Thermodynamic nonequilibrium effects in bubble coalescence: A discrete Boltzmann study

Area of Science:

• Fluid dynamics
• Thermodynamics
• Statistical mechanics

Background:

• Bubble coalescence is a fundamental process in multiphase flow systems.
• Understanding the kinetics and nonequilibrium effects is crucial for predicting system behavior.

Purpose of the Study:

• To investigate thermodynamic nonequilibrium (TNE) effects during the coalescence of two static bubbles under thermal conditions.
• To analyze the spatial and temporal evolution of nonorganized momentum fluxes (NOMFs) and their relationship with bubble morphology and kinetics.

Main Methods:

• Utilized a discrete Boltzmann model to simulate bubble coalescence.
• Employed density-weighted statistical methods to analyze NOMFs.
• Identified characteristic instants in the evolution of averaged NOMFs to define distinct stages of the nonequilibrium process.

Main Results:

• NOMFs exhibit antisymmetrical xx and yy components, with the xy component transitioning from a quadrupole to an octupole structure.
• Two characteristic instants in NOMFs evolution divide the process into three stages, correlating with maximum coalescence speed and bubble aspect ratio.
• TNE intensity, coalescence acceleration, and boundary length slope show high correlation and simultaneous maxima.
• Surface tension and heat conduction accelerate coalescence; viscosity delays it.
• Surface tension and viscosity increase global nonequilibrium intensity, while heat conduction decreases it.

Conclusions:

• The study provides detailed insights into the kinetics of bubble coalescence driven by TNE effects.
• NOMFs serve as a valuable indicator for characterizing the nonequilibrium nature of bubble coalescence.
• The findings highlight the interplay between thermodynamic forces, fluid properties, and bubble dynamics during coalescence.