碳钢_水热虹隙管内部强化传热机理研究

在微层蒸发模型的理论基础上,对热虹吸管内部设置分流管结构强化沸腾传热者机理分析。建立分流管强化热哐吸管内部沸腾传热模型,同时选择七种不同的分流管开孔结构,与光滑管进行了对比实验研究,寻得最佳分流管结构,并综合大量实验数据建立强化沸腾传热准数方程式。     

分离式热管蒸发段的试验研究

该文采用加热石英玻璃管和无缝钢管模拟分离式热管的蒸发段。对例题的充液。流和传热特性进行了系统的试验和理论分析。作者着重分析了核态沸腾传热区及飞溅降膜区的换热原理，试验数据回归整理李相应了换热系数无量钢准则关系式，与试验数据吻合较好；同时将这两个关系式分别与大空间沸腾传热及整体式热管蒸发段降膜传热区传热进行了比较，得出了极为有用的结论。     

垂直下降管内液膜沸腾蒸发及相界面波动研究

采用数值模拟的方法对垂直下降管内液膜沸腾蒸发流动和传热特性进行研究。分析入口雷诺数Re和热流密度的耦合作用对液膜流动和传热的影响,结果表明:壁面生成的汽泡呈现液滴状;大汽泡表面分割、脱离出小汽泡;汽泡生成、脱离强化了沸腾传热效率;热流密度越大,液膜表面的稳定性越差;Re的提高能够增强相界面稳定性;降膜沸腾传热方式的不同对传热系数影响很大;在计算工况范围内,绘制出传热模态分布图,为工程应用提供基础。     

表面润湿性影响池沸腾传热的研究进展

沸腾传热主要受汽泡产生、成长和脱离的控制,而这些汽泡行为与表面润湿性密切相关,通过改变加热面的润湿性,可在一定程度上调节汽泡在加热面上的发展过程,从而影响沸腾传热。介绍了不同润湿性表面及其对沸腾传热的强化机理和气泡动力学,综述了近年来通过表面润湿改性影响沸腾传热的研究进展,并指明表面润湿性与多尺度结构相结合的沸腾传热的研究新动向,为未来利用润湿性强化沸腾传热提供了一个较为清晰、全面的理论基础。     

微层蒸发模型的非平衡热力学分析

基于非平衡热力学理论,以水的沸腾换热时的微层蒸发模型为例,采用国际上流行的IFC公式,对微层蒸发模型的汽泡内部的蒸汽温度分布进行了数值计算.结果表明,要完成微层蒸发模型中的蒸发和凝结两个相变过程,汽泡内部的蒸汽温度并非均匀的,由于汽泡的半径很小,从而导致在汽泡垂直方向上存在有极大的温度梯度,这就使得微层蒸发模型的合理性值得商榷,有必要进一步发展新的模型.     

矩形窄缝流道内过冷沸腾汽泡行为的可视化

采用高速摄像仪,对矩形窄缝流道内过冷沸腾时的汽泡行为进行了可视化实验研究.分析了工况参数对汽泡成核起始点及其脱离直径的影响.结果表明:高过冷沸腾时,窄缝流道内加热面上产生了沿近壁面滑移的汽泡,这种滑移汽泡对窄缝流道内的换热产生了积极的作用,而且汽泡的滑移现象与主流流体的温度有着密切的关系,分析了产生这种滑移现象的原因.     

多壁碳纳米管纳米流体的大容积沸腾

通过在直径为12 mm的沸腾表面进行的多壁碳纳米管阿拉伯树胶水溶液的大容积沸腾实验,研究其沸腾传热特性。纳米流体的沸腾传热效果弱于阿拉伯树胶水溶液,烧毁点的过热度增加而临界热流密度减小,同时,阿拉伯树胶水溶液的传热效果劣于水。纳米颗粒在沸腾加热表面富集、结垢引起液体密度、沸腾表面上活化核心数目的变化,随传热时间的延长,垢层结构包括毛细孔直径、空隙率、垢层厚度不断发生变化,进而引起蒸汽在毛细孔中的流动阻力不断增加、加热表面和垢层间热阻增加,沸腾表面的活化核心数目减小,阿拉伯树胶在蒸发表面的局部富集、黏度大大增加,最终导致沸腾传热恶化。     

满液型海水淡化蒸发器的换热特性研究

海水淡化装置，太阳能或余热吸收式制冷机中的蒸发换热器目前使用管排外降膜式蒸发方式。如将传热管束紧凑排列置于饱和状态液体中则变为满液式蒸发换热器，利用传热管束间受限空间内早期沸腾强化换热机理，将中小热负荷条件下的自然对流换热转化为核沸腾换热，在间隙尺寸适宜时，其换热性能可能优于降膜式蒸发换热器。该研究以盐水为实验工质，对紧凑传热管束受限空间的沸腾换热进行了实验研究，确认了满液式蒸发换热器也具有很好的换热性能，在中小热负荷条件下甚至超过降膜式蒸发换热器。     

热管式真空管集热器热管的传热分析

热管是依靠自身内部液体工质的相变来实现传热的元件,传热机理十分复杂。本文在Nusselt竖直壁面层流膜状凝结理论的基础上,就热管冷凝段和蒸发段的传热分别进行分析,提出了平均换热系数h2^-和Nu^-。     

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。     

竖直矩形窄缝两相同向分相流动沸腾传热模型

首次对竖直矩形窄缝内的汽液分相流动区提出一维两相同向分相流动沸腾传热模型 ,并进行了数值计算 ,得到不同质量流速下液膜厚度变化和沸腾传热系数等结果。沸腾传热系数的模型预测值初步与已有实验关联式进行了比较 ,两者基本吻合 ,偏差在± 1 4% ;从而证实了液膜导热是竖直矩形窄缝内汽液分相流动区沸腾传热的主导机理。     

Critical Heat Flux of Boiling Heat Transfer in a Moderate Narrow Space

INTanDUCTI0NBoilingheattransferandcriticalheatflux(CHF)inaconfinednarrowspacehavebeenstudiedexperi-melltallybyanumberofinvestigatorsinthepastfewdecades.However,thereisnoanypopularlyacceptedmodelintheheattransferinnarrowspaceboiling,althoughsomepopularknowledgeabouttheboilingheattransferinthenarrowspacehavebeenacceptedbymanyresearchers.Theknowledgecanbecon-cludedasthatthenucleateboilingheattransferisenhancedatlowheatfluxregionanddeterioratedathighheatfiuxregi0nespeciallyatCHF.Theenhanceme…     

Subcooled flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. From the measured boiling curves, the temperature undershoot at ONB is found to be relatively significant for the subcooled flow boiling of R-134a in the duct. The R-134a subcooled flow boiling heat transfer coefficient increases with a reduction in the gap size, but decreases with an increase in the inlet liquid subcooling. Besides, raising the imposed heat flux can cause a substantial increase in the subcooled boiling heat transfer coefficient. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are small in the narrow duct. Visualization of the subcooled flow boiling processes reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Moreover, raising the imposed heat flux significantly increases the bubble population, coalescence and departure frequency. The increase in the bubble departure frequency by reducing the duct size is due to the rising wall shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities on the heating surface tend to merge together to form big bubbles. Correlation for the present subcooled flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.     

Marangoni heat transfer in subcooled nucleate pool boiling

The liquid motion induced by surface tension variation, termed the Marangoni effect, and its contribution to boiling heat transfer has been an issue of much controversy. Boiling heat transfer theory, although acknowledging its existence, considers its contribution to heat transfer to be insignificant in comparison with buoyancy induced convection. However, recent microgravity experiments have shown that although the boiling mechanism in a reduced gravity environment is different, the corresponding heat transfer rates are similar to those obtained under normal gravity conditions, raising questions about the validity of the assumption. An experimental investigation was performed in which distilled water was gradually heated to boiling conditions on a copper heater surface at four different levels of subcooling. Photographic investigation of the bubbles appearing on the surface was carried out in support of the measurements. The results obtained indicate that Marangoni convection associated with the bubbles formed by the air dissolved in the water which emerged from solution when the water was heated sufficiently, significantly influenced the heat transfer rate in subcooled nucleate pool boiling. A heat transfer model was developed in order to explain the phenomena observed.     

水平窄空间沸腾传热的实验研究

通过对五种尺寸的窄空间试验元件分别以水和乙醇做工质进行实验。研究了窄空间间距、窄空间尺寸、不同工质及不同热流密度对窄空间沸腾性能的影响。结果表明：当窄空间尺寸与热流通等因素组合恰当时。其换热系数可比大空间池沸腾提高3～6倍；临界热流密度有所降低。     

Saturated flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct

Experiments are conducted here to investigate how the channel size affects the saturated flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured heat transfer data indicate that the saturated flow boiling heat transfer coefficient increases with a decrease in the gap of the duct. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are milder. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases slightly at increasing R-134a mass flux. Moreover, the bubble departure frequency increases at reducing duct size mainly due to the rising shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Correlation for the present saturated flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated.     

Contribution of thin-film evaporation during flow boiling inside microchannels

Flow boiling through microchannels is characterized by nucleation and growth of vapor bubbles that fill the entire channel cross-sectional area. As the bubbles nucleate and grow inside the microchannel, a thin film of liquid or a microlayer gets trapped between the bubbles and the channel walls. The heat transfer mechanism present at the channel walls during flow boiling is studied numerically. It is then compared to the heat transfer mechanisms present during nucleate pool boiling and in a moving evaporating meniscus. Increasing contact angle improved wall heat transfer in case of nucleate boiling and moving evaporating meniscus but not in the case of flow boiling inside a microchannel. It is shown that the thermal and the flow fields present inside the microchannel around a bubble are fundamentally different as compared to nucleate pool boiling or in a moving evaporating meniscus. It is explained why thin-film evaporation is the dominant heat transfer mechanism and is responsible for creating an apparent nucleate boiling effect inside a microchannel.     

竖直矩形狭缝通道内环状流沸腾换热分析模型

对竖直矩形狭缝通道内有液滴卷吸环状流阶段流动沸腾进行分析。以液膜紊流的动量方程和能量方程为基础,加上相应的边界条件和使控制方程组封闭的经验关系式,建立了环状流的教学模型并进行数值计算,得到了矩形狭缝通道内的液膜厚度分布、沸腾传热系数等结果;将模型预测的换热系数同实验关系式作比较,最大相对误差为17．8％。     

Nucleate boiling in two types of vertical narrow channels

To explore the mechanism of boiling bubble dynamics in narrow channels, we investigate 2-mm wide I- and Z-shaped channels. The influence of wall contact angle on bubble generation and growth is studied using numerical simulation. The relationships between different channel shapes and the pressure drop are also examined, taking into account the effects of gravity, surface tension, and wall adhesion. The wall contact angle imposes considerable influence over the morphology of bubbles. The smaller the wall contact angle, the rounder the bubbles, and the less time the bubbles take to depart from the wall. Otherwise, the bubbles experience more difficulty in departure. Variations in the contact angle also affect the heat transfer coefficient. The greater the wall contact angle, the larger the bubble-covered area. Therefore, wall thermal resistance increases, bubble nucleation is suppressed, and the heat transfer coefficient is lowered. The role of surface tension in boiling heat transfer is considerably more important than that of gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of the I-shaped type, and the pressure drop of the former is clearly higher.     

Modeling of the microconvective contribution to wall heat transfer in subcooled boiling flow

In the present work, the two-phase turbulent boundary layer in subcooled boiling flow is investigated. The bubbles in the near-wall region have a significant effect on the dynamics of the underlying liquid flow, as well as on the heat transfer. The present work develops a single-fluid model capable of accounting for the interactions between the bubbles and the liquid phase, such that the two-phase convective contribution to the total wall heat transfer can be described appropriately even in the framework of single-fluid modeling. To this end, subcooled boiling channel flow was experimentally investigated using a laser-Doppler anemometer to gain insight into the bubble-laden near-wall velocity field. It was generally observed that the streamwise velocity component was considerably reduced compared to the single-phase case, while the near-wall turbulence was increased due to the presence of the bubbles. Since the experimentally observed characteristics of the liquid velocity field turned out to be very similar to turbulent flows along rough surfaces, it is proposed to model the near-wall effect of the bubbles on the liquid flow analogously to the effect of a surface roughness. Incorporating the proposed approach as a dynamic boundary condition into a well-established mechanistic flow boiling model makes it possible to reflect adequately the contribution of the microconvection to the total wall heat transfer. A comparison against the experimental data shows good agreement for the predicted wall shear stress as well as for the wall heat flux for a wide range of wall temperatures and Reynolds numbers.