Experimental investigation of Taylor bubble acceleration mechanism in slug flow

Measurements of the total drag force on a solid Taylor bubble placed in a liquid flowing downward in a vertical tube have been performed. Variations in the drag force with the solid bubble displacement from the tube axis were studied in single bubble and dual bubble systems. The former arrangement included measurements for 7.5 cm and 15 cm long bubbles with normal and deformed shape noses. The latter experiment has been performed for different separation distances between the two 15 cm long, normal nose bubbles.; The experimental results showed a significant decrease in the drag force with bubble displacement from the tube axis for both 7.5 cm and 15 cm long bubbles (larger for the latter one). A comparison between the drag forces for the two bubble nose shapes showed lower values for the deformed nose than the normal nose bubble. The difference was obviously greater in the case of shorter bubbles.; In the two bubble system, the drag force measured on a trailing bubble at the center of the pipe decreased gradually for separation distances smaller than two or three pipe diameters, while outside this region it remained essentially constant. Further experiments to study the drag force variation with the bubble radial displacement for separation distances greater than two or three pipe diameters indicated similar values and variations as in single bubble experiments.; Experimental observations suggest that a new mechanism regarding Taylor bubble acceleration at separation distances smaller than a critical value should be considered in order to better understand the hydrodynamic phenomena preceding Taylor bubble coalescence.