Abstract
The interfacial wave structure of the liquid film in both upward and downward annular gas-liquid flows in an 11.7 mm pipe were investigated using the Brightness Based Laser Induced Fluorescence technique (BBLIF). Film thickness measurements were carried out with high spatial and temporal resolution between 330 and 430 mm from the inlet, where the properties of disturbance waves are almost stabilised. Using a tracking algorithm to detect disturbance waves, a full characterisation in terms of their velocity, frequency, longitudinal size and spacing was carried out. Direct comparison between both flow orientations while testing the same flow conditions shows that although the flow orientation does not affect the velocity of disturbance waves, the fraction of film surface occupied by the disturbance waves is smaller in upwards flow. Thus, more liquid travels in the base film in upwards flow, which is consistent with the base film thickness measurements. These observations, together with qualitatively different behaviour of ripple wave velocity in upwards and downwards flows, studied using 2D Fourier analysis, indicate that the role of gravity is much more important on the base film than on disturbance waves. This supposedly occurs due to a local decrease in the interfacial shear stress on the base film surface because of the resistance of the disturbance waves to the gas stream in upward flow.
Highlights
In two-phase gas-liquid flows, different flow regimes can be encountered depending on input parameters such as the flowrates and the properties of both phases, the dimensions of the duct and on the orientation of the flow
The regime of interest in this paper is the annular flow regime, which generally is observed at high gas flowrates and encountered in a wide range of industrial applications, such as transfer pipelines and risers present in the oil-and-gas industry; condensers and evaporators encountered in nuclear reactors and chemical reactors encountered in chemical processing plants [1]
The Brightness Based Laser Induced Fluorescence technique was used to acquire spatiotemporal records of local film thickness at axial distances that range between 330 and 430 mm (28–37 pipe diameters) from the inlet, where it was shown that the flow is close to a fully developed stage
Summary
In two-phase gas-liquid flows, different flow regimes can be encountered depending on input parameters such as the flowrates and the properties of both phases, the dimensions of the duct and on the orientation of the flow. Antipin et al [18] conducted measurements of time-averaged film thickness and pressure gradient in upward and downward orientations of the same pipe at the same flow parameters Both quantities were shown to be slightly higher in the upward configuration, though the difference decreased with increasing gas velocity. A quantitative study of the effect of flow orientation on the properties of disturbance waves using only the literature data would be complicated due to varying conditions – such as ranges of phases flow rates, pipe diameters, distance from the inlet, system pressure and physical properties of the phases – in the works of different authors To conduct such a study, a new cycle of experiments was performed in both upwards and downwards annular flow situations in the same pipe, at the same distance from the inlet, with the same gas and liquid flow rates. The results will be useful in the improvement of existing semi-analytical models, and to provide validation data for potential computational fluids dynamics (CFD) simulations that aim to predict such flows
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