Wave instabilities of falling liquid films are crucial in many applications to enhance heat and mass transfer. Despite the importance of this issue, the interplay between the heat transfer and the wavy dynamics of falling films is still not completely understood. To get more insight, a planar laser-induced fluorescence technique has been developed for imaging the temperature distribution in the cross section of thin liquid films (approximately 0.5–1 mm thick), which are falling down an inclined heated surface. This study reports on the implementation of this imaging technique. It also discusses its advantages and limitations for the investigation of the heat transfer in the falling liquid films. Two-dimensional flow conditions and regular waves are considered for the reconstruction of a complete temperature field in the waves. Measurements provide new understanding of the wave ability to generate mixing within the film. Temperature maps reveal preferential regions where mixing occurs first, before eventually spreading to the rest of the film if the wave amplitude and the travel distance are large enough. The increase in the heat transfer coefficient is directly related to the internal mixing observed in the temperature images.

Graphic abstract

中文翻译：

---

波浪状和下落状液体薄膜横截面的温度分布

下落的液膜的波不稳定性在许多应用中对于增强热量和质量传递至关重要。尽管此问题很重要，但对于降膜的传热和波浪动力学之间的相互作用仍然没有完全了解。为了获得更多的见识，已经开发了一种平面激光诱导的荧光技术，用于对落在倾斜的加热表面上的液体薄膜（约0.5–1 mm厚）的横截面中的温度分布进行成像。这项研究报告了这种成像技术的实施。它还讨论了其优点和局限性，以研究下落的液膜中的热传递。考虑二维流动条件和规则波来重构波中的完整温度场。测量提供了对在薄膜内产生混合的波动能力的新理解。如果波幅和传播距离足够大，则温度图显示优先发生混合的区域，然后最终扩散到其余薄膜。传热系数的增加与温度图像中观察到的内部混合直接相关。

图形摘要