【作者】 姜波；

【导师】 田茂诚；

【作者基本信息】 山东大学 ， 热能工程， 2010， 博士

【摘要】 面对日益严重的能源短缺与环境污染,开发新能源、提高能源利用率已成为国际社会有效缓解这一双重压力的重要途径,与之相关的理论与方法必将继续成.为研究的热点。振动强化传热因其良好的效果曾在上世纪五六十年代引起了广泛的关注,但因难以在换热器内实现振动而停止研究。随着流体诱导振动传热元件的提出,实现振动的方法有了变革性的拓展,这一课题也在逐渐兴起。本文从理论分析、数值模拟与实验研究三方面对振动强化传热的机理进行了较为细致的分析,提出了流体诱导振动强化传热元件——新型弹性管束,并对其固有振动特性、传热特性进行了系统研究。建立了振动管外流动传热的CFD动网格模型,得到了1／4周期内不同时相位的管外速度矢量图。与无振动工况对比,振动能够增加管壁与近壁区流体的相对速度,能够在平行振动方向上的管壁两侧形成有效地冲刷。得到了有、无振动工况的温度场分布图,对比两者可以发现在相同的传热温差下振动工况的热边界层薄、温度梯度大,说明振动能够有效强化传热。提出了有效速度的概念,并采用该参数作为强化传热性能优劣的评价指标。计算了振动圆管近壁区流体在不同时相位、面相位的场协同角余弦值及有效速度值。在每一个确定的时相位下,面相位β=0-360°区间内其协同角余弦值与有效速度值均存在两个变化周期。其中,两峰值分别对应于平行振动方向的管壁左右两侧,两谷值则对应于垂直振动方向的上下两侧。在1／4周期内,随着时相位的升高,其管外平均场协同角余弦值及有效速度值均逐渐增加。计算了不同管型在不同振动频率、振幅下的管外平均场协同角余弦值及有效速度值。对于同一管型与振幅,随着频率的升高,其管外平均场协同角余弦值变化较小,但其管外流体的有效速度值呈近似线性增加。在相同振动频率与振幅下,沿椭圆长轴、圆管径向及椭圆短轴方向振动的管外平均场协同角余弦值、有效速度值均为依次增加,证明除频率和振幅等参数外,管型也是影响换热性能的重要因素。在半周期内,时相位为90°时振动管外的有效速度值最大。建立了振动管外流场的PIV实验台,对不同管型的振动管外流场进行了可视化实验研究,得到了不同工况下的管外流场速度矢量图,其结果与同参数的数值模拟结果吻合较好。建立了单管振动传热实验台,对实验装置与测试系统的可靠性进行了系统分析。得到了不同振幅、频率及振动管型下的表面传热系数,总结了三因素对管外换热性能的影响规律。在相同振动频率与振幅下,沿椭圆长轴、圆管径向及椭圆短轴方向振动的换热性能依次增强,采用三因素三水平正交表对实验数据进行极差及方差分析,结果证明除频率和振幅等参数外,管型也是影响换热性能的重要因素,与数值模拟结果吻合。同时,得到了表面传热系数和振动参数之间的拟合关联式。对实验中出现的共振现象进行了分析讨论,共振能够引起表面传热系数成倍提高,在实验装置可靠性允许的范围内,共振有利于强化传热。提出了一种新型的弹性管束,与原管束相比,该管束与立柱相连接的端部所受弯矩小于原管束的1／6,受力特性有了较大的改善；同时单位容积的换热面积约增加24.7%。对新型弹性管束进行模态实验研究,结果表明,新型弹性管束的振型较为复杂,为面内、面外振动相结合的三维振动：与原管束相比,其固有频率相对较低,并且其中间2#、3#两弹性管的振动较其它两根强烈,原因是其有着相对自由的边界条件。对新型弹性管束进行有限元分析,模拟结果与模态实验结果吻合较好,证明本文采用的实验模拟方法具有较高的可靠性。建立了新型弹性管束换热器电加热恒热流传热实验台,对实验装置和测试系统进行了可靠性验证。在管外流体诱导振动条件下,弹性管束的管外平均表面传热系数基本为固定管束的3倍以上,强化传热效果显著。设计了电机驱动及流体诱导脉动装置,得到了新型弹性管束在不同流体脉动频率下的管外平均表面传热系数,并对各脉动工况下的流动阻力及综合传热性能进行分析,得出低频脉动工况有利于强化传热。得到了不同布置方式下弹性管束的管外平均表面传热系数,通过比较可以发现,大多数工况下,管束换热效果的优劣顺序为：两侧分布-错排>单侧分布-错排>单侧分布-顺排。同时,拟合得到了不同工况下的实验关联式,与实验数据对比最大误差小于5.0%。得到了新型弹性管束的局部表面传热系数,中间两根弹性管的传热系数要明显高于边缘两管,将四根弹性管各位置所测传热系数平均可以得到四管换热性能的强弱次序,即2#>3#>1#>4#。其中,中间两弹性管小自由端的管外表面传热系数要略高于换热元件的其它位置。建立了新型弹性管束换热器水-水、汽-水传热实验台,对两种条件下的新型弹性管束进行了传热实验研究,并对弹性管束管内、管外传热系数进行分离。在新型弹性管束管内流体入口安装了脉动装置,得到了水-水换热条件下不同流量、脉动频率的弹性管束管内外表面传热系数、传热系数。实验结果表明,该工况下管内流体脉动对弹性管束的传热特性基本无影响。水-水与汽-水换热条件下,弹性管束的管外表面传热系数与恒热流条件相比有较为明显的提高,其中,汽-水换热条件下提高最大,原因为管内介质能够在很大程度上改变弹性管束的振动特性、从而强化传热。建立了流固耦合(FSI)简化模型,计算得出了流向最大形变随流体与结构各参数的变化规律曲线。通过极差分析得出了流体与结构参数对结构流向形变的影响大小,其中管长、流体速度、管厚与流体密度为影响结构形变的主要因素。通过数据拟合得到了最大流向形变与流体、结构参数间的计算关联式,与计算结果的平均误差为8.2%。同时,可以通过合理选择各参数的具体数值来有效控制结构形变的大小,为工程设备的运行设计提供参考依据。更多还原

【Abstract】 In face of increasingly serious energy shortage and environmental pollution, to develop new energy sources and to improve energy utilization efficiency have been internationally recognized as important approaches to solve the problem. Theories and methods on this topic have been research focus in the field of energy and power engineering. In the 1950s and 1960s much attention was put to vibration enhanced heat transfer, which was suspended by the difficulty in implement of controllable vibration within heat exchangers. However, with the recent emergence of flow-induced vibration heat transfer components, transformative development has appeared for the implementation of vibration, which draws new attention to this topic. This paper conducts detailed research on the mechanism of vibration enhanced heat transfer from the aspects of theorectical analysis, numerical simulation and experimental study. A flow-induced vibration enhanced heat transfer component—a new type of elastic tube bundle is put forward, and systematic study is carried out on its inherent vibration, heat transfer and fluid-structure interaction characteristics.In this paper, CFD moving mesh model is established on the characteristics of the flow and heat transfer outside the vibrating tube and velocity vector chart outside the tube at different time phase is obtained during a quarter cycle. Compared with vibration-free conditions, it is found that vibration can enhance the relative velocity of fluid in near-wall region and can form effective erosion on the both sides of the wall in the direction parallel to vibration. By comparing the temperature distribution of vibration and vibration-free condition it is found that, with the same heat transfer temperature difference, the thinner thermal boundary layer and the greater temperature gradient occur under vibration, showing that the vibration can enhance heat transfer.The concept of effective velocity is put forward, which is regarded as the evaluation index of heat transfer performance. Then cosine of the field synergy angle and effective velocity at different time phase and surface phase are calculated near the wall of the vibrating tube.The results show that at every certain time-phase in the surface phase intervalβ=0-360°, there exists two variation periods for both cosine of the field synergy angle and the effective velocity, during which two peak values respectively appear at the left and right side of the direction parallel to vibration while two valley values at upside and downside of the direction vertical to vibration. In a quarter period, cosine of the average field synergy angle outside the tube and effective velocity increase with the time-phase goes up.Effective velocity and cosine of the average field synergy angle outside the tube under different vibration frequencies and amplitudes are calculated in this paper. For the same tube type under the same amplitude, cosine of average field synergy angle outside the tube varies little with frequency, while the effective velocity of fluid outside the tube varies approximately linearly. Under the same vibration frequency and amplitude, cosine of average field synergy angle and effective velocity both goes up in the order following:elliptic tube vibrating in the long-axis direction, tube vibrating in diameter direction and elliptic tube vibrating in the short-axis direction, showing that the tube type is also an important factor on heat transfer besides frequency and amplitude. In a half period, the effective velocity reaches the maximum when the time phase is 90°.PIV test bench is set up for visualization research on flow field outside different types of vibrating tube. Flow field vector charts outside tubes under different conditions are obtained, the results of which are in moderate agreement with that of numerical simulation.Single-tube vibration heat transfer experiment bench is set up and reliability analysis is carried out on the instruments and test system. The convective heat transfer coefficient is obtained under different amplitudes, frequencies and the vibration tube type, and the influence rules of the three factors on heat transfer outside the tube are summarized. Under the same vibration frequency and amplitude, the heat transfer performance improves in the order following:elliptic tube vibrating in the long-axis direction, tube vibrating in diameter direction and elliptic tube vibrating in the short-axis direction. Adopting the range method and variance method, it is shown that the tube shape is also an important factor on heat transfer and is consistent with the simulation results. Meanwhile, the fitting formula of vibration parameters for convective heat transfer coefficient is obtained. Analysis is carried out on resonance phenomenon in the experiment, which can increase the convective heat transfer coefficient by times. Within permissible range of experimental instruments credibility, resonance enhances heat transfer.A new elastic tube bundle is presented in this paper. Compared with the original one, the load conditon of the new bundle improves moderately. The heat transfer area per unit volume increases by 24.7%. Modal analysis on the new elastic tube bundle shows that its vibration mode is more complex, which is three-dimensional combined with in-plane and lateral vibration. Compared with the original one, the new bundle has lower natural frequency and the vibration of 2# and 3# tube of the new bundle is stronger than the other two tubes, the reason for which is that the intermediate two tubes possess a relatively free boundary condition. Finite element analysis on the new bundle reveals that simulation results are moderately consistent with experiment data, showing that the methods adopted in experiment and simulation have good reliability.The constant heat flux electric-heated heat transfer experiment bench for the new elastic-bundle heat exchanger is set up and reliability verification of the instruments and test system is carried out. Under the condition of flow-induced vibration outside the tube, the average convective heat transfer coefficient of the new bundle reaches more than three times as high as that of fixed one, which means remarkable improvement of heat transfer performance.Electric motor driven and flow induced pulsation devices are designed, and average convective heat transfer coefficients outside the tube of the new bundle at different frequency are obtained. Analysis on flow resistance and comprehensive heat transfer performance under different pulsation conditions is carried out, the results of which show that low frequency pulsation condition is benificial to heat transfer enhancement. Average convective heat transfer coefficients outside the tube of the elastic tube bundle in different arrangement mode are obtained. By comparing the results it is found that in most conditions, the order of heat transfer performance follows:bilateral-distribution-staggered arrangement>unilateral-distribution-staggered arrangement>unilateral-distribution-in-line arrangement. Meanwhile, fitting formulas for different experiment conditions are obtained, the error of which compared with experiment data is lower than 5%. Local convective heat transfer coefficients are obtained and it is found that those of the two intermediate tubes are apparently higher than those of the other two. The average heat transfer performance order can be briefed as follows:2#>3#>1#>4#. Among which the convective heat transfer coefficient of the small free end of the two intermediate tubes is a bit higher than those of other sections.Water-water and steam-water heat transfer experiment benches are set up for the new elastic-bundle heat exchanger and heat transfer experimental study is conducted for the two conditions. The in-tube and external-tube heat transfer coefficients are separated and the experiment results support the reliability of the separation method. The pulsation device is installed at the flow entrance of new elastic tube bundles and the in-tube and external-tube convective heat transfer coefficients under different flow and pulsation frequency are obtained in water-water condition. The results show that pulsation inside the tube has no effect on heat transfer characteristics of the tube bundle. Compared with the constant heat flux condition, the external-tube convective heat transfer coefficient increases significantly under both water-water and steam-water conditions, between which that of steam-water condition is even higher. This shows that the medium in tube exerts important influence on both vibration and heat transfer characteristics.Simplified model for fluid-solid interaction (FSI) is set up to obtain the change curve for flow maximum deformation with other parameters of fluid and structure. By range analysis, the influence order of fluid and structure parameters is obtained, among which the tube length, fluid velocity, tube thickness and fluid density are the main factors on structural deformation. The fitting formula for maximum structural deformation with fluid and structure parameters is obtained, the average error of which with calculation results is 8.2%. Meanwhile, proper selection of specific values of the parameters can make an effective control of structural deformation, which provides beneficial reference for the operation and design of engineering equipment.更多还原