Liu Lei, Liu Dongxu, Huang Na
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, China.
Entropy (Basel). 2021 Jan 27;23(2):156. doi: 10.3390/e23020156.
It is widely accepted that the frictional pressure drop is impossible to be negative for pipe flow. However, the negative frictional pressure drops were observed for some cases of two-phase slug and churn flows in pipes, challenging the general sense of thermodynamic irreversibility. In order to solve this puzzling problem, theoretical investigations were performed for the entropy generation in slug and churn flows. It is found that the frictional pressure drop along with a buoyancy-like term contributes to the entropy generation due to mechanical energy loss for steady, incompressible slug and churn flows in vertical and inclined pipes. Experiments were conducted in a vertical pipe with diameter as 0.04 m for slug and churn flows. Most of the experimental data obtained for frictional pressure drop are negative at high gas-liquid ratios from 100 to 10,000. Entropy generation rates were calculated from experimental data. The results show that the buoyancy-like term is positive and responsible for a major part of entropy generation rate while the frictional pressure drop is responsible for a little part of entropy generation rate, because of which the overall entropy generation due to mechanical energy loss is still positive even if the frictional pressure drop is negative in vertical slug and churn flows. It is clear that the negative frictional pressure drops observed in slug and churn flows are not against the thermodynamics irreversibility.
普遍认为，对于管道流动，摩擦压降不可能为负。然而，在管道中两相弹状流和 churn 流的某些情况下观察到了负的摩擦压降，这对热力学不可逆性的一般概念提出了挑战。为了解决这个令人困惑的问题，对弹状流和 churn 流中的熵产生进行了理论研究。研究发现，对于垂直和倾斜管道中稳定、不可压缩的弹状流和 churn 流，摩擦压降与一个类似浮力的项共同导致了由于机械能损失而产生的熵。在直径为 0.04 m 的垂直管道中对弹状流和 churn 流进行了实验。在高气液比（从 100 到 10000）下获得的大部分摩擦压降实验数据为负。根据实验数据计算了熵产生率。结果表明，类似浮力的项为正，并且是熵产生率的主要贡献部分，而摩擦压降仅占熵产生率的一小部分，因此即使在垂直弹状流和 churn 流中摩擦压降为负，由于机械能损失产生的总熵仍然为正。显然，在弹状流和 churn 流中观察到的负摩擦压降并不违背热力学不可逆性。
