Abstract:

The flow pattern of CO₂ boiling in a horizontal smooth tube with an inner diameter of 5 mm is visualized under diabatic conditions. The flow pattern maps of CO₂ are obtained for the experimental evaporation temperatures of 7—15℃, heat flux of 5—35 kW·m^-2, and mass flux of 100—500 kg·m^-2·s^-1. The two-phase flow patterns in the tube include bubbly flow, plug flow, slug flow, stratified-wave flow, annular flow and mist flow. The experimental results show that the liquid film at the top of the annular flow dries out prematurely at low to medium quality and the heat transfer coefficient gradually decreases. The reduction of evaporation temperature and the increase of mass flow rate help to stabilize the liquid film around the tube, which can effectively inhibit the decrease of heat transfer coefficient due to early dry-out. When the mass flux is 400 kg·m^-2·s^-1 and the heat flux is 30 kW·m^-2, the heat transfer coefficient at the top of the tube at the evaporation temperature of 7℃ is about 40% higher than that at 15℃.

Key words: CO₂, flow pattern map, flow boiling, visualization, diabatic conditions

摘要：

通过对内径5 mm的水平光滑管内流动沸腾的流型进行非绝热可视化研究，获得了实验蒸发温度为7~15℃，热通量为5~35 kW·m^-2，质量流速为100~500 kg·m^-2·s^-1条件下CO₂的流型图。实验结果显示，管内的两相流流型涵盖了泡状流、塞状流、弹状流、分层波浪流、环状流以及雾状流。环状流顶部液膜在中低干度会提前干涸，导致传热系数逐渐下降。降低蒸发温度或提升质量流速有助于稳定管周液膜，有效抑制因提前干涸而导致的传热系数降低现象。当质量流速为400 kg·m^-2·s^-1，热通量为30 kW·m^-2时，蒸发温度为7℃时的管顶部传热系数相较于15℃时高约40%。

关键词: 二氧化碳, 流型图, 流动沸腾, 可视化, 非绝热条件