ranges of 50 to 150 and –1 to 1, respectively, and the blockage parameters of 2% and 25%. The flow exhibits unsteady periodic characteristics in the chosen range of Reynolds numbers (except for Reynolds number of 50 and blockage parameter of 25%) for the forced convective cases (Richardson number of 0). However, the vortex shedding is observed to stop completely at some critical value of Richardson number for a particular Reynolds number, below which the shedding of vortices into the stream is quite prominent. Representative streamlines and isotherm patterns for different blockage parameters are systematically presented and discussed. The critical Richardson and average Nusselt numbers are plotted against the Reynolds and Richardson numbers, respectively, to elucidate the role of thermal buoyancy on flow and heat transfer characteristics. It is observed that the vortex shedding frequency (Strouhal number) increases with increased heating and suddenly reduces to zero at the critical Richardson number. The critical Richardson number is again found to increase with Reynolds number for a particular blockage ratio, and the higher the blockage ratio, the less is the critical Richardson number. The results obtained from the commercial solver are extensively validated with the available numerical results in the literature and an excellent agreement is observed.     

Laser schlieren measurement of vertical flow past a heated horizontal circular cylinder

Flow past a heated horizontal circular cylinder in the vertically upward direction has been experimentally studied using a monochrome schlieren technique. Both free convection ((Gr)^1/3Re)=0 and mixed convection ((Gr)^1/3Re)=1011, 1055, 1095 and 1133 cases have been studied. The Reynolds number based on the cylinder diameter is set at 102 for the mixed convection, and four heating levels have been utilized with Grashof numbers of Gr=975, 1105, 1240 and 1370. The temperature distribution of the plume, the Strouhal number and the schlieren images have been reported. The vortex shedding frequency decreases with increasing Grashof number and a complete suppression of vortex shedding takes place at Grashof number of 1370. The wake is seen to become visibly narrow during the suppression of vortex shedding. The nondimensional temperature profile inside the plume is a strong function of Grashof number for free convection in comparison to that of mixed convection.     

Triggering Vortex Shedding by Superimposed Thermal Buoyancy Around Bluff Obstacles in Cross-Flow at Low Reynolds Numbers

Influences of superimposed thermal buoyancy on the initiation of vortex shedding process behind bluff obstacles (such as circular and square cylinders in 2-D) in cross-flow at low Reynolds numbers (10 ≤ Re ≤ 40) are discussed. The flow which is steady and separated at this Reynolds number range eventually becomes unsteady periodic with the introduction of thermal buoyancy. The aim here is to numerically predict the critical value of the buoyancy parameter (Richardson number, Ri) for the onset of vortex shedding. The critical Ri is found to have a decreasing tendency for both types of cylinder geometries with increasing Re.     

Effect of channel-confinement and aiding/opposing buoyancy on the two-dimensional laminar flow and heat transfer across a square cylinder

The effect of channel-confinement of various degree (blockage ratio of 10%, 30% and 50%) on the upward flow and heat transfer characteristics around a heated/cooled square cylinder is studied by considering the effect of aiding/opposing buoyancy at −1 Ri 1, for Re = 100 and Pr = 0.7. With increasing blockage ratio, the minimum heating (critical Ri) required for the suppression of vortex shedding decreases up to a certain blockage ratio (=30%), but thereafter increases. The influence of buoyancy and channel-confinement on the recirculation length, drag and lift coefficient, pumping power, Strouhal number and heat transfer from the cylinder, is also investigated.     

On the linear heat transfer correlation of a heated circular cylinder in laminar crossflow using a new representative temperature concept

Determination of the reference temperature for the kinematic viscosity of Reynolds number is the key problem for describing the forced convection of a heated bluff-body. The heat transfer of a heated circular cylinder in laminar cross airflow is experimentally investigated in this paper. New correlation equations based on the effective temperature have been firstly derived. All experimental data collapsed excellently on the correlation curves. Moreover, the derived Nusselt and Reynolds number relations are independent of the vortex shedding patterns. The correlation equations have further been transformed to an explicit form without temperature ratio. By introducing a newly proposed representative Reynolds number concept, linear correlation equations have been successfully derived.     

Heat transfer enhancement in a slot channel via a transversely oscillating adiabatic circular cylinder

Heat transfer enhancement in a uniformly heated slot channel due to vortices shed from a transversely oscillating adiabatic circular cylinder is investigated. Effects of the cylinder motion and vortex shedding on heat transfer are systematically assessed by varying the cylinder oscillation frequency from 75% to 125% of the natural vortex shedding frequency of a fixed cylinder within the same domain. Numerical simulations at Re = 100 and 0.1 ? Pr ? 10 are performed using spectral element discretization of Navier-Stokes and energy equations in a moving domain based on an arbitrary Lagrangian-Eulerian formulation. Results within the thermally developing flow region show heat transfer enhancement due to the placement of a stationary cylinder compared to the straight channel case. Transverse oscillations of the cylinder further increase the wall heat transfer coefficient. Pumping power in the channel and the power necessary to oscillate the cylinder is also provided for comparisons. Cylinder oscillations with 75% of the natural vortex shedding frequency is shown to yield the best results with only 10% more power to pump the fluid, compared with the fixed cylinder case.