多套型套管式换热器传热与流阻性能试验研究

多管型套管式换热器是在大尺寸外管的内部布置多根仙管所构成的换热设备，与单根内管的套管式换热器相比，流量大幅增加，选用螺纹内管和管间折流板可以强化传热。对于多管型套管式换热器的传热性能试验，采用修正威尔逊法进行试验计算，得出了两种多套型套管式换热器的传热与流动阻力性能试验结果。     

带多孔隔板的多支管联箱内两相流流动的数值分析

针对平行流换热器中设置的多孔隔板气液分离联箱,研究制冷剂R134a气液两相流在联箱中的气液分离特性,讨论多孔隔板对气液分离效率和多支管中气相分布均匀性的影响。利用FLUENT软件对无孔隔板、3孔隔板的多支管联箱中气液两相流在进口气相质量流量0.75~1.00g/s、液相质量流量1.00g/s下的流动特性进行模拟计算。结果显示:当气相质量流量0.75~0.90g/s时,3孔隔板联箱可进行有效气液分离,与无孔隔板的多支管联箱相比,平均出口干度最大可提高35%,多支管中气相的分配均匀性最大可提高81%。但当气相质量流量增大到1.00g/s时,气液分离失效。表明在一定工况下多孔隔板可实现多支管联箱内的气液分离,且有助于提高联箱出口气体的分配均匀性。     

板翅式换热器注液封条气水两相流分配特性实验研究

气液两相流分配不均匀是板翅式换热器换热性能下降的因素之一,在入口处设置注液封条结构能够改善两相流分配的不均匀性。为了研究在不同流量条件下注液封条对两相流的分配情况,本文以空气和水为介质通过实验研究了注液封条结构的两相流分配特性。研究表明:注液封条结构的气相均匀性好于液相。在给定的气液流量条件下,随着气液流量的减小,注液封条均匀性变差。     

分配联箱气液两相流流型对垂直并联管分配特性的影响

对垂直Ｕ型单联管系统低质量流量的流量分配特性进行了理论和实验研究,获得气叮和气液两质量流量分配特性,以及各分支管的流型记录,分析了分配联箱中流型对流量分配以及各分支管中流的型的影响。     

换热器各流路对壳侧气液两相流动特性的影响

针对工业中广泛应用的管壳式换热器，应用空气-水两相混合物实验研究了壳侧旁路，泄漏流对气液两相流体流动特性的影响，以Ishihara两相流动模型为基础，建立了以横掠管束的主流路为基础的错流区通用两相压降计算关联式，通过错流区，泄漏流的分相流动模型，分析计算了主流路，旁路，泄漏流中气液分布，也分析了泄漏流对壳侧单相，两相总流量在各个分流路的流量分配影响，研究表明，主流路和旁路中气液各自占相应总流量的比例在不同的流型下明显不同，且比例值的波动范围较大，气液流量的分布在壳侧是不均匀的，折流板/换热管之间的泄漏流对壳侧的两相流动特性影响较小，而折流板/壳体之间的泄漏流影响较大。     

多管型套管式换热器传热与流阻性能试验研究

多管型套管式换热器是在大尺寸外管的内部布置多根内管所构成的换热设备 ,与单根内管的套管式换热器相比 ,流量大幅增加 ,选用螺纹内管和管间折流板可以强化传热。对于多管型套管式换热器的传热性能试验 ,采用修正威尔逊法进行试验计算 ,得出了两种多管型套管式换热器的传热与流动阻力性能试验结果。     

水平管降膜蒸发器中不同形式的布液器对内部流场流动影响的数值模拟

应用Fluent两相流VOF模型,数值模拟水平管降膜蒸发器内部流场流动的过程。提出3种不同结构的布液器,分别对蒸发器内部流动特性进行了比较分析,得到气液两相图、速度云图、XY图等。模拟结果表明:中间开孔布液器使两部分蒸发管阵产生的蒸汽通过中间蒸汽排气口疏导出去,换热面大,使液体分配较均匀,更为合理,有利于蒸发器内部整体流场的优化设计。研究结果可为降膜蒸发器的优化设计提供理论依据。     

倾斜安装对注液封条两相流分配特性的影响

板翅式换热器作为天然气液化(LNG)工艺中主低温换热器,其内部两相冷剂分配不均影响了换热性能,并导致天然气液化效率降低。为了探究不同倾斜安装形式的LNG板翅式换热器内部两相流动均布特性,以注液封条作为LNG板翅式换热器内的两相均布结构,采用数值模拟的方法研究两相冷剂在不同倾斜安装位置注液封条内的流动均布规律。对比分析40个工况下的流量不均匀度以及离散系数,结果表明:(1)注液封条内气液两相的分配特性为中间通道流量大于两侧通道流量,不同安装形式对注液封条内两相分配的总趋势影响较小;(2)不同偏移角度下,不同安装形式的注液封条两相均匀性变化规律不同,封条竖直向上安装均匀性随偏移角度增加而优化,竖直向下安装左偏均匀性优于右偏,水平安装均匀性随偏移角度变化较小;(3)侧倾对于不同安装形式的注液封条均匀性影响一致,侧倾角度越大,均匀性越差。因此,在实际工程中,应避免侧倾安装形式。     

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。     

定壁温条件下竖直管内沸腾传热特性研究

文章建立了应用于蒸发器的满液式竖直管三维物理模型,并采用多相流混合模型对满液式竖直管内的沸腾传热特性进行数值模拟。而后根据模拟结果得到管内静压、管壁加热温度和管长对满液式竖直管内流体的温度、含气率以及该竖直管沸腾传热系数的影响规律,并分析管壁加热温度、管长、管内静压和蒸发温度对满液式竖直管内沸腾传热特性的影响。分析结果表明:满液式竖直管的长度越长,蒸发器的总换热量越大;当满液式竖直管的壁面温度由376 K升高至388 K时,若该竖直管的长度为1.6 m,则其沸腾传热系数提高了7.4%,若该竖直管的长度为1.2,1.0 m,则其沸腾传热系数均升高了约3.3%;在蒸发器竖直管沸腾传热过程中,其换热量和壁面温度呈正相关;当蒸发温度较低时,满液式竖直管内的静压对管内流体的含气率以及该竖直管的沸腾传热系数影响较大。     

气固两相Y型分支管网流量分配特性的试验研究与数值模拟

在水平Y型分支管道中,采用压缩空气作为输送动力,小米作为输送介质进行气力输送试验,对分支管道的固体流量分配特性进行了研究.试验表明,变动支管与主管中轴线夹角与气体表观速度对固相分配特性具有较大影响.同时,采用Euler-Lagrange两相流研究方法,固相采用离散相(DPM)模型,采用Fluent软件对3种不同夹角的Y型分支管内气固两相流动进行了数值模拟.模拟结果较好地预估了颗粒在分支处的流动形态、颗粒在分支管内的运动轨迹,以及重新实现颗粒相流场均匀分布所需的距离.通过对分支管内固体颗粒质量分配的数值模拟结果与试验结果比较,发现两者之间相对误差较小.     

凝汽器水侧流动的三维数值模拟

建立了某一凝汽器实际管束的流动计算模型,运用计算流体力学的方法,对该凝汽器水侧流场进行了三维数值模拟。采用分区对称计算方法,大大降低网格数量,从而详细预测了凝汽器水侧进出口水室以及其连接管和冷却水管束内的流动特性。计算结果可以清楚地表明:该凝汽器进口水室存在大量漩涡,使流动阻力增加,流动恶化;水室速度分布不均匀,进口水室管板中心区域流体流速较高而边缘区域较低,结构上存在一定问题;而出口水室的结构较为合理。这与采用多孔介质模型模拟的结论一致,进一步验证了采用多孔介质模型对凝汽器进行计算是正确可行的。计算结果同时表明,冷却水管束内流量和流速的分布是不均匀的,位于中心位置的冷却水管流量较大,而周边区域较小,最大流量差别可达到38%,且相邻管路的流量减小幅度与冷却水管布置有关。冷却水管内冷却水流量和流速的差异将会影响换热器的换热性能。计算结果可为分析研究管排流动不均而引起的换热效率问题提供条件,也可为凝汽器设计和结构优化提供依据。     

振动状态下水平管内气液两相流流型转变的实验研究

对振动状态下水平管内气液两相流流型进行了实验研究,分析了不同振动频率、振动幅度对流型转变的影响。研究发现:随着振动频率或振动幅度的提高,在液相折算速度相同的情况下,环状流的形成需要更高的气相折算速度,而在气相折算流速相同时,泡状流的形成需要更高的液相流量;振动频率或幅度的提高使各个流型的走势分布发生变化,整体看是以弹状-波状流为中心向外有不同程度的扩张;振动频率与振动幅度两者对流型转变的影响基本相同。     

Experimental study on the operating characteristics of a flat bifacial evaporator loop heat pipe

There is growing demand for highly efficient heat transfer devices having excellent performance, operational stability and low power consumption. Although loop heat pipes satisfy these requirements, conventional loop heat pips have limited application to flat heat sources due to their mostly cylindrical evaporator shape. To overcome this limitation, various types of flat evaporator loop heat pipes have been developed, though their operational reliability is still uncertain. In this work, we focused on the development of a flat bifacial evaporator loop heat pipe, and its operating characteristics at transient and steady states are discussed in detail.     

联箱式分离型热管实验研究

联箱式分离型热管的多支管并联可能导致流量分配不均,造成部分管道干涸,降低换热效果。搭建了7根管道并联的U型联箱式分离型热管实验平台,以R134a为工质,在800～2 200 W加热功率下,分析热管高效工作的充液率上下限、最佳充液率、热管系统换热特性及管道流量分配。结果表明:热管在充液率为31%～46%时可正常工作,最佳充液率约为42%;确认了各换热支管出现的流量分配不均匀特征,最靠近入口的分支管换热性能最佳,第4～6根换热管流量分配最少、性能最差。     

Thermal and hydraulic analysis of a brazed aluminum evaporator

This paper presents an analytical/computer model to predict the performance of a brazed aluminum evaporator operating under dehumidifying conditions. The evaporator uses small hydraulic diameter, flat multi-channel tubes and louver fins. The in-tube refrigerant flow was divided into three regions including the two-phase, liquid deficient and superheat regions. For each region, correlations were selected from the open literature to calculate the local heat transfer and pressure drop. The effects of refrigerant pressure drop along tube and pressure losses at the tube entrance and exit were accounted for in the heat transfer calculations. The air-side fins were assumed to operate at the fully wet condition and the sensible heat transfer coefficient of the wet fins was assumed to be equal to that of the dry fins. The overall heat transfer coefficient was calculated using the enthalpy driving potential method. The total heat transfer rate and refrigerant pressure drop depend on the ratio of the number of tubes in the first and second passes. Parametric studies were done to illustrate selection of the preferred number of tubes per pass. The average refrigerant side heat transfer coefficient is sensitive to the dry-out vapor quality. However, the total heat transfer rate is relatively insensitive to the dry-out vapor quality. As the air inlet humidity increases, the latent and total heat transfer rates increase, but the sensible heat transfer rate decreases. The program was used to design an R-404A evaporator, for which a prototype was built and tested. The program over-predicted the evaporator capacity by 8%. The over-prediction is believed due to flow mal-distribution in the branch tubes.     

Heat transfer characteristics of the evaporator section using small helical coiled pipes in a looped heat pipe

An experimental study was carried out for the heat transfer characteristics and the flow patterns of the evaporator section using small diameter coiled pipes in a looped heat pipe (LHP). Two coiled pipes: the glass pipe and the stainless steel pipes were used as evaporator section in the LHP, respectively. Flow and heat transfer characteristics in the coiled tubes of the evaporator section were investigated under the different filling ratios and heat fluxes. The experimental results show that the combined effect of the evaporation of the thin liquid film, the disturbance caused by pulsation and the secondary flow enhanced greatly the heat transfer and the critical heat flux of the evaporator section. In final, two dimensionless empirical correlations were proposed for predicting the heat transfer coefficients of the evaporator section before and after dryout occurs.     

A three-dimensional thermal-fluid analysis of flat heat pipes

A detailed, three-dimensional model has been developed to analyze the thermal hydrodynamic behaviors of flat heat pipes without empirical correlations. The model accounts for the heat conduction in the wall, fluid flow in the vapor chambers and porous wicks, and the coupled heat and mass transfer at the liquid/vapor interface. The flat pipes with and without vertical wick columns in the vapor channel are intensively investigated in the model. Parametric effects, including evaporative heat input and size on the thermal and hydrodynamic behavior in the heat pipes, are investigated. The results show that, the vertical wick columns in the vapor core can improve the thermal and hydrodynamic performance of the heat pipes, including thermal resistance, capillary limit, wall temperature, pressure drop, and fluid velocities due to the enhancement of the fluid/heat mechanism form the bottom condenser to the top evaporator. The results predict that higher evaporative heat input improves the thermal and hydrodynamic performance of the heat pipe, and shortening the size of heat pipe degrades the thermal performance of the heat pipe.     

Drop-in Test of Microchannel Evaporators Using R-22 for Residential Air Conditioners

The drop-in test of microchannel evaporators using R-22 for residential air conditioners was conducted. Five prototype microchannel evaporators were manufactured and tested in psychrometric calorimeter test facilities. Each evaporator was made with two parallel flow heat exchangers connected with several return pipes. The parallel flow heat exchanger had 41 microchannel tubes with eight rectangular ports. Performance comparisons were made between microchannel evaporators with and without hydrophilic coating to assess the effect on condensate on evaporator surface. Also, various performances of prototype microchannel evaporators were presented. Note that the cooling capacities of prototype 2, with a flow area ratio of 73/58%, and prototype 3, with six return pipes, were larger than prototype 1 by 15% and 9%, respectively. The cooling capacity of prototype 4 with three rows was larger than prototype 1 by 14%. Both refrigerant-side and air-side pressure drops for prototype 2 with the best cooling capacity were 29 kPa and 2.5 mmAq, which are acceptable for residential air conditioners.