Falling films are used in many technical processes for cooling, condensation and evaporation. The hydrodynamics and the heat transfer of these wavy liquid films are important for the design of machines. The literature describes the hydrodynamics of thin films on vertical walls in detail, revealing that the waves of the film have a positive impact on the heat transfer. However, extensive studies on the impact of the stimulation frequency of the waves and their amplitude on the heat transfer of the film have not yet been sufficiently performed.
Due to this, the present work investigated the influence of wave stimulation on heat transfer. Thus, a solid basis for following work was created for this topic. For this purpose, a test stand was set up with which the flow characteristics and the heat transfer of a defined, periodically stimulated falling film can be investigated in a high-resolution manner. The film thickness was recorded with a chromatic-confocal distance measuring system and the film surface was photographed and evaluated with a camera system. The heat transfer was analyzed with wall temperature measurements.
The experiments were carried out in a wide parameter range with the specifically varied parameters frequency and amplitude of stimulation, as well as mass flow, heat flux and angle of inclination of the wall. First, it was shown that the wave forms develop depending on the stimulation frequency along the flow direction. The amplitude of the periodic stimulation, which was not analyzed in previous work, has a significant impact on the development of the waves. The other investigated wave characteristics include the average film thickness, the wave frequency, the wave maxima and minima, the film thickness distribution, the wavelength and velocity.
A comparison of the experimental results with extrapolated correlations did not provide a sufficient agreement and the parameters of the correlations were adapted to the experimental data. The analysis of the wall temperature measurements also revealed a strong frequency-dependent influence of the wave stimulation on the heat transfer. In certain frequency ranges, the heat transfer could be improved. These frequency ranges correspond to the occurrence of pronounced wave minima in the film flow.
- Schröder, Timm (2019): Strömungscharakteristika und Wärmeübertragung eines angeregten Fallfilms. Dissertation, Technische Universität Darmstadt, https://tuprints.ulb.tu-darmstadt.de/id/eprint/8742