Abstract:

The corrugated channel can be used to break the accumulation of non-condensable gas (NCG) layers and reduce the resistance to mass transfer in the cryogenic removal of volatile organic compounds (VOCs). In this study, a two-dimensional model for the cryogenic removal of low concentration R134a from nitrogen on a sawtooth corrugated plate channel was established, and the impact of the sawtooth bottom angle on the removal performance of R134a were revealed. The results showed that the isosceles triangular sawtooth corrugated plate can significantly improve the removal efficiency of R134a on the cold wall surface per unit area compared with the flat plate channel. The spatio-temporal average phase change rate (α) of R134a on the sawtooth plate under the conditions of present study was 0.1268g/(m²·s), which increased by 144.6% compared with flat surface. The NCG flow near the cold wall surface will convert from the flowing adjacent to the wall, to off-wall mixing with the bulk flow, and finally to forming the flow blocking zone with stable vortex with the increase in the bottom angle (θ) of the sawtooth corrugated plate. Significant off-wall mixing with the bulk flow of the NCG layer occurred when θ was 13.30°—35.45°, with α ranged from 0.082—0.1268g/(m²·s). The α reached the maximum value of 0.1268g/(m²·s) when θ was 17.46°. The effect of the off-wall mixing with the bulk flow of the NCG layer will decay and the α will keep at a lower level when θ deviated the aforementioned range. The present study can provide a reference for the design of sawtooth corrugated plate structure for cryogenic condensation-freeze removal of low concentration VOCs.

Key words: waste treatment, mass transfer, cryogenic removal, numerical simulation, corrugated plate

摘要：

针对可挥发性有机物（VOCs）的低温脱除，采用波纹通道可破坏不凝气（NCG）层积聚、减小低温相变脱除传质阻力。本文建立了针对锯齿波纹板通道内少量R134a与氮气混合气的低温相变脱除的二维模型，揭示了锯齿齿板底角角度对R134a相变脱除效果的影响特性。结果表明，与平板通道相比，本文采用的对照组等腰三角锯齿波纹板可显著提升单位面积冷壁面上R134a的脱除效率，在本研究条件下R134a时空平均相变速率（α）为0.1268g/(m²·s)，提高了144.6%；锯齿波纹板底角（θ）增加时，NCG层在近冷壁面处会依次发生贴壁流动、脱壁掺混，最终形成稳定漩涡的流动阻滞区。当θ为13.30°~35.45°时，NCG层发生显著的脱壁掺混，α在0.082~0.1268g/(m²·s)之间；当θ=17.46°时，α达到最大值0.1268g/(m²·s)。当θ过大或过小时，脱壁掺混效果都将衰减，α在较低的水平。本文对用于低浓度VOCs的冷凝-冻结脱除的锯齿波纹板结构设计具有参考意义。

关键词: 废物处理, 传质, 低温脱除, 数值模拟, 波纹板