ISSN Print: 2169-2785
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Interfacial Phenomena and Heat Transfer
APPLICATION OF PLANAR LASER-INDUCED FLUORESCENCE FOR THE INVESTIGATION OF INTERFACIAL WAVES AND RIVULET STRUCTURES IN LIQUID FILMS FLOWING DOWN INVERTED SUBSTRATES
Clean Energy Processes (CEP) Laboratory, Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
Imperial College London
We investigate the interfacial topology of liquid-film flows falling under an inverted planar substrate by conducting space- and time-resolved film-height measurements. A planar laser-induced fluorescence (PLIF) technique is employed for this purpose, with a two-camera arrangement that allows us to image a region of the flow extending ≈ 40 mm on either side of the center of the film span, at a distance 330 mm downstream of the flow inlet. The substrate inclination angle is set to β = −30°, the working fluid comprises 82% glycerol and 18% water (by weight), and the flow Reynolds number, Re, is varied in the range Re = 0.6−8.2. The uncertainty associated with the instantaneous film-height measurement is estimated at less than 3%. Depending on the flow Re, we observe a range of interesting flow regimes typically characterized by pronounced rivulet formation and spatiotemporal coherence, which deviate from expectations of liquid-films flows falling over planar substrates. Over the range Re = 0.6−3.5, a series of regime transitions takes place, followed by the generation of regular, in both space and time, 3D solitary pulses "riding" over rivulet flow structures. These waves grow with increasing flow Re, as more liquid is drawn away from the rivulet troughs due to gravity. Finally, the wave frequencies and rivulet wavelengths are investigated by employment of power spectral density (PSD) and wavelet analyses. The application of PSD analysis offers superior resolution in the frequency domain when performed on temporally varying film-height data, whereas wavelet analysis is preferred when considering the spatially varying film-height data due to the limited spatial extent and low number of captured rivulets in the imaged region.
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