Bubble formation from a free-standing tube in microgravity

The bubble characteristics and the bubble detachment mechanisms during injection of air from a free-standing capillary tube submerged in water were studied in microgravity. The experiments were conducted in the 2.2-s drop tower at the NASA Glenn Research Center. A tube, 0.51 mm in diameter and 150 mm long, in a rectangular test section ( long) served as the injector. Images of the bubbles during the drops were acquired using a high-speed camera for various gas flow rates. Bubble detachment was observed for all the Weber numbers tested (0.28-31.12). This observation was different from previous studies using plate orifices in microgravity when bubble detachment was observed only for Weber numbers larger than 8. In order to resolve these differences, experiments were carried out using plate orifices. It was found that the bubbles detached from the orifice for all Weber numbers and that the bubbles formed were larger than those formed with the tube injector, particularly at low gas flow rates. The availability of a large area for the bubble to anchor itself and the presence of the chamber underneath the orifice could cause these differences. The effects of the chamber volume on the unsteadiness of bubble formation in plate-orifices at low gas flow rates are discussed.