document: In general, liquid film flows of practical relevance are turbulent and, hence, are associated with the presence of broadband interfacial waves on the film surface. A thorough understanding of the characteristic profiles, scales and dynamics of these interfacial waves is of essential importance in making accurate and reliable predictions of heat and mass transfer rates (Mathie and Markides, 2013a; Mathie et al., 2013). Previous efforts in downwards annular flow have focused on the spatio/temporal measurement of liquid film thickness, followed by in-depth statistical analyses of this film thickness (Webb and Hewitt, 1975; Belt et al., 2010; Alekseenko et al., 2012; Zhao et al., 2013). These efforts have contributed to a much improved understanding of the interfacial topology observed in downwards annular flows and also to the subsequent proposal of a series of correlations for the quantification of the mean film thickness, wave amplitudes and liquid entrainment rates into the gas phase (Ambrosini et al., 1991; Karapantsios and Karabelas, 1995; Azzopardi, 1997). On the other hand, less has been published on the velocity distribution and the flow structure within the liquid films, underneath the film surface. This can be related to the relative difficulty of these measurements caused by: (i) the extremely restricted measurement space, due to the small thickness of the liquid films (in the order of and often sub-mm), (ii) the highly disturbed and intermittent nature of the gas–liquid interface, (iii) the entrainment of gas inside the liquid film and of liquid into the gas core, and (iv) the relatively high velocities of both the gas and liquid phases.
doc_bio_tags: O O B B O O O O O O O O O O O O O O B I I O O B O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O B B O O O O B I O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O B I I O O O O O O O O O O O O O O O O O B O B O O B O O O O B O O O O O O O O O O O O O O O O O O O O O B I O O B I I I I I O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O B I O O B I I O O B B O O B O O B O O O O O B I O O O B O B O
extractive_keyphrases: broadband interfacial waves downwards annular flow downwards annular flows entrainment of gas film flow structure within the liquid films gas gas–liquid interface high velocities interfacial topology interfacial waves liquid statistical analyses velocity distribution wave
abstractive_keyphrases: film surface gas core gas phase liquid film liquid film flows liquid films liquid film thickness mean film thickness quantification of the mean film thickness, wave amplitudes and liquid entrainment rates spatio/temporal measurement statistical analyses of this film thickness thickness of the liquid films understanding of the characteristic profiles, scales and dynamics of these interfacial waves wave amplitudes