微细通道纳米与非纳米流体传热和压降对比研究

分别以0.2%、0.5%、1%质量分数的Al2O3-H2O纳米流体和去离子水为实验工质,在高2mm,宽1mm的矩形微细通道内进行纳米流体与非纳米流体两相沸腾传热和压降对比研究。实验结果表明:增加质量通量对两种工质换热系数影响都较小,但增加热流密度可提高换热系数;在相同工况下,与水基液相比,采用Al2O3-H2O纳米流体换热系数明显增大,且随着纳米流体质量分数的增加而增加,对于该实验换热系数可提高8%～17%;随着纳米颗粒质量分数和质量通量的增加,两相摩擦压降显著增大。     

微槽道中纳米流体沸腾换热特性研究

为研究纳米流体在微槽道中的沸腾换热特性及规律,分别以去离子水和体积浓度为0.2%、0.5%的水基Al2O3纳米流体为工质进行试验,研究质量流速、热流密度、进口过冷度、槽道尺寸等因素对沸腾传热系数的影响及其两相摩擦压降与出口干度的关联分析和沸腾换热关联式对比拟合.试验结果表明:在一定热流密度和质量流速下,传热系数随槽道尺...     

矩形微槽道纳米流体饱和沸腾临界热流密度特性

针对纳米流体在微小尺度传热领域的应用,在常压下对微槽道中纳米流体的流动沸腾临界热流密度进行实验研究。分别以体积浓度为0.2%、0.5%的水基Al2O3纳米流体为工质进行试验,研究不同质量流速、槽道尺寸以及体积浓度等因素对沸腾CHF的影响。对比水为工质实验结果,表明:槽道尺寸、质量流速对于水-Al2O3纳米流体和纯水的CHF影响一致。其它参数一定的工况下,纳米流体CHF比纯水大,且随着纳米流体体积浓度增大,出口壁面过热度会增大。最后介绍一个微槽道沸腾CHF的预测模型,在评价其不足的基础上提出一个关于CHF的预测公式,与实验数据进行对比,验证该公式的适用性。     

微槽道纳米流体饱和沸腾CHF特性研究

以水和三种不同质量分数(0.2%、0.5%和1.0%)的Al2O3纳米流体作为实验工质,在三种不同尺寸微槽道中进行饱和沸腾传热实验,研究沸腾传热过程中临界热流密度(Critical Heat Flux,简称CHF)的变化特性。主要分析了微通道水力半径、纳米流体浓度、进口过冷度和临界热力学干度等因素对CHF的影响。实验结果发现:在水力半径较小的槽道内CHF发生得比较早;CHF随纳米流体浓度的增大而增大;CHF随进口过冷度增大有细微增大的趋势;CHF随临界热力学干度的增大而减小。文中还将实验结果与现有的、工况条件与本实验相近的理论模型进行了拟合比较,发现理论模型能较好预测本实验。     

矩形微通道内两相流流阻压降特性的可视化研究

以去离子水和质量分数为0.3%的水基纳米流体为工质,在当量直径为1.241mm的矩形微通道内进行了两相流流动沸腾的实验研究,并借助高速摄像仪对矩形微通道内流型随着质量流量及热流密度的变化进行了观察分析。实验结果表明:单位长度上的两相摩擦压降会随着质量流速的提升而提高;单位长度上的两相摩擦压降会随着热流密度的增大而升高;减小质量流速和提高其热流密度均会加快气泡的产生并提高气泡的脱离直径,当热流密度增大到一定程度时,通道内几乎为环状流与液态单相流交替出现,且环状流占周期中的较长时间。     

微通道内沸腾流动与传热特性耦合机理研究

以水为工质,模拟研究不同条件下水平矩形微通道沸腾流动过程中气泡发生发展及流型演变与温度、压力、传热系数的耦合关系。结果表明：持续吸热的弹状流会占据通道大部分流动区域,易造成堵塞和局部高温;泡状流区域压力波动幅度较小,较长弹状气泡和大雷诺数均会导致较大的局部压降;升高热流密度减小了单相区长度,强化了核态沸腾,提高了通道整体传热性能;增大雷诺数使得通道内气泡尺寸减小,减少了两相流动的不稳定与堵塞现象,通道整体传热性能得以提升。     

不同纳米流体在微通道内的流动传热特性研究

采用数值模拟的方法研究了不同工质在微通道内流动传热特性的差异。对比了去离子水、纳米流体Al₂O₃/Water、CuO/Water、TiO₂/Water、Cu/Water等工质在微通道内的流动传热特性,并研究了纳米颗粒的浓度对流动换热特性的影响。结果表明：CuO/Water作为冷却工质时的对流换热系数比水增加了9.6%,微通道底面平均温度降低了2.6 K,换热性能明显优于其他几种纳米流体。由于纳米颗粒的加入,纳米流体的粘度比水大,进出口的压降比水大。纳米颗粒的体积分数越大,对流换热系数越大,纳米流体在微通道内的换热性能越好。     

纳米流体微通道流动换热数值模拟研究

采用计算流体动力学方法,对两种不同浓度的水-Al2O3纳米流体以及五种不同高宽比的微通道热沉的流动换热特性开展了数值模拟研究.结果 表明,提高纳米颗粒体积分数可降低流固换热面的平均温度,从而提升纳米流体的换热能力,但同时也会显著提升系统的泵功率;通过改变微通道高宽比可有效提升热沉的换热能力,增大高宽比能够有效降低热沉受...     

微通道内流动沸腾换热特性实验研究

采用实验方法对制冷剂R134a在内径为1.98mm的水平光滑铜管内的流动沸腾换热特性进行研究。试验中,质量流速范围720~900kg/(m~2·s),热流密度范围19~28k W/m~2,系统压力0.7MPa和0.81MPa(饱和温度为26.8℃、31.4℃)和干度范围0~0.65。结果表明:质量流速对换热系数的影响较大,随着质量流速的增大而增大;在低干度区,热流密度对换热系数的影响较大,换热系数随干度的增加近似成单调增加;系统压力对换热系数也有明显的影响;将试验结果与Sun-Mishima公式和Liu-Winterton公式进行比较,发现试验结果与Sun-Mishima公式计算值吻合度较高,最大误差为14.1%。     

Experimental analysis of conical baffles with rifts on heat transfer and pressure drop

In the present study, convective heat transfer to the air from a heating tube attached to conical baffles with rift was experimentally examined. The air entering the test section first contacts the large surface of the conical baffle. Therefore, the conical baffle both directs the air toward the heating surface and increases the heat transfer surface area. In the experiments, baffles with inclination angles of 45°, 60°, and 80° were used. The baffles were placed on the heating tube at the pitch of 15 mm. The temperature of the heating fluid (water) was kept fixed at 65°C. In addition to the riftless baffles, the experiments were carried out by using baffles with a rift spacing of 1.5 and 3.5 mm so that the boundary layer separation mechanism could be accelerated. Experimental results for eight different velocities of airflow (2–20 m/s) were presented. For the inclination angle of 60°, the increase in the heat transfer of the baffle with rift was 13% at a rift spacing of 1.5 mm and 4% at a rift spacing of 3.5 mm according to the riftless baffle. In addition, for the inclination angle of 60°, the pressure drop values of the riftless and the rift spacing of 1.5 and 3.5 mm were almost the same.     

Condensation heat transfer and pressure drop characteristics of R-134a inside the flattened tubes at high mass flux and different saturation temperature

In this study, heat transfer coefficients and pressure drops of R-134a inside round and flat tubes are investigated experimentally with mass flux of 450, 550, and 650 kg m^?2 s^?1 at saturation temperatures of 35°, 40°, and 45°C. The effects of mass flux and saturation temperature on heat transfer coefficient and pressure drop are examined. The maximum enhancement factor and pressure drop penalty are obtained by flat tube (FT-2) up to 2.101 at 450 kg m^?2 s^?1 and 3.01 at 650 kg m^?2 s^?1, respectively. The correlation for flat tubes is proposed to predict the heat transfer coefficient within ±20% error.     

The use of enhanced surface in flow boiling heat transfer in a rectangular minichannel

The results of flow boiling heat transfer in a 1-mm-deep vertical minichannel are presented. The heating element for an FC-72 laminar flow in a minichannel is a single-sided enhanced foil with various depressions; liquid crystal thermography was used for measuring temperature distribution of the foil. The void fraction and vapor quality were determined for some cross-sections of two-phase flow images. Results presented in the form of boiling curves were analyzed. Both typical and untypical shapes of boiling curves were found. The suitability of classical methods of two-phase pressure drop determination using experimental verification was confirmed.     

Flow and heat transfer characteristics over rectangular blocked surfaces

Heat transfer and flow measurements were recorded over blocked surfaces to investigate the effects of flow velocity and block shape on flow separation and heat transfer. The results indicated that the flow separation occurred particularly on the first block and after the last blocks. The flow separation and block thickness resulted in higher turbulence and heat transfer, particularly in laminar flow. The average Stanton number increased as high as 125% in laminar, 80% in transitional, and 50% in turbulent flows above those of flat plate values. A new empirical equation indicated a good agreement with the experimental and numerical results.     

微小型矩形槽道平板热管传热性能的数值模拟

在充分考虑槽道内汽-液界面剪切摩擦力下,建立了微小型矩形槽道平板热管的数学模型,通过迭代计算得出工质质量流量、流体和蒸汽平均流速、汽一液界面弯月面半径及蒸汽和液体压力沿轴向分布规律,最后对热管外壁面温度值的计算结果和实验结果进行了比较,两者数值基本一致,证明了所建数学模型与实际相符,对热管的理论分析具有指导意义。     

池沸腾传热的数学分析

在统计方法的基础上,对于池沸腾换热的传热机理提出了一个数学模型. 在没有增加新的经验常数的条件下,从该模型中可得到池沸腾热流密度是壁面过热度、活化穴最小与最大尺寸、流体的接触角与流体物理特性的函数. 该模型可以较好地解释润湿性如何影响沸腾热流密度. 对不同的接触角,模型预测的结果与实验相符合. 关键词： 池沸腾 传热 数学模型     

双液滴撞击平面液膜的流动与传热特性

采用耦合的水平集-体积分数法(CLSVOF)对双液滴连续撞击恒定壁温壁面上的热液膜的流动和换热特性进行了数值模拟及分析,得到了双液滴撞击热液膜后形态演变的过程.分析了液滴垂直间距、撞击速度、液膜厚度以及液滴直径对双液滴撞击液膜后的流动与传热特性的影响,结果显示,壁面平均热流密度随液滴撞击速度的增大而增大,液滴垂直间距、液膜厚度和液滴直径对平均热流密度的影响较小,但会对热流密度在撞击区域和交界区的分布产生重要影响.