Bubble Formation, Diameter, and Coalescence Mechanism and Motion Trajectory of Microporous Aeration in Non-Newtonian Fluids

The diameter and concentration of bubbles in non-Newtonian fluids are of great significance for gas–liquid mass transfer. This work combines high-speed visual measurement and VOF-CFD simulation. 3D structured meshes, the pressure-implicit with splitting of operators (PISO) and the continuum surface force (CSF) models were adopted to simulate the full bubble lifecycle. Multivariate dimensionless power-law correlations for bubble formation time and equilibrium diameter were established to quantitatively evaluate the coupled influences of aeration pore diameter, inlet gas velocity, fluid viscosity and surface tension. Results indicate that aeration pore diameter (Di), gas velocity, transmembrane pressure, fluid viscosity (μ) and surface tension (σ) affect bubble formation time and diameter, with Di as the dominant factor. The bubble-growth resistance coefficient is inversely proportional to Di and increases with μ. Single-bubble ascent is mainly controlled by bubble size, followed by fluid concentration and σ. In static liquid, pore spacing, Di and the gas flow rate (Q) dominate bubble coalescence by regulating inter-bubble interference, bubble size and surrounding flow field, respectively. At a fixed spacing of 3.0 Di, Sauter mean diameter (d32) is 2.07, 2.61 and 3.12 mm for Di of 50, 200 and 500 μm. For Di = 200 μm, d32 reaches 1.72, 2.07 and 2.49 mm at spacings of 1.5 Di, 3.0 Di and 5.0 Di; under Q of 20, 60 and 100 mL·min−1, d32 is 2.07, 3.24 and 2.32 mm. This work reveals cross-scale rheological effects of shear-thinning fluid on single-bubble dynamics and multi-bubble coalescence and elucidates the underlying mechanisms of gas–liquid mixing in shear-thinning power-law fermentation media, thereby contributing to the optimization of mass transfer in bioreactors.