ANALYSIS OF NONLINEAR DYNAMIC STABILITY OF LIQUID-CONVEYING PIPES

Nonlinearly dynamic stability of flexible liquid-conveying pipe in fluid structure interaction was analyzed by using modal disassembling technique. The effects of Poisson, Junction and Friction couplings in the wave-flowing-vibration system on the pipe dynamic stability were included in the analytical model constituted by four nonlinear differential equations. An analyzing example of cantilevered pipe was done to illustrate the dynamic stability,characteristics on the pipe in the full coupling mechanisms, and the phase curves related to the first four modal motions were drawn. The results show that the dynamic stable characteristics of the pipe are very complicated in the complete coupling mechanisms, and the kinds of the singularity points corresponding to the various modal motions are different.

Exact Solution of Unsteady Flow of Viscoelastic Fluid in a Pipe with Fractional Maxwell Model

The unsteady flow of viscoelastic fluid in a cylindrical pipe was investigated using the fractional Maxwell model. Two special cases of unsteady pipe flow were expressed. The first is start-up flow, and the second is oscillating flow. The exact solution of start-up flow under a constant pressure gradient was obtained by using the theories of Laplace transform and Fourier-Bessel series for fractional derivatives. The exact solution of oscillating flow was obtained by utilizing the separation of variables.

New insight into the stability and dynamics of fluid-conveying supported pipes with small geometric imperfections

In several previous studies,it was reported that a supported pipe with small geometric imperfections would lose stability when the internal flow velocity became sufficiently high.Recently,however,it has become clear that this conclusion may be at best incomplete.A reevaluation of the problem is undertaken here by essentially considering the flow-induced static deformation of a pipe.With the aid of the absolute nodal coordinate formulation(ANCF)and the extended Lagrange equations for dynamical systems containing non-material volumes,the nonlinear governing equations of a pipe with three different geometric imperfections are introduced and formulated.Based on extensive numerical calculations,the static equilibrium configuration,the stability,and the nonlinear dynamics of the considered pipe system are determined and analyzed.The results show that for a supported pipe with the geometric imperfection of a half sinusoidal wave,the dynamical system could not lose stability even if the flow velocity reaches an extremely high value of 40.However,for a supported pipe with the geometric imperfection of one or one and a half sinusoidal waves,the first-mode buckling instability would take place at high flow velocity.Moreover,based on a further parametric analysis,the effects of the amplitude of the geometric imperfection and the aspect ratio of the pipe on the static deformation,the critical flow velocity for buckling instability,and the nonlinear responses of the supported pipes with geometric imperfections are analyzed.

Dynamical Stability of Cantilevered Pipe Conveying Fluid with Inerter-Based Dynamic Vibration Absorber

Cantilevered pipe conveying fluid may become unstable and flutter instability would occur when the velocity of the fluid flow in the pipe exceeds a critical value.In the present study,the theoretical model of a cantilevered fluid-conveying pipe attached by an inerter-based dynamic vibration absorber(IDVA)is proposed and the stability of this dynamical system is explored.Based on linear governing equations of the pipe and the IDVA,the effects of damping coefficient,weight,inerter,location and spring stiffness of the IDVAon the critical flow velocities of the pipe system is examined.It is shown that the stability of the pipe may be significantly affected by the IDVA.In many cases,the stability of the cantilevered pipe can be enhanced by designing the parameter values of the IDVA.By solving nonlinear governing equations of the dynamical system,the nonlinear oscillations of the pipe with IDVA for sufficiently high flow velocity beyond the critical value are determined,showing that the oscillation amplitudes of the pipe can also be suppressed to some extent with a suitable design of the IDVA.

Characterizing pressure fluctuation into single-loop oscillating heat pipe

The pressure characteristics inside single loop oscillating heat pipe(OHP) having 4.5 mm inner diameter copper tube with the loop height of 440 mm were addressed.Distilled water was used as working fluid inside the OHP with different filling ratios of 40%,60% and 80% of total inside volume.Experimental results show that the thermal characteristics are significantly inter-related with pressure fluctuations as well as pressure frequency.And the pressure frequency also depends upon the evaporator temperature that is maintained in the range of 60?96 °C.Piezoresistive absolute pressure sensor(Model-Kistler 4045A5) was used to take data.The investigation shows that the filling ratio of 60% gives the highest inside pressure magnitude at maximum number of pressure frequency at any of set evaporator temperature and the lowest heat flow resistance is achieved at 60% filling ratio.

Analysis of Heat Transfer of a Flat Plate Oscillating Heat Pipe, for Different Surfaces

对更大的力量密度和电子系统的更小的尺寸的最近、经常的要求刺激了象热管子那样的不同的被动的热转移方法的新图案的生长。特别地， OHP (震荡的热管子) 是相对新奇的设备，能够在长、短的距离上移开高热率与不温度落下。这研究专注于设计，造并且装配一个测试钻塔以便完全分析通过 5 拐弯的 deionised 水的流动模式板震荡在不同的热输入下面的热管子，它被二精力 M.Sc 在工程的学院和玛丽女王伦敦大学的材料科学里做。学生们。水的充满的比率是 40% 。而且， OHP 热表演上的试验性的研究被执行以便检验不同表面的效果湿条件：超级吸水的、吸水、清洗的黄铜。它被表明液体蛞蝓的形成和沿着隧道的水的蒸汽塞子。试验性的结果证明吸水的表面趋于是更多的精力有效。热转移表演超级吸水、吸水分别地比由 5-12% 和 15-20% 的黄铜高。

定常振荡流施肥模式对灌溉管道混合均匀性的影响

【目的】探究柱塞泵提供的定常振荡注肥模式对灌溉管网首部主干管道水肥混合均匀性的影响,寻求不同运行工况下分干管安全布设距离。【方法】采用室内试验与数值仿真相结合的方法,分析相同工况下管道内肥液质量分数,利用数值仿真方法探究温度、管径、主管流量以及注肥比对水肥混合均匀性影响。【结果】室内试验与数值仿真所得出的水肥混合均匀性规律吻合,随着混合距离的增加混合均匀性逐渐增大。其他工况相同时,当主管管径从63mm增大到110mm,有效混合距离平均增加了0.769m;温度从10℃增加到30℃时,主管流量为6、8 m^(3)/h处理的有效混合距离平均下降了0.499 m和1.681 mm;主管流量从6 m^(3)/h增加到8 m^(3)/h时,63、90、110 mm管径处理的有效混合距离平均增加了1.598、1.064、1.132 m;注肥比从2%增加到4%时,63、90、110 mm管径处理的有效混合距离平均减少了0.534、0.824、0.826m。【结论】水肥混合均匀所需的有效混合距离随管径与主管流量的增加而增加,随温度与注肥比的增加而减少;采用63、90、110mm的灌溉主管,分干管的开口位置应距注肥口5~5.5 m处。

材料特性对输送脉动流体的粘弹性轴向功能梯度管非线性动力行为的影响

The nonlinear dynamic behaviors of viscoelastic axially functionally graded material(AFG)pipes conveying pulsating internal flow are very complex.And the dynamic behavior will induce the failure of the pipes,and research of vibration and stability of pipes becomes a major concern.Considering that the elastic modulus,density,and coefficient of viscoelastic damping of the pipe material vary along the axial direction,the transverse vibration equation of the viscoelastic AFG pipe conveying pulsating fluid is established based on the Euler-Bernoulli beam theory.The generalized integral transform technique(GITT)is used to transform the governing fourth-order partial differential equation into a nonlinear system of fourth-order ordinary differential equations in time.The time domain diagram,phase portraits,Poincarémap and power spectra diagram at different dimensionless pulsation frequencies,are discussed in detail,showing the characteristics of chaotic,periodic,and quasi-periodic motion.The results show that the distributions of the elastic modulus,density,and coefficient of viscoelastic damping have significant effects on the nonlinear dynamic behavior of the viscoelastic AFG pipes.With the increase of the material property coefficient k,the transition between chaotic,periodic,and quasi-periodic motion occurs,especially in the high-frequency region of the flow pulsation.

Stability and nonlinear vibrations of a flexible pipe parametrically excited by an internal varying flow density

Pipes are often used to transport multiphase flows in many engineering applications.The total fluid flow density inside a pipe may vary with time and space.In this paper,a simply supported pipe conveying a variable density flow is modeled theoretically,and its stability and nonlinear vibrations are investigated in detail.The variation of the flow density is simulated using a mathematical function.The equation governing the vibration of the pipe is derived according to Euler-Bernoulli beam theory.When the internal flow density varies with time,the pipe is excited parametrically.The stability of the pipe is determined by Floquet theory.Some simple parametric and combination resonances are determined.For a higher mass ratio(mean flow mass/pipe structural mass),higher flow velocity,or smaller end axial tension,the pipe becomes unstable more easily due to wider parametric resonance regions.In the subcritical flow velocity regime,the vibrations of the pipe are periodic and quasiperiodic for simple and combination resonances,respectively.However,in the supercritical regime,the vibrations of the pipe exhibit much richer dynamics including periodic,multiperiodic,quasiperiodic,and chaotic behaviors.

Visualization Investigation on Temperature Oscillation and Two-Phase Behaviors of A Flat Loop Heat Pipe

To better analyzing the temperature oscillation and the two-phase behavior inside a flat loop heat pipe,visual studies were conducted.Under the 20℃ water cooling and horizontal orientation,the effects of the filling ratio and heat loads on the temperature oscillation were analyzed.Based on the experimental data,the results indicate that owing to the increased system pressure,the temperature oscillation decays as the filling ratio increases from 34%to 58%.Meanwhile,during the startup process,temperature oscillation tends to occur during the boiling and steady stages due to the more violent two-phase behavior,while the temperature curves are smooth during the slow evaporation stage.Moreover,as the heat load increases,the evaporation becomes more intense at the active zone of evaporator,leading to a faster startup process and a higher oscillation frequency.Besides,owing to the synergistic effect of two-phase flow in the compensation chamber caused by heat leak and subcooled liquid backflowing,a“breathing”oscillation behavior of the vapor-liquid interface is observed at the compensation chamber,which further leads to the unstable operation behavior of the loop heat pipe system.

开口圆管气体的非线性振荡研究及雾化应用

主要研究开口圆管内的非线性振荡。圆管受迫端由一活塞驱动，活塞为小谐波振荡，另一端为完全开口状态。理论和实验均发现存在非线性现象。实验中还发现活塞激振频率和管内气体的固有频率接近时会发生共振，有强烈的非线性效应。同时在管口附近产生强烈的整流效应。发现利用此效应对水流有较好的雾化功能。

振荡热管内的振荡及传热传质特性

通过分析振荡热管内部气液两相系统的受力和传热传质过程,对振荡热管进行了合理的简化和假设,建立了详细的理论模型.以n型单液塞振荡热管为例,采用显示差分法求解建立的控制方程,并预测传热传质特性和液塞的振荡行为,结果显示:液塞在毛细管路内的运动既有宏观振荡又有局部振荡,振荡的幅度和频率在增大和减小中交替变化,没有固定的周期;液塞两侧气泡间的压差是其振荡的主要驱动力,当振荡热管非水平工作时,需考虑重力的影响,与其他力相比,毛细阻力对液塞振荡的影响可以忽略;液膜短时间内的干涸是诱发新一轮振荡以及强化传热的源动力.

微槽道脉动热管的启动及传热特性

对竖直和水平放置情况下微槽道脉动热管（当量直径2.82 mm）的启动及传热性能进行了实验研究,并与内径分别为3.4 mm（1~#）、4.0 mm（2~#）和4.8 mm（3~#）的3个光管脉动热管进行了比较。实验工质为去离子水,充液率为50%。实验结果表明,竖直放置（底部加热）时,微槽道结构可以显著降低脉动热管的最小启动功率和启动温度,在约305 W的加热功率下其热阻分别比1#、2#和3#光管脉动热管下降41.7%、35.6%和30.9%,蒸发段壁面平均温度分别下降12.1、11.8和7.6℃;水平放置时,微槽道脉动热管在一定加热功率下能够正常启动,光管脉动热管难以有效运行。使用微槽道结构后,脉动热管显热和潜热传热能力的提高以及微槽道毛细作用利于冷凝液向蒸发段回流可认为是实现热管传热强化的主要原因。

基于绝对节点坐标法的输流管道非线性动力学分析

基于绝对节点坐标法,建立一种新的一维二节点输流管道单元。应用Irschik提出的适用于含非材料体系统的Lagrange方程推导输流管道单元的运动方程。采用Euler梁来模拟管道,并完全采用非线性Green应变张量和第二Piola Kirchhoff应力张量,没有任何量级近似,也没有假设悬臂输流管道的轴线不可伸长,并考虑材料的泊松效应对流速的影响,因此通过该方法得到的运动方程比传统的通过量级近似得到的输流管道的运动方程更合理。通过数值计算,分析不同边界条件下的输流管道非线性行为,并与经典输流管道运动方程的计算结果比较,结果表明该方法更合理。

表面浸润程度对脉动热管传热性能的影响

为了深入理解表面浸润性对脉动热管传热性能的影响,利用碱性辅助表面氧化技术,通过控制氢氧化钠和过硫酸铵溶液与铜表面反应浓度的变化,制备了具有不同浸润程度表面的脉动热管,其表面接触角分别为0°、33.9°、55.7°、84.3°。通过实验研究了表面浸润程度对脉动热管传热性能的影响。结果显示,随着脉动热管内部浸润性的增强,其脉动频率及幅度增加,脉动热管热阻及温差降低,传热性能得到强化。

我国木材干燥节能减排技术研究现状

介绍了我国木材干燥节能减排技术现状。太阳能干燥技术适于木材预干,太阳能与热泵联合干燥是比较理想的组合干燥方式,其节能率可达70%左右。常规木材干燥的排气余热回收目前常用热泵干燥机,其节能率在40%左右。将华北电大与山东省科学院联合研制的振荡流热管换热器用于排气余热回收,其节能率可达20%左右。这种振荡流热管的突出优点是热管内不需要毛细芯,有利于实现小型化,它可以随意弯曲而且启动迅速;而且与热泵干燥机相比,它投资不太高、不耗能又容易维护管理,因此适于木材干燥行业用于余热回收,同时也可以用于其它工业余热的回收。

脉冲加热对振荡流热管传热性能的影响

本文采用脉冲热源替代常规连续热源,通过实验观察加热功率、脉冲宽度及脉冲间隔对振荡流热管传热性能的影响,并对热管壁面温度信号进行了分析。实验结果表明,在绝大多数情况下,脉冲加热振荡流热管的传输热流量均大于连续加热时。振荡流热管的传热性能受脉冲宽度与脉冲间隔的影响,脉冲加热振荡流热管运行存在一最佳的脉冲加热参数。

振荡流热管自激强化传热的可行性分析

围绕自激振荡流热管的工作特性,联系目前国内外纳米流体及非均匀截面结构在强化传热研究方面的最新进展,分析了在自激振荡流热管内的复杂相变换热条件下,通过采用纳米流体工质与非均匀管截面以实现振荡流热管自激强化传热的可行性,并提出了纳米流体的浓度、相容性、充液率以及管截面结构等需要解决的关键性技术问题。为下一步有针对性的强化传热实验研究和新型自激振荡流热管换热器的设计与开发提供了必要的理论依据。

等截面和变截面通道硅基微型脉动热管传热特性比较

为了解截面结构对微型脉动热管传热性能和内部工质流动特征的影响,采用可视化和温度测量的方法对比研究了等截面和变截面硅基微型脉动热管内的工质运动和传热特性。实验工质为R141b,充液率为40%～60%。实验结果发现,变截面通道结构的设计不仅有利于降低微型脉动热管的热阻,提高其传热能力,同时可有效改善其启动性能。随着充液率的增大,该优势更加明显,当充液率约60%时变截面微型脉动热管的启动功率和启动蒸发段平均温度比等截面热管分别下降约0.4 W和17.3℃。另外,因变截面结构而形成的附加毛细作用可使微型脉动热管内工质发生短暂定向运动,而在等截面热管中则未观察到上述现象。

非线性气体振荡整流效应对翼尖涡的影响

以圆管内气体的非线性振荡理论和实验研究成果为基础 ,利用开口圆管中气体非线性振荡的整流效应 ,通过翼面开缝以及翼尖开口引入气体振荡 ,主要进行翼尖涡控制的实验研究。实验结果表明 ,翼面开缝和翼尖开口引入气体振荡在大迎角时对提高升力系数、增大机翼的稳定性有一定作用。同时 ,翼尖开口引入气体振荡能较好改善翼尖涡的位置和强度。对比翼面开缝以及无气体激振状态 ,翼尖涡在翼尖气体振荡条件下向翼尖外部移动了近 3/4个弦长 ,向上翼面方向移动了近