Critical heat flux enhancement by a two-layer structured honeycomb porous plate in a saturated pool boiling of water
Critical heat flux enhancement by a two-layer structured honeycomb porous plate in a saturated pool boiling of water
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DOI：
10.1016/j.ijheatmasstransfer.2017.10.100
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发表时间：
2018-03
影响因子：
5.2
通讯作者：
S. Mori;N. Maruoka;Kunito Okuyama
中科院分区：
文献类型：
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作者：
S. Mori;N. Maruoka;Kunito Okuyama
在本文中，我们提出了一种新的临界热通量（CHF）的增强技术，使用两层结构的蜂窝多孔板（HPP），可以应用在原则上，无论加热器的方向。在以前的研究中，CHF在饱和池沸腾的水进行了实验研究，使用HPP连接到一个加热的表面，并被证明是增强到一个平面的两倍以上（2.0 MW/m2）。根据所提出的毛细极限模型，CHF可以通过减小HPP的厚度来增加，因为多孔介质中的液体流动引起的摩擦压降减小。然而，当HPP的厚度与在聚结蒸汽气泡下方形成的薄液膜的厚度（约100 μm）（宏观层厚度）相当时，CHF不能被大大增强。通过对CHF附近沸腾形态的观察，发现在加热表面上形成了一个大的聚并气泡，并周期性地离开。因此，当包含在多孔材料中的水由于气泡悬浮期间的蒸发而消失时，可能发生CHF。为了防止大聚并气泡悬停期间的干涸现象，提高CHF，本文提出通过两种HPP的叠加来改善HPP的结构，并且每种HPP必须满足两个条件。首先，简单地附接到加热表面的HPP应该具有非常细的孔，以由于强的毛细作用而将水供应到加热表面，并且HPP应该尽可能薄，以减小由内部水流引起的压降。第二，堆叠在薄HPP顶部上的另一HPP必须构造成保持足够量的水，以防止HPP的内部在板上方的气泡悬停期间变干。此外，对于进一步的CHF增强，我们发现由表面粗糙度引起的HPP和加热表面之间的差距是重要的，因为蒸汽通过该间隙逸出，使得液体容易地供应到传热表面。
In the present paper, we propose a novel critical heat flux (CHF) enhancement technique using a two-layer structured honeycomb porous plate (HPP) that can be applied in principle regardless of heater orientation. In a previous study, the CHF during saturated pool boiling of water was investigated experimentally using an HPP attached to a heated surface and was shown to be enhanced to more than twice (2.0 MW/m2) that for a plain surface. According to the proposed capillary limit model, the CHF can be increased by decreasing the thickness of the HPP because of the decrease in the frictional pressure drops caused by the liquid flow in the porous medium. However, the CHF could not be greatly enhanced when the thickness of the HPP was comparable to the thickness (approximately 100 μm) of the thin liquid film (the macro-layer thickness) formed beneath coalescent vapor bubbles. Based on the observation of the boiling configuration near the CHF, a large coalesced bubble forms on the heated surface and departs periodically. Therefore, the CHF may occur when water contained in a porous material disappears due to evaporation during the bubble hovering period. In order to prevent the dry-out phenomenon during the hovering period of a large coalesced bubble and enhance the CHF, we herein propose that the structure of HPPs should be improved by the superposition of two kinds of HPPs and that each of the HPPs must satisfy two conditions. First, an HPP simply attached to a heated surface should have very fine pores to supply water to the heated surface due to strong capillary action, and the HPPs should be as thin as possible in order to decrease the pressure drop caused by internal water flow. Second, the other HPP, which is stacked on top of the thin HPP, must be structured to hold a sufficient amount of water in order to prevent the inside of the HPP from drying out during the bubble hovering period over the plate. Moreover, for further CHF enhancement, we found that the gap between the HPP and the heated surface caused by the surface roughness is important because vapor escapes through this gap so that liquid is easily supplied to the heat transfer surface.