Investigation on film break-up behavior and the Marangoni effect during non-isothermal liquid/gas falling film absorption

Investigating film rupture behavior is significant but remains challenging for non-isothermal falling film absorption such as liquid desiccant dehumidification. This study captured and analyzed more than 15,000 sets of break-up patterns, and the distribution of film thickness and surface temperature across the entire falling film during liquid-gas convective absorption, with special attention to the characteristics near break-up points. The Marangoni effects, caused by surface temperature gradients, were also evaluated. Improved laser-induced fluorescence technique was employed, in which fluorescence images of film thickness were taken from the back of film substrate, and infrared thermal images were taken from the air channel side, avoiding the influence of channel vibration and reflection caused by air flow. The operating temperature of lithium chloride solution and air were 25 similar to 60 degrees C and 25 degrees C respectively. Results show that for non-isothermal falling film, surface solitary waves play a decisive role in film break-up as most rupture occur near the crests of solitary waves. Break-up points can be classified into fixed and unfixed ones. Fixed points and associated dry areas persist in the flow due to the consistent temperature gradients on both sides of the point, which is the main cause of the reduced mass transfer. Unfixed break-up points can be rewetted by flow, but their presence still causes performance fluctuations as their positions shift laterally during falling film. Increasing the liquid temperature can increase the absorption driving force. However, when solution/substrate temperature difference is insufficient high, the enhanced Marangoni effect will promote solitary waves and both types of break-up points increase. The recommended difference in internally heated falling film regenerators should be higher than 15 degrees C to enhance film spreading and mass transfer. Increasing countercurrent airflow can improve mass transfer by reducing the film thickness without exacerbating the film rupture. This study explores falling film rupture behavior with flowing air, improving understanding of film dynamics and liquid-gas transfer, valuable for optimizing operational conditions in practical falling film absorption designs.