竖直矩形流道中泡核沸腾起始点的研究

Studies on the onset of nucleate boiling in vertical rectangular flow channels

竖直矩形窄流道由于具有良好的换热效果和紧凑的结构而被广泛应用于工程领域。针对两个间隙分别为1.8 mm和2.8 mm的竖直矩形流道,以可视化的方法研究了过冷沸腾条件下进口温度、质量流速和流道高度对泡核沸腾起始点(Onset of Nucleate Boiling,ONB)的气泡成长和壁面过热度的影响。以去离子水作为工质,使用高速照相机拍摄气泡的成长过程并使用热电偶记录背板温度。发现两个流道中进口温度和质量流速对ONB点的气泡成长以及ONB点壁面过热度的影响一致;质量流速一定时,流道高度对ONB点的气泡成长无明显影响;流道高度对ONB点壁面过热度与面热流密度的关系无明显影响;ONB点壁面过热度与面热流密度的关系可以由文献中的经验关系式预测。

[Background] Vertical rectangular narrow flow channel has been widely used in engineering field due to its good heat transfer effect and compact structure. While the study on onset of nucleate boiling (ONB) site in vertical rectangular flow channels is very limited.[Purpose] This study aims to figure out the effects of inlet fluid temperature, mass flux and channel height by experiment in the vertical rectangular flow channels with 1.8 mm and 2.8 mm gaps respectively.[Methods] Visualization method was employed to observe the bubble growth and wall superheat at the ONB point under subcooled boiling condition. Deionized water was used as the working medium, a high-speed camera was used to photograph the growth process of bubbles and the temperature of the backing plate was recorded by thermocouples.[Results] It was found that the inlet temperature, mass flux and channel height had the same effects on the bubble growth and the wall superheat at the ONB point in the two flow channels. The bubble size of ONB point in two flow channels increases with the increase of the inlet temperature, and decreases with the increase of mass flow rate.[Conclusions] Under the condition of constant mass flow rate, the flow channel height has no significant effect on the bubble size at ONB point. The superheat degree of the ONB point in two flow channels decreases with the increase of inlet temperature and increases with the increase of the mass flow rate. The relationship between the wall superheat of ONB point and the heat flux is not affected by the flow path height and it can be predicted by the empirical formula in literature.