波纹管降膜蒸发器传热性能数值模拟

为了理解波纹管降膜蒸发过程中涉及的液膜传热过程,本文采用VOF法建立了二维气-液两相分层流动CFD模型,考虑了液相流量,传热温差,蒸发温度,液相粘度等参数对传热效果的影响,根据模拟结果给出了波纹管降膜蒸发器的流量可操作范围。模拟结果和实验数据比较吻合。     

波纹管降膜蒸发器传热性能的研究

本文分析了影响波纹管降膜蒸发器传热性能的主要因素，以葡萄糖溶液为工质，通过实验研究，建立了传热系数准数经验关联式，研究结果表明，准数经验关联式计算值与实验值吻合较好，为工程设计提供了参数依据。     

凹凸变化壁面强化传热机理与传热性能的研究

介绍了一种近年来开发的板式外流降膜蒸发器中的凹凸变化壁面的传热元件;分析讨论了其强化传热的机理及其传热性能;建立了凹凸变化壁面上降落液膜的传热系数准数关联式,为其应用于工业生产提供了参考数据.     

制冷系统水平管降膜蒸发器换热性能的影响因素分析

在理论上对制冷系统中降膜蒸发器进行基本分析,介绍降膜蒸发器的发展背景、发展进程以及工作原理和结构,分析降膜蒸发器的蒸发管上方制冷剂分布器的分配效果和结构、液膜流动特性、制冷剂流量、蒸发器内部的管阵布置方式、蒸发管的表面特性、管束规格和几何排列、气流扰动情况和润滑油等影响因素对降膜蒸发器传热性能的影响,比较降膜蒸发器与其他蒸发器的特点,介绍降膜蒸发的某些技术难题。     

水平外波纹管降膜管间流型转变

相比于光管,外波纹管的壁面凹凸结构可增加换热面积,从而提高水平管降膜蒸发器的传热特性。准确预测溶液管间流型是提高传热传质性能的关键步骤。建立了水平管降膜实验装置,研究管间距、溶液浓度、管型对降膜流动过程管间流型转变的影响。结果表明:随着雷诺数(Re)的增加,管间流动形态依次出现滴状流、滴柱状流、柱状流和柱片状流和片状流;对于每一种流型转变所对应的Re,外波纹管明显低于光管,并随管间距的增大而增大,随NaCl溶液质量分数的增大而减小;相较于光管,两种外波纹管更易在较低Re下获得各稳定流型。流型转变边界可以通过流体Re与伽利略数(Ga)之间的函数关系式描述。通过拟合实验数据,得到了3种管的流型转变关系式。     

海水淡化用波纹管降膜蒸发强化传热实验研究

通过利用水平管降膜蒸发换热试验台分别对Φ19×0.75 mm的波纹管和光滑管进行实验研究。实验在变喷淋密度(0.007~0.130 kg/m·s)、变热通量(52~143 k W/m~2)、变传热温差(1.5~10.0℃)、变蒸发压力(0.020~0.065MPa)条件下进行。通过实验数据得到波纹管和光滑管传热系数与各影响因素(喷淋密度、热通量、传热温差、蒸发温度)之间的变化规律。实验结果表明:在一定范围内,降膜蒸发器的传热系数K随喷淋密度γ、热通量Q的增大、蒸发温度T的升高而增大,随传热温差Δt的增大而降低。当喷淋密度大于0.178 kg/(m·s)时,总传热系数趋于稳定,当热通量大于130 k W/m~2时,总传热系数的增速明显变缓。此外,不凝气含量对传热系数K的影响显著,在同等实验条件下波纹管的传热系数比光滑管提高近30%。     

MVR降膜蒸发器液膜流动特性数值模拟

随着能源价格的上涨,蒸发浓缩过程的节能降耗工作显得非常重要。MVR技术节能潜力巨大,将成为蒸发浓缩技术发展的方向。利用传热学及流体力学的基本理论,在进行合理简化的基础上,建立了适用于降膜蒸发器竖管内蒸发过程的传热模型,模型同时考虑了重力和二次蒸汽剪切力的影响。利用VB语言开发了用于计算蒸发器降膜蒸发管流动状态、传热特性的应用软件。利用所开发的软件定量分析了液膜厚度、传热系数等参数沿蒸发管流动方向的变化规律,比较了两种模型对计算结果的影响。     

低Re数平板降膜瞬态传热特性的研究

在建立低Re数平板降膜流动与传热数学模型的基础上,对平板平滑和波动层流降膜沿整个平板的瞬态传热特性进行了数值计算。平滑层流平板降膜流场按边界层流动进行求解,在此基础上对瞬态温度控制方程进行数值求解。波动降膜则采用坐标变换,在对随时间变化的计算区域进行坐标变换后,对变换后的温度控制方程离散并进行数值求解。平滑层流降膜的计算结果显示,由于平板降膜具有高效的传热特性,传热瞬态过程时间很短。波动层流降膜数值计算结果则显示由于液膜的波动,入口处截面平均温度无论沿流动方向还是随时间变化均呈现波动性,且与平滑层流平板降膜相比,波动降膜明显使液膜温度更快地接近液膜外侧的界面温度,显示了波动对膜内传热的强化作用。     

海水淡化中水平管降膜蒸发器传热传质的研究现状

水平管降膜蒸发器广泛应用于制冷、食品和海水淡化领域,其传热传质过程直接影响到整个蒸发器的性能,因而受到各国研究者的重视。介绍了水平管降膜蒸发器的传热传质过程及影响因素,通过对传热温差、蒸发温度、热通量、喷淋密度、管束布置方式等对液膜厚度、液膜流动状态、热阻、液体黏度、表面张力等的影响分析进而得出其对传热传质过程的影响。     

低温地热双循环式发电系统横管喷淋降膜蒸发器传热性能实验研究

以R245fa为工质,搭建有机朗肯循环（ORC）发电系统横管喷淋降膜蒸发器传热测试平台,研究有机工质喷淋密度,地热水初温及流率等因素对管外换热系数的影响。实验结果表明：随着有机工质喷淋密度、地热水初温、地热水流率的增大,传热系数均先增大后减小。最后,根据实验结果,对现有横管喷淋降膜蒸发器的管外传热系数经验公式的参数进行修正。     

Enhanced Boiling Heat Transfer of the Compact Staggered Tube Bundles under Sub-Atmospheric Pressures

An experimental investigation was carried out on the boiling heat transfer enhancement of water on plain tubes in compact staggered tube-bundle evaporators under atmospheric and sub-atmospheric pressures. The experiment investigated the effects of the tube spacing and positioning and the test pressure on the boiling heat transfer characteristics in restricted spaces of compact tube bundles. The experimental results indicated that for compact tube bundles, the effect of the tube spacing is very significant on the boiling heat transfer. The boiling heat transfer has a maximum enhancement when the tube spacing is so selected as to take an optimum value. The enhanced heat transfer efficiency for the compact bundles would gradually decrease as the test pressure was reduced.     

Film condensation in presence of non-condensable gases over horizontal tubes with progressively increasing radius of curvature in the direction of gravity

A theoretical study is executed to investigate the simultaneous influences of reduction in heat transfer rate on account of condensation in presence of non-condensables and its augmentation by the geometry of an horizontal tube surface with increasing radius of curvature in the direction of gravity. The tube surface profile considered for the present work is an equiangular spiral described in a polar form as: R = ae^mθ (a and m being parametric constants). It is observed that a very small bulk concentration (even less than 1%) of the non-condensable gas reduces considerably the heat transfer coefficient. However, there is an enhancement in heat transfer coefficient for condensation over a polar tube surface, as compared to that over a circular tube surface. This enhancement in heat transfer coefficient, with an increase in the value of m (a surface profile parameter), in presence of non-condensables is more than the corresponding proportional enhancement in the same in absence of any non-condensable species.     

低压条件下紧凑式顺排管束满液型蒸发换热器的强化换热特性研究

提出了一种新型紧凑式顺排光滑管束组成的满液式蒸发换热器。在低压条件下对水平光滑顺排管束的小空间内沸腾强化换热特性进行了实验研究,确认了管距、管位置和运行压力对强化换热性能的影响。实验表明存在一个能得到最大强化换热效果的最佳管距,这一最佳管距接近沸腾气泡的脱离直径。压力对强化换热效果也有重要影响:随着压力降低,强化换热效果也逐步减弱。实验结果对高效节能型蒸发换热器设计提供了设计基础。     

Enhanced boiling heat transfer in restricted spaces of a compact tube bundle with enhanced tubes

In flooded-type tube bundle evaporators with smooth tubes and general tube gaps, both wall superheat and heat flux are generally quite low and boiling cannot occur on the heated tubes. But when the tube gap is quite small or the enhanced heat transfer tubes are employed, the incipient boiling can occur at low heat flux levels and results in a significant heat transfer enhancement effect. This study investigates experimentally enhancement effects by the restricted space comprising the compact tube bundle and the enhanced tubes for boiling heat transfer of pure water and salt-water mixtures under atmospheric pressure. The experimental results show that the small tube gaps can greatly enhance boiling heat transfer for the compact enhanced tube bundle.     

管内插物强化换热性能分析及应用

管内插物的种类很多，扭带、螺旋线圈以及绕花丝是三种较常用的管内插物强化换热技术，对它们的强化换热性能以及应用进行分析比较，是非常有必要的。从综合强化性能来看，螺旋线圈内插物比扭带内插物效果好；绕花丝内插物是一种新型综合强化换热技术，它优于前两种内插物的特点是：可使流体在流动方向上做复杂的三维混合流动，并且所产生的阻力降非常小，所以综合强化换热性能最好。图7表1参10     

Field synergy optimization and enhanced heat transfer by multi-longitudinal vortexes flow in tube

The field synergy equation for steady laminar convection heat transfer was derived by conditional variation calculus based on the least dissipation of heat transport potential capacity. The optimum velocity field with the best heat transfer performance and least flow resistance increase can be obtained by solving the synergy equation. The numerical simulation of laminar convection heat transfer in a straight circular tube shows that the multi-longitudinal vortex flow in the tube is the flow pattern that enhances the heat transfer enormously. Based on this result, a novel enhanced heat transfer tube, the discrete double-inclined ribs tube (DDIR-tube), is developed. The flow field of the DDIR-tube is similar to the optimal velocity field. The experimental results show that the DDIR-tube has better comprehensive heat transfer performance than the current heat transfer enhancement tubes. The present work indicates that new heat transfer enhancement techniques could be developed according to the optimum velocity field.     

Heat transfer enhancement and performance of the molten salt receiver of a solar power tower

This paper investigates the interaction between the heat transfer performance and the thermal efficiency of a molten salt receiver used in the solar power tower plant. A test-bed is built, and a series of experiments of heat transfer enhancement for two types of molten salt receiver tubes, including smooth and spiral tubes, have been carried out under the high temperature and the high heat flux conditions. The experimental results show that the Nusselt numbers of spiral tube with heat transfer enhancement are in the range of 400–1200, which is about 3 times than that of the smooth one on average. The wall temperature of the spiral tube is decreased by about 30 °C comparing with that of the smooth tube under the identical heat transfer conditions. The results of the experiment show that, by using the spiral tube as the heat transfer tube, the heat transfer performance of the molten salt receiver is obviously improved, and the radiation and convection losses are significantly reduced. The results will be helpful for the design of the molten salt receiver.     

Heat transfer characteristics of expanded graphite matrices in metal hydride beds

The thermal conductivity of metal hydride powders and the heat transfer coefficients between metal hydride powder beds and the walls of reaction beds are usually low. When metal hydride powders are used in practical applications, heat transfer enhancement techniques have to be generally applied. Here, expanded natural graphite/metal hydride compacts are described, possessing a high-effective thermal conductivity. Furthermore, the heat transfer coefficient between expanded natural graphite pellets and the inner surface of the surrounding steel tube has been investigated.     

管壳式换热器折流部件优化研究进展

为了提高管壳式换热器的能源利用率,换热器强化传热的研究得到广泛关注。本文从强化传热原理、结构改进和设计优化等三方面对换热器折流部件的优化改进研究进行了分析和总结。其中,强化传热原理主要包括不同折流板通过改变流场的特性影响换热器性能;结构改进包括分段挡板、折流孔板和螺旋挡板的优化进展以及与单弓挡板的对比研究;设计优化包括利用各种新型算法对换热器结构参数的优化和成本的控制。针对管壳式换热器折流部件的强化传热问题,提出了非连续螺旋挡板的研究和结合多目标优化设计的结构改进是未来的重点研究方向。     

Natural convection heat transfer in eccentric horizontal annuli between a heated outer tube and a cooled inner tube with different orientation: The case of an elliptical outer tube

This study conducts numerical analyses and experiments on natural convection heat transfer in eccentric horizontal annuli between a heated outer tube and a cooled inner tube with different orientations. The outer tube is an elliptical tube, and the inner tube is a cylindrical tube. Experiments are conducted with the outer tube oriented to the vertical side and horizontal side, respectively. The numerical analyses were performed by changing the oriented angle and eccentricity parameters. Results obtained for different conditions are presented as temperature profiles, flow patterns, local Nusselt numbers, and total heat transfer. It is found that the heat transfer enhancement effect or heat transfer obstruction effect for the total heat transfer is changed by eccentricity and oriented angle. © 2001 Scripta Technica, Heat Trans Asian Res, 30(8): 624–635, 2001