Abstract:

In order to investigate the local heat and mass transfer characteristics in vapor absorption of falling film flow on a bundle of horizontal tubes in droplet and column flow mode, a transient computational fluid dynamics(CFD) model was developed. The model not only considered the simultaneous hydrodynamic characteristics and mass transfer of LiBr solution flowing on and between tubes, but also considered the real boundary condition for multi-rank tubes and the heat transfer between phases. Simulations were performed for the film Reynolds number varying from 11 to 38. The simulation results showed that the solution outlet concentration and temperature in different mass flow rates agreed well with the experimental data in reference, with deviations less than 2%. In both the droplet and column flow modes, the average concentration and temperature decreased rapidly in the falling-film regions, but increased then decreased in the inter-tube regions. The local absorption rates in the falling-film regions were about 10 times as much as that in the inter-tube regions in the droplet flow mode, and 7 times in the column flow mode. In the falling-film regions, the absorption rate in the column flow mode is obviously less than that in the droplet flow mode. However, in the inter-tube regions, the absorption rates in the two different flow modes were almost the same. When the absorption process reached a steady state, the variations of average concentration and temperature in the droplet flow mode were greater than that in the column flow mode. The average concentration variations of the falling-film regions on the four tubes increased in turn, but the average temperature variations decreased.

Key words: two-phase flow, falling film, heat and mass transfer, absorption, computational fluid dynamics

摘要：

为分析滴状和柱状流型下纯水蒸气水平管外降膜吸收过程的局部传热传质特性，建立非稳态数值模型考虑吸收过程中降膜区和管间区内液相的实际流动特征及气液两相的传质，同时对多管排区域采用实际边界条件，且考虑气液两相的传热过程。溶液的液膜Reynolds数范围为11~38。结果表明，与文献实验对比，相同流量下溶液出口浓度和温度的平均相对误差在2%以内；滴状和柱状流型下，降膜区溶液的平均浓度和温度均迅速下降，管间区先上升后下降，降膜区溶液的局部吸收速率分别约为管间区的10倍和7倍；柱状流型下降膜区的吸收速率明显小于滴状流型，管间区相差很小；吸收达到稳定后，滴状流型下溶液的平均浓度和温度变化均大于柱状流型，四排管降膜区溶液的浓度变化量依次增大，温度变化量依次减小。

关键词: 两相流, 降膜, 传热传质, 吸收, 计算流体力学