Experimental study on rising velocity of nitrogen bubbles in FC-72

In this work, the rising velocities of gas bubbles in a still liquid are measured and compared with available theories. In order to separate the mechanical effects from the thermal and mass exchange ones in bubble dynamics, adiabatic two-phase flow conditions were established by injecting gas (nitrogen) bubbles in a fluoroinert liquid (FC-72) at ambient temperature and pressure through an orifice (about 0.1 mm diameter) drilled on a generatrix of a horizontal tube. Bubble size, aspect ratio, detachment frequency, velocity and frequency of shape oscillations were measured by processing of high speed video images (at 1500 fps). A sensible steady oscillation of velocity, with a amplitude up to 20% of the mean value, was evidenced after the initial acceleration region. This oscillation was well correlated with the one in aspect ratio, thus providing evidence of the separate influence of this last parameter on drag coefficient. Available correlations did not give fully satisfactory results in predicting the mean rising velocity, showing a general tendency to underprediction. Sensible wake effects were excluded. Finally, the frequency of shape oscillation and the mean aspect ratio were compared with available models, evidencing their limitations. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.