换热器管束中的流体弹性不稳定性

文中重点讨论了管壳式换热器管束中的流体弹性不稳定性．从工程实用的观点出发，应用了Ｌｅｖｅｒ与Ｗｅａｖｅｒ的“流管”模型，在建立管子振动的基本方程时则认为管子作周期性的衰减（或发散）运动，以复数解法进行求解．根据所获得的对比流速与质量阻尼参数之间的关系画出的稳定区图，可用来确定管子的流体弹性不稳定性或该时的临界流速．实验数据表明，理论计算结果是令人满意的．     

NEW DESIGN RECOMMENDATIONS FOR FLUIDELASTIC INSTABILITY IN HEAT EXCHANGER TUBE BUNDLES

Design equations are presented to determine the critical velocities for the occurrence of fluidelastic instability in uniform single-phase cross-flow. These equations are an essential part of a new guideline in the “VDI-Wärmeatlas” for estimating vibration excitation in real tube bundle heat exchangers. Six existing guidelines for fluidelastic instability were tested and compared with about 300 experimental data from 34 papers (eight of them being not yet considered in a guideline before). New equations for the stability factor K as a function of the pitch ratio for different tube configurations were derived with statistical methods by a variation of the reference definitions for the structural parameters and the exponents of either the dimensionless mass and the damping or the mass-damping parameter. The criterion used here was first to be on the safe side with a minimum number, and minimum deviation, of experimental data below the recommended threshold line, and second a minimum r.m.s. error for all data considered. The pitch ratio has the strongest influence for the normal triangular array and the influence becomes less for the rotated as well as for the in-line square configuration. This significant gradation of the stability constants as a function of the pitch ratio and the tube bundle configuration enables a reasonable interpolation for non-standard configurations. The exponent of the dimensionless mass-damping parameter for gas cross-flow depends on the tube configuration and is 0·5 for the 30°- and 45°-configuration, and 0·4 for the 60°- and 90°-configuration. For liquid flow, an average exponent of 0·15 has been observed.     

SOME ASPECTS OF HEAT-EXCHANGER TUBE DAMPING IN TWO-PHASE MIXTURES

Two-phase flow exists in many shell-and-tube heat exchangers. To avoid excessive flow-induced vibration, it is necessary to have information on damping. A simple experiment was undertaken to study the effect of several parameters such as void fraction, surface tension, tube frequency and confinement on damping in two-phase mixtures. The experiment consisted of a cantilevered tube immersed in a two-phase mixture generated by bubbling air through water. It is found that void fraction and flow regime are dominant parameters. Surface tension is also important. The results are presented in detail in the paper.     

AN EXPERIMENTAL STUDY ON THE FLUIDELASTIC FORCES ACTING ON A SQUARE TUBE BUNDLE IN TWO-PHASE CROSS-FLOW

A tube in a square tube bundle of P/D=1·42 was oscillated in the lift direction in air–water two-phase cross-flow, and fluidelastic forces acting on the oscillated tube were measured. First, the tube amplitude was fixed to 3 mm (=0·136 D), and added mass, damping, and stiffness coefficients were obtained as a function of two-phase mixture characteristics such as nondimensional gap velocity and void fraction. When reference mixture density and velocity were estimated, the drift–flux model, in which the relative velocity between the gas and liquid phases was estimated, generated better results than the homogeneous model. The added mass coefficient was obtained from quiescent two-phase flow as a function of void fraction. Using the added mass coefficient, the added stiffness coefficient converged to zero with decreasing nondimensional gap velocity. This overcame the contradiction in the added stiffness estimation without added mass, in which the added stiffness coefficient did not converge to zero with decreasing nondimensional gap velocity. Next, the effects of the vibration amplitude on the fluidelastic force coefficients were considered. When the tube amplitude was 3 mm (=0·136 D) or less, the equivalent added stiffness and damping coefficients were almost constant and nonlinearity was small. This showed the validity of the fluidelastic force coefficients obtained based on the data of amplitude of 3 mm. The linearity did not exist when the tube displacement amplitude was 4·5 mm (=0·205 D) or more; a remarkable nonlinearity appeared in the equivalent added damping coefficient. A method to estimate the limit-cycle amplitude of the fluidelastic vibration was proposed when only one tube in the tube bundle was able to vibrate in the lift direction. The amplitude could be obtained from the amplitude at which the equivalent added damping coefficient changed from negative to positive with increase in the tube amplitude.     

FLUIDELASTIC ANALYSIS OF TUBE BUNDLE VIBRATION IN CROSS-FLOW

Tube bundles in cross-flow vibrate in response to motion-induced fluid-dynamic forces; hence, the resultant motions are considered to be a fluidelastic vibration. The characteristics of the vibration depend greatly on the features of the fluid-dynamic forces and the structure of the tube bundle. Therefore, in this study, the equations of motion of the tube bundle are derived. From the viewpoint of vibration, each tube is not independent of the surrounding tubes because its vibration is affected by fluid-dynamic coupling with the neighboring tubes. Thus, the equations are a set of coupled equations and the solution is obtained as an eigenvalue problem. The fluid-dynamic forces, which are indispensable in the calculation, have been obtained by experiments using a vibrating tube in the bundle; it was found that the forces depend strongly on the reduced velocity. Using these equations and the fluid forces, critical velocities of the tube bundle vibration are calculated, and it is found that the critical velocity is strongly dependent on the fluid-dynamic force characteristics, as they vary with the reduced velocity. Vibration tests of the tube bundle have also been conducted, and the critical velocities obtained in the tests are compared with the calculated values; agreement with the calculated values is good, demonstrating that the method of calculation is useful. The effects of mass ratio, frequency deviation and damping deviation of tubes in the bundle on the critical velocity are also examined theoretically. It is found that it is better to treat the mass ratio and the logarithmic decrement separately when the mass ratio is less than 10. Differences in natural frequencies make the critical velocity large. Similarly, differences in logarithmic decrement may distribute the vibration energy to other tubes and make the critical velocity large.     

竖直管道中固-液两相流动流态化实验探讨

主要探讨竖直管道中不同颗粒级配、流体流速条件下的固-液两相流动的流态化规律.首先通过量纲分析获得关键控制参数,然后以玻璃珠(粒径:0.25mm~2.0mm)、粉细砂(d10=0:044mm)两种固体和水为实验介质,开展了两相流动流态化实验,考虑流体流速(相对于管道雷诺数介于640~3300之间)和颗粒级配的影响.通过分析发现:具有均匀粒径分布的玻璃珠床,床层膨胀高度随流速的增加而增加,流速与浓度的对数呈线性关系,与Richardson-Zaki公式一致;具有较宽粒径分布的粉细砂,细颗粒随水流逐渐流出管道,剩余颗粒质量与雷诺数呈指数递减趋势.     

ON THE ADDED MASS AND DAMPING OF PERIODIC ARRAYS OF FULLY OR PARTIALLY POROUS DISKS

Infinite and periodic arrays of porous disks are considered, in oscillatory flow perpendicular to their planes. This configuration is of interest for the offshore oil industry, for instance to be used as dampers to reduce the heave motion of truss SPAR platforms. The hydrodynamic problem is solved by the method of matched eigenfunction expansions, under the assumptions that losses of head, proportional to the square of the traversing velocities, occur at the disks, and that potential flow theory is applicable. Added mass and damping coefficients are derived, as functions of the relative spacings between the disks, and of the parameter (A/a)(1−τ)/(2 μτ²), where A is the flow motion amplitude, a the disks radii, τ the porosity, or open-area ratio, andμ a discharge coefficient, close to 0.5. Results are also given for disks that are partially porous, from their axis to some radial distance. The practical applicability of the obtained results is discussed.     

换热器传热管在静水中的挤压膜阻尼研究

本文通过求解三维Navier-Stokes方程,导出计算挤压膜阻尼的公式,对Mulcahy所提出的模型作了改进,并提出了stokes数的间隙修正因子的概念. 改进后的挤压膜阻尼模型计及管子振幅的影响,在管子振幅很小的情况下,它与Mulcahy提出的模型相似,实际上可把Mulcahy提出的模型看成改进后模型的一个特例. 为了验证改进后的模型,对传热管在空气和静水介质中的阻尼进行了实验研究.结果表明理论计算值与实验测量值符合良好.     

高层框筒和筒中筒结构的整体稳定分析

本文将有限元法用于高层筒 体结构的整体稳定分析。导出了截面含双对称轴的矩形筒的筒单元刚度矩阵和单元几何刚度矩阵。可用于框筒和筒中筒结构的整体稳定计算。计算结果验证了本文方法的精度。     

应用耗散结构理论研究两相流动不稳定性的探讨

本文论证指出:两相流动动力学不稳定性是一种耗散结构。这一结论将非线性非平衡态热力学引入了两相流动不稳定性研究之中,从而阐明了不稳定性发生的原因和机理,并且找到了两相流动不稳定性的直接判据——超熵(1/2δ~2s)(它是非线性非平衡系统的Lyapounov函数)。本文得出了线性非平衡区的判别式和非线性区的一些初步结果,并阐述了在实际应用中对两相流动不稳定性进行判别的具体步骤及本文方法的优越性。     

微重力气液两相流动与池沸腾传热

综述了近年来中国科学院微重力重点实验室(国家微重力实验室)完成的一系列微重力气液两相流动与池沸腾传热方面的地基实验、飞行实验和理论研究等方面获得的主要成果.在微重力气液两相流动方面,提出了半理论Weber数模型用于预测微重力条件下气液两相弹-环状流转换,并采用Monte Carlo方法,针对气泡初始尺寸对泡-弹状流转换的影响进行数值研究.通过俄罗斯"和平号"空间站与IL-76失重飞机实验,获得了微重力下的气液两相流型图,与此同时在地面利用小尺度毛细管模型模拟了微重力气液两相流动特征.实验测量了微重力气液两相流压降,并基于微重力流动特性建立了一个泡状流压降关联模型.在微重力池沸腾传热方面,利用我国返回式卫星完成了两次空间实验,其中,第22颗返回式卫星搭载铂丝表面R113池沸腾实验采用控制温度的稳态加热方式,而实践8号育种卫星搭载平面FC-72池沸腾实验则采用控制加热电压的准稳态加热方式.同时,还进行了地面常重力和落塔短时微重力条件下的对比实验研究.观察到丝状加热表面微重力时轻微的传热强化现象,而平板加热表面微重力核态池沸腾低热流时传热强化、高热流时传热恶化.微重力实验中观察到气泡脱落前存在横向运动现象,据此分析了气泡行为与传热之间关系,并提出了一个预测丝状加热表面气泡脱落直径的半理论模型.旨在对相关领域的进一步发展和空间两相流系统的应用提供数据及理论支持.     

T型微通道反应器内气液两相流动机制及影响因素

基于液滴或气泡的多相微流控是近年来微流控技术中快速发展的重要分支之一.本文利用高速显微摄影技术和数字图像处理技术对T型微通道反应器内气液两相流动机制及影响因素进行实验研究.实验采用添加表面活性剂的海藻酸钠水溶液作为液相,空气作为气相.研究T型微通道反应器内气液两相流型的转变过程,并根据微通道内气泡的生成频率和生成气泡的长径比对气泡流进行分类.研究发现当前的进料方式下,可以观测到气泡流和分层流2种流型,且依据气泡生成频率和微通道内气泡的长径比可将气泡流划分为分散气泡流、短弹状气泡流和长弹状气泡流3种类型,并基于受力分析确定3种气泡流的形成机制分别为剪切机制、剪切-挤压机制和挤压机制.考察不同液相黏度和表面张力系数对不同类型气泡流范围的影响规律.结果表明:液相黏度相较于表面张力系数而言,对气泡流生成范围影响更大.给出不同类型气泡流流型转变条件的无量纲关系式,实现微通道生成微气泡过程的可控操作.      

SKI VIBRATIONS AND DAMPING

Skis and runners sliding on snow like to vibrate. Soft snows tend to dampen ski vibrations but icy surfaces often set up severe oscillations. A feedback situation, such as driving on a gravel road, may also exist since vibrating skis create washboard conditions. Skis are subject to a range of forcing functions from low frequencies in big moguls (bumps) to 200–300 Hz frequencies at high speeds on icy surfaces. Analysis is complicated by random inputs, nonprismatic and anisotropic structures, and varying suspension systems.     

采用压电和粘弹性材料的杂交控制的研究进展

杂交阻尼控制技术是许多工程结构减振和改善性能的重要方法之一.本文主要从杂交阻尼结构的几个特征方面:阻尼结构形式、建模假设、黏弹性材料建模、响应分析方法、控制方法、参数研究和结构选择来研究杂交阻尼结构的研究进展和主要研究成果.研究发现绝大多数的文献只局限于研究梁板的问题.因此很有必要加强对壳体等复杂结构的研究.最后文中指出了有关杂交阻尼结构继续发展的有待深入研究的几个方面.图0参72     

半浮区热毛细对流的不稳定性与转捩

近二十年来,微重力流体开展了半浮区液桥热毛细对流的不稳定性与转捩的研究．文中给出了热毛细振荡对流发生的临界参数,分析了液桥几何位形（尺度比,体积比）、物理参数及传热参数对临界Ｍａｘａｎｇｏｎｉ的影响．报导了有关的地面模拟实验,微重力实验以及本问题的线性稳定性分析、能量分析和数值模拟结果,并介绍了定常轴对称热毛细对流通过非定常振荡热毛细对流到湍流的转捩过程和三种热毛细振荡对流的产生机理．     

软土基坑变形失稳形态模拟试验研究

通过不同土性、地下水条件下软土基坑开挖变形失稳的模拟试验, 研究了软土基坑开挖变形发展直至失稳破坏的全过程。通过试验研究, 初步认为软土基坑坑壁在无支护或支护刚度较小的情况下, 其坑壁破坏形态呈抛物线型; 在基坑开挖范围内若存在有砂性土, 且地下水位较高时, 易于发生流砂渗透破坏, 并导致了地表沉陷, 但侧向变形相对较小; 而对于因承压水引起的坑底土体隆起变形, 若不考虑土体的强度特性, 计算结果是偏于安全的。     

混合层中的流向涡与Rayleigh离心不稳定

用有限差分方法求解三维Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程和连续性方程，对时间发展的平面混合层中流向涡的产生原因进行了分析．将Ｒａｙｌｅｉｇｈ的轴对称无粘离心不稳定理论推广应用于分析混合层的二维基本流，并导出一无量纲量Ｒａｙ＝－（ｒ／νθ）νθ／ｒ，其中νθ为一流体质点相对于平均速度的速度，ｒ为该质点迹线的曲率半径．当Ｒａｙ＞１．０时就会发生离心不稳定．采用这一判别式后发现混合层中展向涡的周围，尤其是在辫带区，的确存在离心不稳定区域，而过去的实验结果也表明三维不稳定产生于展向涡之间的辫带区．因此有理由认为，除非雷诺数特别小，离心不稳定是流向涡产生和发展的主要原因     

聚苯胺体系电流变流体的阻尼机理及阻尼模型研究

对聚苯胺体系电流变流体在正弦信号作用下的阻尼力响应及其频谱特性进行了理论和实验研究.提出电流变流体的非线性阻尼力的频谱是由基频和频率为基频奇数倍的高次谐波分量组成.建立了由粘性阻尼和迟滞阻尼组成的电流变流体阻尼模型,通过对模型的傅立叶变换及数值仿真,验证了理论分析及阻尼模型的正确性.     

平纹织物复合材料的振动能量耗散以及阻尼分析

平纹织物复合材料由于编织结构的特点，对于振动能量有一定的衰减作用．交织纤维束相互间的阻滞作用以及聚合物基体的粘弹性质都会耗损一部分振动能量．本文应用能量原理和阻尼分析的方法解决了平纹织物复合材料的复杂结构损耗因子的计算问题．     

FLOW PERIODICITY AND ACOUSTIC RESONANCE IN PARALLEL TRIANGLE TUBE BUNDLES

Parallel triangle tube arrays with pitch ratios in the range of 1·2–4·2 have been tested at Reynolds numbers up to 9×10⁴to investigate the vorticity shedding and acoustic resonance mechanisms. Three different components of flow periodicities have been observed. The flow periodicity with the highest Strouhal number (S₃) is the weakest component. It is caused by a shear-layer instability in small pitch ratio arrays and at low Reynolds numbers. The S₂component is associated with small-scale vortex shedding at the first row. The third component has the lowest Strouhal number (S₁) and is the strongest. It is generated by large-scale, alternating vortex shedding at deeper rows, and it becomes dominant at all rows at high Reynolds numbers. For tube arrays with pitch ratios less than 3·4, the onset of acoustic resonances could not be related to the natural flow periodicities mentioned above. This behaviour is in contrast with that of normal triangle arrays, but similar to the acoustic behaviour of in-line arrays. Strouhal number charts for the natural flow periodicities and for the onset of acoustic resonances have been developed.