Bubbles in curved tube flows - An experimental study

The heat transfer of boiling flows in helically-coiled tubes has significant relation with the bubble dynamics in the curved flows. To understand the bubble dynamics in curved flows, we experimentally study air bubbles in water flows in a coiled tube, which is approximately considered as a toroidal tube. The torus is perpendicular to the ground. Air bubbles are generated by injecting constant air flow at varied locations of the torus. Focus is put on the relationship of the bubble departure and traveling path with the water flow and injection location. The force analysis includes the gravitational force, the drag forces by the main and secondary flows, and the centrifugal force. Injection at the outer side of the torus shows higher bubble departure rate than the inner side, which is caused by the higher drag force of main flow at the outer side. The displacement of bubble traveling path between the inner and outer sides is mainly affected by the gravitational force for slow curved flows, and is dominated by the centrifugal force for fast curved flows. (C) 2016 Elsevier Ltd. All rights reserved.