Carbon dioxide (CO2/R-744) is receiving renewed interest as a refrigerant, in many cases for systems with microchannel heat exchangers that have high pressure capability, efficient heat transfer, and compact design, A good understanding of two-phase flow of evaporating CO{sub}2 in microchannels is needed to analyze and predict heat transfer: A special test rig was built in order to observe two-phase flow patterns using a horizontal glass tube with ID 0.98 mm, Flow visualization experiments were conducted for temperatures 20℃ and 0℃ and for mass flux ranging from 100 to 580 kg m{sup}(-2)s{sup}(-1), The observations showed a dominance of intermittent (slug) flow at low x and wavy annular flow with entrainment of droplets at higher x. The aggravated dryout problem reported from heat transfer experiments at high mass flux could be explained by increased entrainment. The flow pattern observations did not fit generalized maps or transition lines showed in the literature.
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机译:二氧化碳(CO2 / R-744)作为制冷剂正受到新的关注,在许多情况下,对于具有微通道热交换器的系统,该系统具有高压能力,有效的热传递和紧凑的设计,对蒸发的两相流有很好的了解需要使用微通道中的CO {sub} 2来分析和预测传热:建立了一个特殊的试验台,以便使用ID为0.98 mm的水平玻璃管观察两相流模式,并在20℃的温度下进行流可视化实验在0℃和100至580 kg m {sup}(-2)s {sup}(-1)的质量通量下,观测结果表明低x时间歇性(段塞)流和带夹带的波浪状环形流占优势x较高时的液滴数量。传热实验在高通量下报告的加剧的变干问题可以用夹带增加来解释。流动模式的观察结果不适合文献中显示的广义图或过渡线。
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