Behavior characteristics of micro-bubbles along labyrinth channels under different inlet pressures of micro-nano aeration drip irrigation

Abstract: Behavior characteristics of micro-bubbles are still elusive during operation along the labyrinth channel in micro/nano aeration drip irrigation. This study aims to clarify the presence and movement state of micro-bubbles under various inlet pressures along the labyrinth channels. A Particle Tracking Velocimetry was used to snap the videos for some bubbles at five consecutive positions along the labyrinth channel under the inlet pressure of 0.02, 0.06, and 0.10MPa. Two half units were selected to capture the images along the labyrinth channel (labeled U and D), in order to determine the influence of buoyancy and gravity on the bubble behavior. A total of 10 observation windows were distributed on the labyrinth channel. The behavior characteristics of micro-bubbles were determined to clarify the influence of inlet pressure on the number, diameter distribution, average motion time, and average velocity of micro-bubbles in the labyrinth channel. The results show that the number of bubbles decreased continuously, and the rate of descent slowly decreased from the head to the end of the channel. There was no significant difference among the three inlet pressures at the end of the flow path. The proportion of bubbles with the larger diameter increased significantly, where the maximum and average diameter of micro-bubbles increased along the channel. The maximum bubble diameter appeared at the end of the channel, and then decreased with the increase of inlet pressure. The distribution range of velocity and trace length decreased at 10 observation positions, with the increase of bubble diameter. However, there was no significant change in the average speed and duration of bubble clusters during the shot section. The inlet pressure posed a great influence on the movement characteristics of bubbles in the channel. The number and the maximum diameter of bubbles decreased, as the inlet pressure increased. The average time of bubble swarm passed the channel of the shooting section was shortened, whereas, the average speed increased at the same position of the labyrinth channel. The bubbles with the higher velocity presented a shorter trace, whereas, those with the lower velocity was a longer trace for all the bubbles in the labyrinth channel under three inlet pressures. But, the maximum trace length and velocity of the bubble increased with the increase of inlet pressure. The inlet pressure was provided the minimum velocity (called the "base velocity") for the bubble swarm to pass through the labyrinth channel. The basic velocity increased, as the inlet pressure increased. Even if the traces of some bubbles were very long, the velocity was higher than the base velocity. Therefore, more gas was dissolved in water under the higher inlet pressure, while the undissolved gas presented in the form of micro-bubbles with a smaller diameter and faster speed. As such, better water vapor transportation was achieved in the system. Consequently, the inlet pressure can be as high as possible within 0.10 MPa during micro-nano drip irrigation, in order to effectively improve the uniformity of water and gas transportation for better aeration performance. Besides, there was no significant difference in the behavior characteristics of micro-bubbles at the two half units up and down connected along the labyrinth channel, indicating the very small influence of buoyancy and gravity on the bubble. The finding can provide theoretical support to improve the uniform aeration drip irrigation and gas transportation during operation in the micro-nano aeration drip irrigation system.