Correlation of aspect ratio and drag coefficient for hydrate-film-covered methane bubbles in water

Understanding the law of hydrate-film-covered methane bubbles (MHF-bubbles) rise is a prerequisite for predicting the formation and plugging of hydrate formation in deep water wellbore under bubble flow conditions. The aspect ratio and the drag coefficient are two key parameters for predicting the rising velocity of MHF-bubbles. In this work, an experimental facility is built to simulated deep-water wellbore environment. To effectively process the experimental data, a theoretical model for the rising velocity of a single MHF-bubble was established. The motion and deformation of single MHF-bubbles rising were investigated by injecting methane bubbles into water at 6 MPa and 4 °C. Diameters in vertical and horizontal axes of MHF-bubble, bubble size were recorded by high-speed photography. An image processing software was used to measure aspect ratio and terminal rising velocity. The results showed that due to the formation of the hydrate film the MHF-bubble is less likely to deform, and its aspect ratio remains relatively stable during ascent and decreases as the equivalent diameter increases. The drag coefficient decreases in the region where the equivalent diameter of the bubble is less than 1.5 mm and increase along with equivalent diameter in the range 1.5-7 mm. The experimental results of drag coefficient and aspect ratio were compared with correlations available in literature. The comparison showed that these correlations do not give fully satisfactory results in predicting the drag coefficient and aspect ratio of MHF-bubble rising in water. As a result, two new empirical correlations were proposed to correlate the aspect ratio as a function of Eötvös number (Eo) and the drag coefficient as a function of Morton number (Mo), Eötvös number (Eo), Reynolds number (Re). It was found that the values predicted by new empirical correlations are generally in good agreement with the experiment results. The results of this study have great significance to research into the problem of natural gas-liquid multiphase flow under high pressure and low temperature.