Bubble transport characteristics and their influencing factors in micro-nano aeration drip irrigation system

The transport characteristics of water, air, and oxygen are unclear in drip irrigation systems with micro-nano bubble water, limiting its application in the field of agriculture. This study improved the method of collecting and measuring water and air flowing from an emitter on a drip irrigation system with micro-nano bubble water. The flow rate of water and air was calculated for the emitter at the beginning, middle, and end of the drip tubing. In addition, the dissolved oxygen concentration of the same emitter samples was measured. Then the transport characteristics of water, air, and dissolved oxygen along the drip tubing were analyzed. The uniformity of water, air, and dissolved oxygen for the whole pipe system was calculated. The influence of air intake rate (set by micro nano bubble machine) and system operating pressure on the transport characteristics and uniformity of water, air, and dissolved oxygen was analyzed. The results showed that the micro-nano bubbles had no significant effects on the water flow rate and uniformity (P<0.05), with the average water flow rate and uniformity remaining not less than 1.97 L/h (rated flow of 2.0 L/h) and 96.02%, respectively. The influence of air intake rate and operation pressure on the average air flow rate reached a significant level (P<0.05), as well as the interaction between other factors and operation pressure, while the influence of emitter position on the average outflow flow rate did not reach a significant level (P<0.05).Within a moderate operating pressure range (0.10-0.18 MPa), the average airflow rate and uniformity ranged from 0.13-0.23 L/h and 85.50%-92.41%, respectively, increasing and decreasing with the air intake rate increasing. At high or low operating pressures (0.22, 0.06 MPa), individual emitters in the middle of the drip pipe had abnormally high air flow rates, greatly reducing the air flow uniformity while increasing the average air flow rate of the drip irrigation system. The dissolved oxygen concentration of the drip irrigation system with the micro-nano bubble water was greatly higher than that of tap water. Its mean value increased a little bit along the drip tubing. And there was a single-peak variation trend with the increase in the air intake rate. The drip emitter position, air intake rate, operation pressure, and the interaction between the latter two had significant effects on the mean value of dissolved oxygen concentration for the whole pipe system(P<0.05). When an air intake rate of 1.5 L/min was combined with an operation pressure of 0.14 MPa, the average dissolved oxygen of the drip irrigation system reached the highest of 14.45 mg/L. Taking into account the transport effects of water flow, airflow, and dissolved oxygen, the optimal operating parameters for a drip irrigation system with micro-nano bubble water were determined to be an air intake rate of 1.5 L/min and an operation pressure of 0.14 MPa. The suitable operation pressure range was 0.10-0.18 MPa and the air intake rate range was 1.5-2.4 L/min according to the airflow uniformity>85% and the average dissolved oxygen>12 mg/L. This study provides a theoretical basis for the scientific operation of drip irrigation systems with micro-nano bubble water.