Freon Bubble Rise Measurements in a Vertical Rectangular Duct

摘要

A photographic technique was used to obtain the terminal velocity and shape of isolated Freon-114 bubbles in a 3.3 X 15.24 cm rectangular test section. A 120 mm still camera equipped with a high magnification lens faced the test section, while a 200 W strobe lamp flashing at 200 Hz illuminated the bubbles from the opposite side. This technique allowed the bubble diameter and velocity to be obtained from a single photograph. The bubble rise data were taken over a bubble Reynolds number range of 52-1300 for zero liquid flow. At Reynolds numbers less than 118, the bubble shape was spherical. As the Reynolds number increased, the pressure difference between the front and side surfaces of the bubble increased. This caused the aspect ratio (defined as the ratio of bubble height to width) to drop until a relatively constant value of 0.35 was obtained. The drag coefficient was calculated from the measured values of bubble size and velocity using an expression derived from a force balance on a single rising bubble. This analysis indicates that the drag coefficient varies between 0.12 and 1.69 for the given Reynolds numbers range. Comparisons of bubble velocity and shape data to available correlations show that Freon bubbles behave similarly to bubbles rising in pure liquid. The rise velocity data exhibits a peak of 21.4 cm/s for bubble diameters near 0.42 mm; this peaking is a general characteristic of pure substances.