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。     

Pressure drop in two phase slug/bubble flows in mini scale capillaries

A segmented two phase slug/bubble flow occurs where a liquid and a gas are pumped into the same tube over a range of Reynolds numbers. This segmented two phase flow regime is accompanied by an increase in pressure drop relative to the single phase flow where only one fluid is flowing in a capillary. This work experimentally and theoretically examines the pressure drop encountered by the slug/bubble flow with varying slug lengths in mini channels. In the experimental work the dimensionless parameters of Reynolds number and Capillary number span over three orders of magnitude, and dimensionless slug length ranges over two orders of magnitude to represent flows typical of mini- and micro-scale systems. It is found, in agreement with previous work, that these dimensionless groups provide the correct scaling to represent the pressure drop in two phase slug/bubble flow, although the additional pressure drop caused by the interface regions was found to be ∼40% less than previously reported.     

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...     

Review of two-phase flow and flow boiling of mixtures in small and mini channels

Two-phase flow and flow boiling phenomena of fluidic mixtures in small and mini channels are becoming important in the miniaturization of thermal systems. This paper aims to present a state-of-the-art review in this important area and to identify what have been done so far and what still need to be done in the future. Firstly, various definitions of small and mini channels are described and the criteria based on these definitions are compared with each other. Comments on different viewpoints of the channel size classifications are acknowledged. Secondly, the background of two-phase flow and flow boiling of mixtures is described. Then, the current research status of two-phase flow and flow boiling of mixtures in normal size channels is presented as it is a basis for the study of two-phase flow and flow boiling of mixtures in small and mini channels. Finally, an overall review of two-phase flow and flow boiling of mixtures in small and mini channels is presented. It is concluded that the available study of two-phase flow and flow boiling of mixtures in small and mini channels is rather scarce and a systematic knowledge of two-phase flow and flow boiling of mixtures in small and mini channels is required. Based on this review, the future research directions including both fundamental and applied research in this area have been indicated.     

Flow boiling heat transfer characteristics of R123 and R134a in a micro-channel

Flow boiling heat transfer in a single circular micro-channel of 0.19 mm ID has been experimentally investigated with R123 and R134a for various experimental conditions: heat fluxes (10, 15, 20 kW/m²), mass velocities (314, 392, 470 kg/m² s), vapor qualities (0.2–0.85) and different saturation pressures (158, 208 kPa for R123; 900, 1100 kPa for R134a). The heat transfer trends between R123 and R134a are clearly distinguished. Whether nucleate boiling is suppressed at low vapor quality or not determines the heat transfer trend and mechanism in the flow boiling of micro-channels. High convective heat transfer coefficients in the two-phase flow of micro-channels enables nucleate boiling to be suppressed even at low vapor quality, depending on the wall superheat requirement for nucleate boiling. In the case of early suppression of nucleate boiling, specifically R123, heat transfer is dominated by evaporation of thin liquid films around elongated bubbles. In the contrary case, namely R134a, nucleate boiling is dominant heat transfer mechanism until its suppression at high vapor quality and then two-phase forced convection heat transfer becomes dominant. It is similar to the heat transfer characteristic of macro-channels except the enhancement of nucleate boiling and the short forced convection region.