An experimental investigation on the effect of viscosity on bubbles moving in horizontal and slightly inclined pipes

• The effect of viscosity on drift velocity is investigated.
• The evolution of the bubble drift velocity is monitored along the pipe.
• We show that a relevant reduction of the velocity occurs along the pipe.
• We tested two existing correlation for drift velocity with collected data.

The motion of long bubbles in horizontal and slightly inclined pipes is a thoroughly investigated phenomenon. However, few results are available in literature for the drift velocity of a gas bubble in viscous liquids.The aim of this study is to experimentally analyse the effect of viscosity on the drift velocity of an air bubble in horizontal and inclined pipes. Two different measurement techniques are used: five capacitance probes are placed along the pipe to monitor the velocity evolution and image analysis is used to measure the bubble front velocity at two different distances from the outlet. The results of the measurements show that for horizontal case, as opposite to Benjamin (1968), the bubble drift velocity is not constant while the front displaces along the pipe and, in general, the viscosity slows down the propagation rate of the bubble. Finally, two drift velocity closure equations are tested with collected data.