Evolution of Views on the Wavy Structure of a Liquid Film in Annular Dispersed Gas-Liquid Flow
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EVOLUTION OF VIEWS ON THE WAVY STRUCTURE OF A LIQUID FILM IN ANNULAR DISPERSED GAS–LIQUID FLOW A. V. Cherdantsev∗ and D. M. Markovich
UDC 532.5
Abstract: The wavy structure of a liquid film sheared by high-velocity gas flow with droplet entrainment from the film surface has been investigated. The paper presents an analysis of methods for measuring the local fluid film thickness and a review of classical concepts of the wave hydrodynamics of annular dispersed flow obtained in early studies using rather crude methods. A systematic description of the structure, evolution, and interaction of waves of various types is given based on an analysis of high-resolution spatio–temporal records of film thickness obtained by laser-induced fluorescence. The results are compared with the results of recent experiments, and the necessity of revising some classical concepts is shown. Directions for further research are suggested. Keywords: annular dispersed flow, disturbance waves, ripple waves, droplet entrainment, wave interaction, field measurements of liquid film thickness, laser-induced fluorescence. DOI: 10.1134/S0021894420030037
INTRODUCTION In annular dispersed gas–liquid flow, liquid propagates along the channel walls in the form of a film sheared by high-velocity gas flow in the channel core. At sufficiently high flow rates of the liquid and gas phases, droplets are entrained from the film surface and carried away into the gas-flow core. Annular dispersed flow takes place in a wide range of applications in the atomic, oil and gas, and chemical industries. It occurs when a gas–liquid mixture with a high gas content is supplied to a channel or when a high gas content is achieved due to phase transformations. A feature of this flow is the predominant effect of gas shear friction compared to gravity, resulting in the annular dispersed regime occurring for any flow orientation. Preceding flow regimes are downward liquid film flow, stratified flow in horizontal ducts, and intermittent upward flow. In the absence of intense heat flow, this regime is the last stage of flow development as the gas content increases, and in the presence of heating, the annular dispersed regime precedes the appearance of dry spots on the channel walls. The study of flow integral characteristics, such as average film thickness, dispersed phase flow rate, pressure drop in the channel, and heat transfer coefficient, is of greatest practical interest. The differential pressure and heat transfer rate in dispersed-annular flow can be an order of magnitude higher than those in single-phase flows. In addition, this regime is characterized by a large interfacial area, including the disturbed surface of the liquid film, the surface of droplets in the gas flow, and the surface of gas bubbles entrained in the liquid film. The interaction between turbulent gas flow and initially laminar liquid film occurs by the formation of waves on the liquid film surface. The wavy structure of the liquid film consists of several types of continuously interacting waves with substantially different scales.
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