High-Speed photographic observation of flow boiling of water in parallel mini-channels
High-Speed photographic observation of flow boiling of water in parallel mini-channels
复制标题
DOI：
--
复制
发表时间：
2001
期刊：
影响因子：
--
通讯作者：
S. Kandlikar;M. Steinke;Shu-qing Tian;L. Campbell
中科院分区：
文献类型：
--
作者：
S. Kandlikar;M. Steinke;Shu-qing Tian;L. Campbell
在流动沸腾应用中，使用更小的通道尺寸正变得越来越普遍。液压直径约为1mm的通道提供更高的传热系数，从而使热交换器布置更紧凑。与微通道相比，液压直径为1mm的通道不会造成严重的堵塞或阻塞限制，并且压降损失较小。本文研究了微通道中水的流动沸腾过程。高速摄影可以获得从过冷流沸腾一直到临界热流条件的流动现象的视觉图像。测试包括一个单通道和一组六个平行通道，从通道的三个侧面进行电加热。顶盖由Lexan制成，允许视觉观察。结果用于识别小直径微通道中流动沸腾的具体特征。Ac：截面面积G：通过试验截面的水的质量流量（）c A m m：通过试验截面的水的质量流量（kg/s） Q“：通过通道壁到水的热流密度（kW/m） TA：试验截面入口的体温（℃）TB：试验截面出口的体温（℃）TS：试验截面的平均表面温度（℃）x：在汽车空调、热管、电子设备的直接制冷冷却和燃料电池等许多应用中，增加蒸发器中蒸发液体侧的传热系数的需求变得越来越重要。虽然小直径通道的蒸发在20世纪60年代受到了相当大的关注（例如，Bergles, 1964），但它在具有多个通道的紧凑蒸发器配置中的应用目前正在受到广泛关注。与多通道蒸发有关的复杂性尚不清楚。本研究的重点是深入了解液力直径为1mm的多通道电加热蒸发器蒸发过程中水的两相流动特性。文献综述文献中很少有关于小直径多通道蒸发器流动模式的研究。传统的紧凑型蒸发器是板翅式的，蒸发液体在两个平行的板之间流动，这些板有均匀间隔的凸起，用于将板钎焊在一起。这些障碍物增强了蒸发侧的传热。然而，蒸发液体可以自由地流过平板宽度。使用直径较小的通道，每个1毫米或更小的液压直径呈现出完全不同的场景。在这种情况下，两个管汇之间的压降相当高，蒸发液体不能像板翅式蒸发器那样流过另一个流道。
The use of smaller passage dimensions is becoming more prevalent in flow boiling applications. Passages with hydraulic diameters on the order of 1mm provide higher heat transfer coefficients resulting in more compact heat exchanger arrangements. Passages with 1mm hydraulic diameters do not pose severe clogging or blocking constraints as opposed to micro-channels and have a less pronounced pressure drop penalty. The present paper explores the flow regimes during flow boiling of water in mini-channels. High-speed photography is used to obtain visual pictures of the flow phenomena from subcooled flow boiling all the way to critical heat flux conditions. The tests include one single-channel and a set of six parallel channels with electric heating from three sides of the channel. The top cover is made of Lexan to permit visual observations. The results are used to identify specific features of flow boiling in smaller diameter mini-channels. NOMENCLATURE Ac: Cross-Section area G: Mass flux of water through test section ( ) c A m m : Mass flow rate of water through test section (kg/s) Q”: Heat flux through channel walls to water (kW/m) TA: bulk temperature at the test section inlet (°C) TB: bulk temperature at the test section outlet (°C) TS: Average surface temperature of the test section (°C) x: vapor mass fraction at outlet ( ) fluid vapor m m INTRODUCTION The need to increase heat transfer coefficients on the evaporating liquid side in an evaporator is becoming increasingly important in many applications including automotive air conditioning, heat pipes, direct refrigeration cooling of electronic devices, and fuel cells. Although evaporation in small diameter channels received considerable attention in the 1960’s (for example, Bergles, 1964), its use in a compact evaporator configuration with multiple channels is currently receiving wide attention. The complexities associated with evaporation in multiple channel passages are not clearly understood. The present study focuses on providing an insight into the two-phase flow characteristics during evaporation of water in 1-mm hydraulic diameter, multiple channel, electrically heated evaporator section. LITERATURE REVIEW There are very few publications available in literature addressing the flow patterns in multi-channel evaporators with small diameter channels. Conventional compact evaporators are plate-fin type, with the evaporating liquid flowing between two parallel plates that have uniformly spaced bumps for brazing the plates together. These obstructions provide a heat transfer enhancement on the evaporation side. However, the evaporating liquid is free to flow across the plate width. The use of small diameter channels, each of 1-mm or less hydraulic diameter presents quite a different scenario. Here the pressure drop between the two manifolds is quite high and the evaporating liquid cannot flow across into another flow channel as in the case of a plate-fin type evaporator.