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首页> 外文期刊>International Journal of Refrigeration >Study on two-phase flow pattern of supercritical carbon dioxide with entrained PAG-type lubricating oil in a gas cooler
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Study on two-phase flow pattern of supercritical carbon dioxide with entrained PAG-type lubricating oil in a gas cooler

机译：气体冷却器中夹带PAG型润滑油的超临界二氧化碳的两相流型研究

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摘要

In order to clarify the heat transfer mechanism of supercritical carbon dioxide flowing with a small amount of lubricating oil, visualization experiments were conducted using two sight glass tubes with inner diameters of 2 and 6 mm. The visualization images were recorded using a high-speed CCD camera with shutter speed changing from 500 to 10, 000 pps (pictures per second). PAG-type oil, which is partially miscible with supercritical carbon dioxide, was used. The experiments were conducted with lubricating oil concentrations of 1 and 5 wt%, pressures between 8 and 10 Mpa, and mass fluxes between 200 and 1200 kgm{sup}(-2) s{sup}(-1). The visualization images revealed that the two-phase flow pattern inside the gas cooler was determined by many factors, including the tube diameter, oil concentration, temperature, pressure, and mass flux. For a small size tube of 2 mm ID, the formation of both oil droplets in the bulk region and an oil film along the inner wall of the tube was confirmed. At low temperatures, a large number of oil droplets were observed flowing with CO{sub}2 with a slip ratio of approximately 0.7. With an increase in the bulk temperature, both the dimension and number of oil droplets entrained with CO{sub}2 decreased, and the flow of the oil film became clearly visible. For a large tube of 6 mm ID, the flow pattern at a low mass flow rate was a separated wavy flow; with an increase in mass flow, the flow pattern changed to annular flow, which corresponds to a distinct decrease in the heat transfer coefficient due to the heat resistance of oil layer. The transition of the flow pattern is considered due to the shear stress between the oil layer and bulk CO{sub}2. In addition, the dissolution of CO{sub}2 into PAG oil as well as the change in the solubility and thermodynamic properties of CO{sub}2 with the temperature and pressure makes the prediction of the flow patterns a challenging task. The relationship between the flow pattern and heat transfer characteristics was also discussed.
机译：为了阐明超临界二氧化碳与少量润滑油一起流动的传热机理，使用两个内径分别为2和6 mm的观察镜管进行了可视化实验。可视化图像是使用高速CCD相机记录的，快门速度从500 pps更改为10,000 pps（每秒图片）。使用了可与超临界二氧化碳部分混溶的PAG型油。实验是在润滑油浓度为1和5 wt％，压力在8和10 Mpa之间，质量通量在200和1200 kgm {sup}（-2）s {sup}（-1）之间进行的。可视化图像显示，气体冷却器内的两相流型由许多因素决定，包括管径，油浓度，温度，压力和质量流量。对于内径为2 mm的小尺寸试管，确认了在主体区域中既形成了油滴，又沿着试管的内壁形成了油膜。在低温下，观察到大量油滴以大约0.7的滑移率与CO {sub} 2一起流动。随着整体温度的升高，CO {sub} 2夹带的油滴的尺寸和数量均减小，并且油膜的流动变得清晰可见。对于内径为6 mm的大管，低质量流量下的流型为分离的波状流。随着质量流量的增加，流动模式变为环形流动，这对应于由于油层的耐热性导致的传热系数明显下降。由于油层和大块CO {sub} 2之间的剪切应力，考虑了流型的转变。此外，CO {sub} 2在PAG油中的溶解以及CO {sub} 2的溶解度和热力学性质随温度和压力的变化，使得对流型的预测成为一项艰巨的任务。还讨论了流型与传热特性之间的关系。

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