Abstract:

Under gravity-driven flow, the liquid film on an inclined wall can initially spread completely across the surface, but under heating conditions, a drying phenomenon may occur, forming local dry spots, which significantly reduce heat transfer efficiency and potentially damage equipment performance. Investigating the flow instability and the drying mechanism of the liquid film is crucial for optimizing the falling film evaporation process and the design of heat exchangers. In this study, a micrometer was used to measure the liquid film thickness, and the thickness characteristics of the liquid film on an inclined wall were examined under both heating and adiabatic conditions. The formation and evolution of “dry spots” during the flow were precisely captured, along with the effect of critical spraying density on flow instability. A three-dimensional model was proposed to accurately predict the occurrence of dry spots on the inclined wall, which simultaneously considered the thermal capillarity and evaporation effects of the liquid film. The results showed that under adiabatic conditions, the rate of change in the liquid film thickness was divided into rapid and gradual phases, with liquid viscosity and inclination angle playing key roles in the rapid change phase. The critical spraying density increased significantly with the inclination angle, mainly due to the combined effects of gravity and surface tension fluctuations, which enhanced the flow instability of the liquid film. Under heating conditions, both experimental and model analysis revealed the formation process of the drying mechanism, emphasizing how the surface tension gradient induced by thermal capillarity and evaporation effects work together, causing local rupture of the liquid film at the bottom of the wall, eventually leading to the formation of dry spot region. The findings of this study enriched the understanding of liquid film flow characteristics in falling film evaporation processes and provided technical support for the design of falling film heat exchangers.

Key words: liquid film flow, dry spots, liquid film thickness, spray density, falling film evaporation

摘要：

重力驱动下的倾斜壁面液膜流动，即使初始液膜能够完全铺展在壁面上，但在加热的作用下也可能会出现烧干现象，形成局部干斑，显著降低传热效率并损害设备性能。研究液膜流动不稳定性与烧干机制对于优化降膜蒸发过程和换热器设计至关重要。本文通过使用千分尺测量，研究了倾斜壁面在加热和绝热两种情况下的液膜厚度，精确捕捉了液膜流动过程中干斑的形成与演变，并进行了临界喷淋密度对流动不稳定性的探究，提出了能够精确预测倾斜壁面上液膜“干斑”出现的三维模型，该模型同时考虑了液膜的热毛细效应和蒸发作用。结果表明：在绝热条件下，液膜厚度的变化率划分为快速变化阶段和平缓变化阶段，液体的动力黏度和倾斜角度在快速变化阶段起关键作用；临界喷淋密度随着倾斜角度的增加显著升高，主要是重力和表面张力波动的共同作用导致了液膜流动不稳定性增强。在加热条件下，通过实验与模型分析，揭示了烧干机制的形成过程，重点阐明了热毛细效应引起的表面张力梯度与蒸发效应共同作用，导致液膜在壁面下部局部破裂，进而形成干斑区域。本文的研究内容丰富了降膜蒸发过程中液膜的流动特性，为降膜式换热器的设计提供了技术支撑。

关键词: 液膜流动, 干斑, 液膜厚度, 喷淋密度, 降膜蒸发