µPIV Measurements Of The Phase-Averaged Velocity Distribution Within Wavy Films

Abstract

Thin-film flows are frequently used in process engineering and are still of great interest in research. In most cases, waves occur which are believed to have a positive effect on the material and heat transport and significantly influence the film properties. A combination of μ-particle image velocimetry and confocal chromatic distance measurement is used to measure phase-averaged flow velocities in film waves. A trigger signal is sent to the PIV system and a recording starts when a wave reaches the wave crest threshold. This makes it possible to trigger the μPIV system depending on the temporal film thickness. The functionality and quality of the measurements depend on the correct positioning of the confocal chromatic sensor. Simultaneously to the velocity measurement, the minimum, average, and maximum film thickness, as well as the wave amplitude and wave frequency, are recorded. Film flow is investigated on a glass plate at various angles (5˚-15˚) of inclination, Reynolds numbers (30-200), and measurement positions. The results are compared with the theoretical solutions according to Nusselt. In addition, the influence of the tilt angle and the measuring position is explained. The measuring position, in particular, has a non-negligible influence since the film properties are largely dependent on the distance to the inlet. Compared to the theoretical solutions according to Nusselt, the velocities and film thicknesses may be significantly greater due to the presence of the waves. With increasing inclination angle and distance to the inlet, the measured mean and maximum velocities of the film flow increase. They almost double when the inclination angle increases from 5˚ to 15˚. When comparing the velocities depending on the measuring position (300-500mm), it is noticeable that the velocities increase between 5-15%, depending on the angle, due to wave development and gravity.