Temperature distribution in the cross section of wavy and falling thin liquid films

Abstract

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.

Simultaneous temperature and thickness measurements of falling liquid films by laser-induced fluorescence

“…This region roughly corresponds to the trough of the capillary undulations and the front region of the main wave. For f = 10 Hz, this observation can be related to an ascending liquid flow of higher temperature, which was experimentally evidenced by Collignon et al (2021) in the capillary trough for a very similar case (see figure 24 in Collignon et al (2021)). Outside the red regions mentioned above there is no singular region in the pattern, implying a more global effect on the heat transfer.…”

“…This relatively long distance (almost corresponding to the end of the foil) was chosen to allow the thermal boundary layer that develops from the heated surface to reach a sufficient height to interact with the convective wave structures. As demonstrated by Collignon et al (2021), waves have a very limited influence on the Nusselt number when x is too small. For f = 2 Hz, fluctuations of the film temperature are very small which was expected given the small amplitude of the waves in that case.…”

“…The experimental setup, described in Figure 1, is the same that was presented by Collignon et al (2021). The wall consists of a 100 µm thick titanium foil.…”

“…In an effort to resolve temperature gradients inside the film, Collignon et al (2021) recently developed a planar imaging technique based on the intensity ratio of two spectral detection bands (twocolor PLIF technique). The falling liquid film was illuminated by a pulsed laser sheet perpendicular to the wall while two cameras were used to capture simultaneous images of the fluorescence intensity signal in the two detection bands.…”

“…The two-color LIF technique is implemented as a pointwise technique by focusing the laser beam in a precise region of the flow. Thus, it is not intended to characterize thermal gradients along the film thickness like in Collignon et al (2021), but to monitor the time evolution of the thickness-averaged temperature at a given position in the film flow. Many approaches can be used to determine the local thickness of liquid films, including CCI (Confocal Chromatic Imaging) technique (Cohen-Sabban et al, 2001;Lel et al, 2005;Kofman et al, 2017), side view imaging orLIF intensity method (Adomeit and Renz, 2000;Alekseenko et al, 2005;Lel et al, 2005).…”