Numerical Analysis of Conjugated Heat and Mass Transfer of Helical Hollow Fiber Membrane Tube Bank for Seawater Distillation

A numerical study on the conjugated heat-mass transfer of helical hollow fiber membrane tube bank(HFMTB)for seawater desalination was carried out.Physical and mathematical models of fluid flow,temperature and humidity distribution were constructed to investigate the influences of flow type,Reynolds number,and temperature on the conjugated heat-mass transfer performance of hollow fibers in the distillation membrane module.The conjugated heat-mass transfer characteristics of HFMTB were discussed by utilizing the friction coefficient,Nusselt number(Nu),and Sherwood number(Sh).Results demonstrate that a distillation efficiency enhancement of 29%compared to the straight HFMTB has been detected for four-helical HFMTB configuration,though the friction coefficient of such a module is about 4 times of their straight counterparts.The values of average Nu and Sh numbers are increasing with tube number,which improves distillation efficiency.The effect of flow type has been studied by employing the upstream and downstream flows to the double-helical HFMTB,demonstrating upstream flow type is more conducive to the heat-mass transfer process.Both the outlet air humidity(ω)and distillation efficiency(η)decrease with the air-side Reynolds number(Rea)and inlet air temperature in the helical HFMTB while increasing with the solution-side Reynolds number(Re_(S))and inlet solution temperature.Overall,the obtained results indicate that helical HFMTB applying upstream flow has great potential to achieve high-performance SGMD for seawater desalination.It is anticipated that the present work can assist in a better understanding of the membrane desalination process in HFMTB and thus provide theoretical suggestions for further optimization and development.

Effect of Arrangement of Tube Banks on Acoustic Resonance

In the present paper the attention is focused on effect of arrangement of tube banks on acoustic resonance which occurred in the two-dimensional model of boiler. We have examined the characteristics of vortex shedding and acoustic resonance generated from in-line and staggered tube banks. At the small tube pitch ratio in in-line tube banks, acoustic resonance of third and fourth mode in the transverse direction occurred. As the tube pitch ratio in the flow direction decreased, the vortex shedding frequency became broad-band. The alternative vortex shed from in-line tube banks. The multiple resonance modes were generated within the broad-band vortex shedding frequency. And the acoustic resonances of lower-order modes occurred at the higher gap velocity. On the other hand, at the small tube pitch ratio in staggered tube banks, acoustic resonance did not occurred, although the vortex shed at the resonance frequency in tube banks. The pressure drop at staggered tube banks was larger than that of in-line tube banks. The symmetric vortices were observed inside staggered tube banks at the small tube pitch ratio.

Numerical modeling and multi-objective optimization of a novel cross-flow heat exchanger with rotated aligned tube bank

A novel cross-flow heat exchanger with a rotated aligned tube bank is designed and utilized in a cement plant. The heat exchanger is numerically modeled with various tube pitches in order to obtain correlations of the shell-side average Nusselt number and friction factor. Then, a multi-objective optimization approach is performed based on the genetic algorithm. The goal of this study is to maximize the heat transfer rate and minimize the pressure drop. Pareto optimal solutions are obtained, which indicate that the increase in the heat transfer rate leads to an increase in pressure drop, and vice versa. In addition, heat exchanger effectiveness, total cost, and the ratio of the heat transfer rate to the fan/pumping power demonstrate different variations with the two objective functions. Several selection criteria are discussed to determine the optimal design and to help designers select an appropriate solution based on actual requirements. Two multi-objective optimization design schemes are compared to the original design under the same heat transfer rate. Results show that pressure drop decreases by 67.9% and 69.7%, respectively,and total annual cost decreases by 2.4% and 16.3%, respectively.

Effect of Cavities inside Tube Banks on Acoustic Resonance

In the present paper the attention is focused on the effect of small cavities inside in-line tube banks on acoustic resonance which occurred in the two-dimensional model of boiler. We measured the sound pressure level, the amplitude and the phase delay of acoustic pressures and the gap velocity. As a result, we found many peak frequencies of sound pressure level with different Strouhal numbers, mainly about S1=0.15, 0.26 and 0.52. The variation of SPL for S,=0.26, 0.52 components in the tube banks with cavities was the same as the result of no cavities. The existence of cavities inside in-line tube banks caused the resonance of St=0.15. And the acoustic resonance of the first mode in the transverse direction was generated if the small cavities existed inside the tube banks. This resonance was not generated from the tube banks of no cavities. The resonance onset velocity in the transverse mode was fairly slower than that of no cavities. It was easy to generate acoustic resonance when there were small cavities inside in-line tube banks.

Flow Induced Acoustic Resonance in In-line Tube Banks

In the present paper the attention is focused on the relation between vortex shedding phenomena and acoustic resonance which occurred in the two-dimensional model of boiler. There were tube banks with in-line arrangement for small tube pitch ratio. We measured the sound pressure level, the phase delay of acoustic pressures, the spectrum of velocity fluctuation and the gap velocity. As a result, we found two peak frequencies of sound pressure level with different Strouhal numbers St, mainly about 0.26 and 0,52, The noise of St=0.26 was the resonance of transverse mode and St=0.52 was longitudinal mode. The vortex shedding of St=0.15 was generated inside the tube banks without acoustic resonance. As gap velocity increased, we observed that the peak level of spectrum was weak and broad-banded, The onset velocity of the acoustic resonance of longitudinal mode was lower than that of transverse mode.

H形翅片管束传热和阻力特性的试验与数值模拟

根据大型电站锅炉省煤器的运行工况,对单H形和双H形翅片管束气侧的传热与阻力特性进行了模化试验研究,并利用Fluent软件对H形翅片管束的流场和温度场进行了数值模拟,得到单H形和双H形翅片管束的传热与阻力特性变化规律.结果表明:H形翅片管束的传热和阻力特性与气体的Re有关,随着气体Re的增大,气侧Nu不断增大,传热性能提高,而Eu则逐渐减小,并趋于定值;在相同Re下,单H形翅片管束气侧Nu大于双H形翅片管束气侧Nu,而气侧Eu则小于双H形翅片管束气侧Eu;数值计算结果与试验结果误差较小,采用数值计算方法能比较准确地分析H形翅片管束的流动与传热特性.

H形鳍片管束传热与阻力特性实验研究

为实现H形鳍片管束的性能与结构的优化设计,在对H形鳍片管束的传热机理进行分析的基础上,对优化结构的H形鳍片管束的传热与阻力特性进行了模化实验研究,并将实验结果与有关方法的计算结果进行了比较,为H形鳍片管束的设计提供了可靠的依据.

直接空冷机组空冷系统运行问题分析及对策

直接空冷技术代表了未来空冷系统的发展方向,分析直接空冷系统运行中的主要问题并提出相应防治对策,对直接空冷机组安全高效运行具有重要意义。以山西平朔煤矸石发电有限公司50MW机组直接空冷系统为例,针对直接空冷系统运行中的真空泄漏,管束积灰,严寒环境下的冻裂问题,尤其是环境气温、风速、风向对空冷岛运行的影响进行了分析,重点研究了空冷岛回流率与电厂平面布局、环境风速风向的关系。结合直接空冷系统运行中出现的各种问题,提出了初步防治对策。指出通过合理的空冷散热器结构设计,优化的直接空冷电站平面布局,运行过程中针对不同环境条件的科学风量调节等措施,可有效克服直接空冷系统运行中出现的问题,从而提高机组安全经济运行水平。

横掠管束非稳态周期性充分发展流动的数值模拟

以锅炉省煤器中的流动和换热为应用背景 ,分别采用稳态和非稳态的数学模型对横掠管束的周期性充分发展流动进行了数值模拟 .结果表明 ,在Re大于 15 0到小于 10 0 0的范围内 ,横掠管束的周期性充分发展流动问题是非稳态的 .当采用稳态模型时 ,数值计算结果会随迭代次数的增加而振荡 ,得不到收敛的稳态解 .当采用非稳态模型计算时 ,对节距较小的情况 ,在管的后缘处得到对称的非稳态涡 ;对节距较大的情况 ,在管的后缘处得到周期性脱落的非对称涡 .

螺旋鳍片管束传热与阻力特性研究的现状与发展

螺旋鳍片管束因具有一系列优点 ,在常规锅炉及余热锅炉中得到了广泛的应用。为此 ,国内外有关学者对其传热与阻力特性进行了长期的研究 ,并取得了大量的成果。但是 ,因各种原因 ,现有研究成果存在局限性 ,不能满足实际需要。根据对现有成果的分析与比较 ,指出了有待进一步研究之处。表 2参

非稳态横掠管束周期性充分发展对流换热的数值模拟

本文采用非稳态数学模型对横掠管束的周期性充分发展流动与换热进行了数值模拟。结果显示,在所考虑的参数条件下,Re≥100以后流场和温度场随时间发生振荡,并且振幅随Re增加而增大;进一步增加Re到一定值后,在管束间会有交替出现的涡。计算得到的平均Nusselt数与现有文献中的实验关联式基本是吻合的。平均Nusselt数的计算值随流动方向管间距的增加而增加。

颗粒与顺列管束磨损的数值模拟

采用直接数值模拟方法模拟了在管道中直径80μm的颗粒对顺列10×10管束的碰撞和磨损。圆管与流场流体之间的相互作用采用内嵌边界方法进行计算并得到流场结构。颗粒的运动采用了拉格朗日跟踪方法计算并与流场之间进行了双向耦合。计算分析了沿流向以及垂直于流向各排管束受到颗粒的碰撞产生的磨损量。计算结果表明沿流向第三排管束以后的各排管束应采取防磨措施。

空气横掠叉排三维外肋管束换热及流阻特性的实验研究

对由重庆大学研制开发的新型三维外肋管叉排管束的换热及流动特性进行正交优化实验研究。实验结果表明 :与光管管束相比 ,在Re=2 90 0～ 1 40 0 0范围内 ,三维外肋管束的换热系数增大了 1 .5～ 2 .5倍 ,阻力增大了 1～ 3 .5倍。对实验数据进行逐步回归 ,得出了实验条件下的换热及流阻的准则方程式 ,并对肋几何参数对换热及流阻的影响进行了分析。

纵肋管束换热器特性研究

对纵肋管束换热器的传热、阻力特性进行了试验研究,得出其实验关系式.以此分析管束节距的影响,指出横向节距是决定纵肋管束流阻的首要因素,增加横向节距将大幅度减小流胆;而横向节距和纵向节距对换热的影响程度基本相同.对于1组节距为S_t/D=3.29、S_l/D=1.6的纵肋管束,其实际换热系数可增加9%～12.5%,流阻系数可降低15%～65%.结果表明,纵助管束具有低流阻特性和较强的传热强化作用.

齿型螺旋翅片管束灰污系数试验研究

介绍了齿型螺旋翅片管束灰污系数试验研究的原理、试验系统及所采用的试验研究方法；将所得的试验结果与类似结构的整体型螺旋翅片管束及光管管束灰污系数相比较；得出了适用于工程实际的齿型螺旋翅片管束灰污系数计算公式．

水力式流浆箱稳流室的数值研究

流浆箱是造纸机上非常关键的设备,纸页的成形质量很大程度上取决于流浆箱的性能。水力式流浆箱采用稳流室作为浆料匀整装置,利用淹没射流原理,使流浆箱内的浆流具有均匀的速度剖面。稳流室长度是稳流室的一个非常重要的参数。基于可实现kε-湍流模型,运用Fluent求解水力式流浆箱稳流室内的流场参数,利用PIV测量技术对数值模拟结果进行验证,测量结果与数值模拟结果吻合良好。利用统计学原理计算出稳流室各横截面上的速度均方差,并以速度均方差值的变化趋势来判断稳流室内浆流速度是否达到均匀,对于不同布浆管束直径d和浆料入射速度v,计算出稳流室内浆流达到均匀分布所要求的长度L,结果表明L值随着布浆管束的直径增加和入射速度的提高而增大,并且二者分别符合一次函数关系。利用二元函数拟合得出L值与d和v之间的关系式,该式可作为水力式流浆箱稳流室的设计参考。

绕椭圆柱管束的流动与传热特性

椭圆柱管束换热器在工业领域中得到广泛的应用。采用一种简单合理的自由表面模型深入研究了流体绕流椭圆柱管束的流动与传热问题。通过适体坐标转换方法在非规则流道截面上求解出动量与热量传递控制方程,获得了等壁温条件下的流道内的阻力系数和努塞尔数等准则数,并且和文献值进行了对比。文献值和模拟值之间的最大绝对误差小于3.5%,表明所建立的数学模型是正确的。利用所建立的数学模型分析了流动方向、椭圆柱横截面形状、管束结构参数以及壁面边界条件对绕流椭圆柱管束的流动与传热特性的影响。获得了等壁温和等热流密度条件下的阻力系数和平均努塞尔数。这些准则数为椭圆柱管束的设计和优化提供理论指导。

流过管束的流动换热与结构的综合性能评价

对于流过管束的流动换热过程以给定换热结构使换热量最大为条件导出经济结构尺寸与换热参数间的关系式 ,并借助于换热准则关系得出其与流动参数间的关系式 ;进而以过程中相对火用损失率最小为条件导出该流动换热过程最佳流动参数的表达式 .联立上述两个关系式就可得出使过程的结构尺寸、流动特征与换热性能间综合优化的最佳管排与给定热流和管径间的关系式 ,以此作为性能分析与评估的判据 .

顺排椭圆管束的对流换热及流阻特性

对空气横掠顺排椭圆管束的换热及流阻特性进行了实验研究。讨论了放热系数沿管束纵向及横向的分布规律以及平均阻力系数随管束总排数的变化关系。给出了计算顺排椭圆管束放热系数及阻力的实验关联式。实验结果表明，顺排椭圆管束的管外放热系数略低于相应的叉排椭圆管束及圆管管束，但前者的阻力却比后两者低得多。

螺旋翅片管束灰污系数实验室试验方法

从分析螺旋翅片管束受气固两相流冲刷时其表面上灰污层形成的机理着手，分析各种因数对灰污系数数值的影响。提出了一种切实可行的螺旋翅片管束受含灰气流冲刷时，受热面上灰污系数数值的实验室试验确定方法，以指导本研究项目的实验室试验工作。