16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012 Analysis of spatial and temporal spectra of liquid film surface in annular gas-liquid flow Sergey Alekseenko 1,2 , Andrey Cherdantsev* 1,2 , Oksana Heinz 1 , Sergey Kharlamov 1,2 , Dmitriy Markovich 1,2 1 Kutateladze Institute of Thermophysics, Lavrentiev ave. 1, Novosibirsk, 630090, Russian Federation 2 Novosibirsk State University, Pirogov str. 2, Novosibirsk, 630090, Russian Federation * correspondent author: cherdantsev@itp.nsc.ru Abstract The results of spectral analysis of wavy structure of liquid film in annular gas-liquid flow without entrainment are presented. LIF-technique allows us to perform such analysis in both spatial and temporal domains. Power spectra in both domains are characterized by one-humped shape with long exponential tail. Frequency of maximum power in temporal domain shows parabolic dependence on gas velocity Vg; corresponding spatial frequency shows linear dependence on Vg. Influence of liquid viscosity, liquid Reynolds number and pipe diameter on the main frequency was also investigated. The similarity of power spectra at different gas velocities was observed. Power spectra, linearly normalized along vertical axis by total power, and linearly normalized along horizontal axis by frequency of maximum power, show identical shape independently on gas velocity. Deviations from this universal shape become essential at low gas velocities, where gravity effect is not negligible in comparison to the shear stress. Thus, wavy structure of liquid film in annular flow at high gas velocities is characterized by the universal spectrum. Additional investigation was performed to obtain the frequency of generation of the secondary waves by the primary waves. Combination of spectral analysis and automatic algorithm of characteristic lines identification allowed to measure the generation frequency in the reference system, moving with the primary wave. 1. Introduction Annular gas-liquid flow represents flow of liquid film along channel walls and gas flow along the central part of the channel. Complicated wavy structure appears at the surface of liquid film under the action of gas shear. When liquid flow rate is high enough, interaction of large-scale disturbance waves and small-scale ripples leads to entrainment of liquid from film surface into the gas core (Woodmansee & Hanratty 1969). Using high-speed laser-induced fluorescence (LIF) technique, it was recently shown (Alekseenko et al. 2008) that all the ripples are generated at the back slopes of disturbance waves. Part of the ripples moves slower than disturbance waves and travel along the base film between disturbance waves, until being absorbed by the following disturbance wave. The other part moves faster than disturbance waves, travelling over disturbance wave until being scattered into tiny droplets by the gas shear, contributing to entrainment. When the liquid flow rate is low and no entrainment occurs, no disturbance waves were observed by earlier investigators. On that reason it was concluded that only ripples exist in flow regimes without entrainment. In some cases ripples in regimes with and without entrainment were considered as waves of the same type, characterized by the same properties (e.g., Belt et al. (2010)). In Alekseenko et al. (2009) it was shown that two types of waves exist in flow regimes without entrainment: fast long-living primary waves generate slower short-living secondary on their back slopes, similar to the behavior of disturbance waves and slow ripples in regimes with entrainment. The difference in comparison to the regimes with entrainment consists in absence of fast secondary waves, and, consequently, absence of entrainment. Figure 1 shows evolution of local film thickness (directly proportional to the local brightness of the image) with time (vertical axis) and longitudinal distance (horizontal axis). Working liquid is water- glycerol solution with kinematic viscosity ν=1.9*10 -6 m 2 /s, liquid Reynolds number Re=40 (Re is defined as Re=q/πdν, where q is volumetric liquid flow rate and d is inner diameter of the pipe). - 1 -