Characteristic analysis of fluctuating liquid film flow behavior and heat transfer in nitrogen condensation

摘要

Liquid film fluctuation is supposed to be an important mean of film condensation enhancement for the cryogenic fluids with extremely low surface tension. However, the flow characteristics of the cryogenic liquid film in condensation process are seldom observed and revealed due to the difficulty of cryogenic experiment. In this study, the flow behavior and heat transfer of the fluctuating liquid film in nitrogen condensation are discussed by CFD simulation. A numerical model of nitrogen condensation on a vertical plate is established in accordance to a practical experimental setup, and is further validated with a good accuracy using the experimental data. The liquid film thickness, wave velocity and wall shear force are then analyzed. Statistical tools are employed to reveal the probability distribution of the film thickness and the frequency domain characteristics of the heat transfer coefficient. The FFT analysis of the heat transfer coefficient shows the interface waves around 30 Hz can induce a stronger fluctuation effect and bring about a better heat transfer performance. The velocity in the large solitary wave is regulated by the wall shear force, and a more appropriate correlation is proposed to reveal the physical properties of the solitary wave, which manifests as rolling on the film substrate. In addition, the simulation reveals that an important reason leading to the higher heat transfer coefficient in the experiments than that predicted by the Nusselt's theory is the decrease of film substrate thickness and the increasing probability of thinner liquid film during practical liquid fluctuating. It can be inferred that the condensation process of fluctuating liquid film is an effective way to enhance the heat transfer for cryogenic fluids.