Session: 7.3 - Gas-Liquid flows

Paper Number: 170314

170314 - Inverse Bubble-Mass Cascade in Coalescence-Dominated Turbulent Flows 

Abstract: 

A novel regime in turbulent bubbly flows is investigated experimentally and numerically, in which bubble coalescence, rather than breakup, governs the evolution of the bubble population. At high turbulence intensities, if the bubbles are smaller than the Hinze scale, they will resist turbulent breakup, creating conditions in which coalescence dominates. Therefore, an inverse bubble-mass cascade emerges in which bubble mass is transferred from smaller to larger bubbles through turbulence-enhanced merging. 
 
Experiments are conducted for a duct flow, which is located downstream of a regenerative pump, offering extremely high turbulent flow with bulk Reynolds number of O(10^5)  with void fraction of 0.5–2%. High-speed imaging reveals a steady increase in bubble size in streamwise direction, driven primarily by coalescence. The direct numerical simulations (DNS) of high-intensity homogeneous isotropic turbulence (HIT) at Taylor-scale Reynolds numbers of Re_λ = 350, 450, and 550 were performed using the accurate conservative diffuse-interface approach to capture the bubble dynamics. The results from both DNS and experiments indicate a robust scaling behavior: the maximum bubble diameter grows as D_max(t) ~ (ε^{1/3} t)^3/2, and the bubble size distribution [f(d)] evolves toward a self-similar power-law form, such that f(d)~d^{−3/2}. 
 
The DNS-based scale-by-scale energy flux analysis corresponding with available theoretical predictions suggest that the bubble coalescence rate scales as Rc(d)~d^{7/3}. This scaling implies that larger bubbles grow disproportionately faster, especially in regions where turbulence decays downstream. As turbulent kinetic energy dissipates, breakup remains negligible; coalescence becomes increasingly dominant and further reinforces the upscale transfer of bubble mass to larger sizes. These findings provide a unified theoretical, computational, and experimental validation of a coalescence-dominated regime in turbulent bubbly flows. The results challenge the traditional Kolmogorov–Hinze breakup paradigm and establish a mechanistic foundation for modeling bubble population evolution in multiphase flows, where breakup is intentionally suppressed. Implications extend to industrial processes such as papermaking, froth flotation, and multiphase mixing, in which controlling bubble size is critical. 

Presenting Author: VIVEK KUMAR Georgia Institute of Technology

Presenting Author Biography: Vivek is a Ph.D. student in Mechanical Engineering at Georgia Tech, working with Dr. Cyrus Aidun and Dr. Suhas Jain on multiphase fluid dynamics, particularly bubble coalescence and breakup in turbulent flows. He recently presented at ASME 2024 in California and will attend APS 2024 in Salt Lake City. Vivek completed his fully funded Master’s at the University of Alberta, Canada, publishing five journal papers. Prior to academia, he worked at Robert Bosch GmbH in India and Germany. An IIT Gandhinagar graduate, he is also a recipient of the MITACS, TMF, and MCM fellowships, with a strong interdisciplinary research background.