Bubble characteristics and structure optimization of the swirling micro and nano bubble generator for aerated drip irrigation

Aerated subsurface drip irrigation (ASDI) can improve soil aeration, crop yield, water, and fertilizer use efficiency. In this study, a micronano-bubble generator was designed to stabilize the gas in water, according to the cyclonic shear fragmentation. A gas-liquid mixing flow was also introduced from the inlet runner tangent to the inner cavity. The ellipsoidal inner cavity was used to force the liquid into the cyclone and then spiral outward at the two ends of the spouting holes. As such, a shear effect was formed to crush the bubbles. A 4-factor and 3-level orthogonal test L9 (34) was designed to explore the effects of structural parameters on the generated bubbles and the performance of dissolved oxygen. The following specifications were: the major axis (40, 60, and 80 mm), the minor axis (30, 45, and 60 mm), the inlet diameters (8, 12, and 16 mm), and the spray holes diameter (2, 3, and 4 mm). High-speed photography, image processing, and nano-particle size analysis techniques were used to extract the features, such as the number, particle size, and uniformity of micro-nano-bubbles, in order to monitor the dissolved oxygen in water over time. The results show that the distribution of particle size was basically unchanged in the large bubbles from 60 to 250 μm with the operation of the device, while the average size of the small bubbles from 10 to 60 μm increased by 20% within 2 min of operation. Specifically, the large bubbles above 100 μm were basically dissipated, after the device was stopped for 1 min. Then the count of small bubbles below 60 μm in the water was reduced by about 70% after 3 min of stopping operation. The micro-nano-bubbles with diameters of less than 30 μm were finally suspended in the water. In addition, the performance of the device showed that the bubble generation during the operation depended the most significantly on the spray hole diameter and minor axis. The major axis shared the greatest effect on the average particle size and the uniformity of large bubbles above 60 μm in the water after operation, as well as on the number of small bubbles. The inlet diameter still reached a significant level, although there was the least effect on the number of microbubbles both during and after the operation. In addition, the nanoscale and microscale bubble generation behaved in the same way. Dissolved oxygen tests showed that the oxygen content in the water was divided into three stages: the oxygen enhancement during operation, the attenuation, and stabilization of dissolved oxygen. The inlet diameter dominated the increase and decay of dissolved oxygen. While the stabilized dissolved oxygen depended only significantly on the minor and major axes. The dissolved oxygen of the device was positively correlated with the bubble generation. The more the number of bubbles generated, the smaller and more uniform particle sizes were, and the better the performance of dissolved oxygen was. The dissolved oxygen was varied in water in the different stages, where the bubble indicators were generally consistent with the structural dimensions. Taking the high number of bubbles, small particle size, uniformity, and high dissolved oxygen as the objectives, the optimal combination of parameters was achieved in the 80 mm major axis, 45 mm minor axis, 12 mm inlet diameter, and 3 mm spray hole diameter. The finding can provide the equipment support to promote the application of ASDI.