Bubble formation from a downward-pointing capillary nozzle was investigated in this study. The experiments were conducted at gas flow rate of 40-5400 mL/h and inner nozzle radius of 0.030-0.255 mm. Experimental results show that microbubbles were formed continuously at moderate Weber number, which was not reported in pervious investigations with injecting gas through an upward-pointing capillary nozzle. High-speed visualization indicates that the formation of microbubbles arises from the convergence of the capillary waves induced by the partial coalescence of larger bubbles. A bubbling regime map is given to identify the critical conditions for the formation of microbubbles. In the present air-water experiments, the generated microbubbles are 20-170 μm in diameter. From experimental data, a scaling law for microbubble size is proposed as a function of Weber and Bond numbers.

This study used a Discrete Element Method (DEM) model to investigate the Jenike shear process of flexible, cylindrical particles with different aspect ratios. The model was validated through experiments and analytical solutions. It was found that particle shape and deformation have a significant impact on friction behavior, affecting particle deformation, contact forces, orientation, and internal friction angle. Results indicate an increase in shear stress with normal load, regardless of particle stiffness or shape. Flexible particles showed higher shear stress and internal friction angles than rigid ones, especially for aspect ratios of 6. With aspect ratios of 4 and 5, flexible particles deform significantly during shear with complex reconfiguration, while aspect ratio 6 particles experience a uniform reconfiguration, indicating a solid packing structure that enhances flow resistance. These findings will aid in improving kinetic theories for granular flow of complex irregular particle flows.

It is in urgent requirement to directly measure the droplet coalescence frequency for the application of the population balance model. In this study, a method was firstly developed to directly measure the droplet coalescence frequency in turbulent flow field by using a specially designed mixing tank and a high-speed camera. The effects of the rotating speed and holdup fraction on the droplet coalescence frequency was quantitatively investigated. The increasing of rotation speed promotes first and then inhibits the coalescence, while the holdup fraction has little influence on the coalescence frequency function. The droplet collision frequency was also counted and the coalescence efficiency was calculated. The models in literature were tested with our experimental data and were found failing to predict the coalescence frequency in the stirring tank. Empirical correlations were finally proposed and good agreement was found between the prediction results and the experimental data.